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hi guys, I haven't read in a while so i'm catching up on your posts.

what about retroviral ORIGINS for placental mammals in the first place? any latest news on this?


The viruses that make us: a role for endogenous retrovirus in the evolution of placental species

by Luis P. Villarreal


A main distinction between marsupial embryos and placental embryos is the presence of the outer cell layer of the early placental embryo known as the trophectoderm. This cell layer is to only one to expressing paternal genes and is involved directly in implantation into the uterus then goes on to develop into the placenta. This tissue is the first cell type to differentiate in the placental embryo, yet was also the most recently evolved relative to early mammals. It therefore appears that the trophectoderm is crucial for the biology of placental life strategy.

In terms of implantation and escape from immunological rejection, the trophectoderm appears central to the ability of a placental embryo to prevent immunological recognition. Unlike most any other tissue, mouse trophectoderm can be implanted across strain barriers without being rejected. In addition, the trophectoderm can protect the inner embryo from attack by macrophages. However, it has been unclear what aspect of the trophectoderm protects the embryo. Various models have been proposed including altered expression of antigen presenting molecules (MHC) but these models all have significant problems. However, one activity that is rather unique to the trophectoderm (syncytiotrophoblast) is remarkable; they express extremely large quantities of endogenous retrovirus genes and retroviral particles, which include the envelope gene.

In addition, the envelope gene is generally responsible for the ability of many retroviruses to suppress the immune system of the host...

The retrovirus that are being expressed in the embryoÕs trophectoderm are also highly conserved in all placental species examined so far. It therefore seems possible that this endogenous retrovirus may be providing protection to the embryo from the mothers immune system.

i found a more recent article:

Endogenous retroviruses regulate periimplantation placental growth and differentiation.
Dunlap KA, Palmarini M, Varela M, Burghardt RC, Hayashi K, Farmer JL, Spencer TE.


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