Oocyte development in cattle: physiological and genetic aspects

Oocytes in cattle are formed during embryogenesis and develop within individual follicles in the cortex of the ovary. Dormant primordial follicles become active and undergo progressive development at regular intervals commencing during the late fetal stage and continuing throughout adulthood. Once activated, follicles and oocytes in a cohort either grow to maturation and ovulation or undergo atresia, ultimately depleting the ovaries of germ cells. It takes an estimated 100 days for a follicle and its oocyte to reach the mature ovulatory stage. Within an individual, number of follicles in one ovary is similar to number in the other ovary; however, there are large differences among individuals in total number of follicles present in both ovaries. In follicles that are recruited into the preovulatory pool, the fluid filled antrum enlarges and growth can be monitored by ultrasonography. Preovulatory follicles grow in waves rather than in a continuous stream, and number of follicles detected in a wave is related positively to number of microscopic follicles populating the ovarian cortex. Number of follicles in a wave is highly repeatable and the estimated heritability of number of follicles in heifers is approximately 0.35. Monitoring preovulatory follicle numbers by ultrasonography can be used to identify females that produce more embryos following superovulation or more natural twin births. Number of follicles in a wave can be influenced by energy balance and exogenous hormones, and quality of oocytes within those follicles can be influenced environmental factors, hormones and vitamins. The potential exists to increase genetic progress through repeatedly harvesting oocytes from preovulatory follicles of genetically superior females.