Effect of oviductal secretion components on the fertilizing capacity of amphibian sperm: Biological and ultrastructural studies

The present study was carried out to analyze the fertilization-supporting activity of Rhinella arenarum egg-jelly components. Spermatozoa were treated with diffusible factor (DF) constituted by the components released from the jelly coat into deionized water or with full jelly (FJ) containing all the components secreted by the oviductal pars convoluta (PC) during the transit of the oocytes through the duct, or with washed jelly (WJ) constituted only by structural components. Both jellies were solubilized by ultraviolet irradiation. These gametes were used for ultrastructural and biological studies in order to determine the acrosome state and the fertilizing capacity. Additional experiments were performed by using Ca2+, a diffusible cation present in R. arenarum jelly envelopes. Results demonstrated a marked increase in the acrosome reaction (AR) of sperm treated with FJ or DF compared to the controls (Ringer's solution), no significant differences being observed between both treatments, while WJ showed low AR percentages similar to the ones obtained with the controls. The addition of Ca2+ induced an increase in this parameter in a dose-dependent manner, although the values reached with FJ or DF were not attained. The results of the “in vitro” fertilization show a strong inverse association to the acrosome reaction (AR) rate. Treatment with Ca2+ at the concentration present in the jelly (6.3 ± 0.9 mM) inhibited the fertilizing capacity as a function of incubation time, showing that at 2 min there was a decrease in the fertilization percentages compared to 10% Ringer's. Data indicated that Ca2+ present in jelly is involved in the AR induction but suggests the possible participation of other diffusible and/or structural components of the oviductal secretion in this process.

► The percentage of sperm acrosome reaction (AR) depend of Ca2+ concentration present in the egg jelly.
► Sperm treated with full jelly (6.3 mM Ca2+) or diffusible factors (4.3 mM Ca2+) show high AR percentage.
► Sperm treated with washed jelly (1.7 mM Ca2+) show low percentage of AR.
► Values obtained with full jelly or diffusible factors cannot be reached by Ca2+ addition to washed jelly.
► The presence of Ca2+ is a requisite, but not the only condition, for the successful of AR induction.