Development of naked growing mouse oocytes in vitro

Redox-Induced Apoptosis of Human Oocytes in Resting Follicles In Vitro

BACKGROUND

The age-related depletion of the resting follicle (RF) stock occurs as a result of two processes: atresia and entry in growth phase. Due to difficulties in obtaining sufficient numbers of RFs for study, little is known about the apoptotic mechanisms for RF atresia. The present study was designed to investigate the effects of oxidative stress on the apoptosis of RF oocytes.

METHODS

RFs isolated from human adult ovaries were cultured in vitro, treated with H2O2 at various concentrations (50 microM, 100 microM, 1.0 mM) for 1 hour, and observed for up to 48 hours. The oxidant-induced apoptosis of oocytes were observed by detection of DNA fragments, mitochondria membrane potential (MMP), and cytochrome c release.

RESULTS

Based on nuclear morphology and TUNEL (terminal deoxynucleotidyl transferase-mediated dDTP nick end-labeling), oocyte apoptosis was observed in the RFs treated with 50 microM H2O2 with rates of 35% and 43% at 24 and 48 h after treatment, respectively. But intensive oxidative stress (1 mM H2O2) caused mainly necrosis as measured by quantifying propidium iodide (PI)-positive oocytes (44% within 12 hours), with lower level of apoptosis (17%) being observed at 24 hours after treatment. RFs treated with 100 microM H2O2 showed both apoptosis with the similar rate observed at 50microM and necrosis (13% PI-positive oocytes). Although pre-incubation with cyclosporine A (CsA) could effectively prevent oxidant-induced MMP collapse, but failed to suppress apoptosis of oocytes in RFs.

CONCLUSIONS

Oocytes of RFs in adult ovaries retain their ability to undergo apoptosis under oxidative stress, which is both dose- and time-dependent.

Cells with stem cell characteristics in somatic compartments of the ovary

Antral follicular growth in the ovary is characterized by rapid expansion of granulosa cells accompanied by a rising complexity of their functionality. Within two weeks the number of human granulosa cells increases from less than 500,000 to more than 50 millions cells per follicle and differentiates into groups of cells with a variety of specialized functions involved in steroidogenesis, nursing the oocyte, and forming a functional syncitium. Both the rapid proliferation and different specialized functions of the granulosa cells can only be explained through the involvement of stem cells. However, luteinizing granulosa cells were believed to be terminally differentiated cells. Only recently, stem and progenitor cells with FSH-receptor activity were identified in populations of luteinizing granulosa cells obtained during oocyte collected for assisted reproduction. In the presence of the leukaemia-inhibiting factor (LIF), it was possible to culture a subpopulation of the luteinizing granulosa cells over prolonged time periods. Furthermore, when embedded in a matrix consisting of collagen type I, these cells continued to express the FSH receptor over prolonged time periods, developed globular formations that surrogated as follicle-like structures, providing a promising tool for reproductive biology.

Vitrification at the pre-antral stage transiently alters inner mitochondrial membrane potential but proteome of in vitro grown and matured mouse oocytes appears unaffected

BACKGROUND

Vitrification is a fast and effective method to cryopreserve ovarian tissue, but it might influence mitochondrial activity and affect gene expression to cause persistent alterations in the proteome of oocytes that grow and mature following cryopreservation.

METHODS

In part one of the study, the inner mitochondrial membrane potential (Ψ(mit)) of JC-1 stained oocytes from control and CryoTop vitrified pre-antral follicles was analyzed by confocal microscopy at Day 0, or after culture of follicles for 1 or 12 days. In part two, proteins of in vivo grown germinal vesicle (GV) oocytes were subjected to proteome analysis by SDS polyacrylamide gel electrophoresis, tryptic in-gel digestion of gel slices, and one-dimensional-nano-liquid chromatography of peptides on a multi-dimensional-nano-liquid chromatography system followed by mass spectrometry (LC-MS/MS) and Uniprot Gene Ontology (GO) analysis. In part three, samples containing the protein amount of 40 GV and metaphase II (MII) oocytes, respectively, from control and vitrified pre-antral follicles cultured for 12 or 13 days were subjected to 2D DIGE saturation labeling and separated by isoelectric focusing and SDS gel electrophoresis (2D DIGE), followed by DeCyder(Tm) analysis of spot patterns in three independent biological replicates. Statistical and hierarchical cluster analysis was employed to compare control and vitrified groups.

RESULTS

(i) Mitochondrial inner membrane potential differs significantly between control and vitrified GV oocytes at Day 0 and Day 1, but is similar at Day 12 of culture. (ii) LC-MS/MS analysis of SDS gel fractionated protein lysates of 988 mouse GV oocytes revealed identification of 1123 different proteins with a false discovery rate of <1%. GO analysis assigned 811 proteins to the 'biological process' subset. Thirty-five percent of the proteins corresponded to metabolic processes, about 15% to mitochondrion and transport, each, and close to 8% to oxidation-reduction processes. (iii) From the 2D-saturation DIGE analysis 1891 matched spots for GV-stage and 1718 for MII oocyte proteins were detected and the related protein abundances in vitrified and control oocytes were quantified. None of the spots was significantly altered in intensity, and hierarchical cluster analysis as well as histograms of p and q values suggest that vitrification at the pre-antral stage does not significantly alter the proteome of GV or MII oocytes compared with controls.

CONCLUSIONS

Vitrification appears to be associated with a significant transient increase in Ψ(mit) in oocyte mitochondria, which disappears when oocyte/cumulus cell apposition is restored upon development to the antral stage. The nano-LC-MS/MS analysis of low numbers of oocytes is useful to obtain information on relevant biological signaling pathways based on protein identifications. For quantitative comparisons, saturation 2D DIGE analysis is superior to LC-MS/MS due to its high sensitivity in cases where the biological material is very limited. Genetic background, age of the female, and/or stimulation protocol appear to influence the proteome pattern. However, the quantitative 2D DIGE approach provides evidence that vitrification does not affect the oocyte proteome after recovery from transient loss of cell-cell interactions, in vitro growth and in vitro maturation under tested conditions. Therefore, transient changes in mitochondrial activity by vitrification do not appear causal to persistent alterations in the mitochondrial or overall oocyte proteome.

Regulation of oocyte and cumulus cell interactions by intermedin/adrenomedullin 2

Ovarian folliculogenesis has been studied as a model of hormonal regulation of development and differentiation, cell death, and cell-cell communication. In addition to gonadotropins from the pituitary and follicular paracrine factors, oocyte secreted factors have been shown to play critical roles in the regulation of follicular cell functions. Except for the well characterized BMP family proteins, including GDF9 and BMP15, oocytes are known to secrete oocyte secreted factors that are important for the regulation of cumulus cell survival and the maintenance of tertiary structure of cumulus cell-enclosed oocyte complexes (COCs). Based on genomic screening and studies of COCs cultured in vitro, we showed that intermedin (IMD)/adrenomedullin 2 (ADM2) is a novel oocyte-derived ligand important for the regulation of cell interactions in COCs that functions, in part, by suppressing cumulus cell apoptosis. Consistently, we showed that suppression of IMD/ADM2 signaling in growing rat ovaries in vivo leads to oocyte atresia and aberrant cell cycle progression in follicular cells. Together, our studies indicated that mammalian oocytes deploy a G protein-coupled receptor ligand to coordinate normal interactions of oocytes and cumulus cells and provided a better understanding of how the tertiary structure of a COC is maintained as follicles undergo exponential growth during the late stages of folliculogenesis.

A combination of FSH and dibutyryl cyclic AMP promote growth and acquisition of meiotic competence of oocytes from early porcine antral follicles

Growing porcine oocytes from early antral follicles (1.2-1.5 mm in diameter) do not mature to metaphase II (MII, 4%) under culture conditions which supported maturation (MII, 95%) of fully grown oocytes from large (4-6 mm) antral follicles. We hypothesized that FSH and dbcAMP supported growth and acquisition of meiotic competence. Growing oocytes (113.0 ± 0.4 μm, mean ± SEM) were cultured for 5 d in medium supplemented with 1 mM dbcAMP, 0.01 IU/mL FSH or both; in these media, oocytes reached, 120.5 ± 0.4, 123.5 ± 0.4 and 125.7 ± 0.2 μm, respectively, after 5 d, and then were matured in vitro for 48 h. Oocytes remained enclosed by cumulus cells when cultured with FSH (82%) or both FSH and dbcAMP (80%), but not with dbcAMP alone (0%). Furthermore, oocytes cultured with FSH maintained trans-zonal projections of cumulus cells. Oocytes remained at the GV stage at higher rates when cultured with dbcAMP and FSH (99%), or dbcAMP (97%), than with FSH (64%), or without either (75%). Following in vitro maturation, oocytes reached MII after in vitro growth with dbcAMP (19%), FSH (11%), or both (68%). When oocytes were cultured with both FSH and dbcAMP, activation of Cdc2 and MAP kinases in growing oocytes was similar to fully grown oocytes. In conclusion, growing porcine oocytes grew and acquired meiotic competence in medium supplemented with dbcAMP and FSH; the former maintained oocytes in meiotic arrest, whereas the latter maintained trans-zonal projections of cumulus cells to oocytes during in vitro growth culture.

The biology and technology of follicular oocyte development in vitro

Fully mature oocytes are the rarest cells in the body. A premenopausal woman produces only one during each menstrual cycle and that survives for just a single day. Ovarian productivity is parsimonious in order that the ovulation rate matches the optimal capacity of the uterus for carrying conceptuses to full-term. But, in this new era of assisted reproductive medicine, there are many applications for which spare oocytes are needed (Table 1), and it is desirable to obtain more cells than are routinely available during superstimulated cycles. Since the great majority of ovarian oocytes undergo atresia (> 99.9%), the possibility of tapping the store of immature oocytes before they degenerate and maturing them in vitro is very attractive.

Cell–cell interaction and oocyte growth

In most mammals, oocytes initiate meiosis in late fetal life; by the time of birthe they have already entered the diplotene stage of prophase I of meiosis and becaome arrested thereafter at the dictyate state(Baker, 1972). At this stage they became surrounded by a few nonproliferating flat follicle cells forming a unit called the resting or primordial follicle.

Preservation of fertility in young cancer patients: contribution of transmission electron microscopy

During the last decade, new technologies in reproductive medicine have emerged to preserve the fertility of women whose gonadal function is threatened by premature menopause or gonadotoxic treatments. To offer an individualized approach to these patients, different experimental procedures are under investigation, including oocyte cryopreservation and cryopreservation and transplantation of ovarian tissue in the form of cortical fragments, whole ovary or isolated follicles. This review shows that transmission electron microscopy (TEM), combined with other in-vivo and in-vitro analysis techniques, is a valuable tool in the establishment of new experimental protocols to preserve female fertility. Ultrastructural studies allow in-depth evaluation of the oocyte's unique morpho-functional characteristics, which explain its low cryotolerance, and provide essential information on follicular, stromal and endothelial cell integrity, as well as cellular interactions crucial for normal folliculogenesis. In order to be able to offer appropriate and efficient options in every clinical situation, oocyte in-vitro maturation and ovarian tissue transplantation need to be optimized. Further development of new approaches, such as follicular isolation and whole ovary transplantation, should be encouraged. Fine ultrastructural details highlighted by TEM studies will be useful for the further optimization of these emerging technologies.

Effects of in vitro maturation on gene expression in rhesus monkey oocytes

In vitro oocyte maturation (IVM) holds great promise as a tool for enhancing clinical treatment of infertility, enhancing availability of nonhuman primates for development of disease models, and facilitating endangered species preservation. However, IVM outcomes have remained significantly below the success rates obtained with in vivo matured (VVM) oocytes from humans and nonhuman primates. A cDNA array-based analysis is presented, comparing the transcriptomes of VVM oocytes with IVM oocytes. We observe a small set of just 59 mRNAs that are differentially expressed between the two cell types. These mRNAs are related to cellular homeostasis, cell-cell interactions including growth factor and hormone stimulation and cell adhesion, and other functions such as mRNA stability and translation. Additionally, we observe in IVM oocytes overexpression of PLAGL1 and MEST, two maternally imprinted genes, indicating a possible interruption or loss of correct epigenetic programming. These results indicate that, under certain IVM conditions, oocytes that are molecularly highly similar to VVM oocytes can be obtained; however, the interruption of normal oocyte-somatic cell interactions during the final hours of oocyte maturation may preclude the establishment of full developmental competence.

The Perivitelline Space-Forming Capacity of Mouse Oocytes is Associated with Meiotic Competence

Although mouse oocytes progressively acquire meiotic competence during their growth in the ovaries, only half of full-grown oocytes can accomplish meiosis. Two types of full-grown oocytes have been reported on the basis of their chromatin configuration, the surrounded-nucleolus (SN) type and the non-surrounded-nucleolus (NSN) type. Therefore, full-grown oocytes collected from the ovaries of adult animals comprise a heterogeneous population; some oocytes are meiotically incompetent (NSN-type), and some are competent (SN-type). In the present study, we found that full-grown oocytes could be classified into two groups using the criterion of formation of the perivitelline space (PVS) after culture with 3-isobutyl-1-methylxanthine (IBMX) for 1 h. In oocytes with a PVS, actin-filled processes within zona pellucidae originating from cumulus cells were reduced, while they were rich in oocytes without a PVS, suggesting that a reduction in these processes contributes to PVS formation. PVS formation was highly correlated with meiotic competence and SN-type configuration. The results of this study demonstrate that PVS formation is a useful criterion for easily distinguishing between SN- and NSN-type oocytes, without injury to the cells.

Cross-species hybridizations on a multi-species cDNA microarray to identify evolutionarily conserved genes expressed in oocytes

BACKGROUND

Comparative genomic analysis using cDNA microarray is a new approach and a useful tool to identify important genetic sequences or genes that are conserved throughout evolution. Identification of these conserved sequences will help elucidate important molecular mechanisms or pathways common to many species. For example, the stockpiled transcripts in the oocyte necessary for successful fertilization and early embryonic development still remain relatively unknown. The objective of this study was to identify genes expressed in oocytes and conserved in three evolutionarily distant species.

RESULTS

In this study we report the construction of a multi-species cDNA microarray containing 3,456 transcripts from three distinct oocyte-libraries from bovine, mouse and Xenopus laevis. Following the cross-species hybridizations, data analysis revealed that 1,541 positive hybridization signals were generated by oocytes of all three species, and 268 of these are preferentially expressed in the oocyte. Data reproducibility analyses comparing same-species to cross-species hybridization indicates that cross-species hybridizations are highly reproducible, thus increasing the confidence level in their specificity. A validation by RT-PCR using gene- and species-specific primers confirmed that cross-species hybridization allows the production of specific and reliable data. Finally, a second validation step through gene-specific microarray hybridizations further supported the validity of our cross-species microarray results. Results from these cross-species hybridizations on our multi-species cDNA microarray revealed that SMFN (Small fragment nuclease), Spin (Spindlin), and PRMT1 (Protein arginine methyltransferase 1) are transcripts present in oocytes and conserved in three evolutionarily distant species.

CONCLUSION

Cross-species hybridization using a multi-species cDNA microarray is a powerful tool for the discovery of genes involved in evolutionarily conserved molecular mechanisms. The present study identified conserved genes in the oocytes of three distant species that will help understand the unique role of maternal transcripts in early embryonic development.

Morphological and ultrastructural evaluation of cultured frozen–thawed human fetal ovarian tissue

OBJECTIVE

To investigate a defined culture condition for the culture of frozen-thawed human ovarian tissue.

DESIGN

Prospective laboratory study.

SETTING

Reproductive biology laboratories in university hospitals.

PATIENT(S)

Fetal ovarian tissue from elective termination of pregnancy.

INTERVENTION(S)

Culture of frozen-thawed fetal ovarian tissue for up to 63 days.

MAIN OUTCOME MEASURE(S)

Morphology, morphometry, and survival of follicles in relation to culture times.

RESULT(S)

The proportion of primordial, early primary, and primary follicles in frozen-thawed (day 0) ovarian tissue was 77.5%, 21.7%, and 0.8%, respectively. Pronounced degeneration was found in all cell types, and < or =36% of the follicles had signs of atresia at days 7-14, but this figure improved with culture time to <20% of the total follicular population. After 7-14 and 21-35 days of culture, the relative proportion of the follicles in the different classes remained nearly stable. Morphometric examination of healthy follicles showed a significant increase in both follicle and oocyte diameter compared with control. A few follicles had developed to the early secondary stage. Ultrastructural analysis demonstrated well-preserved morphological integrity of healthy primordial and early primary follicles. Immunohistochemical localization of proliferating cell nuclear antigen was positive in proliferating follicular cells at days 7-14 and 21-35 of culture.

CONCLUSION(S)

The present culture condition leads to good survival and progressive follicular growth and differentiation that is comparable to the physiological pattern of early folliculogenesis.

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.

The ability to develop an activity that transfers histones onto sperm chromatin is acquired with meiotic competence during oocyte growth

Following fertilization, the oocyte remodels the sperm chromatin into the male pronucleus. As a component of this process, during meiotic maturation, oocytes develop an activity that transfers histones onto sperm DNA. To further characterize this activity, we tested whether oocytes at different stages of growth could, upon entry into metaphase of maturation, transfer histones onto sperm DNA, as judged by chromatin morphology and immunocytochemistry. Meiotically competent growing oocytes, which spontaneously enter metaphase upon culture, transferred histones onto sperm chromatin, whereas incompetent oocytes did not, even when treated with okadaic acid to induce germinal vesicle breakdown (GVBD) and chromosome condensation. When incompetent oocytes were cultured until they acquired the ability to undergo GVBD, only a small proportion also developed histone-transfer activity during maturation. However, this proportion significantly increased when the oocytes were cultured as granulosa-oocyte complexes. The failure of histone-transfer activity to develop in incompetent oocytes treated with okadaic acid was not linked to low H1 kinase activity nor rescued by injected histones. Because competent, but not incompetent, oocytes produce natural calcium oscillations, incompetent oocytes were exposed to SrCl2. One-third of treated oocytes produced at least one Ca2+ oscillation and, following insemination, the same proportion transferred histones onto sperm DNA. Histone transfer did not occur in oocytes pretreated with the Ca2+ chelator, BAPTA-AM. These results indicate that the ability to develop histone-transfer activity is acquired by growing oocytes near the time of meiotic competence, that it is separable from this event, and that it may be regulated through a Ca2+-dependent process.

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.

Ontogeny: the evolution of an oocyte

The physiology of oocyte development, maturation, and selection forms the basis for understanding ovarian follicle dynamics, reproductive function and ovulation induction. The number of ovarian follicles with which women are invested from birth and the rate of decline determine their reproductive lifespan. The processes of atresia and apoptosis constantly deplete this finite store. The initiation of follicle growth involves complex chemical signaling and ensures that a cohort of follicles is prepared for recruitment monthly. The effects of gonadotrophins, autocrine signals, and the degree of vascularization further determine dominance and the selection of an oocyte for ovulation.

Gap junctional intercellular communication in the mouse ovarian follicle

A targeted disruption of the gene encoding the gap junction protein connexin37 (Cx37; alpha 4) results in female infertility. Mutant follicles are not observed to develop beyond early antral stages, and there is a lack of both observable mature Graafian follicles and ovulation. The oocytes are unable to acquire meiotic competence. Following oocyte failure, the residual follicular cells do not undergo atresia but rather transdifferentiate into luteal cells, resulting in a mutant ovary populated with numerous, inappropriate corpora lutea. These results indicate that the Cx37-containing gap junctions formed between oocyte and follicular cells permit bidirectional signalling between the two cell types. These junctions are required for oocyte growth and development during preantral stages of the follicle, and for the inhibition of follicle cell luteinization. An additional role for these junctions may be to permit transfer of cytoplasmic signals required to hold oocytes in meiotic arrest. Since the mutant follicles never acquire meiotic competence, this latter role for gap junctional communication cannot be tested in this model.

Mammalian oocyte growth in vitro is stimulated by soluble factor(s) produced by preantral granulosa cells and by Sertoli cells

We have evaluated the possibility that mouse oocyte growth in vitro could be achieved under the influence of soluble compound(s) released by different somatic cell types. For this purpose, zona-free denuded oocytes from 12-day-old mice were cultured on monolayers of NIH-3T3 fibroblasts, which are able to establish gap junctional communications with them, in the presence or absence of media conditioned by preantral granulosa cells or by Sertoli cells, plated at increasing concentrations from 0.3-1 x 10(6) ml-1 cells. After 3 days, no increase in vitellus diameter was recorded from fibroblast-coupled oocytes maintained in culture medium or in the presence of media conditioned by 0.3 x 10(6) ml-1 Sertoli cells. By contrast, increasing proportions of coupled oocytes grew, provided the continuous presence of media conditioned by 0.5 or 1 x 10(5) ml-1 Sertoli cells, or by 0.3, 0.5, and 1 x 10(5) ml-1 preantral granulosa cells. Since the ligand of c-kit, the growth factor KL, promotes the growth in vitro of oocytes cultured in follicles from 8-day-old mice, an antibody against mouse KL was used to evaluate whether in our culture conditions KL might also be responsible for the growth of oocytes from 12-day-old mice. No inhibition of growth was evident in oocytes cultured directly on preantral granulosa or Sertoli-cell monolayers. Furthermore, the growth of fibroblast-coupled oocytes cultured in media conditioned by preantral granulosa cells was not significantly affected by the presence of this antibody during culture. By contrast, a high percentage of oocytes cultured on fibroblasts in the presence of media conditioned by Sertoli cells showed a significant inhibition of growth and no metabolic cooperativity. It was concluded that, besides KL, other bioactive factor(s) released by either preantral granulosa or Sertoli cells can induce a significant stimulation of mouse oocyte growth in vitro.

Mitogen-activated protein (MAP) kinase during the acquisition of meiotic competence by growing oocytes of the mouse

During the growth phase of oogenesis, oocytes acquire the ability to undergo meiotic maturation. Although the molecular basis of this meiotic competence is unknown, specific differences in microtubular organization exist between incompetent and competent mammalian oocytes. Mitogen-activated protein (MAP) kinase has been implicated in microtubular regulation and is present in fully grown competent oocytes of mice, suggesting a possible role for this protein in the acquisition of meiotic competence. We report that the MAP kinase species, p42ERK2 and p44ERK1, were detectable by immunoblotting in incompetent oocytes at the early stages of oocyte growth and throughout subsequent growth and acquisition of competence. In partially competent oocytes, which can enter metaphase but cannot complete the first meiotic division, both p42ERK2 and p44ERK1 became phosphorylated, as judged by retarded electrophoretic mobility, and a morphologically normal spindle was assembled. In incompetent oocytes, which cannot enter metaphase, p42ERK2 and p44ERK1 remained nonphosphorylated. When these oocytes were treated with okadaic acid, an inhibitor of protein phosphatases 1 and 2A, a portion of them entered metaphase and the slow-migrating phosphorylated forms of p42ERK2 and p44ERK1 were observed. These phosphorylated forms appeared more rapidly, relative to the time of entry into metaphase, than during maturation of fully competent oocytes. The remaining incompetent oocytes, which did not enter metaphase during okadaic acid treatment, also did not generate slow-migrating p42ERK2 and p44ERK1. These results suggest that the acquisition of meiotic competence during oocyte growth is not linked to the de novo appearance of p42ERK2 or p44ERK1, that the failure of partially competent oocytes to complete meiosis I reflects a defect acting downstream or independently of MAP kinase phosphorylation, and that the ability of meiotically incompetent oocytes to generate phosphorylated forms of p42ERK2 and p44ERK1 in response to okadaic acid is linked to the ability to enter metaphase.

Endogenous amino acid transport systems and expression of mammalian amino acid transport proteins in Xenopus oocytes

Oocyte amino acid transport has physiological significance to oocytes and practical importance to molecular biologists and transport physiologists. Expression of heterologous mRNA in Xenopus oocytes is currently being used to help clone cDNAs for amino acid transporters and their effectors. A major question to be resolved in many of these studies is whether the injected mRNA codes for a transporter or an activator of an endogenous system. Nevertheless, the cDNAs of several families of amino acid transporters or their activators appear already to have been cloned. One such transporter is the anion exchanger, band 3, which may also transport glycine and taurine under some important physiological conditions such as hypoosmotic stress. Site-directed mutagenesis of band 3 has already shown that an amino acid residue believed to be at or near the active site nevertheless does not appear to influence Cl- transport in Xenopus oocytes expressing the modified band 3 protein. Continuation of such studies along with examination of transport of all possible substrates of band 3 should yield insight into the relationship between the structure and function of this transporter. Each of three other families not only contains amino acid transporters, but also appears to contain members that serve as transporters of neurotransmitters or their metabolites. Because of the distinct structural differences in the preferred substrates of different transporters within some of these families, elucidation of the tertiary and possibly quaternary structural relationships among the members of such families may reveal transport mechanisms. In addition, the grouping of neurotransmitters or their metabolites according to the family to which their transport systems and transporters belong could yield insight into mechanisms of brain development, function and evolution. Another family of transporters for cationic amino acids also serves, at least in one case, as a viral receptor. Hence, these or other transporters also could conceivably function in eggs as receptors for sperm and, more broadly, in cell-cell interactions as well as in amino acid transport. Moreover, a family of apparent amino acid transport activators are homologous to a family of glycosidases, so these activators could also serve to recognize carbohydrate structures on other cells or the extracellular matrix. Some of these activators appear to increase more than one amino acid transport activity in Xenopus oocytes. In other studies, expression of heterologous mRNA in oocytes has led apparently to detection of inhibitors as well as activators of amino acid transport. Some amino acid transport systems also could conceivably contain nucleic acid as well as glycoprotein components.(ABSTRACT TRUNCATED AT 400 WORDS)

Expression of estrogen receptor gene in mouse oocyte and during embryogenesis

Estrogen is required for oocyte maturation and embryonic development in vivo; however, the mechanism involved is not clear. Since the effect of estrogen is mediated through the estrogen receptor (ER), we examined the ontogeny and expression of the ER gene in mouse oocytes and embryos of various gestational stages using the highly sensitive reverse transcriptase-polymerase chain reaction (RT-PCR) technique. Total RNA, extracted from 40 ovulated oocytes, 2-cell embryos, morulae, and blastocysts, was reverse transcribed into cDNA. A pair of primers flanking the 453-bp region encoding the hormone-binding domain of ER was used for 30 cycles of PCR. The identity of the amplified product was confirmed by sizing and Southern blot hybridization. The results indicated that ER gene is expressed in unfertilized oocytes and cumulus-oocyte complexes. The amount of ER mRNA decreases in 2-cell embryos, coincident with degradation of maternal mRNA. No ER transcript can be detected in the morulae or blastocyst stage when the embryonic genome has been activated. Postimplantation embryos do not contain detectable ER mRNA until gestation day 8. The levels of ER mRNA increase from day 10 to day 18 of gestation. These data suggest that estrogen, secreted by granulosa cells, may directly influence oocyte growth and maturation in vivo. Since estrogen is known to stimulate the production of growth factors in mouse uteri, the absence of ER mRNA in periimplantation embryos suggests that the effects of estrogen on early embryogenesis may be indirect, i.e., through estrogen-regulated growth-promoting factors produced by the reproductive tract. In mid- and late-post-implantation embryos which contain ER mRNA, estrogen may affect embryonic development through the receptor-mediated mechanisms.

Centrosome phosphorylation and the developmental expression of meiotic competence in mouse oocytes

Previous studies suggested that the transition from an incompetent to a competent meiotic state during the course of oogenesis in the mouse involved a G2/M-like cell cycle transition (Wickramasinghe et al, 1991. Dev. Biol. 143, 162). The present studies tested the hypothesis that centrosome phosphorylation, an event normally induced by MPF, is required for this developmental transition and the expression of meiotic competence in cultured growing mouse oocytes. Multiple fluorescence labeling techniques were used to evaluate centrosome number, phosphorylation status, and microtubule nucleating capacity in competent and incompetent oocytes. Experimental conditions were established for reversibly altering the phosphorylation status of the centrosomes and the effects of these treatments on meiotic resumption were examined. Phosphorylated centrosomes nucleating short microtubules were observed in competent oocytes, whereas nonphosphorylated centrosomes and interphase microtubule arrays were found in incompetent oocytes. Upon recovery from nocodazole-induced microtubule depolymerization, short microtubules formed from centrosomes in competent oocytes, whereas long microtubules reappear in the cytoplasm of incompetent oocytes. Perturbation of the phosphorylation state of oocytes with activators of protein kinase A or protein kinase C resulted in the formation of long interphase microtubules in competent oocytes while centrosome phosphorylation was maintained. Treatment of competent oocytes with the phosphorylation inhibitor 6-dimethylaminopurine also led to formation of long microtubules, although under these conditions centrosomes were dephosphorylated. When competent oocytes were treated simultaneously with puromycin and the phosphodiesterase inhibitor isobutyl methylxanthine (IBMX) for 6 hr, centrosomes became dephosphorylated; centrosomes were rephosphorylated when competent oocytes were further cultured in IBMX without puromycin. Conditions that induced centrosome dephosphorylation in competent oocytes resulted in the loss of the ability to express meiotic competence in culture, whereas maintenance of centrosome phosphorylation in these oocytes was correlated with the ability to resume meiosis. These results suggest that the G2/M transition that occurs when mouse oocytes progress from an incompetent to a competent state in vivo involves the phosphorylation of centrosomes and that the maintenance of centrosome phosphorylation is required for the in vitro expression of meiotic competence.

Meiotic competence acquisition is associated with the appearance of M-phase characteristics in growing mouse oocytes

To determine whether the acquisition of meiotic competence during the growth phase of oogenesis is associated with the appearance of M-phase characteristics, oocytes obtained from 13- to 30-day-old mice were evaluated by fluorescence microscopy with respect to chromatin and microtubule organization , in vitro maturation ability, and the distribution of M-phase phosphoproteins. Meiotically incompetent oocytes were distinguished from their competent counterparts in displaying elaborate interphase-like arrays of cytoplasmic microtubules and dispersed germinal vesicle chromatin. Meiotically competent oocytes were larger in size, exhibited condensation of chromatin around the nucleolus, and displayed a progressive diminution of cytoplasmic microtubules in conjunction with the appearance of multiple microtubule organizing centers. After 24 hr in culture, medium- to large-sized oocytes exhibiting perinucleolar chromatin condensation resume meiosis whereas smaller meiotically incompetent oocytes retain GVs with diffuse chromatin. Moreover, indirect immunofluorescence studies using the M-phase phosphoprotein specific monoclonal antibody MPM-2 indicate that the appearance of reactive cytoplasmic foci is directly correlated with nuclear changes characteristic of meiotically competent oocytes. Thus, the earliest transition to a meiotically competent state during oocyte growth in the immature mouse ovary is characterized by stage-specific and coordinated modifications of nuclear and cytoplasmic components.

Developmental change in follicular cell-enhanced amino acid uptake into mouse oocytes that depends on intact gap junctions and transport system Gly

Uptake of L-alanine, L-lysine, and choline into both preantral and antral mouse oocytes was enhanced by follicular cells. Follicular cells also enhanced glycine uptake into oocytes at the preantral stage of development, but no effect of these cells was observed at the antral stage. Glycine uptake was predominantly Na+ dependent and inhibited almost completely by 10 mM sarcosine, moderately by proline and its analog pipecolate, and poorly or not at all by other amino acids. By these criteria, glycine transport was mainly via system Gly in follicular cells and the oolemma at both the preantral and antral stages. Moreover, an increase in glycine transport via the oolemma between the preantral and antral stages was more than threefold larger than was the increase in transport of alanine or lysine. This relatively large increase in glycine-specific transport in the oolemma appears to obscure the ability of follicular cells to enhance glycine uptake into antral oocytes. In contrast to other amino acids, leucine uptake into oocytes was not enhanced by follicular cells unless 14 other amino acids were also present at their concentrations in mouse serum. An inhibitor of gap junctional communication, 18-alpha-glycyrrhetinic acid, abolished follicular cell-enhanced uptake of glycine and choline into preantral oocytes. Therefore, the extent to which follicular cells enhance uptake of a particular amino acid into oocytes depends on at least three physiologically important variables. Namely, enhancement may depend on the stage of follicular development, the presence of other amino acids in the environment, and gap junctional communication.

Growth and development of rat oocytes in vitro

Rat oocytes from preantral follicles have been shown to grow and acquire meiotic competence in vitro. Follicles were isolated by enzymatic digestion of ovaries from infant (10- or 11-day-old) Wistar rats. Follicles were cultured for up to 20 days in Minimum Essential Medium (MEM) supplemented with 2 mM hypoxanthine to maintain meiotic arrest. When cultures were begun, oocytes were in midgrowth phase (40-45 microns diameter), and were incapable of undergoing meiotic maturation when placed in hypoxanthine-free MEM. Oocytes grew and acquired meiotic competence during culture for 20 days attaining mean diameters of 62.6 +/- 0.6 microns and 61.1 +/- 0.6 microns in two experiments. Germinal vesicle breakdown (GVB) occurred in 60-70% of oocytes when transferred to MEM without hypoxanthine. Concomitant with oocyte growth and maturation were spontaneous increases in follicular production of progestins, androgens and estrogens. When oocytes grown and matured in this system were inseminated in vitro with epididymal sperm, 36.1% and 25.8% were penetrated by one or more sperm in two experiments. However, fertilization was not generally normal with multiple penetrations and abnormal numbers of pronuclei (one or three) being common, suggesting that in these oocytes cytoplasmic maturation was incomplete or abnormal. In the two experiments, normal fertilization (two pronuclei and one sperm tail in the vitellus) occurred in 34.6% and 47.1% of penetrated oocytes with development of these apparently normal zygotes to two-cell embryos being 66.7% and 62.5%, respectively.

Follicle cell regulation of mammalian oocyte growth

To investigate mechanisms of follicle cell control on mammalian oocyte growth, preantral mouse oocytes free from surrounding follicle cells were individually cocultured with monolayers of different somatic cells competent to form gap junctions, and the rate of in vitro oocyte growth was directly correlated with the level of metabolic coupling on the same cells. The results indicate that 1) at a similar extent of metabolic coupling, mouse oocytes grew on follicle cells but not on 3T3 and Sertoli cell monolayers, and 2) the growth rate of oocytes cultured on follicle cells was dependent on the extent of metabolic coupling. It was concluded that gap-junction-mediated nutrition of ovarian mouse oocytes exerted by somatic cells is necessary but not sufficient to maintain oocyte growth. A specific regulatory role of follicle cells on mammalian oocyte growth is proposed.

Maturation-inhibiting activity in growing mouse oocytes

Growing mouse oocytes incompetent to resume meiosis were fused with fully grown immature mouse oocytes and cultured for 20-24 h. In giant cells that developed, two intact germinal vesicles remained well conserved in all cases. We propose that the cytoplasm of growing oocytes possesses a maturation-inhibiting activity which is able to arrest, after fusion, nuclear maturation in fully grown oocytes competent to mature spontaneously.

Gene expression during oogenesis and oocyte development in mammals

Mouse oocytes progress through early meiotic prophase during fetal life and reach the diplotene stage by birth. During prepubertal and reproductive life, oocytes are continuously selected to grow from the pool of small primordial oocytes. Growing oocytes reach full size in 2 weeks, and full-grown oocytes are present in rapidly enlarging follicles for about 5 days before meiotic maturation and ovulation. RNA synthesis during early meiotic prophase, as estimated from [3H]uridine incorporation followed by autoradiography and from electron microscopic analysis of nuclear components, proceeds at a moderate rate throughout except for a brief period in early pachytene when synthesis is low or absent. RNA synthesis continues in primordial oocytes at a moderate rate. Incorporation studies, electron microscopic analyses, and particularly measurements of ongoing RNA polymerase activity (completion of initiated chains as analysed in tissue sections) indicate a distinctly increased rate of synthesis during oocyte growth over that of primordial oocytes, followed by a decline in full-grown oocytes. During growth, this rate increases severalfold. The absolute rate of synthesis of heterogeneous nuclear RNA (using rRNA as a standard) during mid-growth is very rapid, but nevertheless still much lower than that in typical lampbrush chromosomes. Most of the hnRNA turns over with a half-life of about 20 min, as is typical in somatic cells. Newly synthesized mRNA-like RNA enters the cytoplasm at about one-half the rate of rRNA, and about one-third of the ribosomes and one-fourth of the mRNA appear in polysomes. In full-grown oocytes, the rate of synthesis falls distinctly, but a significant level of synthesis continues until it essentially ceases at breakdown of the germinal vesicle. During meiotic prophase, chromosomes are most compact at pachytene and unfold lateral projections as RNA synthesis increases in late pachytene-early diplotene. In primordial oocytes, the diplotene state of chromosomes is obvious in most mammals, but in rodents the chromosomes are more evenly dispersed and are said to be in a dictyate state, although they are still presumably in a diplotene configuration. The chromosome core, which is present in leptotene through early diplotene stages, apparently disappears in the dictyate stage.(ABSTRACT TRUNCATED AT 400 WORDS)

Early programming of maturation competence in mouse oogenesis

Growing mouse oocytes incompetent to mature were freed of attached granulosa cells at different stages of growth, and cultured in vitro in the presence of fibroblast monolayers and/or their products. In these culture conditions, although growth was arrested, isolated oocytes survived in vitro for several days, and finally resumed meiosis spontaneously, progressing up to metaphase I. The culture time length needed for in vitro acquisition of the capacity to mature was inversely related to the initial oocyte size at the time of isolation from granulosa cells, and closely corresponded to developmental timing of acquisition of such ability in vivo. We conclude that the acquisition of mouse oocyte competence to mature follows a definite time program, which is independent of the presence of granulosa cells and of heterologous cell contacts, at least within the developmental stages studied.

Regulation of mouse oocyte growth: probable nutritional role for intercellular communication between follicle cells and oocytes in oocyte growth

Intercellular communication, as determined by two different assay procedures, was established in vitro between mouse oocytes free of adhering follicle cells and monolayers of either follicle or 3T3 cells. Both of these cell types are known to be able to form homologous gap junctions, and follicle cells naturally form heterologous gap junctions with oocytes in vivo. Monolayers of L cells that are communication deficient did not establish intercellular communication with oocytes as determined by the two different assays for intercellular communication. The diameter of oocytes cultured for 4 days in medium or on monolayers of L cells decreased markedly, 9.7 and 13.1 micron, respectively. In contrast, oocytes cultured for 4 days on follicle cell monolayers increased on the average about 4.7 micron in diameter. Oocytes cultured for 4 days on monolayers of 3T3 cells decreased slightly in diameter, i.e., 2.1 micron. Results from these experiments support a nutritional role for intercellular communication between follicle cells and oocytes in oocyte growth.

Half-lives and relative amounts of stored and polysomal ribosomes and poly(A) + RNA in mouse oocytes

Growing mouse oocytes were labeled in vitro with [3H]uridine and chased for 2 or for 7 days to estimate the relative amounts of RNA appearing in different fractions and to follow their turnover. Oocytes were lysed and thoroughly dispersed in the presence of 1% DOC, and centrifuged on sucrose gradients to separate polysomes from smaller components not engaged in translation. After the short chase, one-third of the labeled ribosomes appeared in EDTA-sensitive polysomes. The proportion of ribosomes in both fractions remained stable during the long chase, demonstrating no net flow from one fraction to the other. When gradient fractions were analyzed by poly(U) Sepharose chromatography, it was found that about 20% of the labeled poly(A)+ RNA appeared in polysomes after the short chase. The half-lives of stored and translated mRNA were followed relative to stable rRNA during the long chase. Stored mRNA was completely stable, but translated mRNA turned over with a t1/2 of about 6 days. Other methods for separating stored from translated components were not successful, including sedimentation of putative large complexes (fibrillar lattices) containing stored components, or chromatography of lysates on oligo(dT)-cellulose. Results presented here combined with our previous results demonstrate that, during meiotic maturation, the percent of labeled stable RNA which is polyadenylated declines from 19 to 10%, suggesting deadenylation or degradation of half of the accumulated maternal mRNA.

Morphology and transcriptional activity of mouse oocyte chromosomes

Lampbrush chromosomes of growing amphibian oocytes carry thousands of lateral loops each of which consists of a chromatin fiber heavily encrusted with nascent ribonucleoprotein fibrils. These are believed to be responsible for the accumulation and maintenance of RNA transcripts found stored in the egg. In the case of mammalian oocytes, lampbrush chromosomes are most likely to occur during the major growth phase and also possibly during pachytene-early diplotene stages of meiosis. We have examined pachytene and early diplotene mouse oocyte chromosomes through the light microscope using sections of plastic-embedded material and air dried spreads stained with either silver nitrate or methyl green pyronin. Our results indicates that the projections radiating from the chromosomal axis are bundles of chromatin fibers rather than single fibers covered with an ribonucleoprotein matrix. These bundles may represent partially unfolded chromomeres. The axis itself could be partially dispersed revealing threads surrounding a fine linear element. -- Little is known about chromosome structure in growing mammalian oocytes, the stage when transcriptional activity is likely to be most rapid. In our preparations chromosomes at this stage appear as partially condensed fuzzy threads of relatively uniform width. In some cases, the fuzzy thread is seen to contain a dense linear core in the center. Thus, during the growth phase, the chromosomes retain a relatively condensed axis, a characteristic of meiotic chromosomes in general. RNA-containing material is found diffusely spread within the nucleus but not specifically associated with the chromosomes. Electron microscopic analysis of spread chromatin from growing oocytes demonstrates that most transcription units possess only one or two nascent ribonucleoprotein fibrils while a few have more. These and other published data indicate that mouse oocytes do not have true lampbrush chromosomes at any stage of their development.

rRNA accumulation and protein synthetic patterns in growing mouse oocytes

The rRNA contents of mouse primordial oocytes, three stages of growing oocytes, full-grown oocytes, and ovulated ova have been measured by hybridization of RNA samples to excess 3H-DNA complementary to rRNA. Since it was known from previous work that rRNA is stable, the results when plotted against days of oocyte growth indicated that rRNA was synthesized at a constant rate over the first 9 days of growth and about 1.5 times faster in the last 5 days. The maximum value of 0.3 ng per oocyte was attained by about 14 days of growth in oocytes 59 micrometers in diameter, well below the maximum diameter of 77 micrometers for full-grown oocytes. The stability of proteins synthesized in mid-growth phase oocytes was measured by labeling for 5 h with 35S-methionine and then following the decline of incorporated label during a 48h chase; 40% of the label decayed with a half-life of 11 h. and 60% was apparently stable. The two-dimensional electrophoretic patterns of labeled proteins synthesized by growing and full-grown oocytes were compared. The principal change was the appearance or great increase in intensity of several spots in full-grown oocytes as compared to growing oocytes. Egg proteins separated on a two-dimensional gel were visualized by silver staining. The cytoskeletal proteins actin, tubulin, and putative intermediate filament protein, as well as putative lactate dehydrogenase, were synthesized in growing and full-grown oocytes, and accumulated to form a significant portion of bulk egg protein.

Ribonucleoside metabolism by mouse oocytes: metabolic cooperativity between the fully grown oocyte and cumulus cells

Ribonucleoside uptake and metabolism were studied in mouse oocytes arrested at dictyate and metaphase II, with or without attached cumulus cells. Uptake of uridine and guanosine decreased after meiotic maturation, whereas that of adenosine and cytidine remained unaltered. Dictyate oocytes free of cumulus cells (denuded) phosphorylated all of the radiolabeled ribonucleosides. Little radiolabeled ribonucleoside-5'-triphosphate was found. This is in contrast to the early mouse embryo, which rapidly converts ribonucleosides to the 5'-triphosphate (Daentl and Epstein, "71; Clegg and Piko, "77). Following meiotic maturation, the major metabolite present when metaphase II oocytes were cultured with cytidine or uridine was the ribonucleoside, whereas the 5'-monophosphate was still the major species for oocyte cultured with guanosine or adenosine. Ribonucleoside uptake was increased and the distribution of phosphorylated metabolites was different for cumulus-enclosed dictyate oocytes compared to their denuded counterparts. This increase in uptake and alteration in metabolite distribution was not observed for cumulus-enclosed metaphase II oocytes. These results are discussed in terms of gap junction-mediated metabolic cooperatively between the cumulus cells and the fully grown oocyte and the possible role this type of cell-to-cell interaction may serve during oocyte growth.