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Birds and Mammals
Cell migration and specification are coordinated by fibroblast growth factors. The cells of the primitive streak appear to be capable of both synthesizing and responding to FGFs (Sun et al. 1999; Ciruna and Rossant 2001). In embryos that are homozygous for the loss of the Fgf8 gene or its receptor, cells fail to emigrate from the primitive streak, so
a very thick streak develops and neither mesoderm nor endoderm are formed. This suggests that the main function of FGF8 (and its receptors) is to drive the mesendoderm out of the streak, perhaps by chemorepulsion, as has been suggested in chick. Fgf8 may also control cell specification by regulating snail,
Brachyury, and Tbx6, three genes that are essential (as they are in the chick embryo) for mesodermal migration, specification, and patterning.
The ectodermal precursors are located anterior and lateral to the fully extended primitive streak, as in the chick epiblast; also as in the chick, a single cell can give rise to descendants in more than one germ layer. Thus, at the epiblast stage, these lineages have not yet become fully separate from one another. Indeed, in mice, some of the visceral endoderm, which had been extraembryonic, is able to intercalate with the definitive endoderm and become part of the gut (Kwon et al. 2008).
Literature Cited
Ciruna, B. and J. Rossant. 2001. FGF signaling regulates mesoderm cell fate specification and morphogenetic movement at the primitive streak.Dev. Cell1: 37–49.
PubMed Link
Kwon, G. S., M. Viotti and A. K. Hadjantonakis. 2008. The endoderm of the mouse embryo arises by dynamic widespread intercalation of embryonic and extraembryonic lineages.Dev. Cell.15: 509–520.
PubMed Link
Sun, B. I., S. M. Bush, L. A. Collins-Racie, E. R. LaVallie, E. A. DiBlasio-Smith, N. M. Wolfman, J. M. McCoy and H. L. Sive. 1999. derriere: a TGF-beta family member required for posterior development in Xenopus. Development 126: 1467–1482.
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