kw: medicine, viruses, embryology, symbiosis
I'll just get this out of the way before I even finish the book. I find the idea behind this image rather unsettling. This shows a portion of the syncytium (pronounced sin-sigh-tee-um), the multinuclear membrane, effectively composed of a single cell, that is the boundary between fetal blood and maternal blood in the placenta of all mammals. It is the reddish layer surrounding the purplish blobs, which are folds of placental tissue.
What you don't see in this light microscope image are the viruses that induce the syncytium to form. Animal tissue doesn't "know" how to form a syncytium, or any multinuclear cell. Its formation is mediated by viruses called HERVs, for Human Endogenous RetroViruses. The retrovirus most of us have heard about is HIV, the cause of AIDS. It is related to HERVs. Our DNA is host to many, many related retrovirus genomes, and certain ones are expressed and work together with "our" DNA at many stages of our life, including setting up the placenta that makes most mammalian pregnancies work.
I'm reading a book on evolutionary virology, which I'll review more fully in a few days. Meantime, I could not get this image out of my mind once I saw it at the author's website. Many of our tissues and organs develop with the help of symbiotic viruses. I never knew viruses could be symbiotic! Not only that, they may be the dominant partner!!
This is one more demotion of our vaunted humanity. First, we were at the center of the Universe. Copernicus and Galileo moved Earth to "third rock from the Sun". Then, we were the peak of creation. Darwin, Wallace and others showed we're smart apes, but apes all the same. In recent decades it has become clear that 9/10 of the living cells in our bodies are bacteria, although each of "our" cells weighs hundreds of times what a bacterium does. Now I read that, while "vertebrate DNA" makes up only 1.5% of our total genome, various total and partial virus sequences make up 45%, or 30 times as much. I am starting to think that if you took away everything that is not "human" from us, we would be nearly weightless shells, ready to collapse under our own negligible mass.
I'm tempted to write more about the book now, but I suspect the author has more surprises waiting in the last few chapters, so I must simply say, "Stay tuned."