DIAPHRAGM

The separation of the pleural and peritoneal cavities is effected by development of the diaphragm, which forms from a portion of the septum transversum mesenchyme above the developing liver (Fig. 59.2A,B). The septum transversum is a population of mesenchymal cells which arises from the coelomic wall of the caudal part of the pericardial cavity. As the population proliferates, it forms a condensation of mesenchyme, caudal to the pericardial cavity and extending from the ventral and lateral regions of the body wall to the foregut. Dorsal to it on each side are the relatively narrow pleuroperitoneal canals. The endodermal hepatic bud grows into the caudal part of the septum transversum, whereas the cranial portion will form the diaphragm.

The oesophagus and stomach are medial to the pleuroperitoneal canals. As development proceeds, the lower portion of the oesophagus inclines ventrally anterior to the descending thoracic aorta. Although the oesophagus has no true ventral or dorsal mesentery, in descriptions of diaphragmatic development the portion of mesenchyme between the oesophagus and aorta at the level of the forming diaphragm is often homologized with part of a dorsal meso-oesophagus. The pleuroperitoneal membranes, which remain small, are dorsolateral to the canals, and the mesonephric ridges, suprarenal glands and gonads are dorsal. Just as the enlargement of the pleural cavity cranially and ventrally is effected by a process of burrowing into the body wall, so is its caudodorsal enlargement. The expanding pleural cavities extend into the mesenchyme dorsal to the suprarenal glands, the gonads and (degenerating) mesonephric ridges. Thus somatopleuric mesenchyme is peeled off the dorsal body wall to form a substantial portion of the dorsolumbar part of the diaphragm (Fig. 59.3C). The pleuroperitoneal canal is closed by the fusion of its edges, which are carried together from posterolaterally to anteromedially by growth of the organs surrounding it, in particular by growth of the suprarenal gland. The right pleuroperitoneal canal closes earlier than the left, which presumably explains why an abnormal communication persisting between the pleural and peritoneal cavities is more frequently encountered on the left.

While these changes are occurring, the septum transversum undergoes a progressive alteration in relative position. The dorsal border of the septum transversum which initially lies opposite the second cervical segment, migrates caudally as the embryo grows and the heart enlarges. At first the ventral border moves more rapidly than the dorsal, but after the embryo has attained a length of 5 mm, the dorsal border migrates more rapidly. When the dorsal border of the septum transversum lies opposite the fourth cervical segment, the phrenic nerve (C3, 4 and 5) and portions of the corresponding myotomes, grow into it and accompany it in its later migrations. The dorsal border of the septum transversum does not come to lie opposite the last thoracic and first lumbar segments, the final position occupied by some of the dorsal attachments of the diaphragm, until the end of the second month. The main derivatives of the central part of the diaphragm lie at considerably more cranial levels.

The diaphragm is therefore composed of the dorsal mesooesophagus and paired pleuroperitoneal membranes (posteriorly); septum transversum mesenchyme (ventrally); and excavated body wall (posteriorly and laterally).

OESOPHAGUS

The development of the oesophagus is described in Chapters 35 and 73. Failure of separation of the oesophagus and trachea is described on page 1036. Oesophageal atresia and tracheo-oesophageal fistula may present antenatally with polyhydramnios due to failure of fetal swallowing, and choking and inability to swallow saliva in the neonatal period.

CELLS THAT GIVE RISE TO THE HEART

The heart is formed from tissues derived from the midline splanchnopleuric coelomic epithelium with later contributions from neural crest mesenchyme. The splanchnopleuric coelomic epithelium gives rise to the myocardium, including the conduction system of the heart, and the endocardium, including its derived cardiac mesenchymal population which produces the valvular tissues of the heart. Splanchnopleuric coelomic epithelium is also the source of the epicardium, coronary arteries and interstitial fibroblasts.

Primitive cardiac myocytes can first be seen in embryos at stage 9. During the onset of neurulation and somitogenesis, the intraembryonic coelom forms across the midline, initially above the endoderm, in a horse shoe-shaped area termed the cardiac crescent (Fig. 59.4). As the head fold emerges, the coelom undergoes a reversal, so that the future pericardial cavity comes to lie ventral to the endodermal foregut (Figs 59.1, 59.5). The splanchnopleuric wall of the pericardial coelom, subjacent to the endoderm, provides a germinal epithelium that produces early cardiac myocytes. It is characterized by the expression of myocardial specific markers, such as cardiac myosin heavy chain. This initial origin of myocardial cells is termed the primary heart-forming field, distinguishing it from later myocardial additions from mesenchyme localized central and peripheral to the cardiac crescent on the embryonic disc, termed the second heart-forming field (Fig. 59.4). It is not yet known whether these two sources of myocardium represent distinct myocardial lineages, or a single part which develops into separate components later on.

Fig. 59.4 A, The early embryonic disc viewed from a dorsolateral aspect showing the position of the primary heart-forming field relative to the surface ectoderm of the trilaminar disc; B, The rostral end of the unfolded embryonic disc from a dorsal aspect showing the position of the mesenchymal populations which will give rise to the primary heart-forming field, septum transversum mesenchyme and second heart-forming field.

Fig. 59.5 A, Median section through part of the embryonic disc to show the position of the future pericardial cavity before head folding. B, Median section through the cranial end of a human embryo during early head folding showing its reversal effect on the position of the pericardium. C, Median section through the cranial end of a human embryo after head folding showing the pericardial cavity and endothelial heart tube now ventral to the foregut. D, Horizontal section through the pericardium and endothelial heart tube shown in C. The position of the dorsal mesocardium which is a source of secondary heart field mesenchyme is indicated.

The endocardium also develops during stage 9 from the coelomic splanchnopleuric epithelium. Cells arise singly close to the ventro-lateral edges of the cranial intestinal portal and form an endocardial plexus between the splanchnopleuric coelomic epithelium and the foregut endoderm. These groups of cells are now termed angioblastic mesenchyme; they are amongst the earliest intraembryonic vascular precursors to appear and express markers for the endothelial cell lineage. The cells aggregate to form an epithelium, the endocardium, which encloses small cavities which coalesce in the vicinity of the developing foregut to establish bilateral, hollow endocardial tubes. The latter fuse across the midline progressively, commencing at the outflow tract, or arterial pole, and extending to the inflow tract, or venous pole (Fig. 59.5). By stage 10 a single endocardial tube is present and is almost completely surrounded by myocardial cells. This arrangement of an outer myocardial sleeve containing an inner endocardial tube constitutes the primary heart tube. The inner endocardial epithelium induces the myocardial cells to synthesize specific extracellular matrix proteins which form a fine extracellular reticulum that holds the endocardial tube apart from the developing myocardium. Close to the foregut endoderm, the myocardial cells at the reflections of the pericardial splanchnopleuric epithelium form the dorsal mesocardium, which may stabilize the developing endothelium and promote the fusion of the bilateral endocardial tubes. The dorsal mesocardium encompasses a mesenchymal population specifically referred to as mediastinal mesenchyme, and is contiguous with the splanchnopleuric mesenchyme surrounding the embryonic foregut.

Much later in heart development, neural crest cells, arising from the region between the otic vesicle and the caudal limit of somite three, grow into the outflow tract of the heart. They are believed to play a role in the spatio-temporal regulation of the division of the outflow tract into the aortic and pulmonary pathways, and the development of the muscular sub-pulmonary infundibulum.

The epicardium, sometimes included in descriptions of the myocardium as ‘epimyocardium’, is not present at the early stages of heart development (but see p. 1027).

HEART TUBE

During the process of embryonic folding, the midline splanchopleuric coelomic epithelium, which is derived from the primary heart field (cardiac crescent) becomes positioned ventral to the foregut. The splanchopleuric epithelium is proliferative and gives rise to mesenchymal populations and particularly early cardiac myocytes which arise from the coelomic wall adjacent to the endoderm of the foregut (Fig. 59.5). After folding, the second heart field contributes cells to both the arterial and venous poles of the heart. It has been suggested that the secondary heart field may contribute cells only to those cardiac components that are required for the pulmonary circulation, namely the right ventricle and outflow tract at the arterial pole, and the atrial septum and the dorsal atrial wall at the venous pole. Extending this suggestion, the original primary heart-forming field would give rise to those components that are required for the systemic circulation, namely the systemic venous sinus and its tributaries, the initial atrium, the left ventricle, and the arterial conus, as seen in the outflow tract of primitive fishes, although this latter structure has no homologue in mammalian hearts.

During the process of folding, the pericardial cavity and concomitantly the myocardium, gradually extend around the forming endocardial tube, leaving the dorsal mesocardium, a transient connection that is analogous to the mesentery of the intestines (Fig. 59.5C,D). The persisting stalk of the dorsal mesocardium connects the venous pole of the heart with the splanchnopleuric mesenchyme around the developing lung buds and with the septum transversum mesenchyme, which will give rise to the liver. The dorsal mesocardium is the site of early mediastinal mesenchyme production. It disappears as a mesenteric entity during the third week of development, when the embryo has from 4 to 12 somites; at the same time, the endocardial heart tube becomes entirely surrounded by the myocardium, and enclosed within the pericardial cavity. The breakdown of the dorsal mesocardium establishes a passage across the pericardial cavity, from side to side dorsal to the heart, which persists as the transverse sinus of the pericardium.

At this stage, the heart is bilaterally symmetrical and has the form of an inverted ‘Y’. The legs of the Y are positioned caudal to each side of the cranial intestinal portal, where they form the venous pole of the heart (59.6A,B). The stem of the Y connects cranially with the great arteries, forming the arterial pole of the heart. From the outset, both poles are connected with the systemic vessels running from the embryo, the yolk sac, and the placenta. The pulmonary veins form later, during the fifth week of development, the initial venous primordium becoming evident in the mesenchyme derived from the dorsal mesocardium caudally. Also at this site, mediastinal myocardium continues to be added to the heart, forming the smooth dorsal wall of the left atrium, and providing the site of formation of the primary atrial septum.

Recent fate maps have shown that the primary heart tube contains cell lines which will contribute to only two compartments. The myocardium of the stem of the inverted Y contains only precursor cells of the left ventricle, and both legs of the inverted Y contain precursor cells of the atrioventricular canal and parts of both atrial chambers. There is no evidence to support the previous, traditional concept of heart development, i.e. that all the putative cardiac segments are already present in the straight heart tube and develop to contribute to the various parts of the definitive cardiac chambers. This hypothesis has now been disproved by elegant experimental studies which have shown that the definitive cardiac chambers are formed by local differentiation and expansion, or ‘ballooning’, of the myocardial walls of the primary heart tube.

Looping of the heart tube

Cranial and caudal recruitment of myocardium and endocardium produces considerable lengthening of the primary heart tube. Recent studies of the mechanism of proliferation in the avian heart have shown that cardiac myocytes stop dividing after overt differentiation. The tube continues to elongate, largely as a result of the recruitment of additional myocytes to the tube, as shown by studies of both morphology and molecular lineages, and to a lesser extent because of an increase in the size of the myocytes which form the walls of the tube. The lengthening heart tube bends ventrally and rightwards, concomitant with the breakdown of the dorsal mesocardium. The bend is called the ventricular loop, since the left ventricle subsequently balloons and expands at its outer curvature, which is the original ventral side of the straight tube. The inner curvature of this loop was originally its dorsal side.

It is often stated that looping of the tube is the first visual evidence of asymmetry in the embryo, although careful examination reveals that the atrioventricular canal has become asymmetric prior to the start of looping. Although the sense of laterality of the developing organs of the body, including the atrial appendages, develops during gastrulation, the pathway of signalling that governs rightward looping of the heart tube remains unknown. However, it is now well established that signalling pathways including Pitx2, nodal, lefty, and cited-2, determine the formation of the morphologically left-sided or right-sided features seen in organs such as the lungs, the bronchial tree, the liver and spleen, and the atrial appendages.

By the time the primary heart tube has formed its ventricular loop, it is possible to distinguish the atrial and ventricular components of the developing heart (Figs 59.6D-G, 59.7A,B) and to recognize an outflow tract connected with the aortic sac and developing pharyngeal arch arteries. The cells which, at this stage, make up the outflow tract, will, at later stages, be found within the right ventricle. At the proximal side of the outflow tract, cells are recruited to the developing right ventricle and, contemporaneously, new cells are recruited to the outflow tract from the second heart-forming field. The atrial and ventricular components are separated one from another by the atrioventricular canal, which at this stage has significant length. The systemic venous tributaries drain directly to the atrial cavity.

Fig. 59.6 Local expansion of the myocardial wall of the early heart tube and heart tube looping A, ventral view; B, ventral wall removed to show inner aspect of the early heart tube; C, left lateral view. The early heart tube is shaped like an inverted ‘Y’. The legs of the Y pass caudal to each side of the cranial intestinal portal. In the early heart tube the myocardium of the stem of the Y (purple) contains only precursor cells for the left ventricle. The remaining myocardium (grey) contains only precursor cells for the atrioventricular canal and atria. The precursor left ventricular cells expand first. D–G: Later heart tube showing looping. D, Ventral view. E, Ventral wall removed to show inner aspect of the folding heart tube. F, Left lateral view. G, Lateral wall removed to show the inner aspect of the heart from the left. In D and E the expansion of the right ventricle can be seen and the heart tube has elongated to the right. The developing atrium can be seen dorsolaterally in D–G.

Fig. 59.7 A, SEM of early developing human heart showing external appearance of the myocardium of the right and left ventricles, and right and left atrial appendages. The truncus arteriosus passes between the atria. B, Reconstruction of heart tube at stage 13 showing the endocardium. At the left and right ventricles the endocardium interdigitates with the myocardium and appears trabeculated; elsewhere it is separated from the myocardium by the cardiac extracellular matrix. C, The heart at stage 13 with the truncus arteriosus reflected to the right. The opening of the sinus venosus into the right atrium can be seen (yellow). The region of the primary heart tube is indicated by a ring.

(A With permission from Oostra RJ, Steding G, Lamers WH, Moorman AFM. Steding’s and Virágh’s scanning electron microscopy atlas of the developing human heart. New York: Springer-Verlag, 2006. B Modified from O’Rahilly and Müller. Developmental Stages in Human Embryos 1987 Carnegie Institution of Washington. Pub 637.)

The sinus venosus with right and left sinus horns, which forms at the confluence of the systemic venous tributaries, is a prominent structure in lower vertebrates. In mammals, it does not become recognizable as a morphologic entity until the systemic veins have excavated themselves from the mesenchyme of the body and been surrounded by myocardium. It then becomes incorporated into the pericardial cavity so that the sinus horns become part of the heart. All of these processes occur up to the 25th day of development in humans: only after this stage is it possible to consider development of the definitive atrial and ventricular chambers.

INFLOW TRACT

The working myocardium of the atrium differentiates locally at the dorsolateral sides of the heart tube (59.6D–G). The developing atrium then expands enormously, in dorsal, lateral and, most prominently, in a cranial direction. The cranial expansion is seen as pouches which become the left and right atrial appendages (Fig. 59.7A,B). The floor of the atrium, including the sinus venosus, and the atrioventricular canal, are made of primary myocardium. Less cellular proliferation occurs in these parts than in the expanding atrial chambers: the primary myocardium marks the inflow to, and the outflow from, the initial atrial chambers as ‘rings’ which are also associated with the formation of the conduction system (see p. 1021). The ring at the inflow defines the sinu-atrial junction, whilst the atrioventricular canal, which forms the atrial outlet during development, will eventually be incorporated into the definitive right and left atrial chambers as the atrial vestibules. Although it is possible to recognize the forming left and right atrial appendages at this stage, the right being more extensive than the left, the atrium itself has a single cavity and there is no evidence of septum formation. The myocardium of the sinus venosus and the newly forming mediastinal myocardium are smooth-walled, whereas the myocardium of the appendages shows ridges, the pectinate muscles, on the inner surface. The formation of the different appendages is under control of the Pitx2 signalling pathway. The myocardium of the appendages has a chamber phenotype, or is working myocardium; it expresses atrial natriuretic factor and connexin40 among other markers.

Right atrium

The further development of the right atrium is characterized by the incorporation of the sinus venosus into the right part of the primary atrium. This process is under control of the T-box transcription factor Tbx18. At about 4 weeks of development, the sinuatrial junction of the looping primary heart tube is positioned symmetrically in the midline (Fig. 59.8; see also Figs 13.1B, 73.8). The left and right common cardinal veins drain directly into the cavity of the primary atrium. The atrial myocardium extends to the margins of the pericardial cavity, and strictly speaking the sinus venosus is not yet formed, because the systemic venous tributaries are embedded within the mesenchyme of the septum transversum.

Fig. 59.8 The changes to the sinus venosus. A, Dorsal view of the early heart tube shown in Fig. 59.6D. B, Changes to the circulation brings the venous circulation to the right. This causes enlargement of the right horn of the sinus venosus and atrium and attenuation of the left horn of the sinus venosus. C, The right horn of the sinus venosus becomes absorbed into the atrium. The left horn of the sinus venosus becomes the coronary sinus. D, Dorsal view of the embryonic heart showing the relative changes to the sinus venosus.

During subsequent development, the pericardial cavity expands to enclose the terminal segments of the systemic venous tributaries, and at the same time their walls differentiate as myocardium. They can now be termed the left and right horns of the sinus venosus; each horn receives the union of the corresponding umbilical vein, vitelline radicles, and common cardinal vein (Fig. 59.8; see also Fig. 73.8). Concomitantly, the constriction between the left horn and the atrium becomes more pronounced. As the dorsal wall of the left atrium is formed from additions of mediastinal myocardium, the left horn becomes incorporated into the developing left atrioventricular junction, its orifice draining to the newly formed right atrium. At the same time, the left-sided venous tributaries diminish in size; the left common cardinal vein forms the oblique vein of the left atrium, and the left sinus horn forms the coronary sinus (Fig. 59.8), maintaining its own myocardial wall as it becomes incorporated into the atrioventricular junction.

The right sinus horn increases rapidly in size with growth of the liver (see Figs 73.8, 73.9). The vitello-umbilical blood flow enters the right horn through a wide but short hepatocardiac channel, which becomes the cranial end of the inferior vena cava. The right horn also receives the right common cardinal vein, draining the blood from the right side of the body (Fig. 59.8B,C). Later, when transverse connections are established between the cardinal veins, the blood from the left side of the body also reaches the heart via the veins draining the right side (see Fig. 13.4). As these changes take place, the right sinus horn, including the proximal parts of the superior and inferior cardinal veins, becomes incorporated into the right atrium, forming the smooth-walled systemic venous sinus, also known as the sinus venarum.

The right sinus horn opens into the right atrium through its dorsal and caudal walls (Fig. 59.8; see Fig. 59.12). The sinuatrial orifice becomes elongated and slit-like, guarded by two muscular folds, the left and right sinuatrial (venous) valves. These two valves meet cranially and become continuous with a fold that projects from the atrial roof, the septum spurium. The valves also meet caudally, and merge with the inferior atrioventricular cushion. With ongoing development, the cranial part of the right sinu-atrial valve loses its fold-like form, but its position is indicated in the adult heart by the site of the crista terminalis of the right atrium. Its caudal part forms the valve of the coronary sinus, also known as the Thebesian valve, and most of the valve of the inferior vena cava (Eustachian valve). The union of the two valvular remnants then passes through the tissue which separates the orifice of the coronary sinus from the fossa ovalis. This area is known as the ‘sinus septum’, but in reality this ‘septum’ is no more than a muscular fold in the dorsal wall of the right atrium. The continuation of the venous valves persists as the tendon of Todaro, an important landmark to the location of the atrioventricular node in the definitive heart. The left venous valve blends with the right side of the atrial septum; there is usually no trace of it in the postnatal heart.

Fig. 59.12 Three representative stages in the development of the atrial septum. A–C, Coronal views from the front; D–F, lateral views from the right side. A and D, The septum primum has not yet obliterated the original communication between the two atria (foramen primum) and the atrioventricular endocardial cushions have not yet fused. B and E, the atrioventricular endocardial cushions have fused with each other and with the septum primum, which has broken down in its dorsal part to form the foramen secundum. C and F, The septum secundum is forming on the right side of the foramen secundum from an interatrial fold.

VENTRICLES

The ventricles develop at the ventral side of the looping primary heart tube during the fourth week of development (Figs 59.6, 59.7). The left ventricle develops from the stem of the Y-shaped heart, the right ventricle develops later, downstream relative to the left ventricle, when more myocardium has been added to the cardiac tube. As a result of the looping of the heart tube, the right ventricle is positioned at the right of the left ventricle, which is a prerequisite for the appropriate connection with the expanding atrial component of the heart. Unlike the atrial chambers, the morphological differences between the right and left ventricles are not part of the general asymmetry between the right- and left-sided organs of the body, but rather are under control of the signalling pathways which determine caudo-cranial differentiation. Retinoic acid, and its downstream transcription factor Tbx5, play a crucial role in this process.

The myocardium at the inner curvature, the original dorsal side of the cardiac tube, remains smooth-walled and maintains its molecular phenotype, whereas the myocardium at the outer curvature of the myocardial tube displays trabeculations in the fifth week of development (Fig. 59.7B). By stage 17, the trabeculations have achieved a typical spatial orientation, giving a sponge-like appearance to the internal aspect of both ventricles (Fig. 59.9). The definitive trabeculations, coarse in the right ventricle but much finer in the left, are first observed about the 40th day of gestation: they appear initially in the walls of both ventricles at the level of the atrioventricular junction and develop towards the apex of the heart. By the time the fetus is 10 weeks old, the trabeculations are much sparser, and are confined to the apical regions. This process of remodeling is accomplished without the intervention of macrophages or inflammatory cells in the immediate interstitium. The ventricular myocardium, encompassing the trabeculations and exterior wall, possesses a chamber phenotype, the myocytes expressing, among other proteins, the gap-junctional protein Cx40 and atrial natriuretic peptide. This myocardium stops proliferating and differentiates into the fast-conducting peripheral ventricular conduction system, whereas the outer layer becomes highly proliferative and forms the compact layer of the ventricular wall.

Fig. 59.9 SEM of human heart at 42 days’ gestation. The right lateral wall of the right atrium, right ventricle and the outflow tract have been removed. The valve of the sinus venosus (V) is prominent. There is no septal cusp of the tricuspid valve guarding the inlet portion of the right ventricle. The parietal cusp has been removed. The atrioventricular cushions are fused. Note the adjacent right lateral tubercles (O). A small interventricular foramen (secundum, see text) is indicated (small arrowhead). There is a well developed right ventricular outflow tract (curved arrow). E, embryonic trabeculae; P, pulmonary trunk

OUTFLOW TRACT

Initially the outflow tract possesses myocardial walls up to the pericardial reflections. It displays a characteristic bend, which has a proximal part arising from the developing right ventricle and a distal part which becomes continuous with the aortic sac beyond the pericardial reflections. Although this bend is a conspicuous morphologic feature, it is highly unlikely that the cells to either side of it retain their position during subsequent development. Instead, it seems probable that cells are recruited from the second heart-forming field, and pass through the outflow tract to contribute to the definitive right ventricle. Subsequently, still more cells are recruited from pharyngeal mesenchyme, passing proximal to the pericardial reflections and forming the intrapericardial components of the arterial trunks. There remains much disagreement as how best to describe these morphological parts of the outflow tract. Some authors have labeled the proximal myocardial part the conus, calling the distal part the truncus, and the intrapericardial portion of the outflow tract the aortic sac. Others have called the entire myocardial outflow tract the bulbus, since the myocardial outflow tract of primitive vertebrates, called the conus arteriosus in primitive fish, and the bulbus cordis in amphibia, is presumed to develop into the right ventricle in higher vertebrates. Because the terms conus and truncus have been inconsistently used in literature, we prefer simply to describe proximal and distal parts of the myocardial outflow tract, and to describe, when it appears, an intrapericardial arterial portion of the outflow tract, this part interposing between the muscular component and the ventral aorta, the distal junction occurring at the pericardial reflections.

Given that different authors use different terminologies, it is not surprising that formation of the outflow tract remains one of the most confusing areas of cardiac development. The recent finding that the stem of the heart tube contains precursor cells exclusively for the left ventricle makes it essential to adopt descriptions which follow dynamic events, rather than continuing to use static names. Thus, as soon as the primary heart tube bends to produce its ventral ventricular expansion, by definition the developing ventricle achieves inlet and outlet components, even though these structures are not the definitive ventricular inlets and outlets. Cells that, initially, are found in the ventricular outflow tract subsequently become cells of the right ventricle. Furthermore, working myocytes can easily be distinguished from the primary myocytes of the straight heart tube. The primary myocardium is smooth-walled, whereas the developing myocardium of the ventricles is trabeculated, and expresses specific markers such as atrial natriuretic peptide and connexin40. Thus, that part of the primary heart tube downstream of the developing ventricular loop is termed the outflow tract, irrespective of the fact that eventually it will largely become incorporated within the definitive right ventricle (Figs 59.6, 59.7). Moreover, within the inner curvature of the ventricular loop, the walls of the outflow tract and atrioventricular canal fade into one another without a clear boundary. Indeed, ventricular working myocardium never develops at the inner curvature. It is within this ventricular part of the primary heart tube that the cushions of the atrioventricular canal and the outflow tract must achieve appropriate connections with the muscular ventricular septum in order to divide this part of the tube into left and right compartments. The cavity of the tube within which these events take place has been called ‘interventricular foramen’. This is incorrect, since it is rather the region between adjacent parts of the primary heart tube. It is, nonetheless, the remodelling of this inner curvature that sets the scene for the completion of cardiac septation.

CARDIAC FUNCTION AND THE CONDUCTION SYSTEM

Although only mammalian hearts have a well-defined conduction system, an essentially similar electrocardiogram can be recorded from animals as diverse as fish and man, indicating that the electrical connections between the cardiac components have been conserved during vertebrate evolution. Indeed, the development of the conduction system of the heart is inextricably associated with the development of the basic building plan for the heart. The conduction system is best defined as the system that initiates and conducts the sinus impulse. In mammals, it encompasses the sinu-atrial node, the atrioventricular node, the atrioventricular bundle, the bundle branches and their terminal ramifications. The nodes and the atrioventricular bundle can be considered the central conduction system, while the bundle branches and their ramifications represent the peripheral ventricular conduction system. The sinus impulse is generated in the sinus node, whence it is rapidly propagated through the atrial myocardium towards the atrioventricular node, where propagation is delayed. After an essential period of atrioventricular delay, the impulse travels rapidly through the atrioventricular conduction axis and the peripheral ramifications to reach the ventricular myocardium, which is then rapidly depolarized. Following repolarization, the sequence recurs in endless fashion throughout the life of the individual. The term ‘conduction system’, as opposed to the working myocardium of the chambers, may suggest that the working myocardium does not conduct, while the conduction system conducts rapidly. In order to produce powerful synchronous contractions, the working myocardium of the chambers must also, of necessity, conduct rapidly, whereas to act as a pacemaker, the cells of the nodes need to be poorly coupled, and hence display slow conduction.

The vertebrate heart is myogenic, implying that all cardiomyocytes have the capacity to generate and conduct the depolarizing impulse. Because cardiac myocytes are electrically coupled, the cells with the highest pacemaking activity take the lead. In the formed normal heart, the leading pacemaker is always within the sinus node. In embryonic hearts, the leading pacemaker is always found at the venous pole. Thus, it is the most recently recruited cells at the venous pole which always have the highest pacemaking activity. All regions of the early embryonic heart tube possess poorly coupled cells and display intrinsic automaticity, thus permitting slow propagation of the depolarizing impulses along the cardiac tube, and producing matching peristaltic waves of contraction which push the blood in an antegrade direction. The myocytes of a developing heart do not have well-developed sarcomeric structures, and have a poorly developed sarcoplasmic reticulum. This tissue has been called primary myocardium, distinguishing it from the rapidly conducting and well-developed working myocardium of the atrial and ventricular chambers.

As development proceeds, the ventricular chambers develop in the cranial part of the heart tube, by ventral expansion, and the atrial chambers in the caudal part by dorso-lateral expansion (Fig. 59.6). An adult type of electrocardiogram can be recorded from such hearts, showing rapid atrial depolarization, a period of atrioventricular delay, and rapid ventricular depolarization. The electrocardiographic tracings reflect the development of fast-conducting components within a slowly conducting heart tube, i.e. the development of a morphologic conduction system. The newly developed working myocardium expresses atrial natriuretic factor, along with the gap junctional proteins connexin40 and 43, which permit fast conduction.

Like the early peristaltic heart, flow through the developing chambered heart remains directional because the dominant pacemaking activity is still at the inlet of the heart. Peristaltic hearts do not need valves, whereas chambered hearts require the presence of one-way valves at the inlets to, and the outlets from, the chambers. The slowly conducting atrioventricular canal, interposed between the atrium and the ventricle, and the outflow tract positioned between the ventricle and the great arteries, are the parts which contain the endocardial cushions. These are able to function as sphincteric valves, as a result of their prolonged duration of contraction. These components retain their function until the definitive leaflets of the one-way valves have been sculpted from the cushions. The primary myocardium of the outflow tract does not regress until the arterial valves have been formed, and does not disappear in its entirety until around the twelfth week of human development. The primary myocardium of the atrioventricular canal is eventually incorporated into the atrial vestibules at the same time as the formation of the atrioventricular valves, becoming sequestrated on the atrial side of the atrioventricular junctions between 6 and 12 weeks of development (Fig. 59.10). An important part of the canal persists dorsally, where it differentiates into the slowly conducting atrioventricular node.

Fig. 59.10 Development of the conduction system of the heart. A, At stage 13 with the truncus arteriosus reflected to the right, the interventricular ring marks the region of the primary heart tube. The purple part of the interventricular ring will give rise to the atrioventricular bundle and bundle branches, whereas the yellow part of the interventricular ring will not participate in the formation of the adult conducting system. B, The truncus arteriosus, now divided into the pulmonary trunk and aorta, is presented between the right and left atria which expand lateral to it The right atrium is now positioned entirely above the right ventricle and the outflow tract has expanded to the left; the left ventricle has access to the aorta. C, The position of the original primary heart tube ring indicated in the formed heart. The pulmonary trunk has been removed.

Considerable progress has been made over recent years in deciphering the pathways involved in establishing the building plan of the heart (Fig. 59.11). The combined action of the transcription factors Tbx5 and Nkx2–5 is required for the formation of the myocardium of the atrial and ventricular chambers. Tbx5 is expressed in a gradient over the heart tube, decreasing in concentration from caudal to cranial and may impose positional information. However, its pattern cannot explain the localized formation of the chambers and the conduction system. Localized expression of the transcriptional repressors Tbx2 and Tbx3 in the inflow tract, the floor of the atrium, the atrioventricular canal, the inner curvature of the ventricular region, and the outflow tract prevents the differentiation of primary into working myocardium. Tbx2 and Tbx3 effectively compete for binding to the promoters of the chamberspecific genes, such as atrial natriuretic factor and connexin40. The remaining primary myocardium of these regions initially induces the endocardium to undergo epithelial-to-mesenchymal transition, by which process the cardiac cushions become filled with mesenchyme. The myocardium also participates in the alignment of the atrial and ventricular chambers (which is essential for proper cardiac septation, and which is guided by the fusing cardiac cushions), and it forms the conduction system, including the sinus and atrioventricular nodes, the atrioventricular bundle, the floor of the developing right atrium (the internodal region), and the entirety of the atrioventricular canal. In the early chamber-forming heart of the mouse, Tbx3 is expressed in the atrioventricular canal, the floor of the atrium and around the orifices of the systemic venous tributaries, but not in the dorsal mesocardium surrounding the entrance of the pulmonary vein. During subsequent development, the domain of expression of Tbx3 expands from the atrioventricular canal to form a crescent on the crest of the ventricular septum. The dorsal aspect of this crescent develops into the atrioventricular bundle, while the atrioventricular node is formed at the dorsal junction with the atrioventricular canal. The ventral parts subsequently disappear during development.

Fig. 59.11 The early heart: distribution of the T-box transcription factors involved in patterning the heart.

The fact that the ventricular conducting system originates from a single primary ring provides a solid base for understanding the disposition of the conducting system in a number of congenital malformations. The concept accounts particularly well for the morphology and disposition of the atrioventricular node and bundle in hearts with straddling tricuspid valves, with double inlet left ventricles, and with congenitally corrected transposition.

SEPTATION OF THE EMBRYONIC CARDIAC COMPARTMENTS

The flow of blood on the right and left sides does not intermingle even in the developing heart, because flow is laminar, and the pressures are similar at the right and left sides. It is only after birth that the pressures in the ‘left’ systemic circulation become higher than those in the ‘right’ pulmonary circulation, and therefore the different flows have to be separated physically in order to prevent admixture of oxygenated and deoxygenated blood. Prior to birth the lungs are not functioning and blood flow to them is small. Even so, normal development of the heart requires that all cardiac compartments receive an adequate amount of blood. To this end, even before birth, the systemic blood of the right atrium is guided to the left atrium so that the left half of the heart and the brain is provided with a normal flow of blood. The right ventricle, in contrast, drains to the dorsal aorta via the ductus arteriosus (see Fig. 35.8). The walls that separate the right and left sides of the heart are largely formed in the fourth and fifth week of development. Some time later, not only the components of the primary heart tube, specifically the atrioventricular canal and the outflow tract, but also the atrial and ventricular chambers, must be separated physically into right and left halves. However, each compartment is separated in markedly different fashion.

Septation and appropriate positioning of the atrioventricular canal

Extracellular matrix accumulates between the endocardium and myocardium of the primary heart tube. However, it mostly disappears in the regions of ballooning of the chamber myocardium of the developing atria and ventricles. The matrix becomes filled with mesenchyme in the persisting regions of the primary heart tube. In conventional accounts of the process, these regions are called the atrioventricular canal and the outflow tract, or conotruncus. The dorsal or inferior atrioventricular cushion continues into the floor of the atrium, which is made of primary myocardium. As the cushion extends dorsally, it meets the mediastinal mesenchyme in the region initially termed the spina vestibuli, where it continues as a mesenchymal cap running along the leading edge of the septum primum until it meets the ventral, or cranial, atrioventricular cushion (Fig. 59.12 and see Fig. 59.14). The dorsal cushion also has a significant ventricular extension in the inner curvature of the heart tube, which comes to lie on top of the developing muscular ventricular septum. The two atrioventricular cushions fuse in the sixth week of development, dividing the atrioventricular canal into its right and left components. The cushions are very large relative to the canal, leaving narrow right and left slits, which increase markedly in size during further development.

Fig. 59.14 Septation of ventricles and outflow tract. A, Frontal view of heart at stage 13 with the outflow tract reflected to the right. Lines show the level of transverse sections of the atrioventricular cushions and outflow tract. Note that blood from both atria pass through the embryonic interventricular foramen at this stage. B, Sagittal section of heart along the dotted line in A showing the atrioventicular junctional region seen from the right side. The atrioventricular cushions extend onto the leading edge of the septum primum (although the mesenchymal cap on the atrial septum, see Fig. 59.12, probably has a different developmental origin).

At the start of stage 10, the venous pole, the atrioventricular canal, the developing left ventricle and the outflow tract are all positioned symmetrically around the midline (Fig. 59.13A). As a result of the subsequent rapid growth of the cardiac tube, the atrioventricular canal moves in its entirety to the left (Fig. 59.13B), as the ventricular part of the heart tube loops to the right, thus placing the developing left ventricle on the left, and the forming right ventricle on the right (Fig. 59.7). This ventricular loop is conspicuous throughout the fourth and fifth weeks of development, and it is at this stage that a deep interventricular groove appears externally. Over this period the atrial floor, including the developing systemic venous sinus (sinus venosus) and the atrioventricular canal, move to the right (Fig. 59.13C). The developing pulmonary vein remains anchored in the midline and, subsequent to these manoeuvres, the atrioventricular canal once more becomes positioned in the midline: this process facilitates the appropriate connections of the developing muscular septum with the atrioventricular and outflow cushions. The orifice of the sinus venosus is by now positioned to the right. From the outset, the muscular ventricular septum develops in line with the right side of the dorsal (inferior) atrioventricular cushion (Fig. 59.14), so that the separating right atrium always has direct access to the developing right ventricle. The process also ensures that space remains ventrally for incorporation of the subaortic component of the outflow tract into the left ventricle.

Fig. 59.13 Movements of the atrioventricular canal relative to the midline during development.

Septation and appropriate positioning of the outflow tract

The length of the myocardial portion of the outflow tract decreases markedly between the fourth and eighth weeks of development. In part, the myocardium becomes incorporated into the ventricles, and in part it disappears by apoptosis: in the definitive heart, there is no distinct myocardial outflow tract present within the left ventricle, because the myocardium which initially separated the developing mitral and aortic valves disappears, leaving the fibrous aortic-mitral continuity distinctive for the postnatal heart. In the right ventricle, the proximal myocardial outflow tract persists as the smooth-walled muscular subpulmonary infundibulum. Formation of the part adjacent to the aortic root requires muscularization of the most proximal parts of the outflow cushions to form the greater part of the supraventricular crest, or crista supraventricularis.

Key to separation of the outflow tract is the appearance of the two outflow ridges, or cushions, which spiral one round another as they run from the distal end of the right ventricle, where they are positioned septally and parietally, to the aortic sac (Figs 59.14, 59.15). Within the aortic sac, a transverse wedge of tissue, termed the aorto-pulmonary septum, separates the origins of the arteries traversing the fourth and sixth pharyngeal arteries. The septum is formed from dorsal mediastinal mesenchyme. Myocardial precursor cells and non-myocardial cells are added to the outflow tract, the latter forming the intra-pericardial components of the arterial trunks. Neural crest cells migrate from the pharyngeal mediastinal myocardium into the outflow cushions. They do not appear to populate the septum itself, but form the larger parts of the walls of the intrapericardial portions of the truncus arteriosus. The precise relationship between the outflow cushions and the newly formed aortic arch arteries, which never possess a septum between them, has still to be assessed.

Fig. 59.15 Septation of outflow tract. A, SEM of human heart at stage 18. The pulmonary trunk is separate from the aorta. B, SEM of human heart at stage 17 showing the luminal aspect of the right ventricle and the outflow tract. The anterior and parietal wall of the outflow tract has been removed along the dotted lines shown in A. C, SEM with added colour to show the structures in B. The outflow cushions are unfused; the distal margins of both outflow tract cushions are contributing to the developing pulmonary valve, the third leaflet and sinus of this valve are derived from the intercalated cushion. The components of the aortic valve develop from the dorsal aspect of the outflow tract cushions.

(By courtesy of Prof G Steding, Göttingen, with permission from Oostra RJ, Steding G, Lamers WH, Moorman AFM. Steding’s and Virágh’s scanning electron microscopy atlas of the developing human heart. New York: Springer-Verlag, 2006.)

The spiral outflow ridges start to fuse from the sixth week onward, initially in the distal part of the outflow tract, and fusions continue gradually in the direction of the ventricular septum (Figs 59.15, 59.16A). The most proximal parts of the cushions remain unfused when the aorta and pulmonary trunk have gained their separate existence within the pericardial cavity. At this time, the proximal part of the outflow tract, upstream to the intrapericardial arterial trunks, remains encased in a myocardial sleeve. The arterial valves and sinuses then develop within the more distal part of this myocardial outflow tract; the outer myocardial wall does not disappear completely until well after the completion of septation. The most proximal parts of the cushions are the last ones to fuse, and they then muscularize and join the muscular ventricular septum, thereby joining the aorta into the left ventricle. The dorsal atrioventricular cushion maintains its mesenchymal character, becoming the membranous part of the interventricular septum in the formed heart. Formation of its interventricular and atrioventricular components does not become evident until the septal leaflet of the tricuspid valve delaminates from the surface of the ventricular septum.

Fig. 59.16 A, SEM of human heart at 32 days (stage 15). The right lateral wall of the right atrium, right ventricle and the outflow tract have been removed. The septum primum (S) and the developing valve of the sinus venosus (white arrow) are at a distance from the unfused ventral (V) and dorsal (D) atrioventricular cushions. The interventricular foramen (primum) is open (O). Its lower margin is made up by the developing interventricular septum (I). Note the left anterolateral ridge (R) and, at the distal margins of the truncus arteriosus, the valve swellings (^*) of the putative aorta and pulmonary trunk. IV, origin of IVth aortic arch; VI, origin of VIth aortic arch. At, atrium; E, embryonic trabeculae. B, Septal aspect of the right ventricle of a human heart at 8 weeks. Delamination of the septal cusp of the tricuspid valve (T) is taking place. A cleft (black arrow) has appeared, marking the posterior margin of the septal cusp. There are no clefts delineating the anterior margins in the region adjacent to the supraventricular crest (C), and the endocardium is continuous. No tension apparatus is present. AV, aortic valve; LA, left atrium; PV, pulmonary valve; RA, right atrium.

NON-MYOCARDIAL COMPONENTS OF THE HEART

The non-myocardial components of the heart are the epicardium, including the coronary vasculature, and the interstitial fibroblasts and the valvular apparatus, which are formed in their larger part from endocardially derived mesenchyme. From the outset, the atrioventricular valves, mitral and tricuspid, are formed at the site of the initial atrioventricular canal, whereas the aortic and pulmonary valves are initially developed within the myocardial outflow tract, and only later achieve the semilunar attachments of the leaflets, which cross the anatomic ventriculo-arterial junctions. All of the leaflets form initially as internal endocardial projections which enclose a myocardial basement membrane, matrix, and mesenchymal cells. The precise mechanisms involved in the formation of these areas have still to be determined.

FETAL CIRCULATION

As the cardiovascular system develops, the embryonic circulation (see Ch. 13) is modified into a fetal circulation which maintains a placental circulation and is also able to establish the pulmonary circulation at birth. Extensive remodeling of the early aortic arch arteries and (initially bilateral) dorsal aortae occurs (see Ch. 35). The dorsal aortae persist on the cranial side of the third aortic arches as continuations of the internal carotid arteries (see Fig. 35.8). The dorsal aorta between the third and fourth aortic arches, the carotid duct, diminishes and finally disappears. From the fourth arch to the origin of the seventh intersegmental artery, the right dorsal aorta becomes part of the right subclavian artery, and caudal to the seventh intersegmental artery, the right dorsal aorta disappears as far as the locus of fusion of the thoracic aortae. After disappearance of the left carotid duct, the remainder persists to form the descending part of the arch of the aorta. The fused right and left embryonic dorsal aortae persist as the definitive descending thoracic and abdominal aorta. A constriction, the aortic isthmus, is sometimes present in the aorta between the final site of origin of the left subclavian artery and the orifice of the arterial duct.

Concomitant changes also occur in the early venous system. The precardinal veins enlarge as the head and brain develop. They are further augmented by the subclavian veins from the upper limb buds, which become the chief tributaries of the common cardinal veins, gradually assuming an almost vertical position as the heart descends into the thorax (see Fig. 13.4). That part of the original precardinal vein rostral to the subclavian vein is now the internal jugular vein, and their confluence is the brachiocephalic vein of each side. The right and left common cardinal veins are originally of the same diameter: by the development of a large oblique transverse connection, the left brachiocephalic vein carries blood across from the left to the right (see Fig. 13.4). The part of the original right precardinal vein between the junction of the two brachiocephalic veins and the azygos veins forms the upper part of the superior vena cava. The caudal part of this vessel, below the entrance of the azygos vein, is formed by the right common cardinal vein. The left precardinal and left common cardinal veins caudal to the transverse branching of the left brachiocephalic vein largely atrophy: the precardinal constitutes the terminal part of the left superior intercostal vein, while the common cardinal is represented by the ligament of the left superior vena cava and the oblique vein of the left atrium (see Fig. 13.4). The remainder of the left superior intercostal vein is developed from the cranial end of the postcardinal vein and drains the second, third and, on occasion, the fourth, intercostal veins. The oblique vein passes downwards across the back of the left atrium to open into the coronary sinus, which represents the persistent left horn of the systemic venous sinus.

The fetal circulation contains a number of relatively large vessels which permit the majority of the blood flow to bypass the liver and lungs. The placenta serves as the organ for fetal nutrition and excretion, receiving deoxygenated fetal blood and returning it oxygenated and detoxified. Fetal blood reaches the placenta via two umbilical arteries and in early fetal life returns by two umbilical veins (Fig. 59.17). The right umbilical vein later disappears, whereas the persisting left umbilical vein enters the abdomen at the umbilicus, traverses the edge of the falciform ligament to reach the hepatic surface, and then joins the left branch of the portal vein at the hepatic portal. Opposite the junction, a large vessel, the ductus venosus, arises and ascends posterior to the liver to join the left hepatic vein near its termination in the inferior vena cava. (For a detailed developmental account of the circumhepatic veins, see Ch. 73, Figs 73.8–73.10.) The portal vein is small in the fetus compared to the size of the umbilical vein. Parts of the left branch of the umbilical vein, proximal and distal to their junctions, function as branches of the portal vein, carrying oxygenated blood to the right and left parts of the liver. Blood in the left umbilical vein therefore reaches the inferior vena cava by three routes: some enters the liver directly and reaches the vena cava via the hepatic veins; a considerable quantity circulates through the liver with portal venous blood before also entering by the hepatic veins; the remainder is bypassed into the inferior vena cava by the ductus venosus.

Fig. 59.17 Plan of the fetal circulation. The arrows indicate the direction of blood flow. The placenta is drawn to a greatly reduced scale.

The refreshed placental blood passes almost directly to the aorta for distribution to the head and upper limbs. Blood from the ductus venosus and hepatic veins mixes in the inferior vena cava with blood from the lower limbs and abdominal wall and enters the right atrium. Because right atrial pressure is much greater than left atrial pressure, it forces the flap-like valve of the septum primum to the left, which permits passage of blood from the right to the left atrium. The valve of the inferior vena cava is so placed as to direct 75% of the richly oxygenated blood from the umbilical vein to the foramen ovale and left atrium, where it mingles with the limited venous return from the pulmonary veins. From the left atrium, blood enters the left ventricle and thence the aorta, by which it is probably distributed almost entirely to the heart, head and upper limbs, which means that little reaches the descending aorta.

Blood from the head and upper limbs returns to the right atrium via the superior vena cava, flows through the right atrioventricular orifice into the right ventricle (together with the small amount of blood that is returned via the inferior vena cava), and then passes into the pulmonary trunk. However, because the fetal lungs are largely inactive, only a little of the blood from the right ventricle flows through the right and left pulmonary arteries and returns to the left atrium via the pulmonary veins. The greater part of the outflow through the pulmonary trunk is carried by the ductus arteriosus directly to the aorta, where it mixes with the small quantity of blood that passes from the left ventricle into this part of the aorta. The mixture descends in the aorta and most is returned via the umbilical arteries to the placenta: some is distributed to the lower limbs and the organs of the abdomen and pelvis.

CHANGES IN THE FETAL CIRCULATION AND OCCLUSION OF FETAL VESSELS AFTER BIRTH

At birth, as pulmonary respiration begins, increased amounts of blood from the pulmonary trunk flow through the pulmonary arteries to the lungs and return by the pulmonary veins to the left atrium: pressure therefore increases within the left atrium. A decrease in pressure also occurs in the inferior vena cava as a result of the reduction of venous return concomitant with occlusion of the umbilical vein and ductus venosus. Atrial pressures become equal and the valvular foramen ovale is closed by apposition, and subsequent fusion, of the septum primum to the rims of the foramen. Contraction of the atrial septal muscle, synchronized with that in the superior vena cava, may assist this closure, which occurs after functional closure of the ductus arteriosus. Although the foramen ovale closes functionally after pulmonary respiration is established, it does not become structurally closed until some time later. It is obliterated in fewer than 3% of infants 2 weeks after birth, and in 87% by 4 months after birth. Fusion is sometimes incomplete and a potential atrial communication (atrial septal defect) persists throughout life. Almost always this has no functional effect, because the inequality of atrial pressures and the valve-like arrangement of the opening do not favour passage of blood.

Soon after birth, a number of fetal vessels occlude, although the majority remain patent. This differential constriction suggests that the walls of a population of fetal vessels are different to those of the remaining vessels. Bradykinin, one of the kinin polypeptide hormones which induce contraction or relaxation of smooth muscle, forms in the blood of the umbilical cord when the temperature of the cord decreases at or shortly after birth. It is also formed and released by granular leukocytes in the lungs of the neonate after exposure to adequate oxygen. Bradykinin is a potent constrictor of the umbilical arteries and veins and of the ductus arteriosus, and is also a potent inhibitor of contraction of the pulmonary vessels.

Ductus arteriosus

The ductus arteriosus shunts blood from the pulmonary trunk to the arch of the aorta, bypassing the fetal lungs (Figs 59.17, 59.18). It arises as a direct continuation of the pulmonary trunk at the point where it divides into right and left pulmonary arteries. It is 8–12 mm long, and joins the aorta at an angle of 30–35° on the left side, anterolaterally, below the origin of the left subclavian artery. The opening of the ductus arteriosus into the aorta is greatly elongated. Its diameter at its origin from the pulmonary trunk, when distended with blood, is 4–5 mm, which is nearly equal to the diameter of the adjacent ascending aorta (5–6 mm). Both arteries taper to a smaller diameter as they pass inferiorly and the aorta remains slightly larger (4 mm) (Fig. 59.19). In the neonate, the ductus arteriosus is closely related to the left primary bronchus inferiorly and the thymus gland anteriorly.

Fig. 59.18 Anterior view of heart and great vessels in a full-term neonate. The lungs have been displaced to expose the heart and the epicardium dissected off the heart and roots of the great vessels. Note that, after birth, blood flow reverses through the ductus arteriosus prior to its closure. (After Crelin ES 1969 Anatomy of the Newborn. Philadelphia: Lea and Febiger.)

Fig. 59.19 Anterior view with the heart removed to show the relationship between the left primary bronchus, the aortic arch and the ductus arteriosus in a full-term fetus. (After Crelin ES 1969 Anatomy of the Newborn. Philadelphia: Lea and Febiger.)

The ductus arteriosus differs from the other great vessels which arise from the heart because its tunica media contains smooth muscle rather than layers of elastin. It has been suggested that the relationship between the recurrent laryngeal branch of the vagus nerve and the developing ductus arteriosus is responsible for this histological difference on the grounds that the left recurrent laryngeal nerve (which, in a stage 16 embryo, is very large in relation to the aortic arch system) supports the ductus arteriosus as it enwraps it, thereby permitting the ductus to develop as a muscular, and not an elastic, artery.

It is essential that the ductus arteriosus remains patent during intrauterine life. Prostaglandins appear to have a role in maintaining this patency. Fetal and neonatal ductal tissue can produce prostaglandins E₂, I₂ and F₂a, which inhibit the ability of the ductus to contract in response to oxygen.

Closure of the ductus arteriosus

The ductus arteriosus starts to close immediately after birth, although blood probably continues to flow intermittently through it for a week or so. The flow is reversed relative to that occurring in the fetal circulation, reflecting the increased systemic vascular resistance which follows exclusion of the placental circulation, and the decreased pulmonary resistance which follows expansion of the lungs. Initial constriction at birth has been attributed to increased oxygen tension. A neural factor may also be involved: the muscular wall has afferent and efferent nerve endings and responds to adrenaline and noradrenaline.

The first stage of ductal closure is completed within 10–15 hours and the second stage takes 2–3 weeks. The first stage consists of contraction of the smooth muscle cells and development of subendothelial oedema. Destruction of the endothelium and proliferation of the intima subsequently occurs, and leads to permanent closure. Diverse factors which may promote ductal closure have been identified. They include: increased oxygen tension; increased plasma catecholamine concentrations; suppression of prostaglandin I₂ production; switching off prostaglandin E receptors; a synergistic role of prostaglandin F₂a and oxygen concentrations; a decrease in plasma adenosine concentration. After birth, these interrelated events result in the closure of the ductus arteriosus. It has been proposed that the high oxygen tension of the reversed blood flow through the ductus initiates the synthesis of a hydroperoxy fatty acid which suppresses prostacyclin production, thus exposing the ductus to the contractile effects of prostaglandin endoperoxide. After closure, the duct becomes the ligamentum arteriosum, which connects the left pulmonary artery (near its origin) with the aortic arch.

FETAL AND NEONATAL HEART

At full term, the heart is situated midway between the crown of the head and the lower level of the buttocks (see Fig. 14.4). The anterior surface is formed mainly by the right atrium and right ventricle, as it is in the adult. This surface is usually covered by the thymus, which may extend over the base of the right ventricle. The heart is relatively large at birth, weighing around 20 grams. The cardiac output is around 550 ml per minute, and the blood pressure is 80/46 mmHg. The heart rate during fetal life, as term approaches, is around 150 beats per minute. It increases at birth to around 180 beats, decreases over the first 10 minutes after birth to 170 beats, and reaches 120–140 beats per minute from 15 minutes to 1 hour after birth. Any signs of fetal distress will increase this general basic level. The heart rate decreases further with increasing age: it is normally 113–127 beats per minute from 6 months to 1 year, and settles to around 100 beats per minute by the end of the first year.

Considered relative to the thoracic landmarks, the foramen ovale lies at the level of the third intercostal space, with its long axis in the median plane. It is 4–6 mm in vertical length and 3–4 mm wide (see Fig. 14.4). It is almost exactly in the coronal plane of the body, so that blood passes from the anterior, or ventral, right atrium posteriorly and upwards to reach the upper and posterior part of the left atrium. After birth, the intra-atrial pressures are equalized, and the free edge of the flap valve formed by the primary atrial septum is kept in contact with the left side of the rims of the fossa, promoting subsequent anatomic fusion, albeit that the foramen remains probe-patent in up to one-third of all individuals. The initially free crescentic margin of the infolded superior interatrial fold forms the border of the fossa after fusion; the flap valve formed by the primary septum accounts for its floor in the adult heart.

Functionally, the ventricular septum is often considered to be part of the left ventricle: while this is certainly true in terms of the orientation of the myocardial aggregates which make up the ventricular walls, it is not correct from a cellular lineage perspective. The ratio of cardiac weight is usually expressed as the weight of the right ventricle relative to that of the left ventricle and the septum. Calculated in this fashion, the left ventricle at birth weighs about 25% more than the right ventricle. However, the right ventricle has been working against the systemic pressure in the fetus, the pulmonary circulation not yet being active, and there is a preponderance of right ventricular function in the first 2 or 3 months after birth. With the establishment of the pulmonary circulation, the work of the right side of the heart decreases, and the left side of the heart, particularly the ventricle, grows rapidly to meet the demands of the active neonate. By the end of the second year, the left side weighs twice as much as the right, a ratio that continues to middle age. At birth, the average thicknesses of the lateral walls of the ventricles are approximately equal (5 mm). By the end of the third post natal month, the left ventricle has already become thicker than the right, it becomes twice as thick by the second year, and three times as thick by puberty.

CONGENITAL HEART DEFECTS

Defects of cardiac development may not come to light until after birth, although many, if not most, can now be observed on fetal ultrasonic examination. They may affect any part of the developing heart, and include more complex forms with inappropriate connections of cardiac components. They include anomalies such as double inlet ventricle; absence of one atrioventricular connection, commonly described as tricuspid or mitral atresia; and discordant atrioventricular and ventriculo-arterial connections, also known as congenitally corrected transposition.

Congenital cardiac malformations are often multiple, and probably occur more frequently in siblings and in children of consanguineous marriages: there is a low correlation among monozygotic twins. Ventricular septal defects are the most common lesions, making up around one-fifth of all cases, followed by persistent patency of the ductus arteriosus, coarctation, pulmonary stenosis, Fallot’s tetralogy, transposition, aortic stenosis, and hypoplasia of the left heart.

Atrial septal defects

A persistent communication between the atrial chambers within the fossa ovale is common, and results from the failure of the flap valve of the primary atrial septum to fuse with the infolded muscular rims of the fossa. When the flap valve is still able to overlap the rims, the communication is of no functional significance as long as left atrial pressure is greater than right, which is usually the case. However, when the flap valve is smaller than the fossa ovale, or when it is perforate, there is a true atrial septal defect (Fig. 59.20A).

Fig. 59.20 A, Location of the defects that produce an interatrial communication. Only defects within the fossa ovalis are true atrial septal defects. B, Shunting across an atrioventricular septal defect. This can be atrial (left), ventricular (right) or at both levels (middle), depending on the attachment of the bridging cusps. C, Ventricular septal defects. Depending on the structure of the anatomic borders seen from the right ventricle, these defects can be placed into perimembranous, muscular or doubly committed groups.

(B and C by courtesy of Professor RH Anderson, Institute of Child Health, University College, London.)

In normal development, the free leading edge of the septum primum merges with the atrioventricular endocardial cushions, permitting subsequent formation of the atrioventricular septum. When this does not happen, there is a common atrioventricular junction and an atrioventricular septal defect. This can occur when the leaflets of the atrioventricular valves are fused to the crest of the ventricular septum (Fig. 59.20A), producing an interatrial communication at the expected site of the atrioventricular septum. This is the so-called ostium primum defect, and in reality is an atrioventricular septal defect permitting exclusive atrial shunting. Other interatrial communications can be formed in the mouths of the vena cavae, most frequently the superior vena cava, and are usually associated with drainage of the right pulmonary veins into the cavo-atrial junction. Known as sinus venosus defects (Fig. 59.20A), their essential feature is a bi-atrial connection of the involved caval vein. An interatrial communication can also occur through the mouth of the coronary sinus when there is a deficiency or absence of the walls that usually separate the sinus from the left atrium.

Atrioventricular septal defects result from failure of fusion of the endocardial atrioventricular cushions, producing a common atrioventricular orifice and deficiencies of the adjacent septal structures (Fig. 59.20B). The common orifice is guarded by a basically common valve, with superior and inferior leaflets bridging the scooped-out ventricular septum and tethered in both right and left ventricles. Although the left component of the valve thus formed is often interpreted as a ‘cleft mitral valve’, in reality it bears no resemblance to the normally structured mitral valve, because it has three leaflets; the ‘cleft’ forms the zone of apposition between the left ventricular components of the bridging leaflets. Two major subgroups are identified. The more frequent pattern has a common atrioventricular orifice, and the potential for shunting through the septal defect at both atrial and ventricular levels (Fig. 59.20B). The less common form has separate right and left atrioventricular orifices, and shunting occurs only at atrial level. Occasionally, when the bridging leaflets are attached to the underside of the atrial septum, shunting is confined at ventricular level, and is typically small.

ABNORMAL CONNECTIONS OF THE GREAT ARTERIES AND VEINS

A number of outflow anomalies may occur should the outflow cushions either fail to develop, or fuse in inappropriate fashion. When the spiral septum cushions fail to fuse, the result is a common arterial trunk, represented by an undivided arterial channel, guarded by a common arterial valve, positioned above and astride the free margin of the muscular ventricular septum (Fig. 59.21). There is, therefore, a coexisting juxta-arterial deficiency of the ventricular septum. The right and left pulmonary arteries usually arise via a confluent segment, but can take independent origin from the common arterial trunk, which continues as the ascending aorta. The common valve usually has three leaflets, but may have two, four or more. The lesion is almost certainly linked to abnormal migration of cells into the heart from the neural crest.

Fig. 59.21 This heart possesses a common arterial trunk, with a common truncal valve over-riding a juxta-arterial deficiency of the ventricular septum, the result of failure of septation of the arterial pole of the developing heart.

(By courtesy of Professor RH Anderson, Institute of Child Health, University College, London. Specimen prepared by Dr Leon M Gerlis.)

Transposition of the arterial trunks (also referred to as transposition of the great vessels) is the condition in which the aorta arises from the right ventricle and the pulmonary trunk from the left. Better described as having discordant ventriculo-arterial connections, such hearts can co-exist with deficiencies of cardiac septation. They can also be found with discordant connections at the atrioventricular junctions, producing congenitally corrected transposition. The developmental history of discordant connections remains unknown.

Double outlet ventricle exists when the greater parts of both arterial valves are attached within the same ventricle, almost always the right. For the circulation to continue, it is then necessary for the ventricular septum to be deficient, although the septal defect can rarely close as a secondary event. The position of the septal defect serves for subclassification. It is usually beneath the aorta or the pulmonary trunk, but can be doubly committed or even non-committed.

Either the systemic or pulmonary veins can be anomalously connected. Right and left superior vena cavae are present in some animals, and occasionally persist in mankind. The most common systemic anomaly is a persistent left superior vena cava draining into the right atrium through the enlarged orifice of the coronary sinus. More rarely, the persisting left-sided vena cava may connect directly with the superior aspect of the left atrium, usually associated with so-called ‘unroofing’ of the coronary sinus, so that the orifice of the sinus functions as an interatrial communication. The most common lesion of the inferior vena cava is when its abdominal course is interrupted, with drainage to the heart via the azygos or hemiazygos venous system. This lesion is found most frequently with isomerism of the left atrial appendages. The pulmonary veins can be connected to an anomalous site individually or in combination. Totally anomalous connection is of most significance. Usually, the veins form a confluence behind the left atrium that then connects to the superior vena cava, the coronary sinus, or the portal venous system after traversing the diaphragm.

A right aortic arch is found most frequently with Fallot’s tetralogy or with a common arterial trunk. It can also exist, together with a left arch, in various combinations known as arterial rings, which compress the oesophagus, giving so-called dysphagia lusoria. Persistent patency of the ductus arteriosus must be distinguished from delayed closure. The persistently patent ductus can be an obligatory part of the circulation when associated with aortic or pulmonary atresia. Coarctation of the aorta can be found as an isolated lesion when the ductus arteriosus is closed, or with an open duct, when it is more likely to be associated with additional lesions within the heart (see p. 984).

In the adult, the right subclavian artery occasionally arises from the arch of the aorta distal to the origin of the left subclavian artery, and then passes upwards and to the right, behind the trachea and oesophagus. This condition is possibly explained by the persistence of the embryonic right dorsal aorta and the obliteration of the fourth aortic arch of the right side.

NEONATAL ARTERIAL AND VENOUS VESSELS

In the neonate, the blood vessels of the trunk and their associated visceral branches are relatively larger than the vessels in the limbs, an arrangement which favours central pooling of blood. Neonatal peripheral vessels are nearly microscopic: their cannulation consequently poses much more of a problem than is the case with their adult counterparts. Large vessels are in the same relative positions as in the adult, but may correspond to different vertebral levels. Thus, although the bifurcation of the common carotid artery into internal and external carotid arteries occurs at the level of the upper border of the thyroid cartilage, in both neonate and adult, the thyroid cartilage is relatively higher in the neonatal neck than it is in the adult. The renal arteries also arise higher in the neonate, often between T12 and L1, whereas they arise at the upper border of L2 in the adult. The abdominal aorta bifurcates into common iliac arteries at the upper border of L4, rather than at the lower border of L4 as occurs in the adult.

TRACHEA

The trachea starts to develop at stage 12, as a ventral outgrowth from the endodermal foregut into the mesenchyme surrounding the sinus venosus and inflow tract of the heart (Fig. 59.1B,C; see Figs 73.3, 73.5). The point at which the original respiratory diverticulum buds from the foregut, the laryngotracheal groove, remains at a constant level during the embryonic period, and the trachea lengthens distally as the bifurcation point descends. The respiratory diverticulum generally becomes surrounded by angiogenic mesenchyme which connects to the developing sixth aortic arch artery and which is essential for airway branching. By stage 17, the mesenchyme around the trachea is beginning to condense to form cartilage.

Initially, the tracheal mesenchyme is continuous with that surrounding the ventral wall of the oesophagus (Fig. 59.3). Progressive lengthening and continued division of the tracheal bud, together with deviation of the lung buds dorsally, isolates the oesophagus and trachea within tissue-specific mesenchyme and facilitates regional differentiation, not only between trachea and lungs, but also within the lungs themselves, i.e. the number of lobes, or the degree of growth and maturity of a particular lung. Each lung develops by a process of dichotomous branching. For branching to occur, a cleft must develop in the tip (or side) of the epithelial tube. The epithelium then evaginates each side of the cleft, forming new branches which lengthen, and the process is then repeated. At the tips of the developing epithelial buds, the mesenchyme is flattened and densely packed, whereas it forms an ordered row of cuboidal cells along the side of the bud and in the clefts. Cells in both arrangements send processes towards the epithelial basal lamina, which is thicker in the clefts, but so attenuated as to be almost indistinguishable on the tips of the buds where the epithelium and mesenchymal cells form intimate contacts. Tenascin, an extracellular matrix molecule, is present in the budding and distal tip regions, but absent in the clefts. Conversely, fibronectin, an extracellular matrix molecule found commonly in basal laminae, is found in the clefts and along the sides of the developing bronchi, but not on the budding and distal tips.

The control of the branching pattern of the respiratory tree resides with the splanchnopleuric mesenchyme. Experimental recombination of tracheal mesenchyme with bronchial respiratory endoderm results in inhibition of bronchial branching, whereas recombination of bronchial mesenchyme with tracheal epithelium will induce bronchial outgrowths from the trachea. Experimental exposure of rat fetal airway to chick mesenchyme produces a chick airway branching pattern. Broadly, receptor tyrosine kinases and their ligands positively modulate growth and branching morphogenesis, while transforming growth factor β (TGF β) family members have an inhibitory effect.

In the normal neonate, the trachea is relatively small in relation to the larynx (Fig. 14.4). The walls of the trachea are relatively thick and the tracheal cartilages are relatively closer together than in the adult. The trachea begins at the upper border of the sixth cervical vertebra, a relationship that is conserved with growth, and it bifurcates at the level of the third or fourth thoracic vertebra.

LUNGS

Branching of the conducting airways is generally complete by week 16. Airways subsequently increase in size, but not in number. Alveolar development is largely (but not completely) a postnatal phenomenon. The development of pre-acinar blood vessels follows the conducting airway development, whereas development of intra-acinar blood vessels follows the development of the alveolar ducts and alveolar sacs. Lung development is described histologically as progressing through embryonic, pseudoglandular, canalicular, saccular and alveolar phases. (For further details see Rosenthal & Bush 2002; Bush 2005.)

FUNCTIONAL CHANGES IN THE LUNG AROUND THE TIME OF BIRTH

Just prior to birth, the placenta is the organ of respiration. Virtually all the venous return to the right heart is shunted away from the lungs through the foramen ovale and the ductus arteriosus. Pulmonary vascular resistance (PVR) is maintained at a very high level by the muscular precapillary vessels. At birth, the umbilical cord is tied and cut, and within seconds the lungs must take over all respiratory function in order to maintain life. They also must convert from secreting fluid to absorbing it. The mechanisms that mediate these dramatic changes are obscure. However, it is clear that the first breath results in vasodilatation by at least two mechanisms. First, the mechanical effect of traction on the vasculature: as the chest wall expands, it pulls open the pulmonary vessels. Second, oxygen entering the lungs for the first time produces pulmonary vasodilatation. The experimental demonstration that PVR falls even if the fetus is delivered into an atmosphere of pure nitrogen, implies that other mediators, including cyclo-oxygenase metabolites, are undoubtedly important. The gene for cyclo-oxygenase 1 (but not 2) in endothelium and vascular smooth muscle shows enhanced expression in late fetal and early post-natal life. Endothelin receptor expression increases around the time of birth, implying a role for this system in postnatal adaptation. The role of nitric oxide in postnatal adaptation is controversial, and there are likely to be important species differences. Nitric oxide synthase (NOS) is more abundant in young compared with mature animals, and smooth muscle sensitivity to nitric oxide may be greater at birth than in older animals. The effects of mediators may differ ante- and postnatally, e.g. endothelin (ET)-1 causes vasodilatation in fetal sheep, but vasoconstriction postnatally.

Immediately after birth, and before structural remodeling has taken place, PVR may rise steeply if the baby becomes even minimally hypoxic or acidaemic. The fetal shunts (foramen ovale and ductus arteriosus) reopen and right to left shunting causes profound hypoxemia, ‘persistent fetal circulation’.

POSTNATAL LUNG DEVELOPMENT

The amount and type of connective tissue in the lung change after birth. The neonatal lung has abundant Type III and Type IV collagen, but little Type I collagen. Types III and IV collagen are not mechanically strong, suggesting that the neonatal lung has a phenotype that facilitates the changes in cell shape and orientation which characterize adaptation to extrauterine life. It is probable that the rapid deposition of Type I collagen postnatally contributes structural stiffness to the blood vessel walls.

Lung volume increases most rapidly during the first year of life and also increases more rapidly than airway calibre during this time, a finding which is consistent with the concept of dysanaptic lung growth, i.e. a dissociation between the growth of the lung parenchyma and the airways. Most of the alveoli have been formed by two years of age, and subsequent lung growth primarily results from an increase in the size of individual alveoli. Forced vital capacity is reported to be the same in male and female infants and very young children (Jones et al 2000), which is consistent with post mortem morphometric measurements. By the second year of life the rate of increase in forced vital capacity is similar to that reported in older children.

During childhood, lung volumes and flow rates increase linearly with height, with a greater intercept and more positive slope in boys compared to girls (Rosenthal et al 1993a & b). The linearity of these changes is interrupted by puberty, when important shape changes occur in the thorax, particularly in boys, and are mirrored by changes in the size of the underlying lung (and in particular the alveoli) as manifest by an abrupt increase in vital capacity, forced expired volume in one second, and total lung capacity. Thereafter, the change in lung size again proceeds in a linear fashion. High resolution computed tomographic scanning has been used to measure airway dimensions, although this technique should be used with caution because of the radiation exposure. Alveolar size can now be measured using hyperpolarized helium, using magnetic resonance imaging.

Normal postnatal pulmonary arterial development

Immediately after birth, dramatic remodelling of the pulmonary vasculature occurs to effect an abrupt reduction of pulmonary vascular resistance. This process continues at a rapid rate throughout the first 1–2 months, while the lungs adapt to extrauterine life, and then more slowly throughout childhood. Failure to remodel in the presence of an anatomically normal heart leads to persistent pulmonary hypertension. Normal postnatal pulmonary arterial development in the full-term neonate can be divided into three stages:

Stage one

lasts from birth to about postnatal day 4, and concerns the immediate adaptation to extrauterine life. At birth, the endothelial cells of the precapillary arteries are squat and have narrow bases on the subendothelium, a low surface : volume ratio and many surface projections. Five minutes after birth, the endothelial cells are thinner and gradually show less cell overlap, the surface : volume ratio increases, few cell projections are seen, the vessel wall becomes thinner and the lumen diameter increases (Fig. 59.23). The smooth muscle cells show a significant reduction in diameter during this time.

Fig. 59.23 En face views (left) and transverse sections (right) showing the changes in the endothelial and smooth muscle cells of small muscular pulmonary arteries accompanying terminal bronchi from the neonatal period to 3 weeks after birth.

Stage two

lasts from around day 4 to 3–4 weeks, and is the time when the cells deposit matrix around themselves to fix their new positions. At birth, the internal elastic lamina of the small muscular arteries consists only of amorphous elastin in a basal lamina-like matrix. By 3 weeks of age, a definitive elastic lamina is evident, although it is heavily fenestrated, permitting contact between the endothelial cells and the smooth muscle cells.

Stage three

continues into adulthood. The intrapulmonary arteries increase in size and their walls increase in thickness. However, the maturation of all of the pulmonary vascular smooth muscle cells, from the hilum to the precapillary bed, is not advanced until 2 years. As the distal airspaces expand, the capillary nets fuse from one alveolus to another, forming, for a period, an extensive double capillary net: this process can be seen from postnatal day 28, becomes more extensive by 1.5 years, and it is believed to be complete by 5 years.

CONGENITAL MALFORMATIONS OF THE TRACHEA, BRONCHI AND LUNGS

This is an enormous topic and only a small number of abnormalities can be mentioned here (for further information, see Bush 2001; Abel et al 2006). The concept of the lung as six ‘trees’ has been mentioned earlier: all but the systemic venous tree may contribute to a congenital thoracic malformation (CTM). In addition, congenital abnormalities of the heart and great vessels, and of the chest and abdominal walls, including neuromuscular disease, may impact on lung development. It is somewhat artificial to describe airway malformations in isolation, and the possibility of associated vascular abnormalities must always be considered. Moreover, descriptions of what is seen clinically should be kept separate from speculations about the embryological origins of the abnormality.

Disorders of the proximal airways

Tracheomalacia and bronchomalacia

Abnormalities in cartilage development lead to the affected airways being ‘floppy’, and collapsing during inspiration or expiration, depending on whether they are outside or inside the bony thorax, respectively: these abnormalities are termed tracheomalacia when the trachea is affected, and bronchomalacia when the bronchi are involved. The abnormality may involve a small, localized area, or may be more generalized (e.g. Williams–Campbell syndrome – congenital deficiency of the airway cartilage – in which there is diffuse bronchomalacia from the second to the seventh generation of bronchi). Tracheobronchomalacia usually presents in early infancy with cough, tachypnoea, stridor and wheeze; it may also be associated with cardiac or respiratory abnormalities, such as absent pulmonary valve syndrome, or tracheo-oesophageal fistula, respectively. Patients with significant obstruction and apnoea may be treated by the insertion of airway stents if the disease is localized; generalized disease may mandate respiratory support, either with non-invasive mask ventilation during sleep, or via a tracheostomy, depending on severity. Surgical treatment by aortopexy is controversial.

Tracheo-oesophageal fistulae

Tracheo-oesophageal fistulae are the most common abnormalities of the lower respiratory tract, and occur in about 1 in 3000 births. They usually present in the newborn with recurrent respiratory distress and choking spells, although late presentation in adult life has been described. The anomaly may be associated with oesophageal atresia. Normally, the oesophagus lengthens with the rapid elongation of the embryo up to 7 weeks. The rapid proliferation of the oesophageal lumen partially obliterates the mucosa and recanalization is not complete until 8 weeks. The cellular processes that lead to separation of the trachea and oesophagus occasionally produce oesophageal atresia. Tracheo-oesophageal atresia is rare, with an incidence of 1 in 3000 to 1 in 4500 births. Five types of tracheo-oesophageal fistulae may be recognized (Fig. 59.24). In almost all cases, the oesophagus ends blindly and the stomach is connected to the lower end of the trachea. Because of this connection, the abdomen becomes rapidly distended with air once the baby is delivered and starts breathing. Prenatally, polyhydramnios may be a clinical feature, but may not be apparent until the third trimester. In two of the five types of tracheo-oesophageal fistulae there is no communication between the stomach and the upper gut. Such cases are identifiable on ultrasound, because the stomach should always be visible at a 20-week examination, and its absence on ultrasound should prompt further evaluation.

Fig. 59.24 Examples of tracheo-oesophageal fistulae and oesophageal atresia.

Tracheo-oesophageal fistula is commonly associated with other congenital anomalies, including cardiovascular defects (30%), anorectal (15%) and genitourinary anomalies (15%). These defects may be combined in the VATER syndrome, which involves vertebral, anorectal, cardiac and oesophageal anomalies, together with radial aplasia and the presence of a single umbilical artery.

Complete cartilage rings

The normal large airway cartilages are horseshoe shaped. Complete cartilage rings may develop in the trachea and large airways, and the posterior membranous part of the airway may be absent in a single cartilage or in long segments of the airway. This abnormality may be associated with vascular abnormalities such as pulmonary artery sling (where the left pulmonary artery originates from the right pulmonary artery, and not from the main pulmonary trunk). Extensive severe disease may present with neonatal respiratory distress; later presentations include apparently steroid-resistant asthma. Tracheal transplantation has been used to treat long segment tracheal stenosis.