Evidence that the fertilization envelope blocks sperm entry in eggs of Xenopus laevis: Interaction of sperm with isolated envelopes

Abstract

The cortical reaction in eggs of Xenopus laevis results in conversion of the vitelline envelope (VE) into the fertilization envelope (FE). The FE is composed of the former VE plus a new component, the fertilization layer, that forms as a consequence of the cortical reaction. An in vitro test system was developed to compare the VE and FE in terms of their penetrability by sperm. VE's and FE's were isolated from unfertilized eggs and cleaving embryos, respectively. Although they swell during the isolation procedure, the isolated VE's and FE's retain the morphological characteristics by which they can be distinguished in situ. Penetrability of isolated envelopes was tested by exposing suspensions of envelopes to sperm in a saline solution to which solubilized components from the jellycoat of the egg were added. Isolated VE's were penetrated by sperm from both sides. By contrast, isolated FE's proved impenetrable: In no instance did sperm penetrate either the fertilization layer or the VE component of the FE. We concluded that the fertilization envelope blocks sperm entry in Xenopus laevis.