The role of oviducal secretions in mediating gamete fusion in the toad, Bufo bufo japonicus.

A fertilizing sperm of the anuran amphibians has to pass through the jelly envelopes and the vitelline envelope (VE) before making a successful fusion with the egg plasma membrane. Of these the jelly envelopes, secreted by the long pars convoluta (PC) portion of the oviduct, have long been known to be indispensable for the sperm entrance in the egg. The most recent experiments employing dejellied uterine eggs of the toad, Bufo bufo japonicus, revealed that the jelly plays its role in fertilization by its unique capacity of retaining divalent cations (Ca2+ and/or Mg2+) which are essential for a fertilizing sperm. There are other lines of evidence which implicate that the secretions of the uppermost portion of oviduct, p. recta (PR), render the VE penetrable by sperm. We show that the secretory granules (PRG) isolated from PR of ovulating Bufo females by centrifugation in Percoll possess such biological activities as an increase of fertilizability of coelomic eggs and the induction of both the acrosome reaction and a release of the VE lysin from sperm. In addition, the activities of the PRG are inhibited by trypsin inhibitors, and this trypsin-like activity is dependent on Ca2+. These results, combined with the previous immunohistochemical demonstration of the deposition of PR-substance(s) in the VE, lead us to propose that a fertilizing toad sperm is acrosome-reacted in response to the PRG substance deposited in the VE and finds a way of traversing the VE by the released lysin, both of which may be dependent on Ca2+ supplied by jelly envelopes, the product of PC.