|MadSci Network: Development|
Hi Sarah, Immediately prior to gastrulation in humans (~7 days post-fertilization), the blastocyst is comprised of cell populations known as the trophoblast (with extraembryonic cell fates), hypoblast (also with extraembryonic cell fates) and the epiblast (with both embryonic and extraembryonic cell fates). The epiblast goes on to form all embryonic tissue and a subset of extraembryonic tissues (e.g., amniotic ectoderm and extraembryonic mesoderm). When the hypoblast layer delaminates from the inner cell mass, its extraembryonic fate is determined (at this point, it should no longer be considered embryonic). Prior to this cell movement, the inner cell mass (ICM) is comprised of what will become both the epiblast and the hypoblast. Thus, the ICM gives rise to both embryonic and extraembryonic tissues. The hypoblast is defined as extraembryonic, as is the extraembryonic endoderm that will further differentiation to form the yolk sac. Strictly speaking, the adult mammal forms from the cells of the embryonic epiblast, so the answer to your question is: NO. Extraembryonic tissue is, by definition, not part of the embryo proper. Hope this helps, Chris Reigstad P.S. All of this information can be found online at http://www.ncbi.nlm.nih.gov where you can find the book by Scott F. Gilbert "Developmental Biology" (Sixth Edition) is freely available electronically. Chapter 11 is called "The Early Development of Vertebrates" and you should read the section called " Gastrulation in Mammals" for more info. Figures 11.26 and 11.27 may be the most helpful.
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