The evolution of epitheliochorial placentation.

The research article focuses on the evolutionary history and adaptive features of epitheliochorial placentation, a type of placentation observed in certain mammals like strepsirrhine primates, pangolins, whales, and hoofed mammals.

Understanding Epitheliochorial Placentation

• This form of placentation involves the interaction between the epithelial layer of the maternal uterus and the trophoblast cells (fetal cells) of the placenta, without any damage to the maternal tissue or cellular invasion.
• It’s generally associated with long gestation periods, small litters (number of offspring per pregnancy), and precocial young (offspring that are relatively mature and mobile from the moment of birth).
• It’s a distinct form of placentation that emerged separately in two different mammalian lineages: strepsirrhine primates (like lemurs and lorises) and laurasiatherians (which include mammals like pangolins, whales, and hoofed animals).

Oxygen Transfer and Histotrophic Nutrition

• In this type of placentation, oxygen transfer between the mother and fetus is facilitated by the indenting of the uterine and trophoblast epithelia by maternal and fetal capillaries, respectively.
• Epitheliochorial placentation is also characterized by histotrophic nutrition, where nutrients are derived from uterine secretions and maternal blood rather than directly through the placenta.
• Adaptations observed in this context include areolas (local thickenings in the placenta) and hemophagous regions (areas involved in the absorption of maternal blood).

Specific Features in Pigs and Horses

• In pigs and horses, iron transportation to the fetus is facilitated by a protein called uteroferrin, secreted from the uterine glands and absorbed by areolas.
• In horses, invasive trophoblast cells form structures called ‘cups’ within the endometrium (lining of the uterus), which secrete the hormone equine chorionic gonadotropin.

Features in Ruminants

• In ruminants like cows, sheep, and deer, another unusual feature is observed where binucleate trophoblast cells (containing two nuclei) fuse with uterine epithelial cells to form trinucleate cells or plaques. These structures can secrete pregnancy hormones.
• Studies indicate immunosuppressive activity in the context of these more invasive types of trophoblasts, likely aiding in preventing maternal immune rejection of the fetus.

Advantages of Epitheliochorial Placentation

• The study concludes by suggesting that the epitheliochorial form of placentation may confer certain advantages for species with long pregnancies, particularly large animals. The precise nature of these benefits, however, requires further research.