Washington, Apr 15 : The first clues about the ancient origins of the placenta - a mother's intricate lifeline to her unborn baby which delivers oxygen and nutrients critical to its health - have been uncovered, say scientists.
The study, led by researchers at the Stanford University School of Medicine, suggests that the placenta of humans and other mammals evolved from the much simpler tissue that attached to the inside of eggshells and enabled the embryos of our distant ancestors, the birds and reptiles, to get oxygen.
"The placenta is this amazing, complex structure and it's unique to mammals, but we've had no idea what its evolutionary origins are," said Julie Baker, PhD, assistant professor of genetics and study's senior author.
The placenta grows inside the mother's uterus and serves as a way of exchanging gas and nutrients between mother and fetus; it is expelled from the mother's body after the birth of a baby.
Baker said that it is the only organ to develop in adulthood and is the only one with a defined end date, making the placenta of interest to people curious about how tissues and organs develop.
"The placenta seems to be critical for fetal health and maternal heath," Baker said.
Despite the organ's major impact, almost nothing was known about how the placenta evolved or how it functions.
Baker and Kirstin Knox, graduate student and the study's first author, began addressing the question of the placenta's evolution by determining which genes are active in cells of the placenta throughout pregnancy in mice.
From the analysis, the research team found that the placenta develops in two distinct stages. In the first stage, which runs from the beginning of pregnancy through mid-gestation, the placental cells primarily activate genes that mammals have in common with birds and reptiles.
This suggests that the placenta initially evolved through repurposing genes the early mammals inherited from their immediate ancestors when they arose more than 120 million years ago.
In the second stage, cells of the mammalian placenta switch to a new wave of species-specific genes. Mice activate newly evolved mouse genes and humans activate human genes.
It makes sense that each animal would need a different set of genes, Baker said.
"A pregnant orca has different needs than a mouse and so they had to come up with different hormonal solutions to solve their problems," she said.
The study is published in Genome Research.