An immunocytochemical localization of the cortical granule lectin in fertilized and unfertilized eggs of xenopus laevis

At fertilization, the vitelline envelope surrounding the egg of Xenopus laevis is modified by the addition of an electron‐dense component termed the “F layer.” The F layer functions as a block to polyspermy and as a block to the escape of macromolecules from the perivitelline space, thereby causing an osmotically driven envelope elevation. F‐layer formation has been hypothesized to result from interaction between a cortical‐granule lectin, released in the cortical reaction, and a jelly‐coat ligand. Evidence for this hypothesis was sought by determining the location of the cortical‐granule lectin both before and after fertilization, using a specific antibody conjugated to horseradish peroxidase. The cortical‐granule lectin was localized only in the cortical granules of the unfertilized egg and was located predominantly in the perivitelline space and the F layer of a fertilized egg. These observations support the hypothesis that the F layer is formed by a cortical‐granule‐Iectin–jelly layer‐ligand interaction.