Binucleate trophoblast cell of bovine placenta from Benirschke
The basic structure of the ruminant placenta is epitheliochorial. Yet the binucleate trophoblast cells of ruminants are able to fuse with uterine epithelial cells to form trinucleate cells (in cattle) or a syncytium (in sheep).To me this is an enigma.
Now we know how it is done. Cornelis and co-authors have found a syncytin in ruminants. It is expressed in the binucleate cells of the placenta. In a previous post it was explained that syncytins are encoded by env genes captured from retroviruses. In a retrovirus, the envelope protein is responsible for fusion with a host cell. In the ruminant placenta, as a syncytin, it enables the binucleate cell (fetal) to fuse with a uterine epithelial cell (maternal). The envelope protein has an immunosuppressive domain and this is retained in ruminant syncytin.
Binucleate cells make a number of interesting proteins such as placental lactogens and growth hormone. By fusing with the uterine epithelium they can pass these products to the mother thereby circumventing the epitheliochorial barrier.
A chevrotain from Singapore Zoo Wikimedia Commons
Cornelis and colleagues were able to find the syncytin gene in all families of ruminant save one – the chevrotains or mouse deer. This family (Tragulidae) differs from other ruminants in having a diffuse placenta – all the others have cotyledons (collectively they are the pecoran ruminants). However, it has been clearly shown that chevrotains have binucleate cells that fuse with maternal ones to form syncytial plaques.
It would be nice to think that capture of an env gene was a prerequisite for emergence of the ruminant type of placentation, but it should then be present in chevrotains. Perhaps it is but just could not be found with primers that worked in other ruminants. That might be because of sequence divergence during the 50 million years since tragulids separated from pecorans. That possibility needs to be explored further.