In mammals, the existence of primitive endoderm and its association with both the extra-embryonic yolk sac and the embryo proper had been noted at the end of the 19th century. The primitive endoderm underlying the primitive ectoderm/epiblast became known as visceral endoderm, whereas that on the maternal side became parietal endoderm, as first recognized by Duval and Sobotta (Duval, 1891; Sobotta, 1911)
However, many years passed before it became clear that the primitive endoderm is replaced by definitive endoderm, which mainly gives rise to the lining of the gut. The first clear experimental demonstration came from the careful experiments of Bellairs, who combined electron and light microscopy, cell marking and experimental embryology. Bellairs established that the deep layer of ‘endoderm’ present in chick embryos before the appearance of the primitive streak contributes to the extra-embryonic yolk sac stalk (Bellairs, 1953a; Bellairs, 1953b). Thus, the early chick embryo contains a transitory, extra-embryonic cell layer at its ventral surface. Its name, ‘endoderm’, was replaced by the term ‘hypoblast’, to distinguish it from definitive gut endoderm, which, as shown by Bellairs, is derived from the epiblast via ingression through the primitive streak, replacing the hypoblast.
Hypoblast cells form an extraembryonic cell layer on the surface of the inner cell mass and faces the blastocoelic cavity. It gives rise to the visceral and parietal endoderm. The hypoblast cells are separated from the epiblast layer by an extracellular basement membrane. Notes These cells are evident from E7 in mouse development. Lineage marker transcription factors are: SOX17, GATA4, GATA6, PDGFRa. Human hypoblast differentiation differs from that observed in mouse, rat and cow embryos in their absence of appreciable Laminin expression in the presumptive hypoblast and downregulation of GATA6 in a subset of SOX17-expressing cells. This, in addition to different reactions to FGF/Erk signaling inhibitors, suggests that the human hypoblast may segregate by unique mechanisms.
No evidence of involvement of E-cadherin in cell fate specification or the segregation of Epi and PrE in mouse blastocysts Katarzyna Filimonow,Nestor Saiz,Aneta Suwińska,Tomasz Wyszomirski,Joanna B. Grabarek,Elisabetta Ferretti,Anna Piliszek,Berenika Plusa ,Marek Maleszewski Published: February 8, 2019 PLoS One. 2019; 14(2): e0212109. https://doi.org/10.1371/journal.pone.0212109
Blastocyst development embryology.med.unsw.edu.au (Greek, blastos = sprout + cystos = cavity) or blastula, the term used to describe the hollow cellular mass that forms in early development.
胞を作っている部分が、trophoblast 栄養外胚葉で、その内側に細胞の塊が存在しており、inner cell mass 内細胞塊と呼ばれます。inner cell mass 内細胞塊は、embryoblast 胚芽（はいが）とも呼ばれます。将来、ヒトの体になるのがこのinner cell mass 内細胞塊です。
The hypoblast forms a squamous epithelium covering the epiblast58 and expands beyond the epiblast margin. At this stage, hypoblast cells diversify into visceral and parietal endoderm (Fig. 2). Visceral endoderm overlies the epiblast and gives rise to a cuboidal epithelium. The peripheral hypoblast cells become parietal endoderm, which forms an inner lining of the trophoblast. Both visceral and parietal endoderm contribute to the primary yolk sac55,59.
Carlson FIG.5.2 および本文 “The epiblast contains the cells that make up the embryo itself, but extraembryonic tissues also arise from this layer. The next layer to appear after the hypoblast is the amnion, a layer of extraembryonic ectoderm that ultimately encloses the entire embryo in a fluid-filled chamber called the amniotic cavity (see Chapter 7).”
Implantation is the process of the blastocyst embedding into the endometrial lining of the uterus, which typically occurs in Week 2 of development. For implantation to occur, the blastocyst must completely hatch from the zona pellucida once the conceptus enters the uterine cavity. https://www.ncbi.nlm.nih.gov/books/NBK554562/
https://pubmed.ncbi.nlm.nih.gov/32310425/ The yolk sac, or umbilical vesicle, is a small membranous structure outside the embryo with various functions during embryonic development. The yolk sac reduces in size, communicates ventrally with the developing embryo via the yolk stalk, and later regresses.
Legier, T., Rattier, D., Llewellyn, J. et al.Epithelial disruption drives mesendoderm differentiation in human pluripotent stem cells by enabling TGF-β protein sensing.Nat Commun14, 349 (2023). https://doi.org/10.1038/s41467-023-35965-8
The extraembryonic coelom, also called the chorionic cavity, is continuous with the intraembryonic coelomalong the lateral edge of the embryo, where the lateral plate mesoderm has split into splanchnopleuric and somatopleuric layers.
