Oocyte regulation of kit ligand expression in mouse ovarian follicles

Paracrine effects of oocyte secreted factors and stem cell factor on porcine granulosa and theca cells in vitro

Oocyte control of granulosa and theca cell function may be mediated by several growth factors via a local feedback loop(s) between these cell types. This study examined both the role of oocyte-secreted factors on granulosa and thecal cells, cultured independently and in co-culture, and the effect of stem cell factor (SCF); a granulosa cell derived peptide that appears to have multiple roles in follicle development. Granulosa and theca cells were isolated from 2-6 mm healthy follicles of mature porcine ovaries and cultured under serum-free conditions, supplemented with: 100 ng/ml LR3 IGF-1, 10 ng/ml insulin, 100 ng/ml testosterone, 0-10 ng/ml SCF, 1 ng/ml FSH (granulosa), 0.01 ng/ml LH (theca) or 1 ng/ml FSH and 0.01 ng/ml LH (co-culture) and with/without oocyte conditioned medium (OCM) or 5 oocytes. Cells were cultured in 96 well plates for 144 h, after which viable cell numbers were determined. Medium was replaced every 48 h and spent medium analysed for steroids. Oocyte secreted factors were shown to stimulate both granulosa cell proliferation (P < 0.001) and oestradiol production (P < 0.001) by granulosa cells throughout culture. In contrast, oocyte secreted factors suppressed granulosa cell progesterone production after both 48 and 144 hours (P < 0.001). Thecal cell numbers were increased by oocyte secreted factors (P = 0.02), together with a suppression in progesterone and androstenedione synthesis after 48 hours (P < 0.001) and after 144 hours (P = 0.02), respectively. Oocyte secreted factors also increased viable cell numbers (P < 0.001) in co-cultures together with suppression of progesterone (P < 0.001) and oestradiol (P < 0.001). In granulosa cell only cultures, SCF increased progesterone production in a dose dependent manner (P < 0.001), whereas progesterone synthesis by theca cells was reduced in a dose dependent manner (P = 0.002). Co-cultured cells demonstrated an increase in progesterone production with increasing SCF dose (P < 0.001) and an increase in oestradiol synthesis at the highest dose of SCF (100 ng/ml). In summary, these findings demonstrate the presence of a co-ordinated paracrine interaction between somatic cells and germ cells, whereby oocyte derived signals interact locally to mediate granulosa and theca cell function. SCF has a role in modulating this local interaction. In conclusion, the oocyte is an effective modulator of granulosa-theca interactions, one role being the inhibition of luteinization.

Ultrastructural localisation of calcium deposits in the mouse ovary

Follicle-enclosed mouse oocytes contain numerous calcium deposits. The ultrastructural distribution of calcium deposits in the nuclei, mitochondria and cytoplasm of mouse oocytes and granulosa cells of primary, secondary and antral follicles was examined using the combined oxalate–pyroantimonate method. The mitochondria of oocytes from all types of follicles had the highest levels of calcium deposits of all oocyte compartments, with the exception of primary follicles, in which oocyte nuclei contained the same level of calcium deposits as the mitochondria. Calcium deposits in the cytoplasm of oocytes from primary follicles were significantly lower than those in the cytoplasm of oocytes from secondary and antral follicles. Calcium deposits in the cytoplasm of granulosa cells were significantly lower than calcium deposits in the mitochondria of granulosa cells and this difference persisted throughout all categories of follicles. Calcium deposits in the nuclei of granulosa cells did not differ from levels in the mitochondria in primary and secondary follicles. In contrast, the nuclei of granulosa cells from antral follicles had lower levels of calcium deposits than the mitochondria. The differences observed in calcium deposits in various cellular compartments in oocytes and granulosa cells in the follicles of ovaries of adult mice can be attributed to their acquisition of meiotic competence and follicular development.

Growth and differentiation factor-9 stimulates progression of early primary but not primordial rat ovarian follicle development

The ovary contains a pool of primordial follicles containing oocytes arrested in meiosis that are the source of developing follicles for the female. Growth and differentiation factor-9 (GDF-9) is a member of the transforming growth factor beta superfamily of growth factors, and follicles of GDF-9 knockout mice arrest in the primary stage of development. The effect of GDF-9 treatment on the primordial to primary follicle transition and on subsequent follicle progression was examined using a rat ovary organ culture system. Ovaries from 4-day-old rats were cultured under serum-free conditions in the absence or presence of growth factors. GDF-9 treatment caused a decrease in the proportion of stage 1 early primary follicles and a concomitant increase in the proportion of stage 2 mature primary follicles. GDF-9 did not effect primordial follicles or stage 0 to stage 1 follicle transition. GDF-9 also did not influence stage 3 or 4 secondary follicle numbers. Isolated antral follicle granulosa and theca cell cultures were used to analyze the actions of GDF-9. GDF-9 treatment did not directly influence either granulosa or theca cell proliferation. The ability of GDF-9 to influence the expression of another growth factor was examined. GDF-9 treatment increased kit ligand (KL) mRNA expression in bovine granulosa cells after 2 days of culture. Ovaries from 4-day-old rats were also cultured with or without GDF-9 treatment, and total ovary expression of KL mRNA was increased by GDF-9. In summary, GDF-9 was found to promote the progression of early primary follicle development but did not influence primordial follicle development. The actions of GDF-9 on specific stages of follicle development may in part be mediated through altering the expression of KL.

A negative feedback system between oocyte bone morphogenetic protein 15 and granulosa cell kit ligand: its role in regulating granulosa cell mitosis

Although the existence of a regulatory paracrine feedback system between oocytes and follicular somatic cells has been postulated for some time, there has not yet been any definitive evidence that such a communication system exists. Herein we present a previously undescribed oocyte-granulosa cell (GC) feedback communication system involving an oocyte-derived factor, bone morphogenetic protein-15 (BMP-15) and a GC-derived factor, kit ligand (KL), both of which have been shown to be crucial regulators of female reproduction. We used a coculture system of rat oocytes and GCs and found that BMP-15 stimulates KL expression in GCs, whereas KL inhibits BMP-15 expression in oocytes, thus forming a negative feedback loop. Moreover, KL, like BMP-15, exhibited mitotic activity on GCs in the presence of oocytes. Because c-kit (KL receptor) is expressed in oocytes but not GCs, the oocytes must be involved in mediating the KL-induced GC mitosis. Furthermore, the blockage of c-kit signaling in oocytes by using a c-kit neutralizing antibody markedly suppressed BMP-15-induced GC mitosis, suggesting that the oocyte must play a role in the GC responses to BMP-15. In contrast, the c-kit antibody had no effect on the mitotic activities of two other known GC mitogens, activin-A and BMP-7. Altogether, this study presents direct evidence of a negative feedback system governed by oocyte-derived BMP-15 and GC-derived KL, and demonstrates that the mitotic activities of BMP-15 and KL for GCs depend on this oocyte-GC communication system. We hypothesize that the negative feedback system most likely plays a pivotal role in early folliculogenesis.

The mammalian oocyte orchestrates the rate of ovarian follicular development

The development of both the mammalian oocyte and the somatic cell compartments of the ovarian follicle is highly coordinated; this coordination ensures that the ovulated oocyte is ready to undergo fertilization and subsequent embryogenesis. Disruption of this synchrony results in oocyte developmental failure. Communication between the oocyte and companion somatic cells is essential for successful development of both follicular compartments. However, it was not previously known whether one cell type, either the somatic or the germ cell compartment, determines the overall rate of follicular development. To test the hypothesis that the oocyte orchestrates the rate of follicle development, mid-sized oocytes isolated from secondary follicles were transferred back to primordial follicles, the earliest stage of follicular development. This transfer doubled the rate of follicular development and the differentiation of follicular somatic cells. Oocyte development in these accelerated follicles appeared normal; recovered oocytes were competent to undergo fertilization and embryonic development. These results demonstrate that oocytes orchestrate and coordinate the development of mammalian ovarian follicles and that the rate of follicular development is based on a developmental program intrinsic to the oocyte.

Involvement of steroid hormones on in vitro maturation of pig oocytes

The purpose of this study was to determine if the addition of steroid hormones into the culture medium could influence the in vitro maturation of pig oocytes. The cumulus-oocyte complexes (COCs). collected from follicles of 2-5 mm diameter, were matured in steroid-free medium supplemented with various concentrations of estradiol-17beta (0-3000 ng/ml), progesterone (0-5000 ng/ml) and testosterone (0-300 ng/ml). The COCs were cultured for 42 h, then fertilized in vitro. We analyzed nuclear and cytoplasmic maturation with lacmoid stain 20 h after in vitro insemination. We observed no significant effect (P > 0.05) on the percentage of oocytes completing nuclear or cytoplasmic maturation or the number of sperm penetrating each oocyte for any concentration of progesterone, estradiol-17beta or testosterone. Similarly, adding a combination of those hormones to the medium did not significantly (P > 0.05) affect any of the criteria. In order to determine if there was a possible secretion of steroids during maturation, we added COCs, denuded oocytes and stripped cumulus cells to drops of a steroid-free medium and cultured them for 42 h, after which we analyzed the medium, before and after culture, for the presence of progesterone, estradiol-17beta and testosterone by radioimmunoassay (RIA) analysis. COCs, as well as cumulus cells alone, secreted similar amounts of estradiol (43.3 and 37.5 pg/ml, respectively) and progesterone (4.24 and 4.79 ng/ml, respectively) into the maturation medium. A small amount of estradiol (28.8 pg/ml) was also detected when oocytes were cultured alone. These results indicate that no steroids need to be added to the maturation medium of pig oocytes and that the COCs secrete steroids during maturation. It is possible that the amounts produced by the COCs fulfill any requirement for steroids if these steroids are required for either nuclear or cytoplasmic oocyte maturation.

Notch pathway genes are expressed in mammalian ovarian follicles

Folliculogenesis is the process of development of ovarian follicles that ultimately results in the release of fertilizable oocytes at ovulation. This is a complex program that involves the proliferation and differentiation of granulosa cells. Granulosa cells are necessary for follicle growth and support the oocyte during folliculogenesis. Genes that regulate the proliferation and differentiation of granulosa cells are beginning to be elucidated. In this study, the expression patterns of Notch receptor genes and their ligands, which have been shown to regulate cell-fate decisions in many systems during development, were examined in the mammalian ovary. In situ hybridization data showed that Notch2, Notch3, and Jagged2 were expressed in an overlapping pattern in the granulosa cells of developing follicles. Jagged1 was expressed in oocytes exclusively. Downstream target genes of Notch also were expressed in granulosa cells. These data implicate the Notch signaling pathway in the regulation of mammalian folliculogenesis.

Mouse oocyte mitogenic activity is developmentally coordinated throughout folliculogenesis and meiotic maturation

Oocytes secrete soluble factors that regulate the growth and differentiation of follicular cells, including maintenance of the distinctive cumulus cell phenotype. This study determines whether the mitogenic activity of oocytes is developmentally regulated and examines the responsiveness of follicular cells to oocytes at different stages of follicular development. Prepubertal SV129 mice of varying ages were primed with 5 IU equine chorionic gonadotropin (eCG) and oocytes/zygotes collected either 46 h post-eCG (immature oocytes), 12 h after administration of 5 IU human CG (hCG; ovulated ova), or 12 h post-hCG and mating (zygotes). Mural granulosa cells (MGC) from antral follicles and GC from preantral follicles were cultured +/- denuded oocytes (DO) for 18 h, followed by a 6-h pulse of [(3)H]thymidine as an indicator of cellular DNA synthesis. Coculturing MGC with meiotically maturing oocytes led to a dose-dependent increase in [(3)H]thymidine incorporation (20-fold above control levels at 0.5 DO/microl). However, [(3)H] counts remained unchanged from control levels when cultured with meiotically incompetent DO from 11- to 15-day-old mice (3% germinal vesicle breakdown; GVB), irrespective of dose of DO or developmental status of GC (MGC or preantral GC). In some treatments, spontaneous meiotic resumption of competent oocytes was prevented by culturing with 5 microM milrinone, a selective inhibitor of oocyte-specific cyclic nucleotide phosphodiesterase. The mitogenic capacity of oocytes was found to decline during and after oocyte maturation. [(3)H]Thymidine incorporation in MGC was highest (11-fold above controls) when cultured with meiotically inhibited (milrinone-treated) GV DO, stimulated 5.5-fold by culture with maturing oocytes, 3-fold with ovulated ova, and unstimulated by zygotes. [(3)H]Thymidine incorporation in MGC was not altered by the dose of milrinone, either in the presence or absence of DO. Metaphase I marked the beginning of the decline in the capacity of oocytes to promote MGC DNA synthesis. These results demonstrate that the capacity of oocytes to promote proliferation of granulosa cells follows a developmental program, closely linked to oocyte meiotic status, increasing with the acquisition of meiotic competence and declining during and after oocyte maturation.

Stage-dependent role of growth differentiation factor-9 in ovarian follicle development

GDF-9 was shown to be essential for follicle progression and is the only factor secreted by the oocyte shown to increase the number of primordial and primary follicles in vivo. Furthermore, GDF-9 is a major growth factor involved in the oocyte control of granulosa cell differentiation. A concentration gradient of the paracrine factor GDF-9 established by the oocyte could provide the basis to explain the stratification of granulosa cells in antral and preovulatory follicles. The stimulatory effects of GDF-9 on early follicle development provide a basis for the use of GDF-9 in the treatment of infertility.

Regulation of prostaglandin-endoperoxide synthase 2 messenger ribonucleic acid expression in mouse granulosa cells during ovulation

Normal ovulation in mice requires PG-endoperoxide synthase 2 (cyclooxygenase-2; COX-2) expression. This study examined the role of the oocyte and other factors in regulating steady state levels of COX-2 messenger RNA (mRNA) in granulosa cells. Multiphasic changes in the expression pattern of COX-2 mRNA were found, with peaks of expression 4 and 12 h after hCG treatment. Changes in relative expression levels in cumulus cells and mural granulosa cells occurred over time, with similar mRNA levels at 4 h, but higher levels in cumulus cells compared with mural granulosa cells at 8 and 12 h post-hCG. In cultured mural granulosa cells, LH, FSH, and oocytes promoted COX-2 mRNA expression concurrent with the first expression peak in vivo. At the same time, FSH, but not LH, treatment of cultured cumulus-oocyte complexes (COC) promoted COX-2 mRNA expression in cumulus cells. This response of cumulus cells to FSH treatment was largely dependent on the presence of either fully grown germinal vesicle stage or maturing oocytes, but not growing oocytes. At 8 h, COX-2 mRNA expression in FSH-stimulated COC was lower than at 4 h; however, oocyte coculture promoted COX-2 mRNA expression in cumulus cells. No second peak in expression occurred in cultured COC. However, coculture of COC with follicle walls promoted COX-2 mRNA expression in cumulus cells 12 h post-hCG; an effect augmented by oocytes. Therefore, the oocyte resident within ovulatory follicles produces a factor(s) that promotes expression of COX-2 mRNA by cumulus cells and possibly by mural granulosa cells. Thus, the oocyte probably plays an important role in promoting ovulation. However, the multiphasic changes in the pattern of COX-2 expression appear orchestrated by non-oocyte-derived factors.

Differential Display and Suppressive Subtractive Hybridization Used to Identify Granulosa Cell Messenger RNA Associated with Bovine Oocyte Developmental Competence1

The main objective of this study was to identify mRNA expressed in the granulosa cells characterizing differentiated follicles bearing developmentally competent bovine oocytes. Analytical comparisons were made on mRNA pools of granulosa cells using differential display reverse transcription polymerase chain reaction (DDRT) analysis and suppressive subtractive hybridization (SSH). With DDRT, mRNA patterns of granulosa cells from small (< 4 mm) and large (> 8 mm) follicles cultured in the presence or absence of LH were compared to identify mRNA associated with follicular size or with the LH response. Nine clones were sequenced, and two were identified. One of the clones, DRAK 1, was associated with the presence of LH in the medium. Other comparisons directed toward the identification of mRNA associated with the presence of a competent oocyte were done on granulosa cells collected in vivo from superstimulated heifers. With the DDRT analysis, four clones associated with the oocyte developmental competence status were identified. With the SSH analysis, four clones specific to the presence of an incompetent oocyte were sequenced and none were identified, whereas 49 clones specific to the presence of a competent oocyte were sequenced and 18 were identified. Among these clones, early growth response 1, sprouty 2, cytochrome C oxidase, matrix metalloproteinase inducer, matrix metalloproteinase, epiregulin, prostaglandin receptor, and progesterone receptor were the most relevant to the ovarian physiology being examined.

Mouse antral oocytes regulate preantral granulosa cell ability to stimulate oocyte growth in vitro

In this study we evaluated whether mouse oocytes derived from early antral or preovulatory follicles could affect the ability of preantral granulosa cells to sustain oocyte growth in vitro. We found that early antral oocytes with a diameter > or =75 microm did not grow any further during 3 days of culture on preantral granulosa cell monolayers in vitro, while most of the oocytes with a smaller diameter increased significantly in size. Similarly, about 65% of growing oocytes isolated from preantral follicles grew when cultured on preantral granulosa cells. By coculturing with growing oocytes fully grown early antral or preovulatory oocytes, a small proportion (about 10%) of growing oocytes increased in diameter, and changes in granulosa cell morphology were observed. Such effects occurred as a function of the fully grown oocyte number seeded and were not associated with a decrease in coupling index values. By avoiding physical contact between antral oocytes and granulosa cells, the proportion of growing oocytes undergoing a significant increase in diameter was about 36%. These results indicate that fully grown mouse oocytes can control preantral granulosa cell growth-promoting activity through the production of a soluble factor(s) and the maintenance of functional communications with surrounding granulosa cells.

Defective zonae pellucidae in Zp2-null mice disrupt folliculogenesis, fertility and development

All vertebrate eggs are surrounded by an extracellular matrix. This matrix is known as the zona pellucida in mammals and is critically important for the survival of growing oocytes, successful fertilization and the passage of early embryos through the oviduct. The mouse zona pellucida is composed of three glycoproteins (ZP1, ZP2 and ZP3), each encoded by a single copy gene. Using targeted mutagenesis in embryonic stem cells, Zp2-null mouse lines have been established. ZP1 and ZP3 proteins continue to be synthesized and form a thin zona matrix in early follicles that is not sustained in pre-ovulatory follicles. The abnormal zona matrix does not affect initial folliculogenesis, but there is a significant decrease in the number of antral stage follicles in ovaries isolated from mice lacking a zona pellucida. Few eggs are detected in the oviduct after stimulation with gonadotropins, and no two-cell embryos are recovered after mating Zp2-null females with normal male mice. The structural defect is more severe than that observed in Zp1-null mice, which have decreased fecundity, but not quite as severe as that observed in Zp3-null mice, which never form a visible zona pellucida and are sterile. Although zona-free oocytes matured and fertilized in vitro can progress to the blastocyst stage, the developmental potential of blastocysts derived from either Zp2- or Zp3-null eggs appears compromised and, after transfer to foster mothers, live births have not been observed. Thus, in addition to its role in fertilization and protection of early embryos, these data are consistent with the zona pellucida maintaining interactions between granulosa cells and oocytes during folliculogenesis that are critical to maximize developmental competence of oocytes.

Comparison of recombinant growth differentiation factor-9 and oocyte regulation of KIT ligand messenger ribonucleic acid expression in mouse ovarian follicles

Oocytes secrete factors that regulate the development of the surrounding granulosa cells in ovarian follicles. KIT ligand (KL) mRNA expression in granulosa cells is thought to be regulated by oocytes; however, the factor(s) that mediate this effect are not known. One candidate is the oocyte-specific gene product growth differentiation factor-9 (GDF-9). This study examined the effect of recombinant GDF-9 (rGDF-9) on steady-state KL mRNA expression levels in preantral and mural granulosa cells in vitro. Furthermore, the study compared the effect of rGDF-9 with that of coculture with oocytes at different developmental stages. As determined by RNase protection assay, both KL-1 and KL-2 mRNA levels in preantral and mural granulosa cells were suppressed by 25-250 ng/ml rGDF-9. Fully grown oocytes also suppressed both KL-1 and KL-2 mRNA expression levels. Partly grown oocytes isolated from 7-, 10-, or 12-day-old mice either had no effect on KL mRNA levels or promoted KL-1 mRNA steady-state expression. It is concluded that GDF-9 is likely to mediate the action of fully grown, but not partly grown, oocytes on granulosa cell KL mRNA expression.

Loss of oocytes in Dazl knockout mice results in maintained ovarian steroidogenic function but altered gonadotropin secretion in adult animals

Within 2 days of birth, the mouse ovary is mainly composed of oocytes surrounded by a few pregranulosa cells forming primordial follicles that remain quiescent until they are recruited by intraovarian or other unknown factors to initiate growth of the oocyte and proliferation of the attendant granulosa cells. However, the role of the oocyte in this early development and organization of the follicle is poorly understood. The Dazl knockout (-/-) mouse in which there is total ablation of oocytes in fetal life has allowed us to address this issue. Ovaries from -/- females lack any follicular structure and have no cells positive for either Mullerian inhibiting factor or sulfated glycoprotein-1, indicating a lack of small follicles or corpora lutea. However, by immunocytochemistry, there are cells positive for 3beta-hydroxysteroid dehydrogenase, 17alpha-hydroxylase, and aromatase, indicating the presence of steroidogenically active cells capable of producing estrogen. This was confirmed by the presence of hypertrophied uterine endometrium expressing both estrogen receptor alpha (ER alpha) and ER beta together with normal levels of plasma estradiol. In addition, these steroidogenically active cells contain ER beta, inhibin alpha, and betaB-subunits, and -/- mice have low measurable plasma inhibin A and B levels. The ovarian steroids and inhibins had no significant effect on either plasma or pituitary gonadotropin levels, with significantly (P < 0.01) lower LH and FSH in intact +/+ and +/- females. However, significantly (P < 0.05) increased plasma inhibin B together with significantly (P < 0.05) lower FSH were observed in the +/- females. In conclusion, our data showed that despite oocyte loss in fetal life, the adult ovaries contained steroidogenically active cells capable of producing estradiol and inhibin. Furthermore, in the +/- mice, the enhanced plasma inhibin B implies a role for Dazl protein within the oocyte either from more small follicles or increased inhibin B production from each follicle.

Oocyte-granulosa cell heterologous gap junctions are required for the coordination of nuclear and cytoplasmic meiotic competence

Homologous gap junctions are generally recognized as a means of coordinating cellular behavior under developmental and homeostatic conditions. In the mammalian ovary, heterologous gap junctions between the oocyte and the granulosa cells have been widely implicated in the regulation of meiotic maturation late in oogenesis. However, the role of oocyte-granulosa cell gap junctions at earlier stages of oogenesis is poorly understood. Stage-specific defects in both oocyte and follicle development have been identified in juvenile mice deficient in heterologous oocyte-granulosa cell gap junctions due to targeted deletion of Gja4, the gene encoding connexin-37. Follicle development arrests at the type 4 preantral stage and although oocytes commence growth, oocyte growth ceases at a diameter of 52 microm (74.3% of control size). Analysis of cell cycle and cytoskeletal markers indicates that oocytes arrest in a G(2) state based on uniform decondensed GV chromatin, interphase microtubule arrays, and nonphosphorylated cytoplasmic centrosomes. Functional assays of meiotic competence confirm that oocytes from connexin-37-deficient mice are unable to enter M phase (initiate meiotic maturation) unless treated with the phosphatase inhibitor okadaic acid (OA). Unlike growing oocytes from heterozygous control animals, OA-treated oocytes from connexin-37-deficient mice respond acutely and progress rapidly to the circular bivalent stage of meiosis I and upon removal from OA rapidly revert to an interphase state. In contrast, OA-treated control incompetent oocytes are slow to respond, exhibit a lower proportion of chromosomal bivalent stage oocytes, but remain in and progress into meiotic M phase upon removal from OA. This study demonstrates that heterologous gap-junctional communication is required for the completion of oocyte growth and the acquisition of cytoplasmic meiotic competence.

Development of mouse and rat oocytes in chimeric reaggregated ovaries after interspecific exchange of somatic and germ cell components

The germ cell and somatic cell compartments of newborn rat and mouse ovaries, which contain only primordial stage follicles, were completely exchanged and reaggregated to produce xenogeneic chimeric ovaries. The reaggregated ovaries were grafted beneath the renal capsules of ovariectomized SCID mice to develop for periods up to 21 days. Xenogeneic follicles developed with essentially normal morphological characteristics. Both rat and mouse oocytes with species-specific characteristics grew within follicles that were composed of somatic cells exclusively of the alternative species. Rat oocytes grown in mouse follicles became competent to resume meiosis, and progressed to metaphase II when they were removed from follicles and cultured. In addition, mouse oocytes grown in rat follicles underwent fertilization and preimplantation development in vitro, and developed to term after embryos were transferred to pseudopregnant mouse foster mothers. Therefore, despite an estimated 11 million years of divergent evolution, oocytes and somatic cells of rat and mouse ovaries can be exchanged and can produce functional oocytes. It is concluded that factors involved in oocyte-somatic cell interactions necessary to support oocyte development and appropriate differentiation of the oocyte-associated granulosa cells are conserved between rats and mice. Moreover, although granulosa cells play important roles in oocyte development, the development of species-specific characteristics of oocytes occurs without apparent modification by a xenogeneic follicular environment.

Oocyte-secreted factor(s) determine functional differences between bovine mural granulosa cells and cumulus cells

Cumulus cells and mural granulosa cells (MGC) are phenotypically different and there is now evidence suggesting that the oocyte plays an active role in determining the fate of follicular somatic cells. This study investigates the role of oocyte-secreted factor(s) in the regulation of the growth and differentiation of cumulus and MGC. Bovine cumulus-oocyte complexes (COC) and MGC were cultured with various hormones for 18 h followed by a further 6-h pulse of [(3)H]thymidine as an indicator of follicular cell DNA synthesis. The COC incorporated 11 to 14 times more [(3)H]thymidine than MGC in either the absence or presence of 50 ng/ml insulin-like growth factor (IGF)-I. Purified porcine FSH (450 ng/ml) added together with IGF-I marginally increased (3)H incorporation in MGC relative to IGF-I alone but dramatically decreased incorporation in COC sixfold. Conversely, mean progesterone production in the presence of IGF-I + FSH was 13-fold higher from MGC than from COC, confirming a distinctive phenotype of cumulus cells. However, this phenotype was found to be dependent on the presence of the oocyte, as microsurgical removal of the oocyte (oocytectomy) resulted in an 11-fold decrease in [(3)H]thymidine incorporation in cumulus cells treated with IGF-I, elimination of the inhibitory effect of FSH on IGF-I-stimulated DNA synthesis, and led to a 2-fold increase in progesterone production in medium with IGF-I and FSH. All of these markers were completely restored to COC levels when oocytectomized complexes were cocultured with denuded oocytes (DO) at a concentration of 0.5 oocytes/microl, demonstrating that oocytes secrete a soluble factor(s) that promotes growth and attenuates cumulus cell progesterone secretion. In the presence of IGF-I, [(3)H]thymidine incorporation in MGC increased ninefold above control levels with the addition of DO. The addition of FSH to IGF-I-increased (3)H counts in MGC, however, led to a decrease in counts in MGC + DO as is also observed in COC. Furthermore, progesterone production was halved when DO were added to MGC cultures, most notably in the presence of IGF-I and/or FSH. These results provide further evidence that MGC and cumulus cells have distinctive phenotypes and that the oocyte is responsible for some of the characteristic features of cumulus cells. Bovine oocytes secrete a soluble factor(s) that simultaneously promotes growth and attenuates steroidogenesis in follicular somatic cells.

Metaphase I arrest and spontaneous parthenogenetic activation of strain LTXBO oocytes: chimeric reaggregated ovaries establish primary lesion in oocytes

Oocytes of strain LT mice, and related strains such as LTXBO, exhibit a high incidence of arrest in the progression of meiosis at metaphase I (MI) and in spontaneous parthenogenetic activation. Activation of these oocytes within the ovary leads to the formation of ovarian teratomas. In this study, the role of the oocyte's companion granulosa cells, the cumulus cells, was investigated using fully grown oocytes matured in vitro after isolation from LTXBO mice. Results showed that the role of cumulus cells in MI arrest is dichotomous. Cumulus cells temporarily helped to sustain MI arrest, but they also promoted a delayed progression to metaphase II. Cumulus cells also promoted parthenogenetic activation that occurred in association with the delayed progression to metaphase II. Next, the question of whether the lesion(s) promoting MI arrest and spontaneous activation is due to defects in the somatic cells or is intrinsic to the oocyte was addressed using chimeric reaggregated ovaries. An improved method for completely exchanging the germ cell and the somatic cell compartments of ovaries from newborn mice is described. These chimeric reaggregated ovaries, grafted beneath the renal capsule of SCID mice, allowed the complete development of LTXBO oocytes to occur in association with somatic cells from control (B6SJLF(1)) ovaries and development of control oocytes in association with LTXBO somatic cells. Oocyte growth and follicular development appeared generally normal in reaggregated ovaries. High incidences of MI arrest and spontaneous activation of LTXBO oocytes occurred regardless of the genotype of the somatic cells. Moreover, there was a low incidence of MI arrest and spontaneous activation of control oocytes, even though they underwent complete development and maturation associated with LTXBO somatic cells. It is concluded that the phenotypes of MI arrest and parthenogenetic activation in LTXBO oocytes are defects caused by lesions intrinsic to the oocyte. Nevertheless, the oocyte's companion somatic cells play crucial roles in the expression of these lesions.

Conditions that affect acquisition of developmental competence by mouse oocytes in vitro: FSH, insulin, glucose and ascorbic acid

The simplest unit required for the support of oocyte growth and development is the oocyte-granulosa cell complex. Therefore, a culture system was established that utilizes these complexes to assess mechanisms promoting nuclear, cytoplasmic and genomic maturation in mammalian oocytes. Deletion of serum from the culture, results in increased apoptosis in oocyte-associated granulosa cells (OAGCs), however, addition of ascorbic acid (0.5 mM) significantly reduced the level of apoptosis in the OAGCs, although no improvement of oocyte developmental competence was detected. The effects of reducing glucose during oocyte growth were studied since, under some culture conditions, glucose has deleterious effects on early preimplantation development. Reducing the glucose concentration to 1 mM resulted in the production of oocytes with greatly reduced developmental competence. Deleterious effects of FSH plus insulin during oocyte growth in vitro on preimplantation development are reviewed and discussed in terms of the communication of oocytes with inappropriately developing granulosa cells. Evidence that oocytes promote the appropriate differentiation of OAGCs in intact follicles in vivo is also discussed. It is hypothesized that oocytes control the differentiation of these cells, in order to promote intercellular signaling essential for the acquisition of competence to undergo normal embryogenesis.