Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/23/1999
Publication Date: 6/1/2000
Citation: Cushman, R.A., Hedgpeth, V.S., Echternkamp, S.E., Britt, J.H. 2000. Evaluation of numbers of microscopic and macroscopic follicles in cattle selected for twinning. Journal of Animal Science. 78(6):1564-1567. Interpretive Summary: Long-term genetic selection of cattle at the Roman L. Hruska U.S. Meat Animal Research Center for natural dizygotic twins has enhanced development of ovarian antral follicles and increased annual ovulation rate and twinning rate to 1.77 and 1.52, respectively. Results of the present study suggest that the enhanced follicular development begins at the secondary stage of preantral follicular development as the result of Twinner cows being able to maintain a large pool of healthy secondary preantral follicles. Ovaries of the two genetic lines of cattle (i.e., Twinner vs Control) did not differ in the density of primordial or primary follicles, but the ovaries of Twinners had a higher density of healthy secondary follicles. Previous results have identified the secondary preantral follicle stage and the small antral follicle stage as the primary stages when follicular atresia occurs. Because this atresia can be reduced by treatment with exogenous insulin-like growth factor-I (IGF-I), we speculate that the greater IGF-I concentrations in both blood and ovarian follicular fluid of Twinner cows contribute to the increased number of secondary preantral follicles and of antral follicles in the ovaries of Twinner cows.
Technical Abstract: We hypothesized that the number of microscopic follicles present in the ovaries of cattle selected for twin births (Twinner) would be greater than in the ovaries of contemporary Controls. Ovaries were collected from seven Control and seven Twinner cows at slaughter. The number of small (1 to 3.9 mm), medium (4 to 7.9) and large (>8 mm) surface follicles was counted and one ovary was fixed for histological evaluation. Fifty to sixty consecutive 6 um slices were taken from a random piece of cortical tissue approximately 1 cm x 1 cm in area. Microscopic follicles were classified as primordial-oocyte surrounded by a single layer of pregranulosa cells, primary-oocyte surrounded by a single layer of granulosa cells, secondary-oocyte surrounded by two or more layers of granulosa cells or tertiary follicles-oocyte surrounded by multiple layers of granulosa cells within initiation of antrum formation to </= 1 mm in diameter. The total number of follicles was counted in 200 fields (2 mm x 2 mm). A field containing no follicles was classified as empty. There were significantly more secondary follicles in Twinner compared to Control ovaries (6.3 vs 12.9; P < .05). Twinners also tended to have more small surface follicles (49.0 vs 35.4; P < .1). We conclude that ovaries of Control and Twinner cows do not differ in the number of primordial follicles or in the number of follicles activated into the growing pool; however, Twinner cows are able to maintain more growing follicles at the secondary and subsequent stages of development.