Effect of post-ovulatory age and calcium in the injection medium on the male pronucleus formation and metaphase entry following injection of human spermatozoa into golden hamster oocytes

The occurrence of parthenogenetic activation is a major hurdle in obtaining sperm chromosome metaphases after heterospecific intracytoplasmic sperm in jection (ICSI) of golden hamster oocytes with human spermatozoa. We address ed two potential contributors to parthenogenetic activation namely, post-ov ulatory age of the oocyte and Ca2+ content of the injection medium, In seri al experiments, hamster oocytes were retrieved at 11.5, 13, 16 and 21 h aft er the ovulatory dose of human chorionic gonadotrophin (HCG) and microinjec ted with human spermatozoa suspended alternately in a regular (1.9 mM Ca2+) or a Ca2+-free medium. A progressive decrease in the rates of male pronucl eus (MPN) formation and metaphase entry and increase in the rates of parthe nogenetic activation without male pronucleus occurred with increasing post- ovulatory age. The favourable influence of Ca2+-free injection medium on th e mean rates of MPN and metaphase entry was restricted to the relatively ol der oocytes (MPN 16 h: 49.5 versus 32.3%, P < 0.008; 21 h: 22.2 versus 11.1 %, P < 0.001; metaphase entry 16 h: 36.8 versus 25.1%, P < 0.02; 21 h: 13.3 versus 5.2%, P < 0.01 in the Ca2+-free and regular groups respectively). O ur data confirm the increased activation sensitivity with post-ovulatory ag eing and its adverse influence on the MPN formation and metaphase entry aft er heterospecific ICSI of hamster oocytes.