. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology. Figure 4. Whole-mount staining of embryos with antibody to Distal-less protein. Methods follow Hanken et al. (1992). (A) Monodelphis after days of gestation (10-12 somites); (B) Mus at 10- to 12-somite stage; (C) Monodelphis after 10 days, 20 hours of gestation (16 somites); (D) Mus at 14- to 16-somite stage. (A) and (B) represent early stages in neural crest accumulation in branchial arches. Note that although the anterior neural tube is open in tjoth embryos, the fore- and midbrain are much more robust (both in
. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology. Figure 4. Whole-mount staining of embryos with antibody to Distal-less protein. Methods follow Hanken et al. (1992). (A) Monodelphis after days of gestation (10-12 somites); (B) Mus at 10- to 12-somite stage; (C) Monodelphis after 10 days, 20 hours of gestation (16 somites); (D) Mus at 14- to 16-somite stage. (A) and (B) represent early stages in neural crest accumulation in branchial arches. Note that although the anterior neural tube is open in tjoth embryos, the fore- and midbrain are much more robust (both in thickness of neuroepithelium and length of region) in the mouse, but that much less accumulation of neural crest has occurred in the frontonasal and first arch regions in this animal. In (C) and (D) the same pattern continues— in Monodelphis the neural tube is still open anteriorly, whereas in the mouse the neural tube is complete and regional differ- entiation is beginning. However, massive accumulations of neural crest occur in the facial region, particularly in the maxillary process, in Monodelphis relative to the mouse. Key: arrows represent approximate forebrain-midbrain and midbrain-hindbrain junctions; o, otic vesicle; mx, maxillary process; md, mandibular process. before any closure of the tube occurs at any point along its length. Further, al- though the hindbrain is well differentiated at this stage, with evidence of all rhom- bomeres, little proliferation or differenti- ation occurs in either the fore- or midbrain regions. In eutherians significant prolifer- ation of tissues in the fore- and midbrain regions occurs before neural crest migra- tion begins, and rhombomere subdivision occurs after this period. These preliminaiy studies do indicate that the neural crest arises from the same rhombomeric seg- ments that have been reported in other vertebrates (, Noden, 1991). However, because little differentiation of fore- and midbrain regions occurs at this stage,
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