Tetraspanin CD9 is the only protein of the oocyte membrane (oolemma) known to be required for the fusion of gametes during fertilization in the mouse. Using electron microscopy and immunostaining we examined the differences in localization of CD9 between ovulated oocytes, zygotes and parthenogenetically activated eggs (parthenogenotes). Changes in ultrastructure of oolemma, which take place in oocytes after fertilization or artificial activation, were also assessed. We demonstrated that after fertilization the level of CD9 present on microvilli of zygote was two times lower than its level on the oolemma of the oocyte. In addition, we showed that the distribution of microvilli is less uniform in the zygotes than in the unfertilized oocytes. We propose that the changes of microvilli distribution and their CD9 content are responsible for the development of the oocyte membrane block to sperm penetration.

Mouse oocytes activated parthenogenetically do not generate a plasma membrane block against spermatozoa over the first three cell cycles. We show that they lose this fusibility spontaneously at the 8-cell stage. Insemination of 1-cell parthenogenetic embryos induces loss of fusibility earlier, at the 2-cell stage. This observation suggests that incorporation of the sperm cell membrane components into the oolemma may be responsible for the development of the membrane block.

We have investigated the oolemma block to polyspermy in the mouse. Zona-free and zona-intact eggs were fertilised and subsequently re-inseminated (the latter following zona pellucida removal). The ‘perivitelline’ block to polyspermy in zona-intact eggs renders motile sperm in the perivitelline space unable to bind to the oolemma. This is not connected with irreversible changes in the egg plasma membrane, because freshly added sperm can still fuse with such eggs freed from the zona. Fertilised eggs eventually lose the ability to fuse with sperm within 1 h, while still being able to bind many sperm.