Effect of Proteolytic Enzymes on the Ultrastructure of Antibody-treated Sea Urchin Eggs

Immunofertilization studies have shown that anti-sera have a number of effects on sea urchin eggs. An unusually interesting feature of the action of antibody on the fertilizing capacity of the eggs is the dependence on the bivalent character of the antibody. Treatment of the eggs with univalent antibody prepared by papain digestion has no effect on fertilizability (Metz and Thompson, 1967; Graziano and Metz, 1967). The failure of univalent antibody to inhibit the fertilizability of eggs is interpreted to mean that there are no essential fertilization antigens at or near the egg surface, at least in the sense of anti-genic combining sites that must be free to function in initial stages of fertilization. Inhibition of /ertilization by bivalent antibody must then depend upon secondary or tertiary effects associated with the capacity of bivalent antibody to cross-link neighboring antigens. It then becomes of interest to determine what such secondary factors might be. One possibility (Baxandall, 1966) is that the 0.1 # thick surface layer of electron opaque material reported by Baxandall et al. (1964) in electron micrographs of bivalent antibody-treated eggs is a barrier to sperm-egg interaction. This view is consistent with the fact that the layer is not found on univalent antibody-treated eggs (Metz et al., 1967; Metz, 1968). Finally, bivalent antibody-treated eggs recover fertitizability when posttreated with proteolytic enzymes (Metz and Thompson, 1967; Graziano and Metz, 1967). In the present study such eggs were examined with the electron microscope in a further attempt to correlate the electron-opaque layer with fertilization inhibition by bivalent antibody. MATERIAL AND METHODS The sea urchins, Lytechinus variegatus, were collected in Biscayne Bay, Dade County, Florida. Gametes were obtained by KCl-indueed shedding. Antisera were prepared in three different rabbits by injecting them with homogenate of acid dejellied eggs in Freund's adjuvant as previously described (Metz and Thompson, 1967). The T-globulin fraction was