Expression of the ovine stem cell factor gene during folliculogenesis in late fetal and adult ovaries

The local regulation of folliculogenesis by members of the transforming growth factor superfamily and its relevance for advanced breeding programmes

Regulation of the growth and maturation of the ovarian follicle is critical for normal reproductive function. Alterations in this growth can lead to pathological conditions, such as cystic follicles, reduced oocyte quality, or an abnormal endocrine environment leading to poor fertility. Alterations in follicular growth also influence the number of follicles ovulating and thus can change litter size. Both endocrine factors, such as follicle stimulating hormone and luteinizing hormone, as well as local factors, are known to regulate follicular growth and development. This review will focus on the role of local factors in regulation of ovarian follicular growth in ruminants, with a focus on members of the transforming growth factor superfamily. The potential role of these factors in regulating proliferation, apoptosis, steroidogenesis and responsiveness to gonadotrophins will be considered.

Expression of kit ligand and insulin-like growth factor binding protein 3 during in vivo or in vitro development of ovarian follicles in sheep

Expression of Kit ligand (KL) and insulin-like growth factor binding protein (IGFBP3) genes was studied at different in vivo and corresponding in vitro stages of development of the ovarian follicles in sheep. The expression of both KL and IGFBP3 was significantly higher in the primordial follicles relative to any other stage of development. Compared to the other stages, the KL expression in the cumulus cells from in vivo grown large antral follicles and that of IGFBP3 in COCs' isolated from large antral follicles matured in vitro for 24 hr were significantly higher. In the oocytes from in vivo grown ovarian follicles, the expression of KL was the same at all the stages of development. Insulin-like growth factor binding protein 3 expression was also the same in the oocytes at all the stages of the development except for a significantly lower expression in those from antral follicles. The expression of KL in the cumulus cells decreased significantly in the in vitro grown early antral follicles but did not change further as the development progressed. The expression of IGFBP3 in the cumulus cells from in vitro grown ovarian follicles appeared to increase as the development progressed although the increase was not significant between any two consecutive stages of development. In the oocytes in in vitro grown ovarian follicles, the expression levels of KL and IGFBP3 genes did not change with development. It is concluded that (i) KL and IGFBP3 genes follow specific patterns of expression during ovarian folliculogenesis and (ii) in vitro culture of preantral follicles compromises the development potential through alterations in the stage-specific patterns of expression of these and other developmentally important genes.

Association of polymorphisms at the microRNA binding site of the caprine KITLG 3'-UTR with litter size

This study identified three novel single nucleotide polymorphisms (SNPs) (c.1389C > T, c.1457A > C and c.1520G > A) in the caprine KITLG 3'-UTR through DNA sequencing. The three SNP loci were closely linked in Guanzhong dairy (GD) goats. Two alleles of the c.1457A > C SNP introduced two miRNA sites (chi-miR-204-5p and chi-miR-211). Individuals with combined genotype TT-CC-AA had a higher litter size compared with those with combined genotypes CC-AA-GG, TC-CC-GA and CC-AC-GG (P < 0.05). Luciferase assays showed that chi-miR-204-5p and chi-miR-211 suppressed luciferase expression in the presence of allele 1457A compared with negative control (NC) and allele 1457C (P < 0.05). Western blot revealed that KITLG significantly decreased in the granulosa cells (GCs) of genotype AA compared with that in the GCs of genotype CC and NC (P < 0.05). The KITLG mRNA levels of the CC-AA-GG carriers significantly decreased compared with those of the TT-CC-AA, TC-CC-GA and CC-AC-GG carriers. In addition, cell proliferation was reduced in haplotype C-A-G GCs compared with that in haplotype T-C-A GCs. These results suggest that SNPs c.1389C > T, c.1457A > C and c.1520G > A account for differences in the litter size of GD goats because chi-miR-204-5p and chi-miR-211 could change the expression levels of the KITLG gene and reduce GC proliferation.

Ovarian follicle development in vitro and oocyte competence: advances and challenges for farm animals

During the last 2 decades, research on in vitro preantral follicle growth and oocyte maturation has delivered fascinating advances concerning the knowledge of processes regulating follicle growth and the developmental competence of oocytes. These advances include (1) information about the role of several hormones and growth factors on in vitro activation of primordial follicles; (2) increased understanding of the intracellular pathway involved in the initiation of primordial follicle growth; (3) the growth of primary and secondary follicles up to antral stages; and (4) production of embryos from oocytes from in vitro grown preantral follicles. This review article describes these advances, especially in regard farm animals, and discusses the reasons that limit embryo production from oocytes derived from preantral follicles cultured in vitro.

Expression and localisation of c-kit and KITL in the adult human ovary

BACKGROUND

The c-kit/kit ligand (KITL) signalling axis is an essential component of ovarian folliculogenesis in mammals, but little is known about expression and localisation of its key components in the ovaries of reproductive age women. This study aimed to characterise mRNA expression of c-kit and KITL isoforms and the localisation of c-kit and KITL proteins in adult human premenopausal ovaries.

METHODS

This study utilised granulosa cells obtained from the preovulatory follicles of women undergoing assisted reproduction, pieces of ovarian tissue obtained from premenopausal women undergoing gynaecological surgeries and archival paraffin-embedded premenopausal ovarian tissues. Methodology included PCR for gene expression and Western blot or immunohistochemistry for protein expression.

RESULTS

Both c-kit mRNA isoforms, known as GNNK+ and GNNK-, were detected in human ovarian cortex, while KITL protein isoforms (KITL1 and KITL2) were present in ovarian cortex and human granulosa cells. Immunohistochemistry showed expression of KITL and c-kit protein in multiple cell types within follicles throughout development, from primordial follicles to large antral follicles, in addition to atretic follicles. Oocytes of all follicle stages expressed c-kit protein exclusively. Interestingly, unlike animal models, expression of both proteins displayed a less cell-type specific distribution with immunostaining present in granulosa, theca and stromal cells, suggesting that autocrine signalling occurs within the human ovary.

CONCLUSION

The results of this study indicate that c-kit/KITL signalling also occurs in the human ovary, as established in various animal models, and may involve previously unknown autocrine signalling.

Two mutations in the 5'-flanking region of the KITLG gene are associated with litter size of dairy goats

In this study, Xinong Saanen (SN) and Guanzhong (GZ) dairy goat breeds were used to detect single nucleotide polymorphisms (SNPs) in the 5'-flanking region of the KITLG gene by DNA sequencing and primer-introduced restriction analysis-polymerase chain reaction. Two novel SNPs (g.13090G>T and g.13664C>A) were identified (GenBank Accession no. KM658964). Furthermore, g.13090G>T and g.13664C>A loci were closely linked in SN and GZ breeds (r(2) > 0.33). Association analysis results showed that g.13090G>T and g.13664C>A SNPs significantly affected litter size (P < 0.05). The litter size of individuals with the combined genotype GG/CC from both dairy goat breeds was greater than that of individuals with TT/AA in average parity (P < 0.05). Known biochemical and physiological functions, along with our results, indicated that GG/CC could be used in marker-assisted selection to choose individuals with greater litter size from both breeds. These results extend the spectrum of genetic variation in the caprine KITLG gene and may contribute to genetic resources and breeding of goats.

Association analysis between variants in KITLG gene and litter size in goats

Xinong Saanen (SN) and Guanzhong (GZ) goat breeds were used to detect single nucleotide polymorphisms (SNPs) in the coding regions with their intron-exon boundaries and the proximal flanking regions of KITLG gene by DNA sequencing and genotyped by PCR-restriction fragment (PCR-RFLP). Four novel SNPs (g.12654G>A, g.12772G>A, g.12829T>C and g.23683C>T) were identified (GenBank accession No. KM609289). It was shown that Xinong Saanen and Guanzhong goat breeds were in Hardy-Weinberg disequilibrium at g.12654G>A, g.12772G>A and g.12829T>C loci (P<0.05). The g.12654G>A, g.12772G>A and g.12829T>C loci were closely linked in both goat breeds (r(2)>0.33). Results of an association analysis indicated that SNPs g.12654G>A, g.12772G>A and g.12829T>C had significant effects on litter size (P<0.05). The combined genotypes of four SNP loci also affected litter size with the C7(GG/GG/CC/CC) genotype in the SN goat breed and C1(AA/GG/CC/CC) and C7(GG/GG/CC/CC) genotypes in the GZ goat breed having the highest litter size. The biochemical and physiological functions, together with the results obtained in our investigation, suggest that C7(GG/GG/CC/CC) could be used in marker-assisted selection to select the individuals with higher litter size in both goat breeds. The results extend the spectrum of genetic variation of the caprine KITLG gene, which might contribute to goat genetic resources and breeding.

Skipping of exons by premature termination of transcription and alternative splicing within intron-5 of the sheep SCF gene: a novel splice variant

Stem cell factor (SCF) is a growth factor, essential for haemopoiesis, mast cell development and melanogenesis. In the hematopoietic microenvironment (HM), SCF is produced either as a membrane-bound (-) or soluble (+) forms. Skin expression of SCF stimulates melanocyte migration, proliferation, differentiation, and survival. We report for the first time, a novel mRNA splice variant of SCF from the skin of white merino sheep via cloning and sequencing. Reverse transcriptase (RT)-PCR and molecular prediction revealed two different cDNA products of SCF. Full-length cDNA libraries were enriched by the method of rapid amplification of cDNA ends (RACE-PCR). Nucleotide sequencing and molecular prediction revealed that the primary 1519 base pair (bp) cDNA encodes a precursor protein of 274 amino acids (aa), commonly known as 'soluble' isoform. In contrast, the shorter (835 and/or 725 bp) cDNA was found to be a 'novel' mRNA splice variant. It contains an open reading frame (ORF) corresponding to a truncated protein of 181 aa (vs 245 aa) with an unique C-terminus lacking the primary proteolytic segment (28 aa) right after the D(175)G site which is necessary to produce 'soluble' form of SCF. This alternative splice (AS) variant was explained by the complete nucleotide sequencing of splice junction covering exon 5-intron (5)-exon 6 (948 bp) with a premature termination codon (PTC) whereby exons 6 to 9/10 are skipped (Cassette Exon, CE 6-9/10). We also demonstrated that the Northern blot analysis at transcript level is mediated via an intron-5 splicing event. Our data refine the structure of SCF gene; clarify the presence (+) and/or absence (-) of primary proteolytic-cleavage site specific SCF splice variants. This work provides a basis for understanding the functional role and regulation of SCF in hair follicle melanogenesis in sheep beyond what was known in mice, humans and other mammals.

Booroola BMPR1B mutation alters early follicular development and oocyte ultrastructure in sheep

Booroola ewes homozygous (BB) for a mutation in the bone morphogenetic protein receptor-1b (BMPR1B) gene exhibit higher ovulation rates, have larger diameter oocytes at earlier stages of follicular development (i.e. Type 3) and smaller diameter follicles at ovulation than wild-type (++) sheep. However, it is not known when BMPR1B is first expressed in the developing ovary or the cell types involved. In addition, the effects of the BMPR1B mutation on primordial (Type 1) follicles or during growth to the Type 3 stage are unknown. In the present study, BB and++fetal ovaries at Days 30-135 of gestation were screened by in situ hybridisation for BMPR1B mRNA. Ovaries from BB and++lambs were examined by microscopy to measure follicular and oocyte ultrastructural characteristics in Type 1-3 follicles. BMPR1B mRNA was observed in ovaries from Day 35 of gestation and was evident in oocytes of newly forming and fully formed Type 1 follicles. In BB animals, the Type 1 follicles had larger mean follicular and oocyte diameters, a greater volume of mitochondria, smooth endoplasmic reticulum and ribosomes and a greater surface area of junctions with the granulosa cells compared with++animals. It is concluded that the BMPR1B mutation alters follicular development from the onset of follicular formation.

Localization of c-kit and stem cell factor (SCF) in ovarian follicular epithelium of a lizard, Podarcis s. sicula

We performed an immunohistochemical study to determine the immunolocalization of c-kit and stem cell factor (SCF) in ovarian follicles during the reproductive cycle of the lizard, Podarcis s. sicula. Follicles were serially cut and used for histological and histochemical characterization and also for immunohistochemical detection of both c-kit and SCF. C-kit and SCF were localized in the follicles with a differing pattern with regard to the stage of sexual cycle or the cell type forming the follicular epithelium (granulosa). In pre-reproductive follicles, where the granulosa consists of three main different cytotypes, the c-kit receptor was prevalently localized on the plasmalemma of small cells, although some pyriform and intermediate cells also appeared positive. C-kit was also localized in the theca. In pre-reproductive follicles, SCF was markedly observed in the cytoplasm of some pyriform cells. Small cells and theca also stained moderately positive, whereas the intermediate cells were mostly negative. In reproductive follicles, where granulosa cells are morphologically rearranged, c-kit was observed in small cells and in some thecal elements, while SCF was weakly immunostained. At the site of follicular layer invaginations evident c-kit/SCF immunostaining was observed in the granulosa epithelium and in the theca. These observations suggest that the expression of c-kit and SCF changes as a function of follicular development and may reflect the involvement of this system in the maturation of the oocyte.

Circulating concentration of stem cell factor in serum of stimulated IVF patients

Stem cell factor (SCF) plays a major role in haematopoiesis and spermatogenesis, and possibly female fertility. This study investigated the role of changes in SCF concentrations in 74 assisted conception patients. In group 1 (n=74) SCF concentration was assessed in serum and follicular fluid (FF) on the day of follicular puncture (FP) and compared in serum and FF in response to ovarian stimulation between low (n=25), moderate (n=26) and high (n=14) responders. In group 2 (n=30) serum for SCF assessment was collected throughout the menstrual cycle until gestation. SCF concentration related to the number of follicles in serum and in FF decreased from low to moderate and high responders (P<0.001); pregnancy rates were 20.0%, 34.6% and 50.1%, respectively (P=0.05). SCF in serum increased from stimulation days 6-8 to 9-11 and peaked on the day of human chorionic gonadotrophin injection (P=0.03). The SCF concentrations dropped slightly on the day of FP, increased significantly to the day of pregnancy confirmation and reached highest concentration (P=0.02) during gestation. SCF is involved in follicle development and may be a predictor of IVF outcome.

The Kit ligand/c-Kit receptor system in goat ovaries: gene expression and protein localization

Relatively little information is available on the local factors that regulate folliculogenesis in goats. To examine the possibility that the Kit ligand (KL) system is expressed throughout the folliculogenesis, we studied the presence and distribution of KL and its receptor, c-Kit, in goat ovaries. Ovaries of goats were collected and either fixed in paraformaldehyde for immunohistochemical localization of KL and c-Kit proteins, or used for the isolation of follicles, luteal cells, surface epithelium and medullary samples to study mRNA expression for KL and c-Kit, using the reverse transcriptase polymerase chain reaction (RT-PCR). KL protein and mRNA were found in follicles at all stages of development, i.e. primordial, primary, secondary, small and large antral follicles, as well as in corpora lutea, surface epithelium and medullary tissue. Antral follicles expressed both KL-1 and KL-2 mRNAs, while earlier staged follicles expressed KL-1 transcript only. KL protein was demonstrated in granulosa cells from the primordial follicle onward. Its mRNA could be detected in granulosa cells isolated from antral follicles and occasionally in their theca cells. c-Kit mRNA was expressed in all antral follicular compartments and at all stages of follicular development. c-Kit protein was predominantly found in oocytes from the primordial follicle stage onwards, in theca cells of antral follicles, as well as in corpora lutea, surface epithelium and medullary tissue, particularly in the wall of blood vessels, which may indicate these cells as the main sites of action of KL. It is concluded that the KL/c-Kit system, in goat ovaries, is widespread and that it may be involved in the regulation of various local processes, including folliculogenesis and luteal activity.

KIT/KIT Ligand in Mammalian Oogenesis and Folliculogenesis: Roles in Rabbit and Murine Ovarian Follicle Activation and Oocyte Growth1

In rodent ovaries Kit ligand (KITL) and its receptor KIT have diverse roles, including the promotion of primordial follicle activation, oocyte growth, and follicle survival. Studies were undertaken to determine whether KITL and KIT carry out similar activities in rabbits. KitlandKitmRNA and protein were localized to oocytes and granulosa cells, respectively, in the rabbit ovary. Ovarian cortical explants from juvenile rabbits and neonatal mouse ovaries were subsequently cultured with recombinant mouse KITL and/or KITL neutralizing antibody. Indices of follicle growth initiation were compared with controls and between treatment groups for each species. Recombinant mouse KITL had no stimulatory effect on primordial follicle recruitment in cultured rabbit ovarian explants. However, the mean diameter of oocytes from primordial, early primary, primary, and growing primary follicles increased significantly in recombinant mouse KITL-treated explants compared with untreated tissues. In contrast, recombinant mouse KITL promoted both primordial follicle activation and an increase in the diameter of oocytes from primordial and early primary follicles in the mouse, and these effects were inhibited by coculture with KITL-neutralizing antibody. Recombinant mouse KITL had no effect on follicle survival for either species. These data demonstrate that KITL promotes the growth of rabbit and mouse oocytes and stimulates primordial follicle activation in the mouse but not in the rabbit. We propose that the physiologic roles of KITL and KIT may differ between species, and this has important implications for the design of in vitro culture systems for folliculogenesis in mammals, including the human.

Kit ligand and c-Kit are expressed during early human ovarian follicular development and their interaction is required for the survival of follicles in long-term culture

The receptor tyrosine c-Kit and its cognate ligand, c-Kit ligand (KL, stem cell factor, SCF), are involved in ovarian follicular development in several animal species. We studied the expression of KL and c-Kit usingin situhybridization and immunohistochemistry in donated human ovarian cortical tissue. The KL transcripts were expressed in granulosa cells of primary follicles, whereas the expression of c-Kit was confined to the oocyte and granulosa cells in primary and secondary follicles. We employed an ovarian organ culture using firstly serum-containing and then serum-free medium to study the effects of KL and an anti-c-Kit antibody, ACK2, on the development and survival of ovarian folliclesin vitro. Culture of ovarian cortical slices for 7 days resulted in a 37% increase in the number of primary follicles and a 6% increase in secondary follicles. The proportion of viable follicles decreased in all cultures. The addition of KL (1, 10 and 100 ng/ml) into the culture media did not affect the developmental stages of the follicles or the proportion of atretic follicles. Inclusion of ACK2 (800 ng/ml) in the culture medium significantly increased the proportion of atretic follicles on days 7 (49 vs 28% in control cultures) and 14 (62 vs 38%) of culture. In conclusion, c-Kit and KL are expressed in human ovaries during follicular development. Blocking the c-Kit receptor induces follicular atresia. The KL/c-Kit signaling system is likely to control the survival of human ovarian follicles during early follicular development.

Kit ligand and c-Kit have diverse roles during mammalian oogenesis and folliculogenesis

Paracrine signalling between the oocyte and its surrounding somatic cells is fundamental to the processes of oogenesis and folliculogenesis in mammals. The study of animal models has revealed that the interaction of granulosa cell-derived kit ligand (KL) with oocyte and theca cell-derived c-Kit is important for multiple aspects of oocyte and follicle development, including the establishment of primordial germ cells within the ovary, primordial follicle activation, oocyte survival and growth, granulosa cell proliferation, theca cell recruitment and the maintenance of meiotic arrest. Though little is known about the specific roles of KL and c-Kit during human oogenesis, the expression profiles for KL and c-Kit within the human ovary suggest that they are also functionally relevant to female fertility. This review details our current understanding of the roles of KL and c-Kit within the mammalian ovary, with a particular focus on the functional diversity of this receptor-ligand interaction at different stages of oocyte and follicle development.

In vitro growth of oocytes from primordial follicles isolated from frozen-thawed lamb ovaries

A study was conducted to develop an in vitro culture system for growing sheep oocytes from isolated primordial follicles. Enzymatically isolated neonatal sheep primordial follicles were cultured in Waymouth MB752/1 medium containing BSA (3 mg/ml) + ITS (1%, v/v) over 28 days. In Experiment 1, primordial follicles (average diameter 40.2+/-0.60 microm) were cultured at densities of 20, 50 and 100 follicles per well. Less than 20% of the oocytes survived to day 28 but there was a significant (P < 0.05) increase in median oocyte diameter from day 2 to day 28 for oocytes cultured at the higher densities of 50 and 100 follicles. In Experiment 2, two methods to improve oocyte:granulosa cell associations were tested. Altering the fibronectin coating regime did not improve oocyte survival and growth. In contrast lectin-aggregated primordial follicles cultured on non-coated wells showed significantly (P < 0.05) improved oocyte survival to 50% and increased median oocyte diameter compared to non-aggregated follicles. In Experiment 3, the effect of KIT ligand (KL) at 0 ng/ml, 10 ng/ml and 100 ng/ml, on lectin-aggregated primordial follicles cultured on non-coated wells was tested. KL at 100 ng/ml significantly (P < 0.05) increased median oocyte diameter compared to non-treated controls but had no effect on oocyte survival. In addition, follicles cultured with 100 ng/ml KL expressed mRNA for AMH, a gene expressed only in granulosa cells of growing follicles. In conclusion, culture of lectin-aggregated primordial follicles supported the long-term survival and growth of oocytes from isolated sheep primordial follicles. Culture of lectin-aggregates with 100 ng/ml KL further increased oocyte growth and induced granulosa cell differentiation.

The expression patterns of mRNA-encoding stem cell factor, internal stem cell factor and c-kit in the prepubertal and adult porcine ovary

The receptor, c-kit, and its ligand, stem cell factor (SCF), are important regulators of ovarian follicle growth and development. The aim of this study was to identify the sites of expression of mRNA for c-kit and SCF in prepubertal and mature (pregnant and non-pregnant) animals. Ovaries were recovered from prepubertal animals, non-pregnant sows and five sows at approximately 3 months of gestation. Ovine SCF and c-kit DNA were cloned into plasmid vectors to produce RNA probes. Expression of mRNA encoding SCF and c-kit were detected via in situ hybridization. Both mRNA were detected throughout ovaries from all animals. This study provides evidence that the growth-factor complex is required throughout follicle development, and also for continued maintenance of the corpus luteum (CL) in the mature animal. SCF mRNA was localized to the granulosa cell layer and was also extensively expressed in endothelial tissue and throughout the CL. c-kit mRNA was detected in the theca layer, oocytes and also in CL. In conclusion, expression of SCF and c-kit mRNA in granulosa and theca cells, respectively, indicate an important interaction between somatic cells throughout follicle development and that in the mature animal, SCF and c-kit potentially have a role in maintaining progesterone secretion by the CL. The observations of continued expression of SCF and c-kit throughout development suggest that there may be differences in the role of this receptor-ligand complex between large mono- vs. poly ovulatory species, such as the pig.

Expression of stem cell factor and its receptor in human fetal and adult ovaries

OBJECTIVE

To investigate the immunocytochemical expression and presence of mRNA transcripts of stem cell factor (SCF) and its receptor (SCF-R) in ovaries from human adults and fetuses.

DESIGN

Immunocytochemical and reverse transcription polymerase chain reaction (RT-PCR) study.

SETTING

Major tertiary care and referral academic centers.

PATIENT(S)

Seven women and girls undergoing laparoscopic ovarian biopsy and 13 women undergoing second and third trimester pregnancy terminations.

INTERVENTION(S)

None.

MAIN OUTCOME MEASURE(S)

Microscopic morphometric analysis, immunocytochemistry for SCF and SCF-R, and RT-PCR analyses.

RESULT(S)

There was strong to moderate immunocytochemical staining for SCF and its receptor in oocytes from primordial stages onward, but not in granulosa cells, in both fetal and adult ovarian samples. Transcripts of SCF and SCF-R RNA were detected by RT-PCR analyses for SCF and SCF-R.

CONCLUSION(S)

The expression of SCF and its receptor in ovarian tissue from fetuses and women suggests a possible role of SCF in growth initiation of human primordial follicles.

The early stages of follicular development: activation of primordial follicles and growth of preantral follicles

Although enormous progress has been made in understanding the events and regulation of the later stages of ovarian follicular development, the early stages of development, to a large extent and particularly in large mammals, remain a mystery. Mechanisms that regulate the initiation of follicular growth (follicle activation) and the ensuing growth and differentiation of preantral follicles are of considerable interest, since their elucidation is a prerequisite to use of the primordial pool to enhance reproductive efficiency in domestic animals, humans, and endangered species. This review is an attempt to summarize the approaches that have been taken to further this goal and the results thus far of these efforts. Preantral follicular development can be divided into three stages: activation of primordial follicles, the primary to secondary follicle transition, and the development of secondary follicles to the periantral stage. The activation of primordial follicles in vitro has been achieved thus far in rodents, cattle, and primates, where it occurs spontaneously without the addition of growth factors or hormones. The ovaries of rodents are small enough to be cultured intact and, in that experimental situation, some follicles activate, while many remain in the resting pool, and the addition of specific factors can increase or decrease the number of follicles that leave the resting pool in vitro. In contrast, follicular activation in cattle and primates has been studied by culturing small pieces of the ovarian cortex, rich in primordial follicles, and the great majority of the primordial follicles activate in that situation, suggesting the importance of inhibitory factors to the normal, gradual exit of follicles from the resting pool. In cultured rodent ovaries, follicles appear to pass easily and spontaneously from the primary to the secondary stage, whereas few of the activated follicles in cultured cortical pieces from cattle or primates progress from the primary to the secondary stage. Understanding the requirements for the primary to secondary transition is critical for growing follicles activated in vitro to the late preantral and antral stages. In contrast, the requirements for the continued growth of larger preantral follicles, which can be isolated for in vitro studies, have been extensively explored in rodents and to a lesser extent in domestic species. A number of hormones and factors have been implicated and will be discussed. Taken together, the results highlight the need for a better understanding of the earliest stages of follicular development in domestic ruminants, particularly follicle activation and the primary to secondary follicle transition.

Bringing up small oocytes to eggs in pigs and cows

It has been known that the mammalian ovary contains a huge number of non-growing small oocytes, of which only a small number grow to their final size, mature, and are ovulated. Artificial maturation of small oocytes could provide a new source of mature eggs for livestock production and assisted reproduction in humans and in endangered species. Two methods have been used for oocyte growth, in vitro growth (IVG) culture and xenotransplantation. By these methods, oocytes in some species grow up to their final size and acquire developmental competence, although the methods are still at the experimental stage. The experiments remind us of many basic questions in mammalian oogenesis: Does the oocyte require certain stimuli to initiate growth? How are the few oocytes selected to grow to final size? How do they grow up in follicular units? How do they acquire meiotic competence during the growth phase? This paper will give some clues to answer these questions by presenting our recent data from IVG and xenotransplantation experiments, and by illustrating differences between the oocytes of mice and larger animals.

Interactions between the Oocyte and Surrounding Somatic Cells in Follicular Development: Lessons from In Vitro Culture

Mammalian oogenesis occurs concomitantly with folliculogenesis in a coordinated manner in the ovaries. In vitro growth (IVG) culture systems of the oocytes have been developed as a new technology for utilizing incompetent oocytes in the ovary as a source of mature oocytes as well as for studying oogenesis, folliculogenesis, and oocyte-somatic cell interactions. The results of IVG experiments have suggested that direct association of oocytes and surrounding granulosa cells supports oocyte viability and growth through the gap junctions, which are efficient conduits for low molecular weight substances. It has been revealed that granulosa cells metabolize some molecules which are in turn transported into the oocytes. IVG systems have also provided evidence that FSH promotes the development of follicles at secondary or later stages by its stimulation of proliferation and differentiation of granulosa cells, and perhaps by its anti-apoptotic effects. In addition, interactions between granulosa cell-derived KIT ligands and oocyte KIT receptors have been suggested as initiating oocyte growth and follicular development. Furthermore, recent findings suggest there are growth factors derived from oocytes such as GDF-9 and BMP-15. With such factors, oocytes participate in follicular development by regulating the differentiation of surrounding somatic cells. These bidirectional communications between oocytes and somatic cells are important for oocyte growth and follicular development. IVG systems should provide further information regarding oogenesis and folliculogenesis in the ovary.

Gene expression in abnormal ovarian structures of ewes homozygous for the inverdale prolificacy gene

Animals heterozygous (I+) for the Inverdale prolificacy gene (FecX(I)) have an increased ovulation rate whereas those homozygous (II) for FecX(I) are infertile with "streak" ovaries and follicular development arrested at the primary (type 2 follicle) stage. The streak ovaries also contain small oocyte-free nodules with granulosa-like cells and often tumor-like structures. It has been hypothesized that these abnormal structures are of granulosa cell origin, and the aim of this study was to determine whether genes normally expressed in granulosa cells are also expressed in the nodules and tumor-like structures. The mRNAs encoding c-kit and its ligand stem cell factor (SCF), FSH receptor (FSH-R), follistatin, alpha-inhibin subunit, and the beta(A)- and beta(B)-activin/inhibin subunits were localized in ovaries of ewes with 0 (++), 1 (I+), or 2 (II) copies of the FecX(I) gene (n = 4-9 animals per genotype per gene) using in situ hybridization. Ontogeny of expression of all mRNAs examined was similar between ++ and I+ ewes. Expression of c-kit mRNA was observed in the oocyte of all follicular types present in ++, I+, and II ewes. Moreover, granulosa cells of type 2 (II) and type 2 and larger follicles (++, I+) expressed SCF mRNA. The mRNAs encoding FSH-R, follistatin, alpha-inhibin subunit, and beta(B)-activin/inhibin subunit were identified in type 3 and larger follicles of ++ and I+ ewes but not in follicles of II ewes that were only at the type 1, 1a, or 2 stages of development. However, the cells within the oocyte-free nodules of II ewes expressed all of these genes. The mRNAs encoding c-kit and beta(A)-activin/inhibin subunit were not observed in granulosa cells until antrum formation (type 5 follicles) or in the nodules of II ewes. Tumors from 4 ewes were obtained and classified as cystic, semisolid, or solid structures containing granulosa-like cells or as solid structures containing predominately fibroblast- and luteal-like cells. Often, two tumors were present on the same ovary. Tumors containing granulosa-like cells (n = 3-4 per gene) expressed the mRNAs encoding alpha-inhibin subunit, beta(A)-, and beta(B)-activin/inhibin subunits, follistatin, and the FSH-R but did not contain detectable amounts of mRNA for c-kit or SCF. Tumors composed predominately of fibroblast- and luteal-like cells expressed very low levels of SCF mRNA; of the other mRNAs examined, none were detected. Also, none of the genes examined were found to be expressed by the surface epithelium, theca externa, fibroblast, or vascular cells within the ovary of animals of any genotype. These findings are consistent with the hypothesis that the somatic cells in oocyte-free nodules and tumor-like tissue in II ewes originate from the granulosa cells of the small follicles.

Oocyte regulation of kit ligand expression in mouse ovarian follicles

Kit ligand (KL), a product of granulosa cells in ovarian follicles, is a putative regulator of oocyte development. However, the factors that regulate KL mRNA levels in granulosa cells remain unclear. This study tested the hypothesis that oocytes regulate granulosa cell steady-state KL mRNA expression levels and that the characteristics of this regulation are dependent on the stage of growth and development of both oocytes and follicles. Levels of mRNA for the KL splice variants (KL-1 and KL-2) were shown to be high in granulosa cells from preantral follicles and then decline after follicular antrum formation. Preovulatory follicular development was associated with a dramatic increase in steady-state levels of KL-1 mRNA in mural granulosa but not cumulus cells. Regulation of these changes was examined in vitro using partly grown oocytes isolated from preantral follicles and fully grown oocytes isolated from preovulatory follicles. FSH increased the steady-state KL mRNA levels in preantral granulosa cells in vitro. Partly grown oocytes either increased or decreased KL mRNA levels in preantral granulosa cells depending on the absence or presence of FSH stimulation, respectively. Fully grown oocytes reduced the KL mRNA level in preantral granulosa cells and increased the ratio of KL-1 to KL-2 mRNA. In mural granulosa cell culture, FSH augmented testosterone-dependent elevation of the steady-state KL mRNA level, but had no effect alone. Fully grown oocytes reduced KL-2 but not KL-1 mRNA levels in mural granulosa cells treated with testosterone plus FSH, whereas fully grown oocytes reduced levels of both KL transcripts in cumulus cell culture. These effects of oocytes on steady-state KL mRNA expression levels in vitro explain the changes in granulosa cell KL mRNA levels observed during follicle development in vivo. The results therefore support the hypothesis that oocytes regulate granulosa cell kit ligand mRNA levels in a way that is characteristic of the stage of growth and development of the oocyte. Moreover, the results suggest that oocytes play a major role in promoting dynamic changes in gene expression by granulosa cells appropriate to the stage of follicular development.

Transforming growth factor-beta regulates Kit ligand expression in rat ovarian surface epithelial cells

In preparation for ovulation, paracrine communication between the preovulatory follicle and overlying theca/stromal cells and ovarian surface epithelium (OSE) must take place to facilitate the degradative and apoptotic events associated with ovulation. Kit tyrosine kinase receptors and their ligand, kit ligand (KL) are expressed within ovarian follicles, and ligand-induced receptor activation appears to account for some of the cell - cell interactions important for oocyte development. We investigated the expression of Kit receptors and KL in OSE cells and the possibility that modulation of their expression could affect OSE cell activity. KL mRNA and protein were detected in the OSE cell layer of rat ovaries, and primary cultures of rat OSE as well as the immortalized rat OSE cell line, ROSE 199, expressed KL, but not Kit receptors. Both primary and immortalized OSE cells preferentially expressed KL-1, rather than KL-2, transcripts, suggesting that these cells produce predominantly the soluble form of KL. Activation of the cAMP signalling pathway using dibutyryl cAMP decreased proliferation of ROSE 199 cells and elicited a threefold increase in KL expression. TGF-beta similarly inhibited ROSE 199 cell proliferation, but strongly inhibited dibutyryl cAMP-induced KL expression, indicating that changes in KL expression were not directly associated with OSE cell proliferation. The expression of mostly soluble KL in the surface epithelium suggests that this cytokine may be acting in a paracrine fashion, perhaps interacting with nearby Kit receptor-bearing theca cells.

Molecular cloning of the ovine Growth/Differentiation factor-9 gene and expression of growth/differentiation factor-9 in ovine and bovine ovaries

Recently a novel member of the transforming growth factor beta (TGFbeta) superfamily termed growth/differentiation factor-9 (GDF-9) was shown to be expressed in ovaries of mice and humans, and to be essential for normal follicular development beyond the primary (type 2) follicle stage in mice. In the present study, the gene for ovine GDF-9 was isolated and characterized, and expression of GDF-9 mRNA in ovaries of domestic ruminants was examined. The predicted amino acid sequence of ovine GDF-9 is 77% and 66% homologous to human and mouse GDF-9, respectively. Specific hybridization using homologous 35S-antisense probes was restricted to oocytes. In contrast to similar studies in mice in which GDF-9 was first detected beginning at the primary (type 2) follicle stage, in ovine and bovine ovaries GDF-9 mRNA was expressed beginning at the primordial (type 1) follicle stage. The observed timing and pattern of GDF-9 expression in oocytes of domestic ruminants is consistent with a role for GDF-9 in the initiation and maintenance of folliculogenesis in these species, and supports the general concept that early stages of follicular growth and development are regulated by intraovarian factors.