Dutch Friesian heifers (n = 13) and cows (n = 13) were used to obtain information about the number, size and micromorphology of antral follicles (≥ 3 mm in diameter) in cattle after induction of luteolysis with the PGF_2α analogue luprostiol. Special attention was paid to the presence of atypical granulosa cells in these follicles to obtain additional data to help evaluate the hypothesis that these cells are markers of follicular atresia. Animals were injected i.m. with 15 mg of the synthetic prostaglandin on day 10 or day 11 of the oestrous cycle. The ovaries were collected on day 12, that is 48 and 24 h after injection of luprostiol, respectively. After prostaglandin-induced luteolysis, the mean number of medium-sized and large nonatretic follicles and of medium-sized atretic follicles had not changed in heifers and in cows, compared with those of untreated animals. However, in heifers, contrary to cows, the development of a preovulatory-sized follicle was initially accompanied by an increase in the number of large definitely atretic follicles. Atypical granulosa cells can be considered as markers for a lower quality follicle, on the basis of their absence in preovulatory-sized follicles and their presence in large numbers in a high proportion of definitely atretic follicles. If it is assumed that only a nonatretic follicle without atypical granulosa cells will grow to preovulatory size, growth of this follicle within 2 days after prostaglandin treatment was almost 9 mm and over 10 mm in heifers and cows, respectively. In cows, most growth of follicles was observed at the first day after prostaglandin treatment; in heifers, this occurred one day later.

Epidemiological studies in humans linking adult disease to growth in utero indicate that prenatal life is a critical period for the appropriate development of the reproductive axis. The aim of this study was to compare ovarian development in intrauterine growth-retarded and normally grown piglets originating from the same litter. Intrauterine growth-retarded piglets (runts) were identified on the basis of statistical analysis of the birth weight distribution within each litter. At birth, ovaries were collected from runt piglets (n=14) and their respective mean weight (normal, n=14) littermates. Ovaries were weighed and fixed, and development of ovarian germ cells was quantified in haematoxylin-eosin-stained paraffin wax sections using an image analysis system. Germ cell loss, using an in situ TdT-mediated dUTP nick-end labelling (TUNEL) assay for DNA fragmentation, and follicle cell activity, using immunohistochemistry to demonstrate vimentin, were studied in ovarian sections. At birth, body weight and absolute ovarian mass were significantly lower in runt piglets compared with their respective normally grown littermates (body weight: 733+/-38.5 versus 1530+/-39.7 g; ovarian mass: 51+/-3.0 versus 108+/-9.6 mg; P<0.001 for both). In the ovary, the proportion of nests of oogonia, the number of oocytes and TUNEL-positive cells, and the localization and intensity of vimentin immunoreactivity were not different between runt and normal littermates. However, runt piglets had more primordial follicles (268+/-18.6 versus 235+/-20.1 per mm(2) of cortex; P<0.05), fewer primary follicles (11+/-2.0 versus 20+/-3.0 per mm(2) of cortex; P<0.001) and no secondary follicles compared with normal piglets. These findings indicate that intrauterine growth retardation delayed follicular development in pig ovaries at birth.

Summary. Five Dutch–Friesian heifers were injected i.m. with 3000 iu pregnant mares' serum gonadotrophin (PMSG) on day 10 of the oestrous cycle, to study the effects on the number and micromorphological quality of antral follicles (⩾0·3 mm in diameter). The ovaries were collected 48 h after PMSG injection.

As well as the presence of mitotic figures and the absence of pyknotic nuclei in the granulosa, atypical granulosa cells were found in nonatretic follicles. These cells had an oblong nucleus and stained with toluidine blue. They were characterized by their dark cell matrix, and the presence of numerous free ribosomes and intermediate filaments of varying quantity. Atypical granulosa cells were micromorphologically similar to fibroblast-like cells in the theca. Their presence coincided with the occurrence of degenerative changes in the cytoplasm of nearby granulosa cells and they were more frequent in atretic follicles. The presence of atypical granulosa cells in follicles hitherto called nonatretic is therefore probably associated with the onset of follicular atresia.

In the PMSG-treated heifers, the mean number of large (⩾6·0 mm in diameter) antral follicles was greater than in the control group (18·4 ± 4·0 versus 3·0 ± 1·0), because of an increase in the number of large nonatretic follicles (11·8 ± 4·4 versus 0·4 ± 0·2). After hormone treatment, the mean number of medium-sized (3·0–5·9 mm) nonatretic follicles also increased (6·4 ± 1·3 versus 1·8 ± 1·0). PMSG did not change the mean number of nonatretic follicles < 3·0 mm or that of atretic follicles in the different size categories. However, when follicles hitherto called nonatretic, with atypical granulosa cells, were taken together with the group of atretic follicles, PMSG appeared to increase the mean number of large atretic follicles (13·6 ± 2·4 versus 3·0 ± 1·0). The mean number of medium-sized and large nonatretic follicles without atypical granulosa cells was markedly increased (3·8 ± 1·0 versus 0·2 ± 0·2 and 4·6 ± 1·9 versus 0·0, respectively). The data demonstrate that PMSG stimulates the formation not only of nonatretic follicles ⩾3·0 mm, but also of atretic follicles ⩾6·0 mm.

Keywords: ovary; follicle; micromorphology; pregnant mares' serum gonadotrophin; heifer