A study of the population of primordial and small follicles in the sheep

Abstract

The population of primordial and small follicles in adult ewes of high (3.1) and low (1.6) ovulation rate was studied by histological methods. The small follicle population which has a skewed frequency distribution in relation to either follicle or oocyte size can be divided according to morphological features into dormant, transitory and growth phases. It is suggested that initiation of follicle growth involves firstly the passage of follicles from the dormant to the transitory category and (if it occurs) is independent of gonadotrophins; and secondly, the passage from the transitory category to the growth phase. The second step is dependent on gonadotrophins and the rate of such passage is higher in ewes with a high ovulation rate. The overall mean follicle size of small follicles is larger in ewes with a low ovulation rate, suggesting that either they have structurally different small follicles or their reserve of small follicles is less depleted than in ewes with a high ovulation rate.

Evidence that the corpus luteum of pregnancy contributes to the control of tonic secretion of LH in the ewe

“…This finding is consistent with the hypothesis that the passage of follicles from the reserve of primordial follicles to the early growth phase is FSH-independent (Cahill & Mauléon, 1981 (Beai et al, 1986) and CL excision at Day 70 of pregnancy in the present study, results in a small transitory decrease in peripheral concentrations of progesterone. In Group L ewes of the present experiment, the concentration of progesterone at Day 80 (10 days after CL excision) was similar to that during the mid-luteal phase of the oestrous cycle (about 3 ng/ml), and thereafter increased, reaching 5-2 ng/ml at Day 100.…”

Difficulties improving ovarian functional recovery by microvascular transplantation and whole ovary vitrification

“…The folliculogenesis cycle into ewes is around 180 days (48). For Imhof et al (49), follicle-stimulating hormone (FSH) reached physiologic levels 6 months or more after transplantation.…”

Ovarian follicular populations and preovulatory enlargement in Booroola and control Merino ewes

“…Furthermore, this study supports earlier claims of a negative correlation between the number of primordial follicles and the mean ovulation rate of different genetic lines (sheep: Land, 1970;Cahill et al, 1979;rat: Land, de Reviers, Thompson & Mauléon, 1974). However, it is not known whether this is genetic in origin or a consequence of a higher number of fetuses (Tassell, Kennedy, Bindon & Piper, 1983) or fetal ovaries (Cahill & Chamley, 1982) developing during pregnancy. The fact that ageing results in both strains in a progressive decrease in the number of primordial follicles but with unaltered numbers of preantral and antral follicles confirms the findings of Erickson (1966) for the cow.…”