Dynamics and histological observation of first follicular wave in Japanese black cows

AIMS: To determine the turnover of the first follicular wave in Japanese black cows and quantitative immunohistological characteristics of the previously in vivo identified dominant follicle (DF) and largest subordinate follicle (SF) derived from ovariectomy on Day 7 (3 cows) and Day 10 (3 cows) (Day 0=estrus). Six cases of first follicular wave in cows were observed twice daily by ultrasound scanning. The number of follicles, diameter of DF and SF, and prevalence of apoptotic granulosa cells (GC) and theca cells (TC) were studied by TUNEL methods. At follicular wave emergence, 13.5 +/- 9.5 Class I (2-5 mm in diameter follicles) were found 12 hr after ovulation, and increased its number until Day 1 pm. Future DF and SF observed retrospectively were 4.9 +/- 0.8 mm and 4.9 +/- 0.9 mm at wave emergence. Deviation of DF and SF occurred on Day 3 pm with mean diameters of 8.9 +/- 1.3 mm and 6.8 +/- 0.9 mm, respectively. DF developed until Day 8 am with a maximum diameter of 14.4 +/- 1.8 mm (n=3) and then regressed. The follicular wall of the DF had a characteristic image of a healthy follicle on Day 7 and slightly atretic DF on Day 10, whereas SF showed heavy atresia on both Day 7 and Day 10 under HE staining. In the prevalence of apoptotic cells, DF were 4.4 +/- 1.0% and 17.9 +/- 4.9% on Day 7 and on Day 10 in GC, respectively, and 2.4 +/- 0.7% and 8.0 +/- 1.4% on Day 7 and on Day 10 in TC, respectively. These results showed that, 1) the first follicular wave in cows is characterized by 24 hr recruitment of small follicles and a gradual divergence of growth rates in future DF and SF, and 2) early regression of DF on Day 10 was preceded by severe apoptosis.