Studies on the zona pellucida of sheep ova: immune lysis and localization of antibodies in ova with, and without, zonae.

The susceptibility of fertilized sheep ova and oocytes, with and without zonae pellucidae, to lysis by complement plus antibodies in antisera raised against sheep ovaries, oocytes and sperm was examined in vitro. The presence of the zona reduced the proportion of ova lysed (8% v. 57%) and a large proportion of ova with zonae continued development in culture following exposure to antisera. Antisera to ovary and to oocytes lysed a higher proportion of ova than did antisera to sperm (51 and 56% v. 29%). There was no effect of age of fertilized ova (day 2, 4, 6; day 0 = day of oestrus) on susceptibility to lysis. Fluorescein-labelled antibodies from high-titre antisera to ovary and oocytes traversed the zona and attached to blastomeres. Following absorption of the antibody conjugates with sheep serum, fluorescein staining of ova with intact zonae was confined to the zona, whereas with zona-free ova the blastomeres were stained. It appears that the zona may protect embryos from lytic antibodies and it is suggested that the zona may be important in protecting the embryo from immunological attack in vivo.