Preovulatory follicular development in sheep treated with PMSG and/or prostaglandin

Abstract

The patterns of growth and atresia of antral follicles including that of the presumptive preovulatory follicle were examined in sheep ovaries for a 24\p=n-\48-h period after the induction of luteolysis with a prostaglandin analogue, cloprostenol or cloprostenol + PMSG. Ewes were ovariectomized at various times after the initiation of the treatments. All follicles \m=ge\1 mm in diameter were dissected from the excised ovaries and the antral fluid and granulosa cells recovered. Individual follicles were classified as healthy or atretic on the basis of the number of granulosa cells recovered and then subclassified as to whether they contained intrafollicular levels of oestradiol that were \ m=ge\ or < than 100 ng/ml. In another series of similarly treated ewes, the ovarian secretion rates of oestradiol and the intrafollicular concentrations of oestradiol in all large antral follicles (\ m=ge\ 5 mm diameter) as well as the levels of progesterone in peripheral plasma were measured at different times after induction of luteolysis. The results showed that a large 'oestrogenic' follicle (\m=ge\5 mm diameter and secreting \ m=ge\ 1ng oestradiol/min) appears around 10 h after the cloprostenol injection and that this presumptive preovulatory follicle emerges before the corpus luteum has ceased to function. Moreover, the presumptive preovulatory ('oestrogenic') follicle appears to develop from the pool of small 'oestrogenic' follicles (1\p=n-\3 mm diameter) after the onset of luteolysis. The emergence of a large 'oestrogenic' follicle is accompanied by a widespread increase in atresia (> 80%) in all other classes of antral follicles (\ m=ge\ 1 mm in diameter). During the first 10 h of cloprostenol-induced luteolysis, PMSG (a) prevented the normal occurrence of atresia in the large follicle population; (b) enhanced oestrogen secretion in a greater proportion of large antral follicles compared to that in control animals; (c) temporarily 'rescued' and/or prevented small antral follicles (1\p=n-\4 mm diameter) from undergoing atresia; but (d) had little, or no, effect on the overall population of antral follicles (\ m=ge\ 1 mm diameter). After 24 h, the atresia-preventing effects of PMSG were no longer discernible and the only obvious difference noted, compared to the controls, was the number of large oestrogen-secreting follicles.

Morphometric Estimation of the Numbers of Granulosa Cells in Preovulatory Follicles of the Ewe

“…The present findings on absolute numbers of granulosa cells per preovulatory follicle are consistent with the data of McNatty et al (1982), in which granulosa cell numbers were counted in a haemocytometer after mechanical dislodgement from the walls of follicles from Romney ewes treated with cIoprosteno!. These authors derived a formula to express the relationship between maximum number of recoverable granulosa cells and follicular diameter, and observed a maximum of :::: 10 X 10 6 granulosa cells in follicles of the largest sizes.…”

“…This is particularly true when follicles need to be processed intact for morphological studies, eliminating the possibility of functional studies on the follicular cells or of analysis of follicular fluid, which could provide additional evidence of preovulatory status (McNatty et al 1982;Webb and England 1982).…”

Ovarian features contributing to the variability of PMSG-induced ovulation rate in sheep

“…The close correlation between the number of healthy by PMSG. This result is supported by the findings that the number of follicles > 1 mm is unchanged after PMSG administration (McNatty et al, 1982) while the number of follicles >2 mm increases (Dott et al, 1979). Follicles recruited by PMSG are much smaller than those recruited at luteolysis of an unstimulated cycle to undergo preovulatory enlargement ( > 2 mm: Driancourt & Cahill, 1984;Tsonis et al, 1984).…”

Identification at the onset of luteolysis of follicles capable of ovulation in the ewe

“…during luteolysis. Other evidence suggests that the final selection of the ovarian follicle(s) destined to ovulate is from a pool of small follicles (2-4 mm diameter) present at the time of luteolysis (Smeaton & Robertson, 1971;Bhérer, Dufour & Matton, 1976;McNatty, 1982;McNatty et al, 1982). made because the extraction efficiency was in the range of 90-95%.…”

“…Statistical analysis. The LH surge was defined as when LH was > 10 ng/ml for > 6 h, and the onset when concentrations first exceeded 10 ng/ml (Wright, Geytenbeek, Clarke & Findlay, 1980 (Smeaton & Robertson, 1971 ;Bhérer et al, 1976;McNatty, 1982;McNatty et al, 1982) Smeaton & Robertson (1971) observed that, in 1 ewe of 7 studied, 2 medium-large follicles marked with ink on Day 13 subsequently ovulated. Bhérer et al (1976) observed that while the largest follicle marked on Day 14 subsequently ovulated, when there were twin ovulations the second ovulation resulted from a smaller unmarked follicle.…”