Histological changes in the uterus during postpartum in the mare.

This research examines how the uterus in horses changes after giving birth. These transformations were evaluated for 10 days after birth, using daily biopsies from 29 horse mothers.

Study Overview

The study involved 29 mares or female horses that had just given birth. Daily biopsies of the uterus were taken for the first 10 days after giving birth which resulted in a total of 87 samples. The researchers documented the changes observed in the uterus tissue over this period to understand the postpartum recovery process in mares.

Observations – Day 0

• The equine endometrium (the inner lining of the uterus) was characterized by regularly ordered microcaruncles on the surface. Microcaruncles are the specialized regions of the endometrium where the fetus attaches during pregnancy.
• The underlying layer of the endometrium, known as the stratum spongiosum, was swollen or ‘oedematous’, and contained dilated but few glands. This swelling is likely due to the fluid accumulation that typically occurs after delivery.

Observations – Day 1

• The researchers detected degenerative changes in the microcaruncles and endometrial glands, indicative of the recovery process beginning to take place.

Observations – Day 2 to 5

• The tissue of the microcaruncles showed signs of cytoplasmic vacuolization and karyorrhexis—indications of cell damage or death. These changes suggest that the microcaruncles, having fulfilled their function during pregnancy, were now being broken down by the body.
• An inflammatory reaction was observed, which involved the presence of neutrophils and phagocytic cells— immune cells involved in the removal of dead cells and tissue.

Observations – Day 7

• By day 7 postpartum, the researchers noted that the appearance of the endometrium was akin to its state during the normal proestrus phase, which is the period before estrus or “heat” and a sign of the mare returning to its reproductive cycle.
• The cuboidal luminal epithelium was present and the stromal tissue was still edematous, indicating that some degree of swelling was still prevalent.

Observations – Day 9 and 10

• By days 9 and 10, the researchers observed that the changes had usually been completed, and the endometrium had the typical appearance of the estrus phase. Estrus refers to the phase when the female is sexually receptive or “in heat”.
• The recovery of the mare’s uterus after birth seems to be a relatively quick process, usually taking around 9 to 10 days for the organ to resume its typical pre-pregnancy structure.