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.

Mouse and human blastocysts compared

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

Even before implantation, three cell lineages are apparent in the blastocyst of mouse and human. Outermost is the trophectoderm that will contribute trophoblast to the placenta. The inner cell mass has already differentiated into the epiblast and the primitive endoderm or hypoblast. For mouse, this much has been clear since the pioneering studies of Emil Selenka (here).

Gene expression in these three lineages of mouse placenta has been known for some time. What does this tell us about human preimplantation development? A recent paper in Development (here) suggests less than some might like to think.

• For trophectoderm, key lineages expressed in the mouse (e.g. Id2, Elf5, Eomes) either are not expressed in human trophectoderm or are expressed in alternative lineages.
• There are several genes that are exclusively expressed in human epiblast (e.g.the transcription factor KLF17).
• Expression of some genes in primitive endoderm is conserved between mouse and human (e.g. Foxa2/FOXA2).

These findings support other work indicating that the genes and signalling pathways involved in lineage specification differ between mouse and human blastocysts.

Do not marsupials have trophoblast?

Embryonic and trophoblastic areas of the marsupial

blastocyst as envisaged by Hartman 

In the final chapter of her book (previous post), Zofia Kielan-Jaworowska writes on placental mammals, "Their enormous success depends in the first place on the acquisition of the trophoblast, on the basis of which the placenta is formed." In contrast, "Without the trophoblast, marsupials are unable to prolong their gestation period and are born at an extremely early level of anatomical development." This view or a version of it seems to be shared by other zoologists. 

The souce of the misunderstanding can be traced to influential papers by Jason A. Lillegraven (e.g. here). The marsupial blastocyst lacks the inner cell mass of a placental mammal blastocyst. Early on, however, there are two distinct areas that often have been referred to as the embryonic and trophoblastic areas. Lillegraven argues, convincingly in my view, that the "embryonic area" is not equivalent to the inner cell mass and that a considerable portion of the "trophoblastic area" forms part of the developing embryo. In an anatomical sense the trophoblastic area is not the trophoblast as defined by Hubrecht (see previous post).

Nonetheless, Lillegraven also says, "But from a functional point of view (as applied to extra-embryonic parts of the conceptus having direct nutritive significance), the term "trophoblast" may be used to good advantage throughout the Amniota, whether development occurs within a shelled or a shell-free setting."

Thus marsupials do have trophoblast and of course they all have at least a yolk sac placenta and sometimes a chorioallantoic placenta to boot. As shown by Ulrich Zeller and Claudia Freyer (here) there are marsupials where the trophoblast forms  a syncytium and exhibits invasive properties.

As noted in a previous post, there are sound reasons for distinguishing between Eutheria and Placentalia but it is little wonder that marsupial specialists bridle at the term "placental mammals."