A temporary organ may carry an ancient viral gene signature
The placenta is a transient organ mediating exchange between mother and offspring, evolved repeatedly—including multiple times in fish and in some reptiles—and crucial to placental mammals’ rise about 150 million years ago. Human syncytiotrophoblast RNA hints a retroviral past helped enable live birth.
The placenta grows from the blastocyst soon after implantation and exists only for the length of a pregnancy. It keeps mother and fetus in separate circulations while passing nutrients, gases and waste between them, and it works as a hormone factory that tunes both bodies. The umbilical cord links it to the fetus; the other face attaches to the uterus in ways that vary by species. When it is delivered after a human birth, a thin layer of the mother's uterine lining comes away with it.
In people it is dark reddish-blue, averaging about 22 cm across and 2 to 2.5 cm thick, thickest in the centre, and weighs roughly 500 grams. The cord, around 55 to 60 cm long, holds two arteries and one vein, and its vessels branch across the surface into tree-like villi. Eutherian mammals build theirs from two embryonic membranes, the chorion and the allantois. The name comes via Latin from a Greek word for something flat and slab-like, after its round, flat shape; placentas is now the usual plural, though placentae is the classical form.
The organ is not unique to placental mammals. Marsupials have one, and it has evolved independently several times, probably first in fish, where it arose repeatedly, including in the genus Poeciliopsis, and later in some reptiles. The mammalian version appeared roughly 150 to 166 million years ago and helped placental mammals diversify explosively.
Its short life makes it an evolutionary laboratory. A new gene that might be harmful in a permanent organ can be tried out here, since the mother discards the placenta after birth, and its looser gene regulation lets viral genes escape the methylation that would normally silence them. Syncytin, a protein in the outer barrier layer called the syncytiotrophoblast, carries an RNA signature suggesting it came from an ancient retrovirus that may have helped mammals switch from laying eggs to live birth.
Source: Placenta
