The Pattern of Follicular Growth and Atresia in the Ovine Ovary

Abstract

The ovaries of 21 2-year-old Merino ewes at various stages of the oestrous cycle were serially sectioned and all follicles > 0·5 mm in diameter were counted and measured. The mean number of normal follicles > 0·5 mm in diameter was 19· 9 per ewe, of which 8·7 were > 1 mm in diameter. The mean numbers of follicles > 0·5 mm in diameter in 'early', 'advanced' and 'late' stages of atresia were 4· 6, 9· 0 and 12· 3 respectively. Early atresia was rarely seen in follicles < 1 mm in diameter; the greatest incidence was in follicles of diameter 1· 5-2' 5 mm.Volumes of the granulosa, theca interna and antrum were calculated for all follicle sizes and the mitotic index of the granulosa and theca determined. Follicles were classified according to granulosa volume, each class having twice the volume of the preceding class. Estimates of the doubling time of the granulosa were made from the mitotic index and an estimate of mitotic time (0'43 h) obtained by use of colchicine on two other ewes. In all ewes, irrespective of day of oestrous cycle, the rate of growth was slow in follicles < 0·4 mm in diameter; above this size it accelerated to a maximum (doubling time 26-30 h) in follicles O· 7-2 mm in diameter. It was estimated that, on average, a follicle takes about 5 days to grow from 0·5 to 2·2mm diameter, and a further 4 days to reach 4·5 mm diameter. The number of follicles entering the rapid growth phase was estimated to be 3-4 per day. The present data do not support any particular 'wave' theory of follicular development, but indicate that follicles are growing or regressing asynchronously at any given time during the luteal stage of the oestrous cycle.At 48 h after treatment of two ewes with PMS gonadotrophin (PMSG), the number of follicles > 0·5 mm in diameter was increased, the greatest proportional increase being in those > 3·5 mm in diameter. Relatively few follicles were undergoing early atresia. At 18 h after RCG treatment (given 48 h after PMSG to two ewes) all follicles> 3·5 mm in diameter were undergoing luteinization and two-thirds of the follicles between 1 and 3·5 mm in diameter were undergoing early atresia.

Effect of the stage of the oestrous cycle on the follicular population in pony mares

“…Thus, continuous follicular growth occurs in the pony mare in average physical condition in the middle of the breeding season, and one hormonally priveleged period is evident towards the end of luteal phase, as in cows (Marion and Gier, 1971 ;Mariana and Nguyen Huy, 1973), ewes (Turnbull, Braden and Mattner, 1977 ;Cahill et al, 1979) and women (Gougeon, 1979). A similar conclusion was reached by Wesson and Ginther (1981) …”

Morphometric and ultrastructural characterization of Bos indicus preantral follicles

“…Many studies have been done in order to describe the oocyte and follicular structure of several domestic species, including sheep (Turnbull et al, 1977, Cran et al, 1980and Cahill and Mauléon, 1981, goats (Lucci et al, 2001), cats (Jewgenow and Göritz, 1995), dogs (Durrant et al, 1998), pigs (Greenwald and Moor, 1989) and cattle (Hulshof et al, 1994, Beckers et al, 1996and Van Wezel and Rodgers, 1996Fair et al, 1997a andFair et al, 1997b;Hyttel et al, 1997). These studies have shown that many characteristics are similar among species, but others may differ.…”

Qualitative changes in medium and large antral follicles in the human ovary during the menstrual cycle

“…These data are interesting, but we do not know the exact mitotic time (the time during which a mitotic figure is visible) of human granulosa cells. Some results on other species are available, but the values given are sometimes contradictory : 30 min (Lushbaugh, 1956), 26 min (Turnbull et al,1977) …”