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Shh Notochord

Figure 5.11 Stages in development of a somite. A. Mesoderm cells are arranged around a small cavity. B. Cells of the ventral and medial walls of the somite lose their epithelial arrangement and migrate in the direction of the notochord. These cells collectively constitute the sclerotome. Cells at the dorsolateral portion of the somite migrate as precursors to limb and body wall musculature. Dorsomedial cells migrate beneath the remaining dorsal epithelium of the somite to form the myotome. C. Cells forming the myotome continue to extend beneath the dorsal epithelium. D. After ventral extension of the myotome, dermatome cells lose their epithelial configuration and spread out under the overlying ectoderm to form dermis.

Figure 5.11 Stages in development of a somite. A. Mesoderm cells are arranged around a small cavity. B. Cells of the ventral and medial walls of the somite lose their epithelial arrangement and migrate in the direction of the notochord. These cells collectively constitute the sclerotome. Cells at the dorsolateral portion of the somite migrate as precursors to limb and body wall musculature. Dorsomedial cells migrate beneath the remaining dorsal epithelium of the somite to form the myotome. C. Cells forming the myotome continue to extend beneath the dorsal epithelium. D. After ventral extension of the myotome, dermatome cells lose their epithelial configuration and spread out under the overlying ectoderm to form dermis.

sclerotome (the cartilage and bone component), its own myotome (providing the segmental muscle component), and its own dermatome, the segmental skin component. Each myotome and dermatome also has its own segmental nerve component.

Molecular Regulation of Somite Differentiation

Signals for somite differentiation arise from surrounding structures, including the notochord, neural tube, epidermis, and lateral plate mesoderm (Fig. 5.12). The secreted protein product of the gene Sonic hedgehog (Shh), produced by the notochord and floor plate of the neural tube, induces the ventrome-dial portion of the somite to become sclerotome. Once induced, sclerotome cells express the transcription factor PAX1, which initiates the cascade of

Figure 5.12 Expression patterns of genes that regulate somite differentiation. Sonic hedgehog (SHH), secreted by the notochord and floor plate of the neural tube, causes the ventral part of the somite to form sclerotome and to express PAX1, which in turn controls chondrogenesis and vertebrae formation. WNT proteins from the dorsal neural tube activate PAX3, which demarcates the dermomyotome. WNT proteins also direct the dorsomedial portion of the somite to form epaxial (back) muscles and to express the muscle-specific gene MYF5. The middorsal portion of the somite is directed to become dermis by neurotrophin 3 (NT-3) expressed by the dorsal neural tube. Hypaxial (limb and body wall) musculature is derived from the dorsolateral portion of the somite under the combined influence of activating WNT proteins and inhibitory BMP-4 protein, which together activate MYOD expression.

cartilage- and bone-forming genes for vertebral formation. Expression of PAX3, regulated by WNT proteins from the dorsal neural tube, marks the dermomy-otome region of the somite. WNT proteins from the dorsal neural tube also target the dorsomedial portion of the somite, causing it to initiate expression of the muscle-specific gene MYF5 and to become epaxial musculature. Interplay between the inhibiting protein BMP-4 (and probably FGFs) from the lateral plate mesoderm and activating WNT products from the epidermis directs the dorso-lateral portion of the somite to express another muscle-specific gene, MYOD, and to form limb and body wall muscles. The midportion of the dorsal epithelium of the somite is directed by neurotrophin 3 (NT-3), secreted by the dorsal region of the neural tube, to form dermis.

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