During the development and closure of the neural groove, paraxial mesoderm increases in the center of the embryo to form somites (see Figs. 1-5, 1-6). The somites increase in number to approximately 40, and eventually this paraxial mesoderm becomes mesenchyme that, in turn, develops into connective tissue, cartilage, muscle, and bone for the trunk and extremities. The neural segmentation pattern appears to be dependent on the underlying mesoderm. In the region of the brain rostral to the developing inner ear, the mesodermal segments are called somit-omeres, whereas segments caudal to this level are somites.72,75 The somitomeres are mesodermal in origin and give rise to the myoblasts of the extraocular muscles and vascular endothelium in and around the eye. Unlike the trunk and extremities, orbital bone and ocular connective tissue are derived from neural crest cells, not mesoderm.
It is important to point out that mesenchyme is a broad term for any embryonic connective tissue, whereas mesoderm specifically relates to the middle embryonic layer. At one time the middle embryonic layer (the mesoderm) was thought to be responsible for most of the ocular and adnexal tissues. Embry-ologic studies have shown that mesoderm plays a relatively small role in the development of head and neck mesenchyme and is probably responsible only for the striated muscle of the extraocular muscles and vascular endothelium. With respect to the ocular development and development of the head and neck, most of the mesenchyme or connective tissue comes from the neural crest cells (see Table 1-1).
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