Development of preovulatory follicles expected to form short-lived corpora lutea in beef cows

Summary. Oestrus, expected to be followed by a short luteal phase, was induced in post-partum cows by weaning their calves at 35 days after parturition. Ovaries containing the first preovulatory follicles (Type F) formed after parturition were collected 3 h after the onset of oestrus. For comparison, preovulatory follicles (Type C) were collected 3 h after the onset of oestrus in normally cycling cows. The number of granulosa cells was determined and the concentrations of receptors for follicle-stimulating hormone (FSH) and luteinizing hormone (LH) in granulosa cells and for LH in theca cells were measured. Concentrations of oestradiol-17β, testosterone, androstenedione and progesterone in follicular fluid were also measured. Type F follicles contained about twice the number of granulosa cells (based on DNA) as did Type C follicles (45·8 ± 11·3 and 24·5 ± 3·9 μg DNA/follicle, respectively; P < 0·05) but these cells had fewer receptors for LH (0·13 ± 0·02 vs 0·29 ± 0·03 fmol/μg DNA; P < 0·01) and FSH (0·61 ± 0·08 vs 1·3 ± 0·29 fmol/μg DNA; P < 0·08) than did those from Type C follicles. Additionally, there were fewer receptors for LH in theca tissue from Type F than from Type C follicles (28·3 ± 5·2 vs 51·3 ± 6·1 fmol/follicle; P < 0·01). Concentrations of oestradiol-17β (475·8 ± 85·6 vs 112·9 ± 40·0 ng/ml; P < 0·01) and androstenedione (214·1 ± 48·7 vs 24·7 ± 7·7 ng/ml; P < 0·01) in follicular fluid were higher in Type C than in Type F follicles. There were therefore more granulosa cells in Type F follicles which contained fewer receptors for LH and FSH and accumulated less oestradiol-17β in the follicular fluid. We suggest that the characteristics of preovulatory follicles may influence the life-span of the subsequently formed corpus luteum and short luteal phases may de due, in part, to altered follicular development.