Factors regulating the growth of the preovulatory follicle in the sheep and human D. T. Baird Department of Obstetrics and Gynaecology, University of Edinburgh, Centre for Reproductive Biology, 37 Chalmers Street, Edinburgh EH3 9EW, U.K. Summary. Around the time of luteal regression in monovular species a single dominant follicle, which will eventually ovulate, is selected from the population of antral follicles. The dominant follicle is characterized by its progressive increase in diameter due to increase in antral fluid volume as well as an increased number of granulosa cells. The crucial factor in the continued development of the dominant follicle is its ability to synthesize oestradiol under the influence of LH and FSH. In the sheep FSH secretion continues throughout the luteal phase while LH is suppressed. Thus development of large antral follicles continues so that when luteal regression occurs and LH secretion increases the final stages of development of the pre-antral follicle occur within 3 days. In the human, however, both FSH and LH are suppressed during the luteal phase and only rise when the levels of progesterone and oestradiol fall a few days before menstruation. This rise in FSH and LH which occurs at this time stimulates the further development of a small antral follicle (1\p=n-\2mm diameter). Within 7 days the favoured follicle establishes dominance over the other asynchronously developing follicles probably by inhibiting the secretion of FSH. As in the sheep, once aromatase enzyme(s) has been fully activated the dominant follicle is able to utilize the increased androgen precursor produced by the theca under LH stimulation. Introduction In most mammals the final stage of folliculogenesis is characterized by the emergence of a minority of large antral follicles which will eventually ovulate (Mauléon & Marianna, 1977). The mechanism by which individual follicles are selected for ovulation is unknown although it is now clear that once established these follicles suppress further development of other large antral follicles. Even in those species that usually only have a single ovulation the total number of antral follicles > 1 mm diameter present in the ovary during the cycle greatly exceeds the number which ovulate, e.g. women: 6-46 per ovary; sheep: 5-24 (McNatty, 1982). This paper will consider how the pre¬ ovulatory follicle is selected and the way in which it establishes dominance over the other antral follicles in the human and sheep. The preovulatory dominant follicle Immediately before ovulation the preovulatory follicle in ewes and women is easily recognized macroscopically by its size and vascular appearance. The vascularity is due to a large capillary network supplying the theca interna and is presumably induced by some product of the growing 0022-4251/83/050343-10S02-00/0 © 1983 Journals of Reproduction & Fertility Ltd Downloaded from Bioscientifica.com at 10/03/2021 04:55:09PM via free access antral follicle, e.g. oestrogen. In both species there is usually only one large follicle in the late follicular phase of the cycle which contains a full complement of healthy granulosa cells (McNatty, 1982). The dominant follicle secretes large amounts of oestradiol into the ovarian vein. In the human by Day 7 (mid-follicular phase) the largest healthy antral follicle has already established dominance, as indicated by the asymmetry in the concentration of oestradiol in blood draining the two ovaries (Baird & Fraser, 1975) and by the marked fall in oestradiol concentration in peripheral blood following its surgical removal (Nilsson, Wikland & Hamberger, 1982). Similar evidence in the sheep indicates that over 95% of the oestradiol entering the ovarian vein originates from the largest non-atretic antral follicle in both the luteal and follicular phase of the cycle (Moor, Hay & Seamark, 1975; Baird & Scaramuzzi, 1976a). The cellular origin of follicular oestradiol has been the source of some controversy (Armstrong, Weiss, Selstam & Seamark, 1981 ; McNatty, 1982). In women and ewes androgens are the main steroids synthesized by the theca cells while the aromatase activity of the granulosa cells is much higher than that of the theca cells (Hillier, Reichert & van Hall, 1981). It is generally assumed, therefore, that some form of interaction occurs between the two cell types in the ovarian follicle with LH stimulating the theca cells to synthesize androgens which are then converted to oestrogens by the granulosa cells. Although there is strong evidence that oestrogen synthesis is enhanced when the two cell types are mixed in vitro (e.g. Ryan, Petro & Kaiser, 1968), convincing evidence that this occurs in vivo is provided by experiments in which antiserum to testosterone was infused into the ovarian artery of sheep ovaries bearing the preovulatory follicle (Baird, 1977) (Text-fig. 1). Oestradiol secretion was inhibited by over 50% during and immediately after the infusion of antiserum to testosterone (but not during infusion of antiserum to BSA or oestrone). As it seems unlikely that the antibodies could enter the theca cells, these experiments support the hypothesis that androgen precursors leave the theca cells before being aromatized to oestrogens. Control Antiserum Control D Oestrone antiserum Testosterone antiserum 21 60 I120 180 min (12) (12) (12) (12) Text-fig. 1. The effect of antiserum to testosterone on the secretion of oestradiol from the sheep ovary during the follicular phase of the cycle. Control antiserum to BSA was infused via the ovarian artery during 0-60 and 120-180 min in 8 sheep with ovaries autotransplanted to the neck. From 60 to 120 min antiserum to testosterone (4 ewes) or oestrone (4 ewes) was infused. Each bar represents the mean + s.e.m. of 28 or 12 observations in 4 sheep. (Data from Baird, 1977). * Significantly different from basal value and that for animals treated with antiserum to oestrone (P < 005). Downloaded from Bioscientifica.com at 10/03/2021 04:55:09PM via free access In addition to being 'oestrogenic', the preovulatory follicle has other characteristics. The most 'oestrogenic follicle' has more granulosa cells and is more likely to contain measurable amounts of FSH than is the 'non'-oestrogenic' follicle (Text-fig. 2) (McNatty & Baird, 1978; McNatty, 1982). FSH stimulates the aromatase activity of cultured granulosa cells from sheep and human antral follicles. In the few days before ovulation the concentration of androgens (androstenedione and testosterone) falls significantly in the follicular fluid of the dominant follicle in contrast to that of other antral follicles (McNatty et al., 1976). It seems likely, therefore, that the dominant follicle manages to maintain a very high secretion of oestradiol because of the large number of granulosa cells with a maximally activated aromatase system. 5000 1000 100 io Text-fig. 2. Mean ± s.e.m. concentration of oestradiol in follicular fluid throughout the menstrual cycle. The follicles were divided into those with detectable amounts of FSH ( > 1-3 mU/ml: FSH+) and those with undetectable levels (FSH—). The numbers indicate the number of fluid samples in each group. EF, MF and LF, early, mid- and late follicular phases respectively. EL, ML and LL, early, mid- and late luteal phases respectively. (Data from McNatty & Baird, 1978.) In the sheep, receptors for LH are present on thecal cells of healthy and atretic follicles although they are confined to the granulosa cells of large healthy antral follicles (Carson, Findlay, Burger & Trounson, 1979). Evidence from the rat suggests that both oestradiol and FSH play an important role in the acquisition of LH receptors by the granulosa cells (Richards & Midgley, 1976). It seems likely that only those follicles in which the granulosa cells contain LH receptors can ovulate in response to the mid-cycle LH surge. In summary, the dominant preovulatory follicle in both species is characterized by the synthesis of large amounts of oestradiol due to the ability of its granulosa cells to aromatise androgen pre¬ cursors synthesized by the theca interna. Downloaded from Bioscientifica.com at 10/03/2021 04:55:09PM via free access OESTRUS I OVULATION Days 26 1 Stage LL Text-fig. 3. Development of the preovulatory follicle in (a) the sheep and (b) the human. In the sheep, follicular development continues throughout the luteal phase because of the relatively high levels of FSH. In the human, development of large antral follicles is suppressed during the luteal phase and only recommences when the levels of FSH and LH rise at luteal regression. EF = early follicular; MF = mid-follicular; LF = late follicular; EL = early luteal; ML = mid- luteal; LL = late luteal. The broken vertical line indicates the onset of luteal regression. Downloaded from Bioscientifica.com at 10/03/2021 04:55:09PM via free access When is the dominant follicle selected? In both the sheep and the human the preovulatory follicle becomes dominant in the few days following regression of the corpus luteum. In the sheep, ovulation occurs within 3 days of natural or induced luteal regression while the equivalent period in the human is 17 days (Baird & McNeilly, 1981; Nilsson et al., 1982). This difference cannot be explained by differences in the rate of follicular development (Text-fig. 3) but is due to the fact that, in the sheep, in contrast to the human, development of large antral follicles (4-6 mm diameter) continues throughout the luteal phase (Turnbull, Braden & Mattner, 1977; Cahill, Marianna & Mauléon, 1979; Gougeon, 1982). We have previously suggested that this difference in follicular development is related to the different pattern of steroid hormones secreted by the corpora lutea of the two species—with consequent differences in secretion of gonadotrophins (Baird, Baker, McNatty & Neal, 1975). The changes in the concentration of pituitary gonadotrophins and ovarian steroids at luteal regression and the periovulatory period in the sheep are illustrated in Text-fig. 3. The decline in progesterone secretion at luteal regression is followed by a rise in the concentration of LH and oestradiol but no change in the secretion of FSH (Baird, Swanston & McNeilly, 1981).