There is evidence that different regions of the mesoderm produce different inducing signals, which induce the regional differences that pattern the neural tissue (see refs. 30-37). Accordingly, it is useful to know how to identify different regions of the mesoderm. The detailed correspondence of mesodermal and neural fates throughout their relative movements, in both anterior-posterior and mediolateral axes, is complex and best visualized with color diagrams (consult Fig. 4 in ref. 9). However, the archenteron cavity and the BC at its edge are useful landmarks for identifying prospective mesodermal regions until they take on a recognizable character of their own. The leading edge of the mesodermal mantle, the first tissue to involute, consists of migratory meso-derm that spreads across the blastocoel roof in dorsal, lateral, and ventral sectors of the gastrula. It consists of prospective PM dorsally, heart mesoderm dorsolaterally, lateral plate mesoderm laterally, and ventral (blood island) mesoderm ventrally. In general, these early involuting tissues lie ahead of the BC, located at the periphery of the archenteric cavity. The late involuting tissues, the prospective notochord in the dorsal sector and the prospective somites in the lateral and ventral sectors lie behind the BC, immediately above the archenteron roof. For example, in the dorsal midline of the early gastrula, the PM lies ahead of the dorsal BC, found at the tip of the archenteron, and the notochordal and somitic mesoderm lie behind the bottle cells (Fig. 1, stages 10-, 10+). However, the dorsal BC respread in late gastrulation, when they expand the archenteron anteriorly, beneath the PM (15,16), changing this relationship (Fig. 1, stages 11.5-17). At the late gastrula and neurula stages, the PM, notochordal, and somitic mesoderm lie dorsal to and approximately coincident with the roof of the archenteron, and the neural plate lies dorsal to and approximately coincident with this mesoderm. If one cuts through the body wall at the peripheral aspect of the archenteron at the late gastrula through neurula stages, the explant contains the entire neural plate, underlaid with PM, somitic, and notochordal mesoderm, and archenteron roof endoderm.
As one cuts and manipulates different regions of the mesoderm, one should be aware of the dramatic differences in their properties. The posterior, dorsal (notochordal and somitic) mesoderm differs in behavior from the anterior leading edge (PM, heart, lateral plate, and ventral mesoderm) in that the former undergo convergent extension, whereas the latter spread and migrate (38). The cell motility in the posterior mesoderm consists of an intrinsic bipolar, mediolaterally directed protrusive activity (39), whereas in the anterior mesoderm, it consists of monopolar, animally directed, substrate-dependent protrusive activity (40,41). These two types of mesoderm also behave differently with respect to extracellular matrix (40,41), and they express different molecular markers. Goosecoid (24), Otx2 (25), and noggin (4) are expressed in the leading edge, PM/pharyngeal endoderm, whereas brachyury is expressed in the postinvolution notochord (42). Other useful markers for regions of mesoderm include the antibody tor-70 (43), which marks notochord from stage 17
onward (see ref. 44), and 12-101, a monoclonal antibody (MAb) (45) that marks somitic mesoderm from stage 18 onward.
The regional character of the mesoderm is not fixed at the onset of gastrula-tion, but is patterned during gastrulation. The precise anterior to posterior progression of the cell behaviors driving mediolateral cell intercalation (46) is actually organized during gastrulation (47). Likewise, regional neural-inducing properties appear in the early neurula (36).
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