Studies on the equine placenta II. Ultrastructure of the placental barrier.

This research paper examines the ultrastructure of the equine (horse) placenta during pregnancy, noting how it changes over time to meet the fetus’s growing need for oxygen and other nutrients.

Initial Structure of the Equine Placenta

• The study begins by explaining that in the early stages of pregnancy, the equine placenta has a simple structure, where the fetal and maternal epithelia are juxtaposed. This changes between 75 and 100 days of gestation with the formation of microcotyledons, structures that increase the interface between the mother and the fetus for more efficient nutrient and gas exchange.

Changes in the Placental Barrier

• Throughout pregnancy, the placental barrier maintains an epitheliochorial arrangement (a type of placental structure where the mother’s and fetus’s bloodstreams are separated by several layers of tissue). However, as the pregnancy progresses, the researchers observed that the maternal epithelium thins and the fetal capillaries progressively indent the chorionic epithelium.
• These changes make the diffusion pathway shorter and decrease the amount of placental tissue between the fetal and maternal bloodstreams, perhaps in response to the fetus’s increasing need for oxygen and other metabolites.

Cellular Activities During Pregnancy

• The researchers note significant pinocytotic activity in the cells of the trophoblast (the outer layer of the embryo that will form the placenta) during the first 200 days of pregnancy. This involves the ingestion of liquids or very small particles.
• From the 100th day of pregnancy, the cells of the trophoblast show an increase in the development of the smooth endoplasmic reticulum, a network of tubules involved in lipid synthesis and metabolism, among other functions.
• They also noticed changes in the paranuclear cytoplasm (the area around the nucleus) after 200 days of gestation. These changes are suggestive of an increased involvement by the trophoblast cells in synthetic processes as the pregnancy progresses.

Ultastructure Observation at Different Gestation Days

• The paper describes the ultrastructure of the equine placenta at various stages of gestation. The trophoblast—initially a wide band of columnar epithelium with large nuclei—changes over time, showing larger pinocytic vesicles, extensive smooth endoplasmic reticulum, and increasingly indented by fetal capillaries.
• Meanwhile, the maternal epithelium progressively thins over the course of the pregnancy. In the last trimester, the trophoblast also becomes thinner, with nuclei and mitochondria appearing near the indented capillaries.
• The researchers note that by the end, the maternal epithelium is only a third of its original thickness, and the ultrastructural characteristics suggest that the trophoblast may be synthesizing steroids towards the later stages of pregnancy.

The study provides a comprehensive overview of how the equine placenta adapts over the course of pregnancy to cater to the metabolic needs of the developing fetus.