Intrafollicular content of luteinizing hormone receptor, alpha-inhibin, and aromatase in relation to follicular growth, estrous cycle stage, and oocyte competence for in vitro maturation in the mare.

This research paper examines the relationship between the levels of certain proteins (LH receptor, alpha-inhibin, and aromatase) in granulosa and cumulus cells, and follicular growth, the stage of the estrous cycle, and oocyte competence for in vitro maturation in mares. The research found a potential correlation between the level of these proteins and follicular growth, suggesting a link between the acquisition of oocyte competence and the expression of these proteins in follicular cells.

Research Design and Process

• The researchers executed follicular punctures 34 hours after an injection of crude equine gonadotropins. This was done at different stages: during the follicular phase, at the end of the follicular phase, or during the luteal phase.
• The cumulus-oocyte complex, granulosa cells, and follicular fluid of follicles larger than 5 mm were collected.
• The nuclear stage of the oocytes after in vitro culture was determined microscopically.
• Granulosa and cumulus cell amounts of LH receptor, alpha-inhibin, and aromatase were assessed by the semi-quantitative Western blot method and image analysis.
• Follicular fluids were assayed for progesterone (P4) and estradiol-17beta (E2).

Research Findings

• The three factors – LH receptor, alpha-inhibin, and aromatase – were expressed in mural granulosa and cumulus cells from all follicles from the gonadotropin-independent growth period until the preovulatory stage.
• The amounts of aromatase in granulosa cells, as well as the E2:P4 ratios, were higher for follicles punctured during the follicular phase than for the two other groups.
• The E2:P4 ratio and the LH receptor and aromatase contents, but not alpha-inhibin, in granulosa cells increased with an increase in follicular diameter.
• The E2:P4 ratios, LH receptor, alpha-inhibin, and aromatase amounts in granulosa cells were lower in follicles 5-9 mm in diameter.
• In cumulus cells, the amounts of the three factors did not differ between the three groups nor between the follicular diameters.

Conclusions and Implications

• The authors found no apparent relationship to oocyte competence for in vitro maturation. However, they did observe that the influence of the follicle diameter on the content of LH receptors, alpha-inhibin, and aromatase in granulosa cells was similar to the influence of follicle diameter on oocyte competence.
• These results led to the hypothesis that, in mares, there is a link between the acquisition of oocyte competence and the expression of LH receptors, alpha-inhibin, and aromatase in the follicular cells.