Head and Neck

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Chapter 14

Head and Neck

Among the earliest vertebrates, the cranial region consisted of two principal components: (1) a chondrocranium, associated with the brain and the major sense organs (nose, eye, ear); and (2) a viscerocranium, a series of branchial (pharyngeal) arches associated with the oral region and the pharynx (Fig. 14.1A). As vertebrates became more complex, the contributions of the neural crest to the head became much more prominent, and the face and many dermal (intramembranously formed) bones of the skull (dermocranium) were added. With the early evolution of the face, the most anterior of the branchial arches underwent a transformation to form the upper and lower jaws and two of the middle ear bones, the malleus and the incus. Along with an increase in complexity of the face (Fig. 14.1B) came a corresponding increase in complexity of the forebrain (telencephalon and diencephalon). From structural and molecular aspects, the rostral (anteriormost) part of the head shows distinctly different characteristics from the pharyngeal region, as follows:

1. The pharyngeal region and hindbrain are highly segmented (see Fig. 14.3), whereas segmentation is less evident in the forebrain and rostral part of the head.

2. Structural segmentation in the pharyngeal region is associated with complex segmental patterns of gene expression (see Fig. 11.12).

3. Formation of the forebrain and associated structures of the rostral part of the head depends on the actions of specific genes (e.g., Lim1 [see Fig. 5.9], Emx1, Emx2, Otx1, and Otx2) and inductive signaling by the prechordal mesoderm or anterior visceral endoderm.

4. Much of the connective tissue and skeleton of the rostral (phylogenetically newer) part of the head is derived from the neural crest. The anterior end of the notochord, terminating at the hypophysis, constitutes the boundary between the mesodermally derived chondrocranium and the more rostral neural crest–derived chondrocranium. Neural crest cells are also prominent contributors to the ventral part of the pharyngeal region.

In previous chapters, the development of certain components of the head (e.g., the nervous system, neural crest, bones of the skull) is detailed. The first part of this chapter provides an integrated view of early craniofacial development to show how the major components are interrelated. The remainder of the chapter concentrates on the development of the face, pharynx, and pharyngeal arch system. Clinical Correlations 14.1 and 14.2, which appear later in the chapter, present malformations associated with the head and neck.

Early Development of the Head and Neck

Development of the head and neck begins early in embryonic life and continues until the cessation of postnatal growth in the late teens. Cephalization begins with the rapid expansion of the rostral end of the neural plate. Very early, the future brain is the dominant component of the craniofacial region. Beneath the brain, the face, which does not take shape until later in embryogenesis, is represented by the stomodeum (Fig. 14.2). In the early embryo, the stomodeum is sealed off from the primitive gut by the oropharyngeal membrane, which breaks down by the end of the first embryonic month (see Fig. 6.23). Surrounding the stomodeum are several tissue prominences that constitute the building blocks of the face (see Fig. 14.6). In keeping with its origin from the anterior neural ridge and its later serving as the tissue of origin of Rathke’s pouch, the ectoderm of the oropharyngeal membrane is first characterized by its expression of the homeodomain transcription factor Pitx-2. In the rostral midline is the frontonasal prominence, which is populated by mesenchymal cells derived from forebrain and some midbrain neural crest. On either side of the frontonasal prominence, ectodermal nasal placodes, which arose from the anterior neural ridge (see p. 95), develop into horseshoe-shaped structures, each consisting of a nasomedial process, also derived from forebrain neural crest, and a nasolateral process, derived from midbrain neural crest. Farther caudally, the stomodeum is bounded by maxillary and mandibular processes, which are also filled with neural crest–derived mesenchyme.

The future cervical region is dominated by the pharyngeal apparatus, consisting of a series of pharyngeal pouches, arches, and clefts. Many components of the face, ears, and glands of the head and neck arise from the pharyngeal region. Also prominent are the paired ectodermal placodes (see Fig. 13.1), which form much of the sensory tissue of the cranial region.

Tissue Components and Segmentation of Early Craniofacial Region

The early craniofacial region consists of a massive neural tube beneath which lie the notochord and a ventrally situated pharynx (see Fig. 14.2). The pharynx is surrounded by a series of pharyngeal arches. Many of the tissue components of the head and neck are organized segmentally. Figure 14.3 illustrates the segmentation of the tissue components of the head. As discussed in earlier chapters, distinct patterns of expression of certain homeobox-containing genes are associated with morphological segmentation in some cranial tissues, particularly the central nervous system (see Fig. 11.12). The chain of events between segmental patterns of gene expression and the appearance of morphological segmentation in parts of the cranial region remains incompletely understood.

Fundamental Organization of the Pharyngeal Region

Because many components of the face are derived from the pharyngeal region, an understanding of the basic organization of this region is important. In a 1-month-old embryo, the pharyngeal part of the foregut contains four lateral pairs of endodermally lined outpocketings called pharyngeal pouches and an unpaired ventral midline diverticulum, the thyroid primordium (Fig. 14.4). If the contours of the ectodermal covering over the pharyngeal region are followed, bilateral pairs of inpocketings called pharyngeal grooves, which almost make contact with the lateralmost extent of the pharyngeal pouches, are seen (see Fig. 14.4C).

Alternating with the pharyngeal grooves and pouches are paired masses of mesenchyme called pharyngeal (branchial) arches. Central to each pharyngeal arch is a prominent artery called an aortic arch, which extends from the ventral to the dorsal aorta (see Chapter 17 and Fig. 14.2). The mesenchyme of the pharyngeal arches is of dual origin. The mesenchyme of the incipient musculature originates from mesoderm, specifically the somitomeres. Much of the remaining pharyngeal arch mesenchyme, especially that of the ventral part, is derived from the neural crest, whereas mesoderm makes various contributions to the dorsal pharyngeal arch mesenchyme.

Establishing the Pattern of the Craniofacial Region

Establishment of the fundamental structural pattern of the craniofacial region is a complex process that involves interactions among numerous embryonic tissues. Major players are as follows: the neural tube, which acts as a signaling center and gives rise to the cranial neural crest; the paraxial mesoderm; the endoderm of the pharynx; and the cranial ectoderm.

Very early in development, the cranial neural tube becomes segmented on the basis of molecular instructions, based largely on Hox gene expression (see Fig. 11.12), and this coding spills over into the neural crest cells that leave the neural tube (see Fig. 12.8). More recently, investigators have recognized that the pharyngeal endoderm also exerts a profound patterning influence on facial development. Patterning of the pharyngeal endoderm itself is heavily based on its exposure to retinoic acid. Formation of the first pharyngeal pouch does not require retinoic acid, but pharyngeal pouches 3 and 4 have an absolute requirement for retinoic acid, whereas pouch 2 needs some, but not so much, exposure to retinoic acid.

Formation of the pharyngeal arches depends on signals from the pharyngeal pouches. Even though neural crest cells are major contributors to the underlying tissues of the pharyngeal arches, experiments have shown that neural crest is not required for the formation or patterning of the pharyngeal arches. In almost all aspects of lower facial morphogenesis, the development of neural crest derivatives depends on signals from cranial ectoderm, but the ectoderm is prepatterned by signals (importantly fibroblast growth factor-8 [FGF-8]) emanating from the pharyngeal endoderm (Fig. 14.5A).

Development of individual pharyngeal arches depends on various sets of molecular instructions. The first arch, which forms the upper and the lower jaws, is not included in the overall Hox code that underlies development of the remainder of the arches and determines their anteroposterior identity (see Fig. 12.8). Within individual pharyngeal arches, a code based on the homeobox-containing Dlx genes heavily influences dorsoventral patterning (see p. 301). Other molecular influences also strongly affect patterning of aspects of pharyngeal arch development. A major force in the patterning of pharyngeal arch 1 is endothelin-1 (Edn-1), which is secreted by the ectoderm of the arches and combines with its receptor (Ednr) on migrating neural crest cells. Although expressed on all of the pharyngeal arches, Edn-1 exerts its most prominent effect on the development of the first arch through its effects on Dlx expression.

A prominent feature of the early developing face is the unpaired frontonasal prominence, which constitutes the rostralmost part of the face (see Fig. 14.6). Originating over the bulging forebrain, the frontonasal process is filled with cranial neural crest. These neural crest cells are targets of a signaling center in the overlying ectoderm, called the frontonasal ectodermal zone (see Fig. 14.5B). This signaling center, which itself is induced by sonic hedgehog (shh) emanating from the forebrain, is an area where dorsal ectodermal cells expressing FGF-8 confront ventral ectodermal cells, which express shh. This confluence of ectodermal signals acts on the underlying neural crest cells to shape the tip of the snout. Mammals and other species with broad faces have bilateral frontonasal ectodermal zones, located at the tips of the nasomedial processes (see Fig. 14.6). In birds, which have a narrow midface tapering into a beak, the two frontonasal ectodermal zones fuse into a single signaling center. In avian embryos, transplantation of the facial ectodermal zone into an ectopic region results in the formation of a second beak.

Cellular Migrations and Tissue Displacements in the Craniofacial Region

Early craniofacial development is characterized by several massive migrations and displacements of cells and tissues. The neural crest is the first tissue to exhibit such migratory behavior, with cells migrating from the nervous system even before closure of the cranial neural tube (see Chapter 12). Initially, segmental groups of neural crest cells are segregated, especially in the pharyngeal region (see Fig. 14.3). These populations of cells become confluent, however, during their migrations through the pharyngeal arches. Much of the detailed anatomy of the facial skeleton and musculature is based on the timing, location, and interactions of individual streams of neural crest and mesodermal cells. Recognition of this level of detail (which is beyond the scope of this text) is important in understanding the basis underlying many of the numerous varieties of facial clefts that are seen in pediatric surgical clinics.

The early cranial mesoderm consists mainly of the paraxial and prechordal mesoderm (see Fig. 14.3). Although the paraxial mesoderm rostral to the occipital somites has been traditionally considered to be subdivided into somitomeres (see Fig. 6-8), some embryologists now classify it as being unsegmented mesoderm (see Fig. 14.3). Mesenchymal cells originating in the paraxial mesoderm form the connective tissue and skeletal elements of the caudal part of the cranium and the dorsal part of the neck. Within the pharyngeal arches, cells from the paraxial mesoderm initially form a mesodermal core, which is surrounded by cranial neural crest cells (see Fig. 14.5A). Myogenic cells from paraxial mesoderm undergo extensive migrations to form the bulk of the muscles of the cranial region. Similar to their counterparts in the trunk and limbs, these myogenic cells become integrated with local connective tissue to form muscles. Another similarity with the trunk musculature is that morphogenetic control seems to reside within the connective tissue elements of the muscles, rather than in the myogenic cells themselves. In the face and ventral pharynx, this connective tissue is of neural crest origin.

The prechordal mesoderm, which emits important forebrain inductive signals in the early embryo, is a transient mass of cells located in the midline, rostral to the tip of the notochord. Although the fate of these cells is controversial, some investigators believe that myoblasts contributing to the extraocular muscles take origin from these cells. On their way to the eye, cells of the prechordal mesoderm may pass through the rostralmost paraxial mesoderm.

The lateral plate mesoderm is not well defined in the cranial region. Transplantation experiments have shown that it gives rise to endothelial and smooth muscle cells and, at least in birds, to some portions of the laryngeal cartilages.

Another set of tissue displacements important in the cranial region is the joining of cells derived from the ectodermal placodes with cells of the neural crest to form parts of sense organs and ganglia of certain cranial nerves (see Fig. 13.1).

Development of the Facial Region

Formation of the Face and Jaws

Development of the face and jaws is a complex three-dimensional process involving the patterning, outgrowth, fusion, and molding of various tissue masses. The forebrain acts as a mechanical substrate and a signaling center for early facial development, and the stomodeum serves as a morphological point of reference. The lower face (maxillary region and lower jaw) is phylogenetically derived from a greatly expanded first pharyngeal arch. Much of the mesenchyme of the face is neural crest, originating from the forebrain to the first two rhombomeres. Each of the tissue components of the early face is the product of a unique set of morphogenetic determinants and growth signals, and increasing evidence indicates that specific sets of molecular signals control their development along the proximodistal and the rostrocaudal axes.

At a higher level, the building blocks of the face relate to one another in highly specific ways, and clues to their origins and relationships can be derived by examination of their blood supply. Disturbances at this level regularly result in the production of craniofacial anomalies, and an understanding of the fundamental elements of facial morphogenesis is crucial to rational surgical approaches to these malformations.

Structures of the face and jaws originate from several primordia that surround the stomodeal depression of a 4- to 5-week-old human embryo (Fig. 14.6). These primordia consist of the following: an unpaired frontonasal prominence; paired nasomedial and nasolateral processes, which are components of the horseshoe-shaped olfactory (nasal) primordia; and paired maxillary processes and mandibular prominences, both components of the first pharyngeal arches. The upper jaw contains a mixed population of neural crest cells derived from the forebrain and midbrain, whereas the lower jaw contains mesenchymal cells derived from midbrain and hindbrain (rhombomeres 1 and 2) neural crest. The specific morphology of facial skeletal elements is determined by signals passed from the pharyngeal endoderm to the facial ectoderm and then to the neural crest precursors of the facial bones. Narrow zones of pharyngeal endoderm control the morphogenesis of specific portions of the skeleton of the lower face. FGF-8 signaling from the facial ectoderm plays a key role in patterning of the facial skeleton.

Another factor that strongly influences facial form is the responsiveness of the various facial processes to Wnts. In many developing structures, Wnt signaling stimulates cellular proliferation that increases the mass of that structure. In facial development, species with an elongated midface (e.g., the beak of birds) have a midline zone of Wnt responsiveness within the frontonasal process. Other species (e.g., humans) that have flat but broad faces have Wnt-responsive regions in the maxillary and mandibular processes, thus supporting lateral growth in the face.

The frontonasal process is a prominent structure in the earliest phases of facial development, and its formation is the result of an exquisitely sensitive signaling system that begins with the synthesis of retinoic acid in a localized region of ectoderm opposite the forebrain and continues with the action of shh produced by the ventral forebrain. The action of shh, through the mediation of the most rostral wave of neural crest cells, underlies the establishment of the frontonasal ectodermal zone, located at the tips of the nasomedial processes (see p. 298). The signaling molecules (FGF-8 and shh) emanating from this zone stimulate cell proliferation in the neural crest mesenchyme of the frontonasal process. In the absence of such signaling, cell death in the region increases, and cell proliferation decreases, resulting in various midfacial defects (see Clinical Correlation 14.1). Retinoic acid is unusual in that both deficiencies and excess amounts can cause very similar defects. From 4 to 5 weeks, the frontonasal process is a dominant structure of the early face (see Fig. 14.6), but with subsequent growth of the maxillary process and the nasomedial and nasolateral processes, it recedes from the oral region. The nasolateral process develops as a result of FGF signaling emanating from the nasal pit.

The maxillary and mandibular processes have traditionally been considered to be derivatives of the first pharyngeal arch. More recent research has suggested that although some of the cells that form the maxillary process arise from the first arch, many mesenchymal cells of the maxillary process are not first-arch derivatives, but instead come from other areas of cranial neural crest. How these cells are integrated into a unified structure and what controls their specific morphogenesis remain to be determined.

As with the limb buds, outgrowth of the frontonasal prominence and the maxillary and mandibular processes depends on mesenchymal-ectodermal interactions. In contrast to the limb, however, the signaling system (FGF and shh) is concentrated in the apical ectoderm of these processes, where it may act as a morphogenetic organizer and a stimulus for mesenchymal outgrowth of the facial primordia. The homeobox-containing gene Msx1 is expressed in the rapidly proliferating mesenchyme at the tips of the facial primordia. The parallel with expression of Msx1 in the subectodermal region of the limb (see p. 200) suggests that similar mechanisms operate in outgrowing limb and facial primordia. Hox genes are not expressed in the first arch, and the presence of Otx-2 distally, coupled with the absence of Hox proximally, provides the molecular basis for the development of the first arch.

The subdivision of the first arch into maxillary and mandibular regions is controlled to a large extent by endothelin-1. Expressed at the distal (ventral) tip of the arch, endothelin-1 effectively represses the local expression of genes, such as Dlx-1/2, that are heavily involved in formation of the proximal maxilla. Distally, endothelin-1 promotes the expression of distal genes, such as Dlx-5/6 and their downstream targets (Hand-2 and Goosecoid), which pattern the mandible. At an intermediate dorsoventral level within the first arch, endothelin-1 stimulates the expression of Barx-1, which is a prime determinant of the formation of the mandibular joint. When endothelin-1 is mutated or inactivated, the mandible becomes transformed into a structure resembling the maxilla. If endothelin-1 is overexpressed in the proximal part of the first arch, the future maxilla becomes transformed into a mandible. This effect is transmitted through the activation of Dlx-5/6 (see later). In the proximal (dorsal) part of the first arch, the influence of endothelin-1 is reduced, and active patterning genes lay the foundation for the formation of both the maxilla and the middle ear bones (malleus, incus, and tympanic ring).

Despite the relatively featureless appearance of the early mandibular process (first pharyngeal arch), the mediolateral (oral-aboral) and proximodistal axes are tightly specified. This recognition has considerable clinical significance because increasing numbers of genetic mutants are recognized to affect only certain regions of the arch, such as the absence of distal (adult midline) versus proximal structures. The medial (oral) region of the mandibular process, which seems to be the driver of mandibular growth, responds to local epithelial signals (FGF-8) by stimulating proliferation of the underlying mesenchyme through the mediation of Msx-1, similar to the subectodermal region of the limb bud. Growth of the jaws is influenced by various growth factors, especially the bone morphogenetic proteins (BMPs), which, at different stages, are produced in either the ectoderm or the mesenchyme and can have strikingly different effects. Experiments on avian embryos have shown that increasing the expression of BMP-4 in first-arch mesenchyme results in the formation of a much more massive beak than that of normal embryos.

Proximodistal organization of the arch is reflected by nested expression patterns of the transcription factor Dlx (the mammalian equivalent of distalless in Drosophila) along the arch. The paralogues Dlx-3 and Dlx-7 (Dlx-4) are expressed most distally, expression of Dlx-5 and Dlx-6 extends more proximally, and expression of Dlx-1 and Dlx-2 extends most proximally. In mice, only Dlx-1 and Dlx-2 are expressed in the maxillary process. Although mutants of individual Dlx genes produce minor abnormalities, mice in which Dlx-5 and Dlx-6 have been knocked out develop with a homeotic transformation of the distal lower jaws into upper jaws. It seems that Dlx-5 and Dlx-6, functioning downstream of endothelin-1, are selector genes that control the rostrocaudal identity of the distal segments of the first pharyngeal arch.

Through differential growth between 4 and 8 weeks (see Fig. 14.6), the nasomedial and maxillary processes become more prominent and ultimately fuse to form the upper lip and jaw (Fig. 14.7). As this is occurring, the frontonasal prominence, which was a prominent tissue bordering the stomodeal area in the 4- and 5-week-old embryo, is displaced as the two nasomedial processes merge. The merged nasomedial processes form the intermaxillary segment, which is a precursor for (1) the philtrum of the lip, (2) the premaxillary component of the upper jaw, and (3) the primary palate.

Between the maxillary process and the nasal primordium (nasolateral process) is a nasolacrimal groove (naso-optic furrow) that extends to the developing eye (see Fig. 14.6). The ectoderm of the floor of the nasolacrimal groove thickens to form a solid epithelial cord, which detaches from the groove. The epithelial cord undergoes canalization and forms the nasolacrimal duct and, near the eye, the lacrimal sac. The nasolacrimal duct extends from the medial corner of the eye to the nasal cavity (inferior meatus) and, in postnatal life, acts as a drain for lacrimal fluid. This connection explains why people can have a runny nose when crying. Meanwhile, the expanding nasomedial process fuses with the maxillary process, and over the region of the nasolacrimal groove, the nasolateral process merges with the superficial region of the maxillary process. The region of fusion of the nasomedial and maxillary processes is marked by an epithelial seam, called the nasal fin. Mesenchyme soon penetrates the nasal fin, and the result is a continuous union between the nasomedial and maxillary processes.

The lower jaw is formed in a simpler manner. The bilateral mandibular prominences enlarge, and their medial components merge in the midline, to form the point of the lower jaw. The midline dimple that is seen in the lower jaw of some individuals is a reflection of variation in the degree of merging of the mandibular prominences. A prominent cartilaginous rod called Meckel’s cartilage differentiates within the lower jaw (see Fig. 14.36D). Derived from neural crest cells of the first pharyngeal arch, Meckel’s cartilage forms the basis around which membrane bone (which forms the definitive skeleton of the lower jaw) is laid down. Experimental evidence indicates that the rodlike shape of Meckel’s cartilage is related to the inhibition of further chondrogenesis by the surrounding ectoderm. If the ectoderm is removed around Meckel’s cartilage, large masses of cartilage form instead of a rod. These properties are similar to the inhibitory interactions between ectoderm and chondrogenesis in the limb bud. A later-acting influence in growth of the lower jaw is the planar cell polarity pathway, which influences outgrowth of Meckel’s cartilage. If this pathway is disrupted, the mandible will not develop to its normal length.

Shortly after the basic facial structures take shape, they are invaded by mesodermal cells associated with the first and second pharyngeal arches. These cells form the muscles of mastication (first-arch derivatives, which are innervated by cranial nerve V) and the muscles of facial expression (second-arch derivatives, which are innervated by cranial nerve VII). At the level of individual muscles, highly coordinated spatiotemporal relationships between mesodermal and neural crest cells are very important in the determination of muscle attachments and the overall shape of the muscles.

Although the basic structure of the face is established between 4 and 8 weeks, changes in the proportionality of the various regions continue until well after birth. In particular, the midface remains underdeveloped during embryogenesis and early postnatal life.

Temporomandibular Joint and Its Relationship with the Jaw Joint of Lower Vertebrates

Of considerable clinical importance and evolutionary interest is the temporomandibular joint, which represents the hinge between the mandibular condyle and the squamous part of the temporal bone. The temporomandibular joint, which phylogenetically appeared with the evolution of mammals, is a complex synovial joint surrounded by a capsule and containing an articular disk between the two bones. Based on the early expression of Barx-1, this joint is formed late during development, first appearing as mesenchymal condensations associated with the temporal bone and mandibular condyle during the seventh week of development. The articular disk and capsule begin to take shape a week later, and the actual joint cavity forms between weeks 9 and 11.

In lower vertebrates, the jaw opens and closes on a hinge between cartilaginous portions of the mandibular process—the articular bone in the lower jaw and the quadrate bone in the upper jaw, both derivatives of Meckel’s cartilage. During phylogenesis, the distal membranous bone (the dentary bone) associated with Meckel’s cartilage increased in prominence as the jaw musculature became more massive. The dentary bone of contemporary mammals and humans constitutes most of the lower jaw, and Meckel’s cartilage is seen only as a prominent cartilaginous rod within the forming jaw complex during the late embryonic stage of development.

In mammals, over many millions of years, the original jaw-opening joint became less prominent and was incorporated into the middle ear as the malleus (derivative of articular bone of the lower jaw) and the incus (derivative of the ancestral quadrate bone in the skull). The incus connects with the stapes (a derivative of the second pharyngeal arch). The tympanic ring, a neural crest–derived bone that surrounds and supports the tympanic membrane, is derived from the angular bone, one of the first-arch membrane bones that overlies the proximal part of Meckel’s cartilage.

Formation of the Palate

The early embryo possesses a common oronasal cavity, but in humans the palate forms between 6 and 10 weeks to separate the oral from the nasal cavity. The palate is derived from three primordia: an unpaired median palatine process and a pair of lateral palatine processes (Figs. 14.8 and 14.9).

The median palatine process is an ingrowth from the newly merged nasomedial processes. As it grows, the median palatine process forms a triangular bony structure called the primary palate. In postnatal life, the skeletal component of the primary palate is referred to as the premaxillary component of the maxilla. The four upper incisor teeth arise from this structure (Fig. 14.10).

Formation of the palate involves (1) growth of the palatal shelves, (2) their elevation, (3) their fusion, and (4) removal of the epithelial seam at the site of fusion. The lateral palatine processes, which are the precursors of the secondary palate, first appear as outgrowths of the maxillary processes during the sixth week. At first, they grow downward on either side of the tongue (Fig. 14.11). Similar to other facial primordia, outgrowth of the palatal shelves involves ectodermal-mesenchymal interactions and specific growth factors. FGF-10 produced in the mesenchyme of the forming palatal shelf is bound to an FGF receptor in the ectoderm (Fig. 14.12). This process stimulates the release of shh from the ectoderm. Shh causes the release of BMP-2 in the mesenchyme. BMP-2 and Msx-1, which interacts with BMP-4, stimulate proliferation of the mesenchymal cells of the palatal shelf and its resultant growth. During week 7, the lateral palatine processes (palatal shelves) dramatically dislodge from their positions alongside the tongue and become oriented perpendicularly to the maxillary processes. The apices of these processes meet in the midline and begin to fuse.

Despite many years of investigation, the mechanism underlying the elevation of the palatine shelves remains obscure. Swelling of the extracellular matrix of the palatal shelves seems to impart a resiliency that allows them to approximate one another shortly after they become dislodged from along the tongue. Research suggests that the rapid closure of the palatal shelves is accomplished by the flowing of the internal tissues, rather than by a reaction that approximates the closure of swinging doors.

Another structure involved in formation of the palate is the nasal septum (see Figs. 14.8 and 14.11). This midline structure, which is a downgrowth from the frontonasal prominence, reaches the level of the palatal shelves when the palatal shelves fuse to form the definitive secondary palate. Rostrally, the nasal septum is continuous with the primary palate.

At the gross level, the palatal shelves fuse in the midline, but rostrally they also join the primary palate. The midline point of the fusion of the primary palate with the two palatal shelves is marked by the incisive foramen (see Fig. 14.10).

Because of its clinical importance, fusion of the palatal shelves has been investigated intensively. When the palatal shelves first make midline contact, each is covered throughout by a homogeneous epithelium. During the process of fusion, however, the midline epithelial seam disappears. The epithelium on the nasal surface of the palate differentiates into a ciliated columnar type, whereas the epithelium takes on a stratified squamous form on the oral surface of the palate. Significant developmental questions include the following:

The disappearance of the midline epithelial seam after the approximation of the palatal shelves involves several fundamental developmental processes (Fig. 14.13). Some of the epithelial cells at the fusion seam undergo apoptosis and disappear. Others may migrate out from the plane of fusion and become inserted into the epithelial lining of the oral cavity. Still other epithelial cells undergo a morphological transformation into mesenchymal cells. Transforming growth factor-β3 (TGF-β3) is expressed by the ectodermal cells of the distal rim of the palatal shelves just before fusion and loses prominence shortly thereafter. It plays an important role in stimulating apoptosis of the epithelial cells at the fusion seam. In TGF-β3–mutant mice, the lateral palatal shelves approximate in the midline, but the epithelial seam fails to disappear, and the mice develop isolated cleft palate.

Experiments involving the in vitro culture of a single palatal shelf of several species have shown clearly that all aspects of epithelial differentiation (cell death in the midline and different pathways of differentiation on the oral and nasal surfaces) can occur in the absence of contact with the opposite palatal shelf. These different pathways of differentiation are not intrinsic to the regional epithelia, but they are mediated by the underlying neural crest–derived mesenchyme. The mechanism of this regional specification of the epithelium remains poorly understood. According to one model, the underlying mesenchyme produces growth factors that influence the production and regional distribution of extracellular matrix molecules (e.g., type IX collagen). The way these events are received and interpreted by the epithelial cells is unknown.

Formation of the Nose and Olfactory Apparatus

The human olfactory apparatus first becomes visible at the end of the first month as a pair of thickened ectodermal nasal placodes located on the frontal aspect of the head (Fig. 14.14A). Similar to the formation of the lens placodes, the formation of the nasal placodes requires the expression of Pax-6 and the action of retinoids produced in the forebrain. In the absence of Pax-6 expression, neither the nasal placodes nor the lens placodes form. The nasal placodes originate from the anterolateral edge of the neural plate before its closure.

Soon after their formation, the nasal placodes form a surface depression (the nasal pits) surrounded by horseshoe-shaped elevations of mesenchymal tissue with the open ends facing the future mouth (see Fig. 14.6). The two limbs of the mesenchymal elevations are the nasomedial and nasolateral processes. Formation of the thickened nasal processes depends on the retinoid-stimulated production of FGF-8, which stimulates proliferation of the mesenchymal cells within the nasal processes. The source of these retinoids is the epithelium of the nasal pit itself. Meanwhile, production of retinoids by the forebrain diminishes. As a consequence, the frontonasal prominence, which depends on forebrain retinoids for supporting proliferation of its mesenchymal cells, is reduced. As the nasal primordia merge toward the midline during weeks 6 and 7, the nasomedial processes form the tip and crest of the nose along with part of the nasal septum, and the nasolateral processes form the wings (alae) of the nose. The receding frontonasal process contributes to part of the bridge of the nose.

Meanwhile, the nasal pits continue to deepen toward the oral cavity and form substantial cavities themselves (see Fig. 14.14). By image weeks, only a thin oronasal membrane separates the oral cavity from the nasal cavity. The oronasal membrane soon breaks down, thereby making the nasal cavities continuous with the oral cavity through openings behind the primary palate called nasal choanae (see Fig. 14.9). Shortly after breakdown of the oronasal membrane, however, the outer part of the nasal cavity becomes blocked with a plug of epithelial cells, which persists until the end of the fourth month. With the fusion of the lateral palatal shelves, the nasal cavity is considerably lengthened and ultimately communicates with the upper pharynx.

Similar to the other major sensory organs of the head, the epithelium of each nasal pit induces the surrounding neural crest mesenchyme to form a cartilaginous capsule around it. In a three-dimensionally complex manner, the medial parts of the nasal capsules combine with more centrally derived deep neural crest mesenchyme to form the midline nasal septum and ethmoid bones. The lateral region of the nasal capsule forms the nasal bones. During the third month, shelflike structures called nasal conchae form from the ethmoid bones on the lateral wall of the nasal cavity. These structures increase the surface area available for conditioning the air within the nasal cavity. Late in fetal life and for several years after birth, the paranasal sinuses form as outgrowths from the walls of the nasal cavities. The size and shape of these structures have a significant impact on the form of the face during its postnatal growth period.

At 6 to 7 weeks, a pair of epithelial ingrowths can be seen in each side of the nasal septum near the palate. Developing as invaginations from the medial portion of the nasal placode, these diverticula, known as vomeronasal organs (see Fig. 14.11B), reach a maximum size of about 6 to 8 mm at around the sixth fetal month and then begin to regress, leaving small cystic structures. In most mammals and many other vertebrates, the vomeronasal organs, which are lined with a modified olfactory epithelium, remain prominent and are involved in the olfaction of food in the mouth or sexual olfactory stimuli (e.g., pheromones).

The dorsalmost epithelium of the nasal pits undergoes differentiation as a highly specialized olfactory epithelium (see Fig. 14.14). Differentiation of the olfactory organ and the vomeronasal organ requires the action of FGF-8, which is produced in a signaling zone that surrounds the nasal pit. Beginning in the embryonic period and continuing throughout life, the olfactory epithelium is able to form primitive sensory bipolar neurons, which send axonal projections toward the olfactory bulb of the brain. Preceding axonal ingrowth, some cells break free from this epithelium and migrate toward the brain. Some of these cells may synthesize a substrate for the ingrowth of the olfactory axons. Other cells migrating from the olfactory placode (specifically, the vomeronasal primordium) synthesize luteinizing hormone–releasing hormone and migrate to the hypothalamus, the site of synthesis and release of this hormone in adults. The embryonic origin of these cells in the olfactory placode helps to explain the basis for Kallmann’s syndrome, which is characterized by anosmia and hypogonadotropic hypogonadism. Cells of the olfactory placode also form supporting (sustentacular) cells and glandular cells in the olfactory region of the nose. Physiological evidence shows that the olfactory epithelium is capable of some function in late fetal life, but full olfactory function is not attained until after birth.

Formation of the Salivary Glands

Starting in the sixth week, the salivary glands originate as solid, ridgelike thickenings of the oral epithelium (Fig. 14.15). Extensive epithelial shifts in the oral cavity make it difficult to determine the germ layer origins of the salivary gland epithelium. The parotid glands are probably derived from ectoderm, whereas the submandibular and sublingual glands are thought to be derived from endoderm.

As with other glandular structures associated with the digestive tract, the development of salivary glands depends on a continuing series of epitheliomesenchymal interactions. Branching morphogenesis of the salivary glands depends heavily on shh signaling, acting on FGFs. In contrast to most other glandular structures, however, in which epithelially produced shh acts on the underlying mesenchyme, in the salivary glands the entire sequence of shh signaling and of FGF response occurs within the epithelium. The basal lamina that surrounds the early epithelial lobular ingrowths differs in composition, depending on the growth potential of the region. Around the stalk and in clefts, the basal lamina contains types I and IV collagen and a basement membrane-1 proteoglycan. These components are not found in the regions of the lobules that undergo further growth. Under the influence of the surrounding mesenchyme, the basal lamina in growing regions loses the collagens and proteoglycans that are associated with stable structures (e.g., stalks, clefts).

In addition to alterations in the basal lamina, branching is associated with the local contraction of ordered microfilaments within the apices of epithelial cells at the branch points. Continued growth at the tips of lobules of the glands is supported by high levels of mitotic activity of the epithelium and the deposition of newly synthesized glycosaminoglycans in the area. During organogenesis, the parasympathetic innervation, acting through acetylcholine secretion, maintains the population of epithelial progenitor cells. In its absence, the amount of budding of epithelial lobules is dramatically reduced. The structural and functional differentiation of the epithelium of the salivary gland continues throughout fetal life.

Clinical Correlation 14.1 presents malformations of the face and oral regions.

Clinical Correlation 14.1   Malformations of the Face and Oral Region

Cleft Lip and Palate

Cleft lip and cleft palate are common malformations, with an incidence of approximately 1 in 1000 births (cleft lip) and 1 in 2500 births (cleft palate). Numerous combinations and degrees of severity exist, ranging from a unilateral cleft lip to a bilateral cleft lip associated with a fully cleft palate.

Structurally, cleft lip results from the lack of fusion of the maxillary and nasomedial processes. In the most complete form of the defect, the entire premaxillary segment is separated from both maxillae, with resulting bilateral clefts that run through the lip and the upper jaw between the lateral incisors and the canine teeth (Fig. 14.16). The point of convergence of the two clefts is the incisive foramen (Fig. 14.17B). The premaxillary segment commonly protrudes past the normal facial contours when viewed from the side. The mechanism frequently underlying cleft lip is hypoplasia of the maxillary process that prevents contact between the maxillary and nasomedial processes from being established.