Scientists have solved the mystery of the evolution of the placenta

(ORDO NEWS) — The scientists compared gene expression in the wombs of pregnant female mammals from different species and compared these data with the position of the species on the evolutionary tree to trace the history of the placenta.

It turned out that the placenta of the common ancestor of placental mammals was invasive, that is, the embryo was introduced deep into the tissues of the uterus, as in modern insectivores, bats, rodents and monkeys.

The fossil record reveals much about the history of life on Earth by preserving the bones, teeth, and shells of animals. It is much more difficult to investigate the history of soft tissues and organs that decompose without leaving any traces behind.

Scientists from the State University of New York at Buffalo, the University of Chicago (USA) and the University of Sydney (Australia) have studied gene expression patterns to determine the origin of the placenta, an organ that plays a critical role during mammalian pregnancy.

All modern mammals, with the exception of marsupials and monotremes, are placental. Their pregnancy takes a long time, and the exchange of substances between the developing fetus and the mother’s body occurs through the placenta. Marsupials also have a placenta, but placental ones are more perfect.

Placentas can be divided into types according to the degree of invasiveness – how deeply the embryo is embedded in the tissues of the uterus. The most non-invasive placenta is the epitheliochorial placenta, found in tapirs, whales, horses, and pigs.

With this structure of the placenta, the mother’s blood does not mix with the blood of the fetus, and the tissues of the embryo simply adjoin the epithelium of the uterine mucosa.

The most invasive is the hemochorial placenta of insectivores, primates, bats, and rodents. With this structure, the outer shells of the embryo are washed by the blood of matter. There are also intermediate types of placenta.

The scientists compared transcriptomes – the totality of all RNA molecules resulting from transcription (expression of various genes) – in the uterus of 20 different species of marsupial and placental mammals during pregnancy.

It turned out that the gene expression profiles differed depending on the degree of placental invasiveness. The authors of the work used these data to trace the evolutionary history of the organ.

The results of the study showed that the last common ancestor of placental mammals had a hemochorial placenta. And the epitheliochorial placenta is its derivative form, which independently arose several times in various placental groups.

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