Developing Genital Ridge

Dorsal root ganglion

Sympathetic ganglion

Developing suprarenal gland

Dorsal root ganglion

Sympathetic ganglion

Developing suprarenal gland

Urogenital Ridge

Preaortic ganglion

Enteric ganglia

Urogenital ridge

Preaortic ganglion

Enteric ganglia

Urogenital ridge

Figure 5.3 Formation and migration of neural crest cells in the spinal cord. A and B. Crest cells form at the tips of neural folds and do not migrate away from this region until neural tube closure is complete. C. After migration, crest cells contribute to a heterogeneous array of structures, including dorsal root ganglia, sympathetic chain ganglia, adrenal medulla, and other tissues (Table 5.1). D. In a scanning electron micrograph of a mouse embryo, crest cells at the top of the closed neural tube can be seen migrating away from this area (arrow). E. In a lateral view with the overlying ectoderm removed, crest cells appear fibroblastic as they move down the sides of the neural tube. (S, somites).

Schwann cells, and cells of the adrenal medulla (Fig. 5.3). Neural crest cells also form and migrate from cranial neural folds, leaving the neural tube before closure in this region (Fig. 5.4). These cells contribute to the craniofacial skeleton as well as neurons for cranial ganglia, glial cells, melanocytes, and other cell types (Table 5.1). Induction of neural crest cells requires an interaction between adjacent neural and overlying ectoderm. Bone morphogenetic proteins (BMPs), secreted by non-neural ectoderm, appear to initiate the induction

Mouse Embryo Images Lens

Figure 5.4 A. Cross section through the cranial neural folds of a mouse embryo. Neural crest cells at the tip of the folds (arrow) migrate and contribute to craniofacial mesenchyme. B. Lateral view of the cranial neural folds of a mouse embryo with the surface ectoderm removed. Numerous neural crest cells can be observed leaving the neural folds (NF) and migrating beneath the ectoderm that has been removed. Unlike crest cells of the spinal cord, cranial crest exits the neural folds before they fuse.

Figure 5.4 A. Cross section through the cranial neural folds of a mouse embryo. Neural crest cells at the tip of the folds (arrow) migrate and contribute to craniofacial mesenchyme. B. Lateral view of the cranial neural folds of a mouse embryo with the surface ectoderm removed. Numerous neural crest cells can be observed leaving the neural folds (NF) and migrating beneath the ectoderm that has been removed. Unlike crest cells of the spinal cord, cranial crest exits the neural folds before they fuse.

process. Crest cells give rise to a heterogeneous array of tissues, as indicated in Table 5.1 (see p. 95).

By the time the neural tube is closed, two bilateral ectodermal thickenings, the otic placodes and the lens placodes, become visible in the cephalic region of the embryo (Fig. 5.85). During further development, the otic placodes in-vaginate and form the otic vesicles, which will develop into structures needed for hearing and maintenance of equilibrium (see Chapter 16). At approximately the same time, the lens placodes appear. These placodes also invaginate and, during the fifth week, form the lenses of the eyes (see Chapter 17).

In general terms, the ectodermal germ layer gives rise to organs and structures that maintain contact with the outside world: (a) the central nervous

Figure 5.5 A. Dorsal view of a human embryo at approximately day 22. Seven distinct somites are visible on each side of the neural tube. B. Dorsal view of a human embryo at approximately day 23. Note the pericardial bulge on each side of the midline in the cephalic part of the embryo.

system; (b) the peripheral nervous system; (c) the sensory epithelium of the ear, nose, and eye; and (d) the epidermis, including the hair and nails. In addition, it gives rise to subcutaneous glands, the mammary glands, the pituitary gland, and enamel of the teeth.

Derivatives of the Mesodermal Germ Layer

Initially, cells of the mesodermal germ layer form a thin sheet of loosely woven tissue on each side of the midline (Fig. 5.9A). By approximately the 17th day, however, cells close to the midline proliferate and form a thickened plate of tissue known as paraxial mesoderm (Fig. 5.9B). More laterally, the mesoderm layer remains thin and is known as the lateral plate. With the appearance and coalescence of intercellular cavities in the lateral plate, this tissue is divided into two layers (Fig. 5.9, B and C): (a) a layer continuous with mesoderm covering the amnion, known as the somatic or parietal mesoderm layer; and (b) a layer continuous with mesoderm covering the yolk sac, known as the splanchnic or visceral mesoderm layer (Figs. 5.9, C and D, and 5.10). Together, these layers line a newly formed cavity, the intraembryonic cavity, which is continuous with the extraem-bryonic cavity on each side of the embryo. Intermediate mesoderm connects paraxial and lateral plate mesoderm (Figs. 5.9, B and D, and 5.10).

Lateral Plate Mesoderm Mouse Embryo

Figure 5.6 Dorsal (A) and ventral (B) views of a mouse embryo (approximately 22-day human). A. The neural groove is closing in cranial and caudal directions and is flanked by pairs of somites (S). B. The same embryo showing formation of the gut tube with anterior and posterior intestinal portals (arrowheads), heart (H) in the pericardial cavity (asterisks), and the septum transversum (arrow) representing the primordium of the diaphragm (see Chapter 11). The neural folds remain open, exposing forebrain and midbrain regions.

Figure 5.6 Dorsal (A) and ventral (B) views of a mouse embryo (approximately 22-day human). A. The neural groove is closing in cranial and caudal directions and is flanked by pairs of somites (S). B. The same embryo showing formation of the gut tube with anterior and posterior intestinal portals (arrowheads), heart (H) in the pericardial cavity (asterisks), and the septum transversum (arrow) representing the primordium of the diaphragm (see Chapter 11). The neural folds remain open, exposing forebrain and midbrain regions.

Figure 5.7 A 12- to 13-somite embryo (approximately 23 days). The embryo within its amniotic sac is attached to the chorion by the connecting stalk. Note the well-developed chorionic villi.

table 5.1 Neural Crest Derivatives

Connective tissue and bones of the face and skull

Cranial nerve ganglia (see Table 19.2)

C cells of the thyroid gland

Conotruncal septum in the heart

Odontoblasts

Dermis in face and neck

Spinal (dorsal root) ganglia

Sympathetic chain and preaortic ganglia

Parasympathetic ganglia of the gastrointestinal tract

Adrenal medulla

Schwann cells

Glial cells

Arachnoid and pia mater (leptomeninges) Melanocytes

Figure 5.8 A. Lateral view of a 14-somite embryo (approximately 25 days). Note the bulging pericardial area and the first and second pharyngeal arches. B. The left side of a 25-somite embryo approximately 28 days old. The first three pharyngeal arches and lens and otic placodes are visible.

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