Immunocytochemical localization of ZP3 in primordial follicles of rabbit, marmoset, rhesus monkey and human ovaries using antibodies against human ZP3.

Active immunization with zona pellucida (ZP) proteins leads to depletion of primordial and recruited follicles in the ovary by a yet unknown mechanism. Small amounts of ZP present on primordial follicles or granulosa cells may be one of the reasons for this ovarian pathology. This study was undertaken in an attempt to identify the presence of ZP in or on primordial follicles and granulosa cells in ovaries of rabbits, common marmosets (Callithrix jacchus), rhesus monkeys (Macacas mulatta) and humans. Therefore, monoclonal antibodies (mAbs) and rabbit and mouse antisera against recombinant human ZP3 (hZP3) were produced. All these antibodies bound to the ZP of native intact human oocytes. Several fixatives have been tested on pieces of rabbit ovaries. With 4% (w/v) paraformaldehyde in PBS (4% PFA) followed by paraffin embedding and cryostat sections postfixed with 4% PFA, the most intense staining of ZP with one of the mAbs was obtained and the morphology was well preserved. In humans, besides the ZP, the oocyte cytoplasm and granulosa cells of most primordial and recruited follicles were also stained with these antibodies. In rhesus monkey ovaries, all primordial oocytes were also stained, in addition to some granulosa cells of primary follicles. In marmosets, small dots of immunoreactive ZP were found on 60% of the primordial follicles but granulosa cells were not stained. In rabbits, only minor staining of primordial follicles was observed. After passive immunization of rabbits with mAb 4, antibodies were found on both primordial and recruited follicles. These results clearly show the presence of ZP3 on primordial follicles and in granulosa cells, and this could explain the ZP-induced ovarian pathology after active immunization with ZP3.