Involvement of blood serum in amphibian fertilization

A proteolytic enzyme secreted by the first portion of amphibian oviduct, pars recta, called oviductin in Xenopus laevis, causes ultrastructural alterations on the extracellular matrix of coelomic eggs, turning them susceptible to fertilization. Although great advances have been made in the field of reproduction, the molecular mechanisms responsible for the fusion between the egg and the sperm are yet to be understood. We have recently demonstrated the presence of proteins from pars recta fluid in blood serum and extracellular matrix of coelomic eggs in Bufo arenarum. Here we show, using immunofluorescence procedures, that blood serum components are present in the extracellular matrix of coelomic and pars recta fluid-conditioned eggs in Bufo arenarum. Furthermore, by assessing the neutralizing effect on the conditioning activity of pars recta fluid on coelomic eggs we found that antibodies against pars recta secretions and blood serum inhibited the effect of sperm-lysin on the vitelline envelope of conditioned oocytes and impaired fertilization by sperm. Thus, serum proteins appear to be implicated in the molecular events that lead to amphibian fertilization.