Oviducal pars recta as factor in fertilization properties and hormonal regulation in the toad Bufo arenarum

In this communication I have attempted to present an overview of some contributions to the understanding of the oviduct-egg interaction in amphibians. According to data from other authors, the vitelline envelope of the newly ovulated egg constitutes a barrier for the passage of spermatozoa. Our results demonstrated that only after they have been affected by substances released by the first 1-3 cm of the oviduct (pars recta), is the envelope sensitive to spermlysins and the oocytes fertilizable. This functional change is matched by biological, physicochemical and morphological differences in the vitelline envelope. The fact that the pars recta activity is affected by the sexual cycle and that in ovariectomized females - devoid of active pars recta - the biological activity can be restored by steroid hormones, strongly suggests that the molecules involved in fertilization are synthesized and secreted during specific steps of the reproductive cycle. The pars recta-oocyte interaction probably involves more than one type of molecules, considering the observations made on the carbohydrate metabolism of coelomic eggs, which could be altered by the oviducal secretions. Several explanations for the pars recta mechanism of action have been suggested. One is a direct action on the sperm; pars recta molecules - engulfed in the vitelline envelope - would trigger the acrosome reaction. We propose the unmasking of specific vitelline envelope sites for sperm interaction. Material on the outer surface of the VE can be removed or altered by the enzymatic activity - similar to plasmin and trypsin - detected in the pars recta secretions.