Septum transversum and intra-embryonic coelom

Before folding

The septum transversum is a sheet of mesoderm that appears on day 22 rostral to the developing heart (Fig. 3.1). Before folding, the intra-embryonic coelom appears as a horseshoe- or U-shaped cavity (Fig. 3.2). The bend of the U lies anteriorly, and represents the part of the coelomic cavity from which the pericardial cavity will develop. Each limb of the U consists of two cavities: (1) a pericardioperitoneal canal, from which the pleural cavity will develop, and (2) a peritoneal cavity which lies in the future abdomen. In the umbilical region, the peritoneal cavities in each limb open into the chorionic cavity or the extra-embryonic coelom. This communication allows the herniation of midgut loops into the umbilical cord (see Chapter 7).

Fig. 3.1 Sagittal section of a 3-week embryo showing the position of the septum transversum rostral to the pericardial cavity.

Fig. 3.2 The ventrolateral view of the intra-embryonic coelom in a 4-week embryo with surrounding tissues removed.

After folding

As a result of the enlarging head fold, the heart, with its pericardial cavity, swings ventral to the foregut. The septum transversum, initially cranial to the pericardial cavity, finally is wedged between the heart and the neck of the yolk sac. The pericardial cavity now opens into the paired pericardioperitoneal canals, which run on either side of the foregut to reach the peritoneal cavity. These canals end in two openings on the dorsolateral aspect of the foregut; therefore, at this stage the septum transversum is an incomplete partition between the thorax and abdomen (Fig. 3.3). The septum transversum forms the central tendon of the diaphragm.

Fig. 3.3 Longitudinal section of a 25-day embryo showing the head and tail folds. Note the position of the septum transversum between the pericardial cavity and the yolk sac.

Division of the intra-embryonic coelom into four cavities

Paired mesenchymal ridges grow from the lateral body wall into the pericardioperitoneal canals and separate these paired cavities from the pericardial and peritoneal cavities (Fig. 3.4). When invaginated by the lung buds, the pericardioperitoneal canals become the pleural cavities. The pleuropericardial folds separate the pleural cavity from the pericardial cavity, and the more substantial caudal partitions, which separate the pleural cavity from the peritoneal cavity, are known as the pleuroperitoneal membranes. The pleuroperitoneal membranes are at the level of the septum transversum.

Fig. 3.4 The ventrolateral view of the embryo showing the pericardial cavity, pericardioperitoneal canals and their associated folds and membranes with surrounding tissues removed. The arrows indicate communication between the pericardioperitoneal canals and the peritoneal cavity.

As the heart descends, the pleuropericardial folds migrate ventrally and fuse with each other forming the fibrous pericardium. The thoracic cavity is then divided into a definitive pericardial cavity and two lateral pleural cavities. The phrenic nerves, which have been located within the pleuropericardial folds, run through the fibrous pericardium in the adult to supply the diaphragm.

The diaphragm

Initially, the septum transversum forms an incomplete partition between the thoracic and abdominal cavities, due to the presence of the pericardioperitoneal canals. Soon a number of mesodermal structures fuse with each other at the level of the septum transversum to form the definitive diaphragm. Five structures, described below, contribute to the development of the diaphragm (Fig. 3.5).

Fig. 3.5 Formation of the diaphragm as seen from above. (A) At week 5 the pleuroperitoneal membranes are growing towards the septum transversum. (B) At week 7 the membranes have fused with the septum transversum. Note the arrows indicating ingrowth of the body wall. (C) The components of the diaphragm in a newborn. The outlines of the pleuroperitoneal canals may still be seen, and are the potential sites of congenital defects.

The cervical somites

During the fourth week, the heart and septum transversum lie in the cervical region opposite the third, fourth and fifth cervical somites. At this stage the myoblasts from these somites migrate into the septum transversum and differentiate into the diaphragmatic musculature. Because of its origin the muscle of the diaphragm is innervated by the nerves of third, fourth and fifth segments of the cervical spinal cord. The nerve fibres from these segments join to form the phrenic nerve. The final thoracic position of the diaphragm is reached during the sixth week after descent of the heart and formation of the neck. This accounts for the cervical origin of the nerves supplying the diaphragm.

Clinical box

Diaphragmatic herniae

If one of the pleuroperitoneal canals fails to close, the abdominal viscera may enter the pleural cavity (Fig. 3.6). This condition, called a diaphragmatic hernia, is usually on the left side. Because of the presence of the abdominal viscera in the chest, the heart is displaced, and the lungs are reduced in size. Consequently, the affected child may have severe respiratory problems even after the hernia is surgically repaired.

Fig. 3.6 A left posterolateral defect in the diaphragm seen in a newborn from below.

Sometimes, congenital herniae occur through small gaps in the anterior part of the diaphragm near its sternal attachment or through the oesophageal opening. These herniae, known as the parasternal and oesophageal hernia respectively, do not usually have any clinical significance. Acquired diaphragmatic herniae usually occur through the oesophageal opening and are termed hiatal herniae. Such herniae may have a peritoneal covering, and be associated with digestive disturbances.

Summary box

The space formed within the lateral plate mesoderm becomes the intra-embryonic coelom lined by the somatic and visceral layers of lateral plate mesoderm.

The somatic mesodermal layer becomes the parietal serous membrane and the splanchnic mesodermal layer becomes the visceral serous membrane.

After folding of the embryo, partitions divide this coelom into pericardial, pleural and peritoneal cavities.

The pericardial cavity is separated from the peritoneal cavity by the septum transversum.

The pleuropericardial folds separate the pleural from the pericardial cavities and form the fibrous pericardium.

The septum transversum and the pleuroperitoneal membranes contribute to major areas of the diaphragm.

Defects of the diaphragm give rise to congenital herniae in which the abdominal viscera may be pushed into the thoracic cavity.