Maturation-promoting factor (MPF) is responsible for the transformation of sperm nuclei to metaphase chromosomes in maturing bovine oocytes in vitro

Bovine oocytes at the germinal vesicle stage were inseminated in Brackett and Oliphant's medium in the presence of BSA (10 mg ml ^-1), caffeine (5 mmol^-1) and heparin (10 μg ml^-1). When oocytes were transferred into tissue culture medium (TCM)-199 containing 10% fetal calf serum (FCS) with or without 6-dimethylaminopurine (6-DMAP; 2 mmol l^-1) 8 h after insemination and cultured for 15-40 h at 39°C in 5% CO₂ in air, 74-83% of oocytes were penetrated and polyspermy (67-80%) was common. At 40 h after culture in 6-DMAP-free medium, 65% and 63% of unpenetrated and penetrated oocytes, respectively, reached metaphase II or beyond. A few (6%) oocytes were activated and contained both male and female pronuclei. Sperm metaphase chromosomes were observed in 90% of the penetrated oocytes. Penetration by more than four spermatozoa greatly retarded the meiotic maturation of the oocyte. However, sperm chromosomes were never observed in oocytes cultured in 6-DMAP supplemented medium and oocyte maturation did not proceed beyond the stage of prometaphase I. These results demonstrate the possible participation of maturation-promoting factor in metaphase chromosome formation in spermatozoa.