Timing and location of zona pellucida synthesis during oogenesis in domestic cats--an ultrastructural immunohistological investigation.

The application of zona pellucida proteins for contraception of wildlife and feral animals, including stray cats, has been promoted since it was demonstrated to be effective for free-roaming feral mares. Active immunization with zona pellucida proteins leads to either reversible or irreversible infertility. Therefore, knowledge of the timing and location of zona pellucida synthesis during oogenesis in cats is a key aspect of designing an immunocontraceptive for felids. Domestic cat ovaries obtained after ovariohysterectomy were used to produce a specific rabbit antibody against feline zonae pellucidae. Ultrathin sections (70 nm) of cat ovaries were treated with the anti-zona antibody followed by incubation with gold-labelled anti-rabbit IgG for ultrastructural investigation. The gold label was related to the ultrastructure of oocytes and granulosa cells. Cat follicles at different stages of development were examined. The antibody reacted very specifically with zona pellucida proteins in fully grown oocytes with a compact zona pellucida. In secondary follicles, gold labels were found on the zona pellucida and inside granulosa cells in the vicinity of oocytes. Primary follicles were labelled inside their cubic granulosa cells and on fragments of zona pellucida in the cleft between granulosa cells and the oocyte. Some primordial follicles were characterized by labelling of the granulosa cells. In conclusion, these results indicate that cat zona pellucida is produced exclusively by granulosa cells and not by oocytes. Synthesis of zona pellucida takes place at every stage of follicular development.