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

The intrafollicular content of LH receptor, alpha-inhibin, and aromatase ar e known good indicators of follicular status. We investigated the amounts o f these proteins in granulosa and cumulus cells in relation to oocyte compe tence for in vitro maturation, follicular growth, and estrous cycle stage i n the mare. Follicular punctures were performed 34 h after an injection of crude equine gonadotropins, either during the follicular phase, at the end of the folli cular phase, or during the luteal phase. The cumulus-oocyte complex, granul osa cells, and follicular fluid of follicles larger than 5 mm were collecte d. The nuclear stage of the oocytes after in vitro culture was determined m icroscopically. Granulosa and cumulus cell amounts of LH receptor, alpha-in hibin, and aromatase were assessed by the semiquantitative Western blot met hod and image analysis. Follicular fluids were assayed for progesterone (P- 4) and estradiol-17 beta (E-2). The three factors were expressed in mural granulosa and cumulus cells from all follicles from the gonadotropin-independent growth period until the pre ovulatory stage. Considering all the follicles punctured, the amounts of LH receptor and alpha-inhibin in granulosa cells were not different for the t hree physiological stages studied. The amounts of aromatase in granulosa ce lls, as well as the E-2:P-4 ratios, were higher for follicles punctured dur ing the follicular phase than for the two other groups (p < 0.05). Consider ing the data from the three groups, the E-2:P-4 ratio and the LH receptor a nd aromatase contents, but not a-inhibin, in granulosa cells increased with an increase in follicular diameter p < 0.01). The E-2:P-4 ratios and the a mounts of LH receptor, (alpha-inhibin, and aromatase in granulosa cells wer e lower in follicles 5-9 mm in diameter than in larger ones (p < 0.05). In cumulus cells, the amounts of the three factors were different neither betw een the three groups nor between the follicular diameters. Although we could not establish any obvious relationship to oocyte competen ce for in vitro maturation, the influence of the follicle diameter on the c ontent of LH receptors, or-inhibin, and aromatase in granulosa cells was si milar to the influence of follicle diameter on oocyte competence. Therefore , one can hypothesize that, in the mare, there is a link between the acquis ition of oocyte competence and the expression of these factors in the folli cular cells.