Mouse oocytes regulate hyaluronic acid synthesis and mucification by FSH-stimulated cumulus cells

Mucification (or expansion) of the cumulus cells surrounding the oocyte is thought to depend on the direct action of gonadotropins in stimulating production and deposition of hyaluronic acid (HA) in the extracellular matrix. We now report that the oocyte is essential for this process. Either follicle-stimulating hormone (FSH) at 1 micrograms/ml or dibutyryl cAMP at 2 mM induces mucification of intact cumulus cell-oocyte complexes (COCs) in vitro, but fails to stimulate mucification of isolated cumulus cells. HA synthesis by FSH-stimulated cumulus cells is only approximately 3.5% of the value achieved by FSH-stimulated COCs. Isolated oocytes cultured with or without FSH do not synthesize detectable amounts of HA but induce isolated cumulus cells to increase HA synthesis approximately 13-fold in cocultures with FSH. Medium conditioned by isolated oocytes for 5 hr induces nearly the same level of HA synthesis by cumulus cells under the same culture conditions. FSH also stimulates cumulus cells to increase synthesis of dermatan sulfate proteoglycans (DS-PGs) approximately 3-fold, but this stimulation does not depend upon the presence of oocytes. The results indicate that oocytes produce a soluble factor(s) essential in combination with FSH to stimulate HA, but not DS-PG, synthesis by cumulus cells in vitro and that this factor(s) acts independently or downstream from the FSH-induced formation of cAMP.     

The mechanism of cumulus cell-oocyte uncoupling: Evidence for the participation of both cumulus cells and oocytes

Cumulus cells are metabolically coupled to oocytes via heterologous gap junctions. This coupling terminates near the time of ovulation, and the termination appears to be correlated with the mucification of the cumulus cells lying immediately adjacent to the oocytes. The first objective of this project was to determine whether follicle stimulating hormone (FSH) induction of cumulus cell-oocyte uncoupling could occur independently of FSH-stimulated cumulus mucification (expansion). Intercellular coupling was measured as a percentage of radiolabeled choline (or its metabolites) that was incorporated into the oocyte relative to the total amount of radiolabel incorporated into the entire cumulus cell-oocyte complex. It was found that the complete suppression of FSH-stimulated cumulus expansion with chondroitin sulfate B had no suppressive effect on FSH-stimulated cumulus cell-oocyte uncoupling. This finding showed that FSH-stimulated cumulus expansion was not required for cumulus cell-oocyte uncoupling. Since 17β-estradiol, testosterone, or progesterone could not induce maximal cumulus cell uncoupling, it was concluded that the uncoupling-promoting action of FSH was probably not mediated by steroid hormones. A partial uncoupling of cumulus cells and oocytes was found when spontaneous oocyte maturation had occurred in the absence of FSH. This partial uncoupling was prevented by incubation of cumulus cell-oocyte complexes in concentrations of dibutyryl cyclic adenosine monophosphate (dbcAMP) or 3-isobutyl-1-methyl xanthine (IBMX) (0.25 and 0.10 mM respectively) that suppressed spontaneous oocyte maturation without inducing cumulus expansion. These inhibitors also prevented the maximal induction of uncoupling that would have been provoked by biological grade preparations of either FSH or luteinizing hormone (LH). It was concluded that two factors were required to bring about maximal cumulus cell-oocyte uncoupling: one factor was dependent upon the action of gonadotropins on cumulus cell function, the other factor appeared to be a function of the oocytes, since maximal uncoupling could occur only after the germinal vesicles had broken down.     

Localization and synthesis of hyaluronic acid in the cumulus cells and mural granulosa cells of the preovulatory follicle.

Mural and cumulus granulosa cells synthesize hyaluronic acid (HA) and expand in vitro in response to follicle-stimulating hormone and a soluble factor(s) produced by fully grown oocytes. In the present study we examined HA synthesis and extracellular matrix organization by the two cell populations in vivo during the preovulatory period. After injection of human chorionic gonadotropin into pregnant mares' serum gonadotropin-primed animals, a progressive increase in HA synthesis was observed by the cumulus cell-oocyte complex (COC), and by the mural granulosa cells adjacent to the antrum (antral granulosa cells). The outermost layers of mural granulosa cells (peripheral granulosa cells) did not synthesize HA. Net HA synthesis was approximately 4 pg/cell for COCs isolated after full expansion induced either in vivo or in vitro, whereas the total HA content and cell number in the ovulated COC (approximately 11 ng HA and approximately 3000 cells per COC) were about threefold higher than for COCs expanded in vitro (approximately 4 ng HA and approximately 1000 cells per COC). The increased cell content of ovulated COCs appears to be primarily the result of inclusion of proximal mural granulosa cells which synthesize HA in response to the oocyte factor(s) and become incorporated in the expanded COC extracellular matrix mass. Media conditioned by oocytes enclosed in the cumulus cell mass (intact COCs) contained only 10-20% of the HA-stimulatory activity of media conditioned by an equal number of isolated oocytes when tested on mural granulosa cell cultures. Further, HA-stimulatory activity of media conditioned by isolated oocytes was dramatically reduced (approximately 70%) by preincubation for 5 hr with cumulus cells compared to preincubation in the absence of cells. The results suggest that differences in HA synthesis between subregions of membrana granulosa depend on a diffusion gradient of the oocyte factor(s).     

Developmental pattern of the secretion of cumulus expansion-enabling factor by mouse oocytes and the role of oocytes in promoting granulosa cell differentiation

The expansion, or mucification, of the mouse cumulus oophorus in vitro requires the presence of an enabling factor secreted by the oocyte as well as stimulation with follicle-stimulating hormone (FSH). This study focuses on (1) the ability of mouse oocytes to secrete the enabling factor at various times during oocyte growth and maturation, (2) the temporal relationships between the development of the capacity of the oocyte to undergo germinal vesicle breakdown, the ability of the oocyte to secrete cumulus expansion-enabling factor, and the capacity of the cumulus oophorus to undergo expansion, and (3) the role of the oocyte in the differentiation of granulosa cells as functional cumulus cells. Growing, meiotically incompetent oocytes did not produce detectable amounts of cumulus expansion-enabling factor, but fully grown meiosis-arrested oocytes, maturing oocytes, and metaphase II oocytes did. Detectable quantities of enabling factor were produced by zygotes, but not by two-cell stage to morula embryos. The ability of oocytes to secrete cumulus expansion enabling factor and the capacity of cumulus cells to respond to FSH and the enabling factor are temporally correlated with the acquisition of oocyte competence to undergo germinal vesicle breakdown. Mural granulosa cells of antral follicles do not expand in response to FSH even in the presence of cumulus expansion-enabling factor, showing that mural granulosa cells and cumulus cells are functionally distinct cell types. The perioocytic granulosa cells of preantral follicles isolated from 12-day-old mice differentiate into functional cumulus cells during a 7-day period in culture. Oocytectomized granulosa cell complexes grown in medium conditioned by either growing or fully grown oocytes were comparable in size to intact complexes and maintained their 3-dimensional integrity to a greater degree than oocytectomized complexes grown in unconditioned medium. After 7 days, the oocytectomized complexes were stimulated with FSH in the presence of enabling factor, but no expansion was observed whether or not the oocytectomized complexes grew in the presence of oocyte-conditioned medium. These results suggest that a factor(s) secreted by the oocyte affects granulosa cell proliferation and the structural organization of the follicle, but continual close association with the oocyte appears necessary for the differentiation of granulosa cells into functional cumulus cells, insofar as they are capable of undergoing expansion.     

Paracrine actions of growth differentiation factor-9 in the mammalian ovary.

Although the transforming growth factor-beta (TGF-beta) superfamily is the largest family of secreted growth factors, surprisingly few downstream target genes in their signaling pathways have been identified. Likewise, the identities of oocyte-derived secreted factors, which regulate important oocyte-somatic cell interactions, remain largely unknown. For example, oocytes are known to secrete paracrine growth factor(s) which are necessary for cumulus expansion, induction of hyaluronic acid synthesis, and suppression of LH receptor (LHR) mRNA synthesis. Our previous studies demonstrated that absence of the TGF-beta family member, growth differentiation factor-9 (GDF-9), blocks ovarian folliculogenesis at the primary follicle stage leading to infertility. In the present study, we demonstrate that mouse GDF-9 protein is expressed in all oocytes beginning at the type 3a follicle stage including antral follicles. To explore the biological functions of GDF-9 in the later stages of folliculogenesis and cumulus expansion, we produced mature, glycosylated, recombinant mouse GDF-9 using a Chinese hamster ovary cell expression system. A granulosa cell culture system was established to determine the role of GDF-9 in the regulation of several key ovarian gene products using semiquantitative RT-PCR. We find that recombinant GDF-9 induces hyaluronan synthase 2 (HAS2), cyclooxygenase 2 (COX-2), and steroidogenic acute regulator protein (StAR) mRNA synthesis but suppresses urokinase plasminogen activator (uPA) and LHR mRNA synthesis. Consistent with the induction of StAR mRNA by GDF-9, recombinant GDF-9 increases granulosa cell progesterone synthesis in the absence of FSH. Since induction of HAS2 and suppression of the protease uPA in cumulus cells are key events in the production of the hyaluronic acid-rich extracellular matrix which is produced during cumulus expansion, we determined whether GDF-9 could mimic this process. Using oocytectomized cumulus cell-oocyte complexes, we show that recombinant GDF-9 induces cumulus expansion in vitro. These studies demonstrate that GDF-9 can bind to receptors on granulosa cells to regulate the expression of a number of gene products. Thus, in addition to playing a critical function as a growth and differentiation factor during early folliculogenesis, GDF-9 functions as an oocyte-secreted paracrine factor to regulate several key granulosa cell enzymes involved in cumulus expansion and maintenance of an optimal oocyte microenvironment, processes which are essential for normal ovulation, fertilization, and female reproduction.     

Oocytes Preserve the Ability of Mouse Cumulus Cells in Culture to Synthesize Hyaluronic Acid and Dermatan Sulfate

A soluble factor(s) produced by fully grown oocytes is essential, together with follicle stimulating hormone (FSH), to stimulate in vitro hyaluronic acid (HA) synthesis by mouse cumulus cells (CCs). The stability of the response to this stimulus by CCs in culture was investigated. The data showed that preculture for 8 hr in basal medium reduced to approximately 30% the ability of CCs to synthesize HA in response to FSH or dibutyryl cyclic AMP (Bt₂cAMP) and soluble oocyte factor(s). However, if CCs were precultured for the same period of time as intact cumulus cell-oocyte complexes, or in the presence of fully grown oocytes, or in medium conditioned by fully grown oocytes, their ability to synthesize HA was 75-95% preserved. In vitro stimulation of dermatan sulfate (DS) synthesis by CCs does not require oocyte factors and is induced by FSH or Bt₂cAMP treatment alone. However, the preservation of such activity, like that of HA synthesis, depended on the presence of a soluble oocyte factor(s) during preculture. The presence of isolated oocytes or of oocyte-conditioned medium also prevented the spreading of CCs in culture. However, inhibiting CC spreading by culture on agar-coated plates or in serum-free medium did not preserve their HA or DS synthetic activity, thus suggesting that the two oocyte actions on CCs are independent. Growing oocytes were unable both to induce HA synthesis in freshly isolated CCs stimulated with FSH and to preserve the ability to synthesize HA and DS in 8-hr precultured CCs. The results suggest that the stability of the differentiated state of mouse CCs in vitro depends upon continued exposure to a soluble factor(s) produced by fully grown oocytes.     

Secretion of cumulus expansion-enabling factor (CEEF) in porcine follicles

The objective of this study was to find out whether porcine cumulus and mural granulosa cells can secrete cumulus expansion-enabling factor (CEEF). Culture drops of M-199 medium were conditioned with denuded porcine oocytes (1 oocyte/μl), cumulus cells from oocytectomized complexes (1 OOX/μl), pieces of mural granulosa isolated from preantral to preovulatory follicles (1000 cells/μl), or oviductal cells (1000 cells/μl) for 24 hr. The production of CEEF was assessed by the addition of mouse OOX and follicle-stimulating hormone (FSH) (1 μg/ml) to microdrops of the conditioned medium. After 16–18 hr, expansion of the mouse OOX was scored on a scale of 0 to 4 by morphologic criteria. Mouse OOX did not expand in nonconditioned FSH-supplemented medium. Immature porcine oocytes produced +3 to +4 expansion of the mouse OOX. Granulosa cells isolated from preantral and early antral follicles and cumulus cells isolated from all stages of follicle development constitutively secreted CEEF under in vitro conditions. Mural granulosa cells of small, medium, and preovulatory (PMSG) follicles also secreted CEEF in vitro; however, FSH or leutenizing hormone (LH) stimulation was essential for this secretion. Hormonally induced secretion of CEEF was accompanied by expansion of the mural granulosa itself. Granulosa cells isolated from follicles of gilts 20 hr after PMSG and human chorionic gonadotropin (hCG) administration did not produce CEEF and did not expand in response to FSH and LH in vitro. CEEF activity also was found in the follicular fluid of small antral follicles, was reduced in medium follicles, and was not detectable in PMSG-stimulated follicles. However, CEEF activity was reestablished in the follicular fluid of preovulatory follicles by hCG injection, conceivably due to increased production of CEEF by cumulus cells. We conclude that (1) porcine cumulus and mural granulosa cells are capable of CEEF production in vitro and (2) autocrine secretion of CEEF by cumulus cells is involved in regulation of porcine cumulus expansion both in vitro and in vivo. Mol. Reprod. Dev. 49:141–149, 1998. © 1998 Wiley-Liss, Inc.     

Activation of cumulus cell SMAD2/3 and epidermal growth factor receptor pathways are involved in porcine oocyte-cumulus cell expansion and steroidogenesis

Several lines of evidence suggest that in mice the activation of SMAD2/3 signaling by oocyte secreted factors, together with epidermal growth factor receptor (EGFR) activation, is essential to induce cumulus expansion. Here we show that inhibition of EGFR kinase in follicle stimulating hormone (FSH)-stimulated porcine oocyte-cumulus cell complex (OCCs) strongly decreases hyaluronan (HA) synthesis and its retention in the matrix, as well as progesterone synthesis. Although porcine cumulus cells undergo expansion independently of oocytes, we use biochemical and gene expression analyses to show that they do require activation of SMAD2/3 for optimal stimulation of HA synthesis and proteins involved in the organization of this polymer in the expanded matrix. Furthermore, FSH-induced progesterone synthesis by porcine cumulus cells was increased by blocking SMAD2/3 activation. In conclusion, these results support the hypothesis that an FSH-EGF autocrine loop is active in porcine OCCs, and provide the first evidence that the SMAD2/3 signaling pathway is induced by paracrine/autocrine factors in porcine cumulus cells and is involved in the control of both cumulus expansion and steroidogenesis.     

FSH-induced expansion of the mouse cumulus oophorus in vitro is dependent upon a specific factor(s) secreted by the oocyte

Although it has been shown that granulosa cells regulate the growth and meiotic maturation of mammalian oocytes, there is little evidence of a role for the oocyte in the differentiation or function of granulosa cells. To test the hypothesis that the oocyte participates in the regulation of granulosa cell function, oocytes were removed from isolated oocyte-cumulus cell complexes by a microsurgical procedure and oocytectomized complexes were tested for their ability to undergo expansion in response to follicle-stimulating hormone (FSH). FSH increased the levels of intracellular cAMP, the activity of the hyaluronic acid-synthesizing enzyme system, and induced cumulus expansion in intact complexes. In contrast, FSH did not induce increased hyaluronic acid-synthesizing enzyme activity or cumulus expansion in oocytectomized complexes. Therefore, the participation of the oocyte is necessary for the cumulus cells to synthesize hyaluronic acid and undergo cumulus expansion in vitro in response to stimulation with FSH. FSH induced the elevation of intracellular cAMP to the same extent in both intact and oocytectomized complexes and the cAMP analog 8-bromo cyclic adenosine monophosphate (8Br-cAMP) did not stimulate expansion in oocytectomized complexes. Therefore, the influence of the oocyte on cumulus expansion occurs downstream from the elevation of cAMP levels in the cumulus cells. Epidermal growth factor (EGF), a potent stimulator of cumulus expansion in intact complexes, which probably acts by a mechanism at least initially different from FSH, failed to stimulate cumulus expansion after oocytectomy. Next, oocytectomized complexes were either cocultured with germinal vesicle stage denuded oocytes or cultured in medium conditioned by denuded oocytes. In both cases, FSH or EGF stimulated expansion by oocytectomized complexes. The degree of expansion was directly correlated to the number of oocytes used to condition the medium. Contact between the oocyte and the cumulus cells is not necessary for cumulus expansion. Rather, a factor(s) secreted by the oocyte is necessary for the cumulus cells to undergo expansion in response to either FSH or EGF. FSH did not induce expansion of oocytectomized complexes in media conditioned by various somatic cells such as granulosa cells, fibroblasts, and Sertoli cells; by a mixed population of male germ cells; or by spermatozoa. This suggests that the expansion enabling activity is specific to the oocyte. These results demonstrate that the oocyte participates in the regulation of cumulus cell function.     

Oocytectomy does not influence synthesis of hyaluronic acid by pig cumulus cells: retention of hyaluronic acid after insulin-like growth factor-I treatment in serum-free medium.

Mouse oocytes secrete a factor that enables cumulus cells to undergo expansion in response to FSH (1 microg/ml), whereas expansion of the porcine cumulus oophorus has been shown to be independent of the oocyte. The aim of this study was to assess FSH-induced synthesis of hyaluronic acid (HA) by porcine cumulus cells before and after oocytectomy. In addition, we studied the effect of insulin-like growth factor-I (IGF-I) on the ability of cumulus cells to synthesize and retain HA in response to FSH in serum-free medium. Porcine oocyte-cumulus complexes and complexes from which the oocytes had been removed by oocytectomy were cultured for 24 h in the presence of 2.5 microCi of D-[6-(3)H]glucosamine hydrochloride, fetal calf serum (FCS, 5%), and FSH. After 24 h, incorporation of [(3)H]glucosamine into HA was measured either in complexes alone (retained HA) or in medium plus complexes (total HA). Specificity of incorporation of radioactivity into HA was confirmed by the sensitivity to highly specific Streptomyces hyaluronidase. Our results suggest that 1) the synthesis of HA by pig cumulus cells in vitro is stimulated by FSH and that oocytectomy does not change this synthesis; 2) oocytes do not influence retention of HA within the complex; 3) FSH-induced synthesis of HA by cumulus cells is decreased in medium with polyvinylpyrrolidone (PVP)-supplemented (total and retained HA) compared to FCS-supplemented medium; 4) IGF-I enabled cumulus cells to synthesize HA in response to FSH in PVP-supplemented medium in a manner similar to that observed when serum is present in the medium.     

Local roles of TGF-beta superfamily members in the control of ovarian follicle development

Members of the transforming growth factor-beta (TGF-beta) superfamily have wide-ranging influences on many tissue and organ systems including the ovary. Two recently discovered TGF-beta superfamily members, growth/differentiation factor-9 (GDF-9) and bone morphogenetic protein-15 (BMP-15; also designated as GDF-9B) are expressed in an oocyte-specific manner from a very early stage and play a key role in promoting follicle growth beyond the primary stage. Follicle growth to the small antral stage does not require gonadotrophins but appears to be driven by local autocrine/paracrine signals from both somatic cell types (granulosa and theca) and from the oocyte. TGF-beta superfamily members expressed by follicular cells and implicated in this phase of follicle development include TGF-beta, activin, GDF-9/9B and several BMPs. Acquisition of follicle-stimulating hormone (FSH) responsiveness is a pre-requisite for growth beyond the small antral stage and evidence indicates an autocrine role for granulosa-derived activin in promoting granulosa cell proliferation, FSH receptor expression and aromatase activity. Indeed, some of the effects of FSH on granulosa cells may be mediated by endogenous activin. At the same time, activin may act on theca cells to attenuate luteinizing hormone (LH)-dependent androgen production in small to medium-size antral follicles. Dominant follicle selection appears to depend on differential FSH sensitivity amongst a growing cohort of small antral follicles. Activin may contribute to this selection process by sensitizing those follicles with the highest "activin tone" to FSH. Production of inhibin, like oestradiol, increases in selected dominant follicles, in an FSH- and insulin-like growth factor-dependent manner and may exert a paracrine action on theca cells to upregulate LH-induced secretion of androgen, an essential requirement for further oestradiol secretion by the pre-ovulatory follicle. Like activin, BMP-4 and -7 (mostly from theca), and BMP-6 (mostly from oocyte), can enhance oestradiol and inhibin secretion by bovine granulosa cells while suppressing progesterone secretion; this suggests a functional role in delaying follicle luteinization and/or atresia. Follistatin, on the other hand, may favor luteinization and/or atresia by bio-neutralizing intrafollicular activin and BMPs. Activin receptors are expressed by the oocyte and activin may have a further intrafollicular role in the terminal stages of follicle differentiation to promote oocyte maturation and developmental competence. In a reciprocal manner, oocyte-derived GDF-9/9B may act on the surrounding cumulus granulosa cells to attenuate oestradiol output and promote progesterone and hyaluronic acid production, mucification and cumulus expansion.     

Growth differentiation factor-9 stimulates proliferation but suppresses the follicle-stimulating hormone-induced differentiation of cultured granulosa cells from small antral and preovulatory rat follicles

In addition to pituitary gonadotropins and paracrine factors, ovarian follicle development is also modulated by oocyte factors capable of stimulating granulosa cell proliferation but suppressing their differentiation. The nature of these oocyte factors is unclear. Because growth differentiation factor-9 (GDF-9) enhanced preantral follicle growth and was detected in the oocytes of early antral and preovulatory follicles, we hypothesized that this oocyte hormone could regulate the proliferation and differentiation of granulosa cells from these advanced follicles. Treatment with recombinant GDF-9, but not FSH, stimulated thymidine incorporation into cultured granulosa cells from both early antral and preovulatory follicles, accompanied by increases in granulosa cell number. Although GDF-9 treatment alone stimulated basal steroidogenesis in granulosa cells, cotreatment with GDF-9 suppressed FSH-stimulated progesterone and estradiol production. In addition, GDF-9 cotreatment attentuated FSH-induced LH receptor formation. The inhibitory effects of GDF-9 on FSH-induced granulosa cell differentiation were accompanied by decreases in the FSH-induced cAMP production. These data suggested that GDF-9 is a proliferation factor for granulosa cells from early antral and preovulatory follicles but suppresses FSH-induced differentiation of the same cells. Thus, oocyte-derived GDF-9 could account, at least partially, for the oocyte factor(s) previously reported to control cumulus and granulosa cell differentiation.     

Oocyte control of metabolic cooperativity between oocytes and companion granulosa cells: energy metabolism

Intercellular communication between oocytes and granulosa cells is essential for normal follicular differentiation and oocyte development. Subtraction hybridization was used to identify genes more highly expressed in cumulus cells than in mural granulosa cells of mouse antral follicles. This screen identified six genes involved in glycolysis: Eno1, Pkm2, Tpi, Aldoa, Ldh1, and Pfkp. When oocytes were microsurgically removed from cumulus cell-oocyte complexes, the isolated cumulus cells exhibited decreased expression levels of genes encoding glycolytic enzymes, glycolysis and activity of the tricarboxylic acid (TCA) cycle. These decreases were prevented by culturing the cumulus cells with paracrine factors secreted by fully grown oocytes. Paracrine factors from fully grown oocytes exhibited greater ability than those from growing oocytes to promote expression of genes encoding glycolytic enzymes and glycolysis in the granulosa cells of preantral follicles. However, neither fully grown nor growing oocytes secreted paracrine factors affecting activity of the TCA cycle. These results indicate that oocytes regulate glycolysis and the TCA cycle in granulosa cells in a manner specific to the population of granulosa cells and to the stage of growth and development of the oocyte. Oocytes control glycolysis in granulosa cells by regulating expression levels of genes encoding glycolytic enzymes. Therefore, mouse oocytes control the intercellular metabolic cooperativity between cumulus cells and oocytes needed for energy production by granulosa cells and required for oocyte and follicular development.     

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.     

Mouse oocytes promote proliferation of granulosa cells from preantral and antral follicles in vitro.

Evidence is now emerging that the oocyte plays a role in the development and function of granulosa cells. This study focuses on the role of the oocyte in the proliferation of (1) undifferentiated granulosa cells from preantral follicles and (2) more differentiated mural granulosa cells and cumulus granulosa cells from antral follicles. Preantral follicles were isolated from 12-day-old mice, and mural granulosa cells and oocyte-cumulus complexes were obtained from gonadotropin-primed 22-day-old mice. Cell proliferation was quantified by autoradiographic determination of the 3H-thymidine labeling index. To determine the role of the oocyte in granulosa cell proliferation, oocyte-cumulus cell complexes and preantral follicles were oocytectomized (OOX), oocytectomy being a microsurgical procedure that removes the oocyte while retaining the three-dimensional structure of the complex or follicle. Mural granulosa cells as well as intact and OOX complexes and follicles were cultured with or without FSH in unconditioned medium or oocyte-conditioned medium (1 oocyte/microliter of medium). Preantral follicles were cultured for 4 days, after which 3H-thymidine was added to each group for a further 24 h. Mural granulosa cells were cultured as monolayers for an equilibration period of 24 h and then treated for a 48-h period, with 3H-thymidine added for the last 24 h. Oocyte-cumulus cell complexes were incubated for 4 h and then 3H-thymidine was added to each group for an additional 3-h period. FSH and/or oocyte-conditioned medium caused an increase in the labeling index of mural granulosa cells in monolayer culture; however, no differences were found among treatment groups.(ABSTRACT TRUNCATED AT 250 WORDS)     

Oocyte regulation of anti-Müllerian hormone expression in granulosa cells during ovarian follicle development in mice

In the ovarian follicle, anti-Müllerian hormone (Amh) mRNA is expressed in granulosa cells from primary to preovulatory stages but becomes restricted to cumulus cells following antrum formation. Anti-Müllerian hormone regulates follicle development by attenuating the effects of follicle stimulating hormone on follicle growth and inhibiting primordial follicle recruitment. To examine the role of the oocyte in regulating granulosa cell Amh expression in the mouse, isolated oocytes and granulosa cells were co-cultured and Amh mRNA levels were analysed by real-time RT-PCR. Expression in freshly isolated granulosa cells increased with preantral follicle development but was low in the cumulus and virtually absent in the mural granulosa cells of preovulatory follicles. When preantral granulosa cells were co-cultured with oocytes from early preantral, late preantral or preovulatory follicles, and when oocytes from preovulatory follicles were co-cultured with cumulus granulosa cells, Amh expression was increased at least 2-fold compared with granulosa cells cultured alone. With oocytes from preantral but not preovulatory follicles, this was a short-range effect only observed with granulosa cells in close apposition to oocytes. We conclude that stage-specific oocyte regulation of Amh expression may play a role in intra- and inter-follicular coordination of follicle development.     

Covalent transfer of heavy chains of inter-alpha-trypsin inhibitor family proteins to hyaluronan in in vivo and in vitro expanded porcine oocyte-cumulus complexes

Previous studies have shown that the heavy chains (HCs) of serum-derived inter-alpha-trypsin inhibitor (IalphaI) molecules become covalently linked to hyaluronan (HA) during in vivo mouse cumulus expansion and significantly contribute to cumulus matrix organization. Experiments with mice suggest that the incorporation of such proteins in cumulus matrix appears to be rather complex, involving LH/hCG-induced changes in blood-follicle barrier and functional cooperation between cumulus cells, granulosa cells, and oocyte within the follicle. We demonstrate here that HC-HA covalent complexes are formed during in vivo porcine cumulus expansion as well. Western blot analysis with IalphaI antibody revealed that follicular fluids from medium-sized follicles and those from large follicles unstimulated with hCG contain high levels of all forms of IalphaI family members present in pig serum. The same amount of HCs were covalently transferred from IalphaI molecules to HA when pig oocyte-cumulus complexes (OCCs) were stimulated in vitro with FSH in the presence of pig serum or follicular fluid from unstimulated or hCG-stimulated follicles. In addition, HC-HA coupling activity was stimulated in cumulus cells by FSH treatment also in the absence of oocyte. Collectively, these results indicate that IalphaI molecules can freely cross the blood follicle barrier and that follicular fluid collected at any stage of folliculogenesis can be successfully used instead of serum for improving OCC maturation. Finally, pig cumulus cells show an autonomous ability to promote the incorporation of IalphaI HCs in the cumulus matrix.     

Role of the epidermal growth factor network in ovarian follicles

The LH surge causes major remodeling of the ovarian follicle in preparation for the ovulatory process. These changes include reprogramming of granulosa cells to differentiate into luteal cells, changes in cumulus cell secretory properties, and oocyte maturation. This review summarizes published data in support of the concept that LH stimulation of ovarian follicles involves activation of a local epidermal growth factor (EGF) network. A model describing this property of LH signaling and its branching to other signaling modules is discussed. According to this model, LH activation of mural granulosa cells stimulates cAMP signaling, which, in turn, induces the expression of the EGF-like growth factors epiregulin, amphiregulin, and betacellulin. These growth factors function by activating EGF receptors in either an autocrine/juxtacrine fashion within the mural layer, or they diffuse to act on cumulus cells. Activation of EGF receptor signaling in cumulus cells, together with cAMP priming, triggers oocyte nuclear maturation and acquisition of developmental competence as well as cumulus expansion. This model has important implications for ovarian physiology and for the development of new strategies for the pharmacological control of ovulation and for gamete maturation in vitro.