Mesodermo

The embryonic period, or period of organogenesis, occurs from the third to the eighth weeks of development and is the time when each of the three germ layers, ectoderm, mesoderm, and endoderm, givesrise to a number of specifi c tissues and organs. By the end of the embryonic period, the main organ systems have been established, rendering the major features of the external body form recognizableby the end of the second month. DERIVATIVES OF THE ECTODERMAL GERM LAYER At the beginning of the third week of development, the ectodermal germ layer has the shape of a disc that is broader in thecephalic than in the caudal region (Fig. 6.1). Appearance of the notochord and prechordal mesoderm induces the overlying ectoderm to thicken and form the neural plate (Fig. 6.2A,B). Cells of the platemake up the neuroectoderm, and their induction represents the initial event in the process of neurulation. Molecular Regulation of Neural Induction Upregulation of fi broblast growth factor (FGF)signaling together with inhibition of the activity of bone morphogenetic protein 4 (BMP4), a transforming growth factor-b (TGF-b) family member responsible for ventralizing ectoderm and mesoderm, causesinduction of the neural plate. FGF signaling probably promotes a neural pathway by an unknown mechanism while it represses BMP transcription and upregulates expression of chordin and noggin, whichinhibit BMP activity. In the presence of BMP4, which permeates the mesoderm and ectoderm of the gastrulating embryo, ectoderm is induced to form epidermis, and mesoderm forms intermediate and lateral platemesoderm. If ectoderm is protected from exposure to BMPs, its “default state” is to become neural tissue. Secretion of three other molecules, noggin, chordin, and follistatin, inactivates BMP.These three proteins are present in the organizer (primitive node), notochord, and prechordal mesoderm. They neuralize ectoderm by inhibiting BMP and cause mesoderm to become notochord and paraxial...