Figure 5.1 A. Dorsal view of a 16-day presomite embryo. The primitive streak and primitive node are visible. B. Dorsal view of an 18-day presomite embryo. The embryo is pear-shaped, with its cephalic region somewhat broader than its caudal end. C. Dorsal view of an 18-day human embryo. Note the primitive node and, extending forward from it, the notochord. The yolk sac has a somewhat mottled appearance. The length of the embryo is 1.25 mm, and the greatest width is 0.68 mm.
Figure 5.2 A. Dorsal view of a late presomite embryo (approximately 19 days). The amnion has been removed and the neural plate is clearly visible. B. Dorsal view of a human embryo at approximately 20 days showing somites and formation of the neural groove and neural folds. C. Scanning electron micrograph of a mouse embryo (approximately 20-day human) showing the typical appearance of the neural groove stage. Cranial neural folds have segregated themselves into forebrain (F, prosencephalon), midbrain (M, mesencephalon), and hindbrain (H, rhombencephalon) regions.
neural plate. Thus, in the presence of BMP-4, which permeates the mesoderm and ectoderm of the gastrulating embryo, ectoderm becomes epidermis, and mesoderm forms intermediate and lateral plate mesoderm. If BMP-4 is absent or inactivated, ectoderm becomes neuralized. Secretion of three other molecules, noggin, chordin, and follistatin, inactivates this protein. These three proteins are present in the organizer (primitive node), notochord, and pre-chordal mesoderm. They neuralize ectoderm and cause mesoderm to become notochord and paraxial mesoderm (dorsalizes mesoderm). However, these neural inducers induce only forebrain and midbrain types of tissues. Induction of caudal neural plate structures (hindbrain and spinal cord) depends upon two secreted proteins, WNT-3a and FGF (fibroblast growth factor). In addition, retinoic acid appears to play a role in organizing the cranial-to-caudal axis because it can cause respecification of cranial segments into more caudal ones by regulating expression of homeobox genes (see p. 105).
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