IN-VITRO MATURATION OF EQUINE OOCYTES OBTAINED FROM DIFFERENT AGE-GROUPS OF SEXUALLY MATURE MARES

Oocytes were harvested from mare ovaries obtained at slaughter and wer e divided into 3 groups based on the age of the donor. The age groups consisted of young (2 to 7 yr), middle-aged (8 to 14 yr) and aged (gre ater than or equal to 15 yr) mares. There were no differences between age groups in the proportions of follicles available for examination o r the proportions of normal, abnormal or total oocytes collected. Afte r 24 h of culture, the overall maturation rate to the second metaphase (MII) was 52.7%. Maturation rates for oocytes obtained from young and middle-aged mares were similar, but oocytes from aged mares were only similar to 25% as likely to reach MII and they were 3 times more like ly to remain at metaphase I. Twelve oocytes had chromosome spreads sui table for counting; 6 were haploid, 2 were hyperhaploid and 4 were hyp ohaploid. Insufficient numbers of readable spreads precluded compariso ns of chromosome complements between age groups.