Ovarian and Uterine Dynamics During the Estrous Cycle in Criollo Breed Mares.

This research is focused on the reproductive dynamics or ovarian function of the Criollo breed of horses, concentrating on follicular activity, corpus luteum growth and regression, as well as changes in cervical and uterine tone during two consecutive estrous cycles.

Follicular Dynamics

• The researchers observed that the interovulatory intervals (IOI) in Criollo mares lasted for 21 days. These IOIs are the periods between each ovulation during the estrous cycle.
• The dominant and subordinate follicles—structures in the ovary that develop into mature eggs—emerged together on day 5 (with ovulation as day 0). The rate of their growth was slower until the day of deviation, which is when their growth rates start to diverge.
• The researchers observed the divergence in growth rates, which initiated when the dominant and subordinate follicles reached 22 and 21 mm in diameter, respectively.
• The average size of the preovulatory follicle, which is the follicle that will release the egg, was 43 mm the day before ovulation.
• Different combinations of follicular waves, or patterns of follicular development and regression, were observed in each IOI.

Corpus Luteum Dynamics

• The corpus luteum—a structure formed at the ovulation site that secretes progesterone to prepare the uterus for pregnancy—reached its maximum diameter approximately 2 days post-ovulation and then gradually decreased.

Cervical and Uterine Dynamics

• The tone of the cervix and uterus, indicating their physiologic state, showed a positive correlation post-ovulation and stayed elevated until the expected luteolysis (degradation of the corpus luteum) around days 12 to 14.
• After expected luteolysis, an increase in endometrial echotexture was recorded, reaching a maximum 3 to 5 days before ovulation, then it started to decline. The endometrial echotexture refers to the ultrasonographic appearance of the uterus lining, which goes through changes throughout the estrous cycle.

Within-Animal Repeatability and Significance of the Findings

• The study also found moderate to strong positive correlations between IOIs within individual mares for the diameter of the preovulatory follicle, cervical and uterine tones, endometrial echotexture, and corpus luteum diameter, suggesting a level of consistency or repeatability within the same animal.
• These findings help in driving an articulate understanding of the reproductive physiological patterns of the estrous cycle in Criollo breed mares, which may influence both natural and artificial breeding programs.