Meiotic competence of marmoset monkey oocytes is related to follicle size and oocyte-somatic cell associations.

This study was conducted to investigate the relationships between oocyte meiotic competence, follicle size, and occyte-somatic cell associations in the marmoset monkey (Callithrix jacchus). Follicles were excised from ovaries of nonstimulated adult cyclic females (n = 6) collected on Day 7 of the follicular phase. Follicles were separated into size groups: large preantral (260-400 microns), periantral (420-640 microns), small antral (660-1000 microns), large antral (1020-2000 microns), and preovulatory (> 2000 microns). Partially naked and cumulus/granulosa-enclosed oocytes (n = 473) were released from follicles and cultured in Waymouth's medium with 10% fetal calf serum, 1 microgram/ml human (h) FSH, and 10 micrograms/ml hLH. Somatic cells remaining after 46 h were removed, and oocytes were fixed after 48 h and mounted for viewing. Chromatin staining and microtubulin fluorescence labeling were used to assess progression of meiotic maturation and spindle normality. The follicle size distribution and oocytesomatic cell associations are reported. Competencies of oocytes to achieve germinal vesicle breakdown (GVBD) and metaphase II (MII) increased significantly (p < 0.001) with follicular size but not with the association of somatic cells. Marmoset oocytes from antral follicles resumed (GVBD) and completed (MII) meiotic maturation with high frequencies (98% and 72%, respectively), with no significant differences among size groups of antral follicles. GVBD competence was virtually absent in oocytes from preantral follicles (2%) and was acquired coincidentally with antrum formation (60%), although MII competence was attained after the completion of antrum formation. Partially naked oocytes from small antral follicles matured with a high incidence of spindle and meiotic abnormalities (44%). Marmoset oocyte meiotic competencies are notably higher than in any other nonhuman primate species studied, and a possible explanation for this phenomenon in relation to the stage of antrum formation is offered.