Human development - the first 13 days

Human embryo Carnegie Stage 5c (Carnegie Embryo #7700)
Photomicrograph courtesy of Dr. Allen C. Enders

A system created for cultivating mouse blastocysts has been applied successfully to describe the development of the human embryo for 13 days after in vitro fertilization. This is a step towards opening the black box in our understanding of human embryology (reviewed here). Hitherto we have been confined to interpreting the histological sections of embryos in the Carnegie Collection.

Papers by two groups were just published: Shahbazi et al. in Nature Cell Biology and Deglincerti et al. in Nature. They used appropriate molecular markers to identify epiblast, primitive endoderm (hypoblast) and trophectoderm. In addition they used cytokeratin 7 and human chorionic gonadotrophin as markers for cyto- and syncytiotrophoblast.


Day 13 embryo of the rhesus macaque (Macaca mulatta)
Courtesy of Dr. Allen C. Enders

Both groups showed the appearance of cavities corresponding to the amnion and primary yolk sac as known from studies in the rhesus macaque by Enders, Schlafke and Hendrickx. In the macaque, the yolk sac (at bottom in the figure) is outlined by visceral endoderm (beneath the epiblast) and the more squamous parietal endoderm. These tissues were identified by Shahbazi et al. in human embryos and shown to express the endoderm marker GATA6.  Deglincerti et al. found the GATA6 signal was low in the parietal cells and that they expressed the trophectoderm marker CDX2. This is an interesting observation but hardly justifies them calling these cells "yolk sac trophectoderm." The term was criticized by Janet Rossant in the accompanying News and Views (here)  and it must be hoped it does not gain currency.

As in the macaque, amnion formation was by cavitation. This is nicely described by Shahbazi et al. Unfortunately they use the term pro-amnion, which is appropriate in the mouse but not in primates (contrasted here).

Differentiation of trophectoderm into cytotrophoblast and multinucleated syncytiotrophoblast was confirmed with appearance of lacunae in the latter as appropriate for Carnegie Stage 5c.