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Ovarian follicular development during early pregnancy in cattle

Summary. On the day of oestrus (Day 0), 23 cows were assigned randomly to 4 groups in a 2 x 2 factorial experiment. Main effects were cow status [pregnant (P) versus non-pregnant (NP; not mated)] and day (Day 17 versus Day 34) when cows were slaughtered and ovaries collected; i.e. Groups NP-17 (N = 6), NP-34 (N = 5), P-17 (N = 6) and P-34 (N = 6). To mimic CL maintenance associated with presence of a conceptus in Group P-34, cows assigned to Group NP-34 were hysterectomized between Days 4 and 6 after oestrus. Pregnancy in Groups P-17 and P-34 (recovery of conceptus) and CL maintenance between day of surgery and Day 34 in Group NP-34 were confirmed. In Group NP-17, all cows except one were slaughtered before luteolysis. CL-bearing ovaries were serially sectioned (10 µm) and follicles (>0·15 mm) were counted and measured using routine histological techniques. For each size class of follicles, the percentages of follicles were similar (P>0·1) on Days 17 and 34. The percentage of non-atretic follicles (<4 pycnotic bodies) was lower (P<0·002) in pregnant than non-pregnant groups in the smallest class of antral follicles (0·16–0·28 mm) but was higher (P<0·03) in larger classes (0·68–3·67 mm) for pregnant groups. Percentages of atretic follicles were 3·7 and 4·9 (P>0·1) for follicles of 0·16–0·67 mm, 91·1 and 78·9 for follicles of 0·68–3·67 mm (P<0·04) and 5·1 and 16·1 (P<0·05) for follicles of 3·67–78·56 mm in pregnant and non-pregnant groups respectively. The percentage of atretic follicles classified as late atretic was higher in pregnant than in non-pregnant groups for follicles of 0·68–1·57 mm (P < 0·06) and 3·68–8·56 mm (P < 0·05). The results suggest that, as early as Day 17, the presence of a conceptus favours a more rapid turnover of follicles from class 1 (0·16–0·28 mm) to classes 3 and 4 (0·68–3·67 mm) but limits further growth by increasing atresia.