The function of the growing follicle

Rescue of follicle development after oocyte-induced ovary dysfunction and infertility in a model of POI

The mechanisms and aetiology underlying the development of premature ovarian insufficiency (POI) are poorly understood. However, the oocyte clearly has a role as demonstrated by the Double Mutant (DM) mouse model where ovarian dysfunction (6 weeks) is followed by POI (3 months) due to oocyte-specific deletion of complex and hybrid N- and O-glycans. The ovaries of DM mice contain more primary follicles (3a stage) accompanied by fewer developing follicles, indicating a block in follicle development. To investigate this block, we first analysed early follicle development in postnatal (8-day), pre-pubertal (3-week) and post-pubertal (6-week and 3-month) DM (C1galt1 F/F Mgat1 F/F:ZP3Cre) and Control (C1galt1 F/F Mgat1 F/F) mice. Second, we investigated if transplantation of DM ovaries into a "normal" endocrine environment would restore follicle development. Third, we determined if replacing DM ovarian somatic cells would rescue development of DM oocytes. At 3-week, DM primary 3a follicles contain large oocytes accompanied by early development of a second GC layer and increased GC proliferation. At 6-week, DM primary 3a follicles contain abnormally large oocytes, accompanied with decreased GC proliferation. Transplantation of DM ovaries into a 'normal' endocrine environment did not restore normal follicle development. However, replacing somatic cells by generating reaggregated ovaries (ROs) did enable follicle development to progress and thus highlighted intra-ovarian factors were responsible for the onset of POI in DM females. Thus, these studies demonstrate oocyte-initiated altered communication between GCs and oocytes results in abnormal primary follicles which fail to progress and leads to POI.

Three-dimensional and two-dimensional relationships of gangliogenesis with folliculogenesis in mature mouse ovary: a Golgi-Cox staining approach

The present study was set out to investigate two-dimensional (2D) and three-dimensional (3D) evaluations of ovarian nervous network development and the structural relationship between folliculogenesis and gangliogenesis in mouse ovaries. Adult mice ovarian tissue samples were collected from follicular and luteal phases after cardiac perfusion. Ovarian samples were stained by a Golgi-Cox protocol. Following staining, tissues were serially sectioned for imaging. Neural filaments and ganglia were present in the ovaries. In both 2D and 3D studies, an increase in the number and area of ganglia was seen during the follicular growth. The same pattern was also seen in corpora lutea development. However, in some cases such as ratio of ganglia number to follicle area, the ratio of ganglia area to follicular area, 2D findings were different compared with the 3D results. 3D analysis of ovarian gangliogenesis showed the possible direct effect of them on folliculogenesis. Golgi-Cox staining was used in this study for 3D evaluation in non-brain tissue. The results of 3D analysis of the present study showed that, in some cases, the information provided by 2D analysis does not match the reality of ovarian neuronal function. This confirmed the importance of 3D analysis for evaluation of ovarian function.

Effects of isoliquiritigenin on ovarian antral follicle growth and steroidogenesis

Isoliquiritigenin is a botanical estrogen used as a dietary supplement. Previous studies show that other botanical estrogens affect ovarian estradiol synthesis, but isoliquiritigenin's effects on the ovary are unknown. Thus, this study tested the hypothesis that isoliquiritigenin inhibits ovarian antral follicle growth and steroidogenesis. Antral follicles from CD-1 mice were cultured with vehicle control (dimethyl sulfoxide; DMSO) or isoliquiritigenin (0.6μM, 6 μM, 36 μM, and 100 μM) for 48-96h. During culture, follicle diameters were measured daily to assess follicle growth. After culture, media were collected for hormone assays and follicles were collected for gene expression analysis of steroidogenic enzymes. Isoliquiritigenin inhibited antral follicle growth and altered estradiol, testosterone, and progesterone levels. Additionally, isoliquiritigenin altered the mRNA levels of cytochrome P450 steroid 17-α-hydroxylase 1, aromatase, 17β-hydroxysteroid dehydrogenase 1, and steroidogenic acute regulatory protein. These data indicate that exposure to isoliquiritigenin inhibits growth and disrupts steroid production in antral follicles.

Equol inhibits growth, induces atresia, and inhibits steroidogenesis of mouse antral follicles in vitro

Equol is a non-steroidal estrogen metabolite produced by microbial conversion of daidzein, a major soy isoflavone, in the gut of some humans and many animal species. Isoflavones and their metabolites can affect endogenous estradiol production, action, and metabolism, potentially influencing ovarian follicle function. However, no studies have examined the effects of equol on intact ovarian antral follicles, which are responsible for sex steroid synthesis and further development into ovulatory follicles. Thus, the present study tested the hypothesis that equol inhibits antral follicle growth, increases follicle atresia, and inhibits steroidogenesis in the adult mouse ovary. To test this hypothesis, antral follicles isolated from adult CD-1 mice were cultured with vehicle control (dimethyl sulfoxide; DMSO) or equol (600 nM, 6 μM, 36 μM, and 100 μM) for 48 and 96 h. Every 24h, follicle diameters were measured to monitor growth. At 48 and 96 h, the culture medium was subjected to measurement of hormone levels, and the cultured follicles were subjected to gene expression analysis. Additionally, follicles were histologically evaluated for signs of atresia after 96 h of culture. The results indicate that equol (100 μM) inhibited follicle growth, altered the mRNA levels of bcl2-associated X protein and B cell leukemia/lymphoma 2, and induced follicle atresia. Further, equol decreased the levels of estradiol, testosterone, androstenedione, and progesterone, and it decreased mRNA levels of cholesterol side-chain cleavage, steroid 17-α-hydroxalase, and aromatase. Collectively, these data indicate that equol inhibits growth, increases atresia, and inhibits steroidogenesis of cultured mouse antral follicles.

Differences in collagen expression in cumulus cells after exposure to highly purified menotropin or recombinant follicle-stimulating hormone in a mouse follicle culture model

Extracellular matrix (ECM) formation by cumulus cells is an important process that determines fertilization and embryo quality. Several collagen types are present in the ovarian follicular ECM and are related to proliferation, steroidogenesis, and luteinization. In vitro mouse follicles can optimally grow and provide developmentally competent oocytes with 10 IU/L recombinant follicle-stimulating hormone (rFSH). As a model for superovulation, experiments with 100 IU/L rFSH or 100 IU/L highly purified menotropin (HP-hMG) exposure during antral growth were undertaken. Col4a1, Col4a2, and Col6a2 expression levels were analyzed at three time points during antral growth and at a 4-h interval up to 16 h after ovulation induction using quantitative PCR. The presence and induction of the collagen mRNA and protein were confirmed in cumulus from in vivo- and in vitro-grown follicles, and TGFBs 1 and 2 were assayed as potential regulators. The study revealed that exposure to 100 IU/L FSH, as in both superovulation conditions, significantly influenced the follicle morphology and slowed down nuclear maturation and mucification (P < 0.05). This coincided with an increased expression of the three collagens in the cumulus-oocyte complex at the end of antral growth and in the first hours following the ovulatory dose of human chorionic gonadotropin (P < 0.05). The increased expression might reflect a differentiation but is most likely due to a precocious luteinization of the cumulus. Growth in HP-hMG resulted in higher Tgfb1 mRNA and protein levels, fewer COCs with an increased collagen expression and with a more synchronous nuclear maturation. This suggests that the presence of luteinizing hormone activity tempered the effect of the elevated FSH dose.

Regulation of primordial follicle assembly and development

The assembly of the primordial follicles early in ovarian development and the subsequent development and transition of the primordial follicle to the primary follicle are critical processes in ovarian biology. These processes directly affect the number of oocytes available to a female throughout her reproductive life. Once the pool of primordial follicles is depleted a series of physiological changes known as menopause occur. The inappropriate coordination of these processes contributes to ovarian pathologies such as premature ovarian failure (POF) and infertility. Primordial follicle assembly and development are coordinated by locally produced paracrine and autocrine growth factors. Endocrine factors such as progesterone have also been identified that influence follicular assembly. Locally produced factors that promote the primordial to primary follicle transition include growth factors such as kit ligand (KL), leukaemia inhibitory factor (LIF), bone morphogenic proteins (BMP's), keratinocyte growth factor (KGF) and basic fibroblast growth factor (bFGF). Factors mediating both precursor theca-granulosa cell interactions and granulosa-oocyte interactions have been identified. A factor produced by preantral and antral follicles, Müllerian inhibitory substance, can act to inhibit the primordial to primary follicle transition. Observations suggest that a complex network of cell-cell interactions is required to control the primordial to primary follicle transition. Elucidation of the molecular and cellular control of primordial follicle assembly and the primordial to primary follicle transition provides therapeutic targets to regulate ovarian function and treat ovarian disease.

Regulation of follicle-stimulating hormone secretion by estradiol and dimeric inhibins in the infantile female rat

Plasma and ovarian levels of the dimeric forms of inhibin and plasma estradiol-17beta were investigated and compared with changes in plasma gonadotropins from Postnatal Day (PND) 5 to PND 30 in the female rat. The inhibin subunit proteins were localized in follicular granulosa cells of the ovary. Plasma immunoreactive inhibin levels were low until PND 15 and increased thereafter. Plasma levels of inhibin B (alpha and beta(B) subunits) remained very low until PND 15 and then increased by approximately 24-fold. In contrast, plasma levels of inhibin A (alpha and beta(A) subunits) were relatively low and steady until PND 20, then increased by approximately 3-fold at PND 25. Changes in ovarian inhibin A and B levels closely resembled those in plasma levels. Plasma FSH levels were low at PND 10 but started to peak from PND 15 and remained high until PND 20, followed by a remarkable reduction at PNDs 25 and 30. This dramatic fall in FSH coincided with the rise of inhibin A. A significant inverse correlation was observed between plasma FSH and plasma inhibin A (r = -0.67, P < 0.0002), ovarian inhibin A (r = -0.48, P < 0.01), plasma inhibin B (r = -0.48, P < 0.05), and ovarian inhibin B (r = -0.54, P < 0.01). Plasma estradiol-17beta levels were elevated from PND 5 through PND 15, then fell sharply through PND 30. Plasma estradiol-17beta was significantly and positively (r = 0.75, P < 0.0002) correlated with plasma FSH. Plasma LH rose to higher levels at PND 15 and tended to be lower thereafter. The inhibin alpha, beta(A), and beta(B) subunits were localized to primary, secondary, and antral and large antral follicles, but the types of these immunopositive follicles varied with age. It appeared that, at PND 25 and afterward, all three subunits were mainly confined to large antral follicles in the ovary. We conclude that estradiol-17beta likely is the major candidate in stimulation of FSH secretion in the infantile female rat. We also conclude that inhibin regulation of pituitary FSH secretion through its negative feedback in the infantile female rat begins to operate after PND 20. We suggest that this negative feedback is achieved by increases in plasma levels of the two dimeric forms, and that inhibin A appears to be the major physiological regulator of FSH secretion at the initiation of this mechanism. We also conclude that large antral follicles in the ovary are the primary source of these bioactive inhibins that are secreted in large amounts into the circulation after PND 20.

Kit-ligand/stem cell factor induces primordial follicle development and initiates folliculogenesis

Initiation of folliculogenesis through the induction of primordial follicle development in the ovary has an important role in determining the fertility and reproductive fitness of most mammalian species. The factors that control this critical process are largely unknown. The hypothesis tested in the current study was that kit-ligand/stem cell factor (KL) promotes the initiation and progression of primordial follicle development in the ovary. Ovaries from 4-day-old rats were maintained in organ culture for 5 and 14 days and treated with no factor (control), recombinant kit-ligand (KL), or gonadotropins (FSH and hCG). Follicles in ovarian sections were counted and histologically classified as primordial (stage 0), early primary (stage 1), primary (stage 2), transitional (stage 3), or preantral (stage 4). Fresh ovaries from 4-day-old rats contained 68% primordial follicles (stage 0) and 32% developing follicles (stages 1-4) per section. After 5 and 14 days in culture, section from control ovaries contained approximately 41% and 55%, respectively, developing follicles (stage 1-4) per section due to spontaneous development of primordial follicles. Spontaneous primordial follicle development was completely blocked by ACK-2, a c-kit antibody that blocks KL actions. This observation suggests that endogenous KL is necessary for primordial follicle development in vitro. After 14 days of KL treatment, sections from ovaries contained 17% primordial follicles (stage 0) and 83% developing follicles (stage 1-4) per section demonstrating a dramatic induction of primordial follicle development by KL. Gonadotropins (FSH and hCG) did not induce primordial follicle development but did increase the percentage of preantral follicles (stage 4) per section. This small increase in preantral follicles in response to gonadotropins was blocked by ACK-2 suggesting that KL may in part mediate gonadotropin actions after the initiation of primordial follicle development. Ovaries contained an average of 309+/-10 follicles per section. The total number of follicles per section did not significantly vary between treatments suggesting that the effects of KL were not due to an alteration in follicle number (i.e. survival). KL appears to be one of the first factors identified to be involved in the promotion of primordial follicle development. Results suggest that KL is necessary and sufficient to induce primordial follicle development and initiate folliculogenesis.

Protective effects of D-Trp6-luteinising hormone-releasing hormone microcapsules against cyclophosphamide-induced gonadotoxicity in female rats

The possible protective effect of an agonist of luteinising hormone-releasing hormone (LH-RH) against the ovarian damage caused by cyclophosphamide was investigated in rats. D-Trp6-LH-RH microcapsules were injected once a month for 3 months, in a dose calculated to release 25 micrograms day-1. Control animals received the injection vehicle. Sixty days after the first injection of microcapsules, cyclophosphamide was given at a loading dose of 50 mg kg-1 followed by 5 mg kg-1 day-1 for 30 days, while the treatment with D-Trp6-LH-RH was continued. When the ovaries were examined 3 months and 5 months after discontinuation of treatment, a significant reduction in the total number of follicles (P less than 0.01) was found in non-pretreated animals given cyclophosphamide. This reduction affected mainly follicles larger than 100 microns. An irreversible disintegration and destruction of granulosa cells was also observed in this group. In animals pretreated with D-Trp6-LH-RH, administration of cyclophosphamide caused no reduction in the number and diameter of follicles. Thus, the treatment with D-Trp6-LH-RH microcapsules before and during chemotherapy prevented the ovarian injury inflicted by cyclophosphamide. The suppression of gonadal function by LH-RH analogues could be possibly utilised for the protection of the ovaries against damage caused by cytotoxic drugs.