Well, you didn't hatch from an egg, and that has profound effects on our psychology! we get to grow quite large inside our mothers and it's an intense 9 month relationship. But we mammals don't take lightly to having alien creatures living inside us and feeding on our fluids. What I mean by alien is that the fetus has its FATHER'S GENES! The mother's white blood cells are constantly slurping through her body, slipping between tissue cells and searching for foreign invaders. If they get to the fetus... TROUBLE! so...
how is it done? VIRUSES! yes that's right, The fetus uses viruses to build an impenetrable barrier against these white blood cells! As with everything in life, the evil is mixed in with the good.
here is an image of the baby placenta with its impenatrable barrier from
Somehow in the process of proto-mammals evolving to give birth to live young, they invented placentas with the help of scrappy random retroviruses they accumulated in their DNA 'junk drawers'. The placenta is the organ responsible for integrating the fetus with the mother's tissues and keeping the mother's immune system from rejecting the fetus. How do the retroviruses help?
Retroviruses attack mammals by fusing their membranes with mammal cell membranes, that is how they gain entry. In the course of invasion, retrovirus DNA accumulates in their hosts and if it gets into egg cells.. it can survive generation after generation. eventually many of the viral genes decay due to neutral evolution, but the genes for membrane fusion may still remain functional.
in fact by keeping old de-activated viruses around that can still express these fusion genes helps us to fight off new retroviruses by clogging our membrane receptors so they won't fuse with the new viruses! and so mammals occasionally update their junk drawers of retrovirus fusion genes!
eventually proto-mammals figured out how to use them in their placentas. One problem the placenta will face is maternal white blood cells trying to slip in between cells which is their function and explore the growing fetus. If the white blood cells find the fetus, and sense the father's proteins, they will attack. the father's proteins are foreign and don't belong in the mother.
Here is how the placenta uses the retroviruses to defend: the placental cells in a layer called the syncitiotrophoblast use the retroviral genes to fuse together into ONE HUGE CELL. This huge cell completely surrounds the fetus! This means there are NO spaces between cells for the mother's white blood cells to slip between and invade!
some references on this amazing syncytiotrophoblast. so far i only have a suggestion from wikipedia that the function of forming ONE single membrane, is to block entry by white blood cells. I'll keep looking.
I'm not 100% sure this is the whole story on the usefulness of the retroviruses, it's very complicated in there and I thought there were more functions for the retroviral genes.. and I don't have a medical library at my disposal. The placenta also produces hormones that help suppress the mother's immune response in other ways. I'm still searching.
Another fascinating thing is that different groups of mammals continue to use new retroviruses for these purposes. the ones we use are different than the ones mice use. each lineage caught its own retroviruses!
Is it safe to use viruses to build placentas? well... placentas only stay in the body for 9 months, so maybe it's kinda safe!
Here is the full story about the retroviruses: