SPERM-EGG INTERACTION IN THE PAINTED FROG (DISCOGLOSSUS-PICTUS) - AN ULTRASTRUCTURAL-STUDY

The ultrastructure of sperm changes and penetration in the egg was stu died in the anuran Discoglossus pictus, whose sperm have an acrosome c ap with a typical tip, the apical rod. The first stage of the sperm ap ical rod and acrosome reaction (AR) consists in vesiculation between t he plasma membrane and the outer acrosome membrane. The two components of the acrosome cap are released in sequence. The innermost component (component B) is dispersed first. The next acrosome change is the dis persal of the outermost acrosome content (component A). At 30 sec post insemination, when the loss of component B is first observed, holes ar e seen in the innermost jelly coat (J1), surrounding the penetrating s perm. Therefore, this acrosome constituent might be related to penetra tion through the innermost egg investments. At 1 min postinsemination, during sperm penetration into the egg, a halo of finely granular mate rial is observed around the inner acrosome membrane of the spermatozoo n, suggesting a role for component A at this stage of penetration. Gam ete-binding and fusion take place between D1 (the egg-specific site fo r sperm interaction) and the perpendicularly oriented sperm. Spermatoz oa visualized at their initial interaction (15 sec postinsemination) w ith the oolemma are undergoing vesiculation. The first interaction is likely to occur between the D1 glycocalyx and the plasma membrane of t he hybrid vesicles surrounding the apical rod. As fusion is observed b etween the internal acrosome membrane and the oolemma, it can be postu lated that gametic interaction might be followed by fusion of the tatt er with the apical rod internal membrane that extends posteriorly into the inner acrosome membrane. Insemination of the outermost jelly laye r (J3) dissected out of the egg, and observations of the ultrastructur al changes of spermatozoa in this coat, indicate that J3 rather than t he vitelline coat (VC) induces the AR. Interestingly, at the late post insemination stage, VC fibrils are seen crosslinking the inner acrosom e membrane. The role of this binding is here discussed. (C) 1997 Wiley -Liss, Inc.