By the onset of gastrulation, if not before, the AC shows several dorsalventral differences relevant to neural induction. Early in the blastula stage, only the ventral half of the AC expresses the epidermal-specific antigen, Epi-1 (48,49), possibly owing to early signals acting through the plane of the tissue (50). After treatment with activin, the dorsal half of the AC is more likely than the ventral to form mesodermal tissue (51). The dorsal half can be induced to converge and extend (52) and to express neural molecular markers (53) when placed in planar apposition to the Organizer, whereas the ventral side shows little or no response. Dorsal and ventral halves differ in expression of protein kinase C isozymes, which is thought to have a role in biasing the dorsal half to form neural tissue (54). Bone morphogenetic protein (BMP) signaling between ventral cells (prospective epidermal) may prevent neuralization and ensure epidermalization, while blocking such signaling results in neuralization (55; see also ref. 56). Thus, the dorsal side of the AC is biased in the direction of neural development by the early gastrula stage. Later ventrally derived epidermal tissue takes an active role in patterning the dorsal aspects of the neural tube in amphibians (57-59) and in other vertebrates (60,61). Using ACs from UV "ventralized" embryos, which lack an Organizer and a nervous system (62), would presumably avoid this dorso-ventral bias of the AC.
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