Process Of Follicle Selection Research Articles

Transcriptome Profiling of Goose Ovarian Follicle Granulosa Cells Reveals Key Regulatory Networks for Follicle Selection

The selection of follicles determines the reproductive performance of birds, but the process of follicle selection in geese is still elusive. This study focuses on Yangzhou geese during the egg-laying period and divides the follicular development process into three stages: small follicle development, follicle selection, and follicle maturation. Transcriptome sequencing was performed on granulosa cells from large white follicles, small yellow follicles, and F5 and F4 follicles. In addition, we selected the transcripts that remained unchanged during the development and maturation of small follicles but significantly changed during the follicular selection stage as the transcript collection that plays an important role in the follicular selection process. Then, we performed functional analysis on these transcripts and constructed a ceRNA network. The results showed that during the follicular selection stage, the number of differentially expressed mRNAs, miRNAs, and lncRNAs was the highest. In addition, miR-222-3p, miR-2954-3p, miR-126-5p, miR-2478, and miR-425-5p are potential key core regulatory molecules in the selection stage of goose follicles. These results can provide a reference for a better understanding of the basic mechanisms of the goose follicle selection process and potential targets for the precise regulation of goose egg production performance.

An integrative analysis of lncRNAs and mRNAs highlights the potential roles of lncRNAs in the process of follicle selection in Taihang chickens

Taihang chickens are a domestic breed distributed throughout Hebei province in the Taihang Mountains of China and are characterized by their high meat and egg quality. However, the relatively limited egg production by this breed constrains their more widespread commercial utilization. The follicle selection process is closely linked to oocyte development and ovulation, making it a key determinant of laying performance and fecundity in hens. To understand the biological basis for such follicle selection and to identify the associated regulatory pathways, we conducted a genome-wide analysis of long noncoding RNAs (lncRNAs) and mRNAs from the pre-hierarchical follicles and hierarchical follicles of Taihang laying hens. We identified 81 lncRNAs and 528 mRNAs that were differentially expressed during follicle selection, and integrated network analyses suggested that these RNAs were associated with the cell cycle, focal adhesion, oocyte meiosis, peroxisome proliferator-activated receptor signaling, and steroid hormone biosynthesis pathways. The identified lncRNAs were also predicted to influence a series of target genes in cis and trans, suggesting that they may be important regulators of ovarian follicular development. Overall, the present analysis of lncRNA and mRNA expression patterns associated with ovarian follicle development offers a new foundation for future studies of reproductive physiology in Taihang chickens, highlighting new opportunities to improve the laying performance of this important domestic breed.

Profiling of RNA N6-methyladenosine methylation during follicle selection in chicken ovary

Ovarian follicle selection is the critical step which determines the oocyte development and ovulation. In avian species, the somatic cells in the follicles decide the process of follicle selection but the precise molecular regulation is not well defined. N6-methyladenosine (m6A) is a ubiquitous reversible epigenetic RNA modification that plays an important role in the gene expression regulation and cell functions. In this study, we profiled transcriptome-wide m6A methylation in chicken follicles during follicular selection process in order to identify key factors involved in the follicle selection. The chicken follicle transcriptome was extensively methylated by m6A and a negative correlation was found between the m6A methylation enrichment and gene expression levels. Interestingly, both the m6A methylation peaks and the m6A modified transcripts increased during follicle selection, which lead to the dynamic expression of many folliculogenesis relevant genes. Functional enrichment analysis indicated that m6A modification of key factors in Wnt pathway could play a major role in regulating follicle selection. This study is the first to comprehensively characterize the m6A patterns in the chicken transcriptome, and provides deep insights into the m6A topology and relevant molecular mechanisms underlying follicle selection.

Multi‐scale modelling of ovarian follicular development: From follicular morphogenesis to selection for ovulation

In this review, we present multi-scale mathematical models of ovarian follicular development that are based on the embedding of physiological mechanisms into the cell scale. During basal follicular development, follicular growth operates through an increase in the oocyte size concomitant with the proliferation of its surrounding granulosa cells. We have developed a spatio-temporal model of follicular morphogenesis explaining how the interactions between the oocyte and granulosa cells need to be properly balanced to shape the follicle. During terminal follicular development, the ovulatory follicle is selected amongst a cohort of simultaneously growing follicles. To address this process of follicle selection, we have developed a model giving a continuous and deterministic description of follicle development, adapted to high numbers of cells and based on the dynamical and hormonally regulated repartition of granulosa cells into different cell states, namely proliferation, differentiation and apoptosis. This model takes into account the hormonal feedback loop involving the growing ovarian follicles and the pituitary gland, and enables the exploration of mechanisms regulating the number of ovulations at each ovarian cycle. Both models are useful for addressing ovarian physio-pathological situations. Moreover, they can be proposed as generic modelling environments to study various developmental processes and cell interaction mechanisms.

The Role of Luteinizing Hormone in Regulating Gene Expression During Selection of a Dominant Follicle in Cattle

At approximately 8.5 mm in diameter, the future dominant follicle is "selected" for continued growth in cattle. In the present study, cows were treated with a gonadotropin-releasing hormone receptor antagonist, acyline, just before follicle selection (near 7.8 mm) to investigate the role of LH in changing mRNA concentrations during selection of a dominant follicle. The ovaries containing the expected dominant follicle (EDF; first largest follicle) and expected largest subordinate follicle (ESF) were removed after 12 or 24 h of treatment. Real-time PCR was used to determine mRNA concentrations. ELISA was used to measure testosterone and 17beta-estradiol (E(2)) and radioimmunoassay to measure androstenedione (A(4)) in follicular fluid. Concentrations of E(2) were greater in EDF than in ESF of untreated cows near the time of follicle selection (12 h) or at 12 h after selection (24 h). Testosterone, E(2), and A(4) were all dramatically decreased by acyline treatment at both times. In theca cells, acyline treatment reduced CYP17A1 (P450 17alpha) in EDF and STAR (steroidogenic acute regulatory protein) in both EDF and ESF but did not alter CYP11A1 (P450scc). In granulosa cells (GCs), LHCGR (luteinizing hormone [LH] receptor) was much greater in EDF than in ESF at both time of selection (739% greater) and 12 h after selection (2837% greater) and was decreased by acyline in EDF (87% decrease). The mRNA for CYP19A1 (cytochrome P450 aromatase) and PAPPA (pregnancy-associated plasma protein-A) tended to be greater in EDF than in ESF at follicle selection, and both mRNAs were much greater at 12 h after selection, with acyline significantly decreasing PAPPA mRNA after 24 h of treatment. The mRNA for FSHR (follicle-stimulating hormone receptor) was not different in EDF versus ESF and was not altered by acyline. Thus, induction of LHCGR mRNA in GCs is an early event during the follicle selection process, and surprisingly, expression of LHCGR mRNA is dependent on circulating LH. Production of follicular A(4), testosterone, and E(2) are also acutely related to LH but due to changes in expression of STAR and CYP17A1 in TC.

Multi-scale modeling of the follicle selection process in the ovary

The biological meaning of follicular development is to free fertilizable oocytes at the time of ovulation. The ovulation rate results from an FSH-dependent follicle selection process. In this paper, we designed a multi-scale model of follicular development, where selection arises from the endocrine feedback between the ovaries and pituitary gland and appeals to control theory concepts. Each ovarian follicle is described through a 2D density function giving an age and maturity-structured description of its cell population. The control intervenes in the velocity, gain and loss terms of the conservation law ruling the changes in the density. The model accounts for the changes in the total cell number, growth fraction and global maturity of both ovulatory and degenerating follicles for various intensities of the selection rate. The different selection process outputs (mono- or poly-ovulation, anovulation) predicted by the model are consistent with physiological knowledge regarding vascularization, pituitary sensitivity to ovarian feedback and treatment with exogenous FSH.

The physiology of follicle selection

During the follicular phase of the primate menstrual cycle, a single follicle usually matures to the preovulatory stage and releases its oocyte for fertilization and the potential establishment of pregnancy. In assisted reproductive technology procedures, it is desirable to override the natural process of follicle selection to produce many oocytes that are capable of being fertilized and undergoing normal embryo development. The goal of this chapter is to summarize the current views regarding the natural process of follicle selection in primates and to discuss how this process may be amplified to produce a greater number of oocytes.

Hormone secretion in the asian elephant (Elephas maximus): characterization of ovulatory and anovulatory luteinizing hormone surges.

In the elephant, two distinct LH surges occur 3 wk apart during the nonluteal phase of the estrous cycle, but only the second surge (ovLH) induces ovulation. The function of the first, anovulatory surge (anLH) is unknown, nor is it clear what regulates the timing of these two surges. To further study this observation in the Asian elephant, serum concentrations of LH, FSH, progesterone, inhibin, estradiol, and prolactin were quantified throughout the estrous cycle to establish temporal hormonal relationships. To examine long-term dynamics of hormone secretion, analyses were conducted in weekly blood samples collected from 3 Asian elephants for up to 3 yr. To determine whether differences existed in secretory patterns between the anLH and ovLH surges, daily blood samples were analyzed from 21 nonluteal-phase periods from 7 Asian elephants. During the nonluteal phase, serum LH was elevated for 1-2 days during anLH and ovLH surges with no differences in peak concentration between the two surges. The anLH surge occurred 19.9+/-1.2 days after the end of the luteal phase and was followed by the ovLH surge 20.8+/-0.5 days later. Serum FSH concentrations were highest at the beginning of the nonluteal phase and gradually declined to nadir concentrations within 4 days of the ovLH surge. FSH remained low until after the ovLH surge and then increased during the luteal phase. Serum inhibin concentrations were negatively correlated with FSH during the nonluteal phase (r = -0.53). Concentrations of estradiol and prolactin fluctuated throughout the estrous cycle with no discernible patterns evident. In sum, there were no clear differences in associated hormone secretory patterns between the anLH and ovLH surge. However, elevated FSH at the beginning of the nonluteal phase may be important for follicle recruitment, with the first anLH surge acting to complete the follicle selection process before ovulation.

Expression of mRNAs for insulin-like growth factor binding proteins-2, -3 and -4 in the ovine ovary throughout the oestrous cycle.

Ovarian localization of the IGFs and their binding proteins is species-specific. In the ewe we have demonstrated previously that IGF-II mRNA is found at high concentrations in the theca, whereas IGF-I mRNA is undetectable in ovine follicles at any stage of development, although both are present in, the corpus luteum. In this study we have investigated the localization of mRNA for IGF-binding proteins (IGFBPs)-2, -3 and -4 using in situ hybridization to further elucidate the roles of the IGFs and their possible modulation during follicular development. Results were quantified by optical density (OD) measurements of autoradiographs using image analysis. There was intense follicular expression of mRNAs for both IGFBP-2 and -4 although IGFBP-3 mRNA was undetectable. Concentrations of IGFBP-2 mRNA varied significantly (P < 0.01) with respect to size, being highest in small (< 2 mm) follicles but with no variation attributable to health, whereas expression of IGFBP-4 mRNA was significantly (P < 0.01) influenced by health and not size, being higher in healthy than atretic follicles. IGFBP-2, -3 and -4 mRNAs were all present at low concentrations in the corpus luteum (OD 0.04 +/- 0.005, n = 6; 0.05 +/- 0.02, n = 5; 0.1 +/- 0.02, n = 8 respectively), whereas only IGFBP-2 and -4 mRNAs were found in ovarian stroma (OD 0.04 +/- 0.006, n = 4; 0.04 +/- 0.007, n = 9 respectively). These data suggest that changes in follicular distribution of IGFBPs-2 and -4 may modulate the actions of the IGFs during follicular growth thus contributing to the process of follicle selection.

Susceptibility of avian ovarian granulosa cells to apoptosis is dependent upon stage of follicle development and is related to endogenous levels of bcl-xlong gene expression.

Studies were conducted to evaluate the susceptibility of avian ovarian granulosa cells to apoptosis when incubated in vitro and to relate this relative susceptibility to both the stage of follicle development from which granulosa cells were collected (atresia-prone vs. -resistant) and to the expression of a gene previously linked to the regulation of cell viability, bcl-xlong. Granulosa cells from slow growing, prehierarchal (4- to 8-mm diameter; atresia-prone) follicles were found to undergo rapid and progressively extensive apoptosis after incubation in defined medium for 6-24 h (P < 0.05 vs. unincubated controls). By contrast, cells from the largest preovulatory (F1) follicle, as well as from follicles most recently recruited into the follicle hierarchy (9- to 12-mm diameter), showed significantly less low molecular wt labeling at 6 h of incubation (P < 0.05 vs. 4- to 8-mm follicles). Furthermore, the amount of low molecular wt labeling did not significantly increase in cells from either stage of follicle development at 12 or 24 h of incubation (P < 0.05 vs. 6 h incubation). This biochemical indication of ongoing apoptosis in prehierarchal follicle granulosa cells was confirmed by an increased incidence of pyknotic nuclei detected by morphological analysis. Thus, increased susceptibility to apoptosis in incubated prehierarchal follicle granulosa cells is correlated with the high rate of follicle atresia that is known to occur at this stage of development in vivo. Recombinant human FSH (100 mIU) and transforming growth factor-alpha (3.3 nM) partially suppressed apoptosis in prehierarchal follicle granulosa cells after 6 h of incubation (by 46-57%; P < 0.05 vs. control), as did the cAMP analog, 8-Br-cAMP (1 mM; by 59%; P < 0.05). A single form of the bcl-2-related gene, bcl-x, was detected in hen ovarian tissues; this transcript corresponded to bcl-xlong, the death-suppressing form of bcl-x. The highest levels of bcl-xlong messenger RNA were found in granulosa tissue from preovulatory follicles, with significantly lower levels detected in prehierarchal follicle granulosa tissue (P < 0.05). Elevated expression of bcl-xlong in preovulatory follicles was correlated to increased resistance to the process of apoptosis, in vitro, and the virtual absence of follicle atresia at this stage of development, in vivo. We conclude that there is a direct relationship between the inherent susceptibility of avian granulosa cells to apoptosis and the high rate of follicle atresia in follicles not yet selected into the preovulatory hierarchy. Moreover, our results are consistent with the proposal that the expression of death-suppressing genes, including bcl-xlong, is capable of rendering cells resistant to the process of apoptosis. The findings reported herein provide the foundation for a novel model with which to further elucidate molecular mechanisms related not only to the initiation of follicle atresia, but also events associated with the process of follicle selection.

Alternative gonadotrophins for superovulation in cattle

Superovulation requires the administration of a gonadotrophin preparation rich in or mimicking the effect of follicle stimulating hormone (FSH). In addition, it is necessary that gonadotrophins be available long enough for the follicle to continue growing to enable final maturation of the oocyte, which will ensure normal fertilization and embryo development. Several gonadotrophins have been used to increase the ovulation rate in beef and dairy cattle, either in conjunction with progestogen or prostaglandin F2α treatments to regulate the estrous cycle. Ovulation rates of about 15 are frequently recorded; however, fewer than 10 embryos may be recovered, with about six embryos suitable for transfer. Pregnant mare serum gonadotrophin (PMSG) has been used and associated with high numbers of unovulated follicles, because of its long half-life in blood. The use of anti-PMSG has resulted in a decrease in the number of large follicles at the time of embryo recovery. This decrease has not always been accompanied by a corresponding increase in the number of transferrable embryos. FSH, with its shorter half-life, if given frequently, results in equivalent ovulation rates but generally gives a “better” ovarian response in terms of fewer follicles and a higher number of transferable embryos. The use of other preparations, such as human menopausal gonadotrophin (hMG), have resulted in variable ovulatory responses. Progestogen treatment, to regulate the cycle, will facilitate superovulation without reference to the stage of the estrous cycle; however, in general, responses have not matched those obtained with prostaglandin F2α at mid-cycle. One of the major problems with superovulation is the large variability in the ovulatory response, with a relatively high proportion of animals failing to produce any transferable embryos. A better understanding of the endocrine and paracrine factors controlling the process of follicle selection, growth and atresia is required in order to develop more effective systems of superovulation.

Endocrine evaluation of superovulation in water buffaloes

Superovulation requires the administration of a gonadotrophin preparation rich in or mimicking the effect of follicle stimulating hormone (FSH). In addition, it is necessary that gonadotrophins be available long enough for the follicle to continue growing to enable final maturation of the oocyte, which will ensure normal fertilization and embryo development. Several gonadotrophins have been used to increase the ovulation rate in beef and dairy cattle, either in conjunction with progestogen or prostaglandin F2α treatments to regulate the estrous cycle. Ovulation rates of about 15 are frequently recorded; however, fewer than 10 embryos may be recovered, with about six embryos suitable for transfer. Pregnant mare serum gonadotrophin (PMSG) has been used and associated with high numbers of unovulated follicles, because of its long half-life in blood. The use of anti-PMSG has resulted in a decrease in the number of large follicles at the time of embryo recovery. This decrease has not always been accompanied by a corresponding increase in the number of transferrable embryos. FSH, with its shorter half-life, if given frequently, results in equivalent ovulation rates but generally gives a “better” ovarian response in terms of fewer follicles and a higher number of transferable embryos. The use of other preparations, such as human menopausal gonadotrophin (hMG), have resulted in variable ovulatory responses. Progestogen treatment, to regulate the cycle, will facilitate superovulation without reference to the stage of the estrous cycle; however, in general, responses have not matched those obtained with prostaglandin F2α at mid-cycle.One of the major problems with superovulation is the large variability in the ovulatory response, with a relatively high proportion of animals failing to produce any transferable embryos. A better understanding of the endocrine and paracrine factors controlling the process of follicle selection, growth and atresia is required in order to develop more effective systems of superovulation.

Superovulation in beef heifers with PMSG: Effect of dose and monoclonal antibodies to PMSG

Superovulation requires the administration of a gonadotrophin preparation rich in or mimicking the effect of follicle stimulating hormone (FSH). In addition, it is necessary that gonadotrophins be available long enough for the follicle to continue growing to enable final maturation of the oocyte, which will ensure normal fertilization and embryo development. Several gonadotrophins have been used to increase the ovulation rate in beef and dairy cattle, either in conjunction with progestogen or prostaglandin F2α treatments to regulate the estrous cycle. Ovulation rates of about 15 are frequently recorded; however, fewer than 10 embryos may be recovered, with about six embryos suitable for transfer. Pregnant mare serum gonadotrophin (PMSG) has been used and associated with high numbers of unovulated follicles, because of its long half-life in blood. The use of anti-PMSG has resulted in a decrease in the number of large follicles at the time of embryo recovery. This decrease has not always been accompanied by a corresponding increase in the number of transferrable embryos. FSH, with its shorter half-life, if given frequently, results in equivalent ovulation rates but generally gives a “better” ovarian response in terms of fewer follicles and a higher number of transferable embryos. The use of other preparations, such as human menopausal gonadotrophin (hMG), have resulted in variable ovulatory responses. Progestogen treatment, to regulate the cycle, will facilitate superovulation without reference to the stage of the estrous cycle; however, in general, responses have not matched those obtained with prostaglandin F2α at mid-cycle.One of the major problems with superovulation is the large variability in the ovulatory response, with a relatively high proportion of animals failing to produce any transferable embryos. A better understanding of the endocrine and paracrine factors controlling the process of follicle selection, growth and atresia is required in order to develop more effective systems of superovulation.

Time of ovarian follicle selection during the porcine estrous cycle

Two experiments were conducted to: 1) determine the time during the procine estrous cycle when compensation in ovulation rate after unilateral ovariectomy (ULO) ceases to be complete, 2) compare the follicle selection process in gilts selected for high ovulation rate with unselected control gilts and 3) determine the number of follicles on the right ovary at various stages of the estrous cycle. Experiment I included 25 crossbred gilts, while Experiment II included 17 gilts selected for high ovulation rate and 16 unselected control gilts. The right ovary was removed via a mid-ventral laparotomy on either day 13, 15, 17 or 19 of the cycle. In Experiment I, compensation in ovulation rate ceased between days 13 and 15; whereas, in Experiment II, cessation occurred between days 15 and 17. Selected and control gilts responded alike to ULO, indicating similarity in the follicle selection process. Follicle numbers in the right ovary showed a general decline, especially between days 17 and 19, indicating that atresia was occurring during the follicular phase. The results indicate that the selection of ovarian follicles for ovulation at the ensuing estrus occurs before day 17 of the porcine estrous cyle.

Follicle dominance and ovarian asymmetry after luteectomy in rhesus monkeys.

Previous work demonstrated that asymmetrical ovarian activity accompanies morphological asymmetry during the ovarian cycle in rhesus and cynomolgus macaques. This study was designed to determine whether functional ovarian asymmetry could be used to detect the upcoming dominant follicle (DF) even before it was grossly visible. Revealing a latent DF in this manner would permit a better estimate of the time when dominance of the follicle selected to ovulate is attained. To accomplish this, rhesus monkeys were luteectomized at midluteal phase to synchronize subsequent follicle growth, and 4 or 8 days later either the ipsilateral or contralateral ovary was removed. Unilateral ablation at day 4 (when no DF is grossly apparent) of either ovary produced symmetrical responses: the interval from luteectomy (CLX) to the next luteinizing hormone (LH) surge was extended by about 4 days in both groups (P less than 0.01), i.e., from about 12.5 days to 16.7 +/- 1.6 and 17.0 +/- 1.5 days (mean +/- SE). In contrast, hemiovariectomy at day 8 produced markedly divergent asymmetrical responses. Removal of the ipsilateral ovary 8 days after CLX did not affect the timing of the next LH surge (13.2 +/- 0.6 days), which ordinarily occurs about 12.5 days after CLX alone. However, ablation of the contralateral ovary (bearing the next DF) on day 8 extended the interval from CLX to the next LH surge from about 12.5 to 26.6 +/- 1.3 days. These findings indicate that, during the normal ovarian cycle when menses occurs 2--4 days after luteolysis, the follicle destined to ovulate becomes dominant between the 2nd and 6th day and that attainment of dominance signals the completion of a follicle selection process that begins or resumes promptly after luteolysis.