How is the hypoblast related to the endoderm?

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I'm getting contrasting information from different sources about the relationship between the hypoblast (one of the two structures that make up the inner call mass) and the endoderm (which will form a few days later on, after gastrulation, and which will eventually give rise to the GI tract and several body organs of the embryo).

- "The first group of these stem cells (epiblast) superficially might be considered the most precious as they will make the new organism, the baby itself". (Book: 'The dance of life')
- "The epiblast is the more dorsal mass. It will give rise to all three germ layers of the trilaminar embryonic disc in the third week". (Source)

OK, so it's only the epiblast that gives rise to the 3 germ layers. To the embryo, basically. The hypoblast doesn't seem to be involved in the formation of the embryo proper, only in the generation of the yolk sac:
"The yolk sac is a membranous sac attached to an embryo, formed by cells of the hypoblast adjacent to the embryonic disk". (Source)

But if that's the case, I don't understand this sentence:
"On one side there is the primitive endoderm-this in time will generate the yolk sac". (Book: 'The dance of life')
So now the yolk sac is not only formed by the hypoblast, but by the endoderm. I don't understand this.

The hypoblast helps determine the embryo's body axes, and its migration determines the cell movements that accompany the formation of the primitive streak and its orientation. It develops into the endoderm and helps to orient the embryo and create bilateral symmetry”
(Source)
Again, the hypoblast seems to be connected to the endoderm. But how is that possible, if only the epiblast gives rise to the 3 germ layers? Or is there some sort of difference between 'hypoblast' and 'primitive hypoblast'?
 

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BillTre
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These stages frequently annoy me.

I glanced at a wikipedia article on this stuff because I don't remember all the details of these largely extra-embryonic structures.
The term hypoblast seems to be used to describe cells contributing to different structures in different animals.
The hypoblast in fish (but not in birds and mammals) contains the precursors of both the endoderm and mesoderm.[5] In birds and mammals, it contains precursors to the extra-embryonic endoderm of the yolk sac.[3][5]

It could be your confusion is due to descriptions of what it does in different species. This wouldnbe something to check in your sources.
Things might be more clear if you stick to one group at a time.
Birds vs. mammals vs. fish can get confusing very fast. These groups all do things differently even though they have a lot of similarities.

Surprisingly, there is a lot of diversity in these pre-gastrulation situations as well as in gastrulation movements leading to the pharynx stage during/just after gastrulation, as well as later stages.
 
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@BillTre Thanks, but to make matters even more complicated:

"Recent data have shown that the distinction between embryonic and extraembryonic endoderm is not as strict as previously thought due to the integration, and not the displacement, of the visceral endoderm into the definitive embryonic endoderm. Therefore, cells from the extraembryonic endoderm also contribute to definitive endoderm".

Interestingly, the lineage distinction between embryonic and extraembryonic endoderm tissue was marked by the assumption that the visceral endoderm that surrounds the epiblast was displaced by the definitive endoderm. Recently it has been shown that cells from the VE persist within the definitive endoderm layer of the embryo and contribute to the early gut tube. This suggests that the distinction between extraembryonic and embryonic tissues is not as strict as believed and the lineage that was previously considered to be exclusively embryonic has extraembryonic contributions (Kwon et al. 2008)". (Source)
 
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So we don't go too far into the weeds here is an old saying. Not perfect but illuminating.
Ontogeny reflects phylogeny

I think this is what @BillTre is telling you.

And I always found this aspect of embryology almost as annoying as 1950's Cytogenetics. Just because someone gave something a name does not mean anyone actually understands it fully.
 
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BillTre
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This suggests that the distinction between extraembryonic and embryonic tissues is not as strict as believed and the lineage that was previously considered to be exclusively embryonic has extraembryonic contributions (Kwon et al. 2008)". (Source)
Recently, due to new techniques, people have been able to do a lot more lineage tracing which has in some cases revealed that thighs that looked distinctive morphologically, can in fact change to neighboring cell types.
It has resulted in a lot of revisions, some of which you may have run into.

Ontogeny reflects phylogeny

I kind of like the weeds Jim!
The original was ontogeny recapitulates phylogeny.
A rough guide, but with a lot of deviations.

One of the Evo-Devo revelations is that this holds less truth than previously presumed.
It was thought to be a funnel-like relationship. Embryos were considered more similar at early stages and less so later as they divergently developed into their taxon specific morphologies.
It is now widely realized that there are a diversity of pregastrula situations that converge on the post gastrula stages, which best unite different body structures.

The early differences are attributed to things like:
  • big eggs vs. small eggs (large or small yolk for the embryonic cells to navigate around.
  • yolk isolated in a yolk sac vs. distributed though all the cells of the embryo
  • formation of extra-embryonic structures
These differences are thought to have arisen under selective differences favoring things like lots of small (little yolk; some fish can lay more than 100,000 eggs)) eggs or fewer eggs with more yolk (greater individual survival, but requiring greater maternal investment.
 

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