Re: difference in location of intra and extra embryonic ectoderm & endoderm

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.