Chorionic gonadotropin secretion is associated with an inhibition of follicular growth and an improvement in oocyte competence for in vitro maturation in the mare.

The research article indicates that in pregnant mares, levels of circulating equine Chorionic Gonadotropin (eCG) are associated with a decrease in follicular growth and an increase in the competence of oocytes for in vitro maturation and fertilization.

Objectives and Methodology

• The aim of the study was to examine the effects of circulating eCG on the growth of follicles and on the competence of oocytes for in vitro maturation and fertilization.
• Ultrasound-guided punctures were performed repeatedly on 4 pregnant mares and 5 control mares over a period of time. In the control group, the embryonic vesicle was crushed on Day 22.
• The researchers routinely punctured all follicles larger than 5mm in both the pregnant mares and the control mares, 24 hours after the largest follicle reached 18mm in size.
• The researchers cultured the compact cumulus oocyte complexes (COCs) in vitro for 46 hours. After this, they either stained the COCs for analysis or processed them for in vitro fertilization (IVF).

Findings

• In control mares, eCG levels remained stable, but in all pregnant mares, eCG levels increased beyond 100ng/ml from Day 37 onward.
• In pregnant mares, the number of follicles flushed during each puncture attempt decreased significantly over time. However, there was no change in the number of follicles flushed in control mares.
• The in vitro maturation rate of oocytes derived from follicles of size 10-14 mm was significantly higher in pregnant mares compared to control mares (82% vs 43%). No significant difference was found in maturation rates of oocytes from smaller or larger follicles.
• After IVF, no oocyte was fertilized in either group, but this did not diminish the insight into the role of eCG on oocyte competence and follicular growth.
• The study concluded that higher levels of circulating eCG are associated with a decrease in follicular growth and an increase in the competence of oocytes for in vitro maturation, which suggests that eCG plays a crucial role in the reproductive processes of mares.