202 GENETIC INFLUENCE ON FOLLICULAR DEVELOPMENT IN CATTLE

Abstract

In cattle, the development of ovarian follicles >5 mm occurs in waves. During each estrus cycle, a cohort of 5–7 follicles develops either 2 or 3 times, with the last wave containing the ovulatory follicle. It is speculated that follicular wave number within estrus cycle may be genetically determined; however, no data have been published to support this hypothesis. The present study was designed to test this hypothesis by using genetically identical cloned cows and comparing their wave patterns among each other and to those of control animals by studying their ovarian follicular dynamics. Three 5-yr-old cloned cows, derived from somatic cells of a 13-yr-old Holstein dairy cow, were age- and lactational status-matched with contemporary cows produced by AI, and maintained under the same managemental conditions. Ovarian follicular dynamics of all the cows were determined using a real-time, B-mode, linear-array ultrasound scanner equipped with a 7.5 MHz transrectal transducer (Aloka Co., Tokyo, Japan). Ultrasound examinations were carried out every day for 101 days and covered 4 estrus cycles for each animal. For tracking the follicular dynamics, daily ultrasound observations were videotaped and individual follicle dimension and location were drawn. The estrus was determined by the visual observation of a growing follicle over a period of several days, reaching a maximum diameter of about 15 mm or more, and its subsequent disappearance on the following day. The ultrasound observations were corroborated with the visual and behavioral signs of estrus. The average length of estrus cycle in clones and controls was 23.08 ± 0.47 days and 22.67 ± 0.37 days (mean ± SEM), respectively, and were not significantly different. Out of the four estrus cycles studied, all clones showed 3 follicular waves in 3 cycles and 2 waves in one cycle. However, the wave pattern within each cycle did not match for the clones over all the cycles. In controls, one cow had 2 waves in all 4 cycles while the other two cows had both 3- as well as 2-wave cycles. The 3-wave cycles had an average length of 23.89 ± 0.59 days (mean ± SEM) and 24.00 ± 0.71 days in clones and controls, and the average lengths of 2-wave cycles were 20.67 ± 0.33 d and 22.00 ± 0.38 d in clones and controls, respectively. There was no significant difference in the average cycle lengths among the three clones, which were 23.00 ± 1.23, 23.75 ± 1.38, and 22.50 ± 0.65 days. Over the 4 estrus cycles, the clones and the controls had a total of 60.33 ± 7.45 and 63.00 ± 6.12 small (4 mm–<6 mm), 63.74 ± 6.34 and 69.00 ± 9.62 medium (6 mm–10 mm), and 22.00 ± 2.51 and 20.00 ± 1.16 large (>10 mm) follicles, respectively. The total number of follicles for each clone during 4 cycles were different, viz., large (25, 17, 24), medium (60, 76, 55), and small (71, 64, 46). Thus the clones, in spite of their shared genetic makeup, had different cycle lengths, follicle numbers, and follicular wave patterns. This indicates that genetic makeup is not the only major influence determining bovine follicular dynamics.