Ultrastructural and histological characteristics of the endometrium during early embryo development in mares.

The aim of this research is to study changes in the endometrium, which is the lining of the uterus, during the early stage of embryo development in horses. The research is particularly focused on observing the changes on days 7, 10, and 13 after ovulation, comparing mares that are pregnant and ones that are not.

Materials and Methods

• The study was carried out among 30 cyclic mares, monitored through transrectal palpation and reproductive tract ultrasonographic examinations until estrus was detected.
• In the first cycle, endometrial biopsies were taken from the mares on the 7th, 10th, and 13th days post-ovulation.
• In the second cycle, these mares were bred by a fertile stallion and intrauterine biopsies were collected on the same days post-ovulation. The collected samples were then immediately used for flushing the uteri of the horses.
• Mares with successful embryo recovery after flushing were put into the ‘Pregnant group’.
• Progesterone concentrations were measured daily in blood samples collected from both cyclic and pregnant mares from the date of ovulation detection to the day of uterine biopsy.

Findings

• By the 7th day of pregnancy, a noticeable loss of ciliated cells was observed among pregnant mares compared to the cyclic group. Endometrial superficial cells were more bulging and some histotrophic material was seen between the folds.
• By the 10th day, the secretion of histotrophic material from the glands and luminal epithelium became more intense among pregnant mares versus cyclic mares.
• On the 13th day, a large amount of histotroph was observed with large glandular openings surrounded by ciliated cells.
• The pregnant mares also had larger blood vessel calibers compared to the cyclic ones from the 7th to the 13th day post-ovulation.
• The concentration of progesterone was affected by the day of pregnancy (P < 0.001), but not affected by the group they belonged to.

Implications

• These changes in the uterus seem to aid in providing the necessary nutrients for the early stage of embryo development, as well as promoting cellular changes that will interact with embryonic signaling and future fixation, implantation, and placentation.