Epiblast from the past

Mouse blastocyst with epiblast (EPI), trophoblast (TE)
and primitive endoderm (PE) or hypoblast from Selenka 1883

The three cell lineages of the blastocyst are trophoblast, hypoblast (primitive endoderm) and epiblast. The epiblast creates the embryo and contributes extra-embryonic mesoderm to the fetal membranes. An insightful review by Guojun Sheng (OA here) traces epiblast evolution across reptiles/birds, monotremes, marsupials and eutherians.

First he summarizes early development in the mouse as known since Selenka and Sobotta and now understood in terms of gene expression. Some of this translates to early human development (previous post). Yet the behaviour of the epiblast is very different in mammals as diverse as rabbit, pig, cattle and dog. As an example there is loss of the polar trophoblast to expose the epiblast. There are also important differences in gene expression as known from cattle (OA here). 

Meroblastic cleavage of the platypus egg
From Hughes 1993 (here)

Monotremes are more like birds and reptiles starting with the incomplete (meroblastic) cleavage of the yolky egg. Guojun Sheng suggests how the monotreme and reptile/bird patterns of epiblast epithelialization could each have evolved from a hypothetical amniote prototype.

Embryonic and trophoblastic areas of the marsupial

blastocyst as envisaged by Hartman 

Marsupials have a blastocyst-like stage with no inner cell mass (previous post) and it is not fully clear how the cell lineages segregate. Guojun Sheng thinks it can be seen as an intermediate in evolution from a hypothetical therian prototype. 

Serial sections through the blastula of the lowland streaked tenrec
(Hemicentetes semispinosus) from Bluntschli 1937

Finally, he suggests that the blastula stages described in tenrecs by Bluntschli and elephant shrews by van der Horst (discussed here) may reflect the early eutherian prototype.

Altogether a very good read.