Cell proliferation patterns during development of the equine placenta.

The research studies how cells divide and grow during the development of the placenta in horses. It used a proliferation marker called Ki-67 antigen and observed changes in mares during different stages of pregnancy, with results helping to understand factors influencing placental growth.

Methods

• The study used endometrial biopsies from eight pregnant mares between the 14th and 26th days of gestation, and another eight mares in a non-reproductive condition subjected to different combinations of progesterone and estrogen.
• The researchers also used samples of endometrium and fetal membranes taken from 19 pregnant mares with normal horse fetuses at different gestation days, and three mares with failing cross-species (donkey-in-horse) pregnancies.
• Ki-67 antigen, a known marker for proliferation, was used to track cell division during placentation.

Findings

• The proliferation of cells in the superficial layer of the endometrium was observed to increase by day 18 of gestation.
• This effect could be replicated by supplementing mares with oestradiol benzoate during the last six days of an extended period of progesterone administration.
• The girdle cells of fetal chorionic, an embryonic structure that contributes to the formation of the placenta, showed vigorous proliferation during days 30 to 32 of gestation, but ceased dividing after invading the endometrium.
• However, the trophoblast cells of the allantochorion, also involved in placenta formation, displayed an increase in cell division after day 38.

Additional Observations

• The epithelium or the lining of the endometrium only began to proliferate after the formation of the true epitheliochorial placenta’s primary villi.
• This proliferation was only observed in the pregnant horn, the side of the uterus the embryo implanted itself in, from days 58 to 70 of pregnancy where placental development was more advanced.
• During the second half of gestation, most of the cell division activity was localized to the periphery of the growing microcotyledons.
• In the cross-species pregnancies, reduced proliferation rates were observed at day 70 in regions of heavy lymphocyte infiltration in the endometrium and poor placentation.

Conclusion

• The findings of this study provide a foundation for further research on the factors that influence both invasive and non-invasive placentation, understanding the placental development in horses and potentially aiding in addressing complications during gestation.