The acrosomic reaction in stallion spermatozoa: inductive effect of the mare preovulatory follicular fluid.

In the female genital tract, spermatozoa must undergo capacitation and acrosome reaction prior to fertilization. A number of factors may induce physiological acrosome reaction assayed in vitro. The aims of this study are to determine the inductive effect of the preovulatory follicular fluid on the sperm acrosomal status in the equine, once some characteristics of the follicular fluid during folliculogenesis had been evaluated. The spermatozoa were obtained from cauda epididymes of adult stallion. Follicular fluid was taken from mare ovarian follicles classified according to their diameter. In these fluids, total protein, progesterone, estradiol and osmolarity were determined. Afterwards, the effect of preovulatory follicular fluid (50%) upon induction of the acrosomic reaction in stallion capacitated spermatozoa was assayed. Results show that during folliculogenesis the ratio progesterone/estrogen is below 1. In large preovulatory follicles, there is a sharp increase of progesterone, reaching a ratio progesterone/estrogen close to 4. Protein concentration and osmolarity increase together with follicular development, being osmolarity very high at the preovulatory stage. Follicular fluid--in vitro--increases the percentage of spermatozoa with acrosome reaction, maintaining high rates of vitality and motility. The characteristics of follicular fluid undergo dynamic changes during the folliculogenesis, such as steroid level, protein concentration and osmolarity. These events may play a role in the reproductive process in vivo, considering that in vitro the follicular fluid is a very effective inductor of the acrosome reaction, with optimum levels of vitality and motility.