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Somitomeres Somite

Mesenchyme for formation of the head region is derived from paraxial and lateral plate mesoderm, neural crest, and thickened regions of ectoderm known as ectodermal placodes.

Paraxial mesoderm (somites and somitomeres) forms the floor of the brain case and a small portion of the occipital region (Fig. 15.1) (see Chapter 8), all voluntary muscles of the craniofacial region (see Chapter 9), the dermis and connective tissues in the dorsal region of the head, and the meninges caudal to the prosencephalon. Lateral plate mesoderm forms the laryngeal cartilages (arytenoid and cricoid) and connective tissue in this region. Neural crest cells originate in the neuroectoderm of forebrain, midbrain, and hindbrain regions and migrate ventrally into the pharyngeal arches and rostrally around the forebrain and optic cup into the facial region (Fig. 15.2).

In these locations they form midfacial and pharyngeal arch skeletal structures (Fig. 15.1) and all other tissues in these regions, including cartilage, bone, dentin, tendon, dermis, pia and arachnoid, sensory neurons, and glandular stroma. Cells from ectodermal placodes, together with neural crest, form neurons of the fifth, seventh, ninth, and tenth cranial sensory ganglia.

The most typical feature in development of the head and neck is formed by the pharyngeal or branchial arches. These arches appear in the fourth and fifth weeks of development and contribute to the characteristic external appearance of the embryo (Table 15.1 and Fig. 15.3). Initially, they consist of bars of mesenchymal tissue separated by deep clefts known as pharyngeal (branchial) clefts (Figs. 15.3C;

Figure 15.1 Skeletal structures of the head and face. Mesenchyme for these structures is derived from neural crest (blue), lateral plate mesoderm (yellow), and paraxial mesoderm (somites and somitomeres) (red).

see also Fig. 15.6). Simultaneously, with development of the arches and clefts, a number of outpocketings, the pharyngeal pouches, appear along the lateral walls of the pharyngeal gut, the most cranial part of the foregut (Fig. 15.4; see also Fig. 15.6). The pouches penetrate the surrounding mes-enchyme, but do not establish an open communication with the external clefts (see Fig. 15.6). Hence, although development of pharyngeal arches, clefts, and pouches resembles formation of gills in fishes and amphibia, in the human embryo real gills (branchia) are never formed. Therefore, the term pharyngeal (arches, clefts, and pouches) has been adopted for the human embryo.

Pharyngeal arches not only contribute to formation of the neck, but also play an important role in formation of the face. At the end of the fourth week, the center of the face is formed by the stomodeum, surrounded by the first pair of pharyngeal arches (Fig. 15.5). When the embryo is 42 weeks old, five mesenchymal prominences can be recognized: the mandibular prominences (first pharyngeal arch), caudal to the stomodeum; the maxillary prominences (dorsal portion of the first pharyngeal arch), lateral to the stomodeum; and the frontonasal prominence, a slightly rounded elevation cranial to the sto-modeum. Development of the face is later complemented by formation of the nasal prominences (Fig. 15.5). In all cases, differentiation of structures derived from arches pouches, clefts, and prominences is dependent upon epithelial-mesenchymal interactions. In some instances, signals for these interactions are similar to those involved in limb development, including fibroblast growth factor (FGFs) for outgrowth and sonic hedgehog (SHH) and WNTs for patterning.

Electron Micrograph Cranial Ganglia

Figure 15.2 A. Scanning electron micrograph showing cranial neural crest cells migrating into the facial region beneath the ectoderm that has been removed. B. Migration pathways of neural crest cells from forebrain, midbrain, and hindbrain regions into their final locations (shaded areas) in the pharyngeal arches and face. Regions of ectodermal thickenings (placodes), which will assist crest cells in formation of the fifth (V), seventh (VII), ninth (IX), and tenth (X) cranial sensory ganglia, are also illustrated.

Figure 15.2 A. Scanning electron micrograph showing cranial neural crest cells migrating into the facial region beneath the ectoderm that has been removed. B. Migration pathways of neural crest cells from forebrain, midbrain, and hindbrain regions into their final locations (shaded areas) in the pharyngeal arches and face. Regions of ectodermal thickenings (placodes), which will assist crest cells in formation of the fifth (V), seventh (VII), ninth (IX), and tenth (X) cranial sensory ganglia, are also illustrated.

Pharyngeal Arches

Each pharyngeal arch consists of a core of mesenchymal tissue covered on the outside by surface ectoderm and on the inside by epithelium of endodermal origin (Fig. 15.6). In addition to mesenchyme derived from paraxial and lateral plate mesoderm, the core of each arch receives substantial numbers of neural crest cells, which migrate into the arches to contribute to skeletal components of the face. The original mesoderm of the arches gives rise to the musculature of the face and neck. Thus, each pharyngeal arch is characterized by its own table 15.1 Derivatives of the Pharyngeal Arches and Their Innervation

Pharyngeal Arch Nerve

Muscles

Skeleton

Pharyngeal Arch Nerve

Muscles

Skeleton

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