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

In the female genital tract, spermatozoa must undergo capacitation and acro some reaction prior to fertilization. A number of factors may induce physio logical acrosome reaction assayed in vitro. The aims of this study are to determine the inductive effect of the preovul atory follicular fluid on the sperm acrosomal status in the equine, once so me characteristics of the follicular fluid during folliculogenesis had been evaluated. The spermatozoa were obtained from cauda epididymes of adult stallion. Foll icular 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 follicu lar fluid (50%) upon induction of the acrosomic reaction in stallion capaci tated spermatozoa was assayed. Results show that during folliculogenesis the ratio progesterone/estrogen i s below 1. In large preovulatory follicles, there is a sharp increase of pr ogesterone, reaching a ratio progesterone/ estrogen close to 4. Protein con centration and osmolarity increase together with follicular development, be ing osmolarity very high at the preovulatory stage. Follicular fluid -in vi tro- increases the percentage of spermatozoa with acrosome reaction, mainta ining high rates of vitality and motility. The characteristics of follicular fluid undergo dynamic changes during the folliculogenesis, such as steroid level, protein concentration and osmolari ty. These events may play a role in the reproductive process in vivo, consi dering that in vitro the follicular fluid is a very effective inductor of t he acrosome reaction, with optimum levels of vitality and motility.