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Department of Obstetrics and Gynecology, Division of Reproductive Endocrinology, Washington University in St. Louis, St. Louis, Missouri 63110
Address all correspondence and requests for reprints to: Dr. Kelle Moley, Department of Obstetrics and Gynecology, Division of Reproductive Endocrinology, Washington University in St. Louis, 4911 Barnes Plaza Road, Box 8064, St. Louis, Missouri 63110-1094. E-mail: moleyk{at}wustl.edu.
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Introduction |
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In the developing oocyte, granulosa cells surround the oocyte, support its growth, and provide hormonal supplementation. They form a multilayered structure around the oocyte. Paracrine signaling and gap junctional communication occur between the granulosa cells and the oocyte. The presence of normal granulosa cell communication and development are critical for differentiation and oocyte growth to occur (10). Because of the importance of this relationship, we also chose to examine whether the diabetic state influenced granulosa cell intercellular communication. It is possible that the maturational delay suspected in oocytes from diabetic mice may be a result of poor paracrine communication between these two compartments because of poor intercellular talk among granulosa cells. Connexin-43 is a key gap junction protein expressed in granulosa cells that is necessary for intercellular communication between granulosa cells and normal folliculogenesis (10). A lack of connexin-43 expression, however, results in abnormal oocytes as well as poor folliculogenesis. Furthermore, diabetic human retinal pericytes display lower levels of connexin-43 when compared with control pericytes (11). Finally, recent studies have shown that connexin-43 expression in granulosa cells is inversely related to apoptotic cell death in the avian follicle (12). Because of these links between diabetes, decreased connexin-43 expression, and apoptosis, one of our objectives in this study was to see whether connexin-43 levels were altered in diabetic cumulus-enclosed oocytes (CEOs).
It is well established that the vertebrate ovarian follicles can undergo apoptosis, via a process called atresia, and this apoptosis is initiated within the granulosa cell layer. In previous studies, we have established that hyperglycemia induces apoptosis in the murine blastocyst. One objective of this study is to examine whether an external stress such as hyperglycemia could cause higher levels of apoptosis in the granulosa cells of developing follicles that might influence oocyte development, maturation (13), and perhaps communication between the granulosa cells and the oocyte. Poor outcomes have previously been associated with granulosa cell apoptosis. Granulosa cell apoptosis has been found to be accelerated in human patients with unexplained infertility (14). Increased apoptosis of surrounding cumulus cells has been correlated with oocyte maturational delay and poor pregnancy outcomes (15, 16).
Two major pathways are involved with apoptosis initiation in vertebrates. They include the intrinsic apoptotic pathway whereby mitochondrial disruption occurs as a result of death-promoting members of the Bcl-2 family, leading to the release of factors that promote caspase-9 activation and downstream apoptosis. The extrinsic apoptotic pathway involves activation of the cell-death receptors by ligands that belong to the TNF superfamily and downstream activation of caspase-8. The two pathways appear to be connected by the protein Bid, which contains Bcl-2 homology domains. Caspase-8, which is usually activated in the extrinsic pathway, can trigger mitochondrial cytochrome c release and thus cause downstream activation of the intrinsic pathway (17). Previously, our laboratory had determined that the intrinsic pathway was active during hyperglycemia-induced apoptosis in the preimplantation blastocyst-stage embryo via the overexpression of Bax, a death-promoting member of the Bcl-2 family (3, 18). It was also discovered that apoptosis was increased in the blastocysts obtained in vivo vs. those cultured in vitro in high-glucose conditions, suggesting that this apoptosis may be occurring under the combined influence of both the extrinsic and intrinsic pathways. In this study, the effects of hyperglycemia on activation of the extrinsic pathway, specifically the TNF-related apoptosis-inducing ligand (TRAIL) (TNF superfamily) in the ovarian follicle, are examined. TRAIL mediates apoptosis through stimulation of DR5 or murine KILLER receptor.
In this study, we find that hyperglycemia, both short and long term, has a significant impact on the developing preovulatory oocyte, causing slower growth, delayed maturation, increased apoptosis in the surrounding granulosa cells, up-regulation of the extrinsic apoptotic pathway, and diminished levels of a key gap junction protein.
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Materials and Methods |
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Induction of hyperglycemia
To generate a diabetic model, female mice (age 20–24 d) received a single injection of streptozotocin at a dose of 190 mg/kg (dissolved in sodium citrate buffer, pH 4.4; Sigma Chemical Co., St. Louis, MO). Four days after injection, a tail-blood sample was measured for glucose concentrations via a Hemocue B glucose analyzer (Stockholm, Sweden). If glucose levels were more than 240 mg/dl, these mice were selected and received a priming injection of equine chorionic gonadotropin. A few control mice were also randomly selected; their blood sugar was checked to ensure that it was less than 240 mg/dl.
Akita mice
Akita mice have an autosomal dominant mutation, or Mody mutation, resulting in hyperglycemia and notable pancreatic ß-cell dysfunction (19, 20). Diminished levels of both proinsulin and insulin are produced with resultant hyperglycemia. These mice were maintained at our animal facility, received the standard murine chow diet, and kept on a 12-h light, 12-h dark cycle. These mice were not growth retarded at any stage of development compared with control mice. All Akita mice had glucose levels checked at approximately 5–6 wk through a tail-blood sample by a Hemocue B glucose analyzer. If glucose levels were more than 300 mg/dl, these mice were considered to have the mutation. Age-matched controls were C57BL/6 females; these mice also had glucose levels that were checked by the glucometer.
Oocyte maturation and size
Oocytes from the ruptured antral follicles were observed using Hoffman optics on an inverted Nikon microscope (TMS scope). GVBD was assessed, and the oocytes that retained a germinal vesicle and/or the nucleolus failed to demonstrate maturation. Maturation was expressed as a percentage of GVBD. Oocyte diameters were measured excluding the zona pellucida with an eyepiece graticule (x200 magnification). The volume of each oocyte was calculated based on the formula for the volume of a sphere [(4/3)(
r3)]. These measurements were done at 2 and 6 h after hCG.
Ovarian sectioning
Mice were killed, and the ovaries were obtained at specific time points (2 and 6 h). The ovaries were quickly submerged in liquid nitrogen and flash frozen. The ovaries were sectioned using a cryostat machine (Leica 1850, CM) in 11-µm sections. Each ovary was sectioned throughout, and one of every eight ovarian sections was used for immunohistochemical staining.
Evaluation of apoptosis by terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick end labeling (TUNEL) assay
The CEOs and cryopreserved ovarian slices were fixed in 3% paraformaldehyde, permeabilized with 0.1% Tween 20, and then incubated in fluorescein-labeled dUTP and terminal transferase in the dark for 1 h at 37 C to label fragmented 3' DNA (TUNEL, Cell Death in Situ Kit; Roche Molecular Biochemicals, Mannheim, Germany) as previously described. Counterstaining of all nuclear DNA was achieved by incubating the tissue in 4 µM To-Pro-3-iodide (Molecular Probes, Eugene, OR) (blue channel) for 20 min. CEOs were visualized using confocal immunofluorescent microscopy with a Nikon C1 laser scanning confocal microscope at x63 magnification. Apoptosis was expressed as the percentage of TUNEL-positive nuclei per CEO or the number of TUNEL-positive follicles per total number of follicles. Student’s t test was additionally used for statistical analysis.
Immunohistochemistry
CEOs were fixed on glass slides in 3% paraformaldehyde for 20 min and permeabilized with 0.1% Tween 20 for 10 min. The CEOs were first incubated in 20% normal donkey serum (Pierce, Rockford, IL) in PBS containing 2% BSA (PBS/BSA) for 1 h to block any nonspecific binding. They were then washed in PBS/BSA and then incubated in the primary antibody [anti-DR5 (Santa Cruz Biotechnology, Santa Cruz, CA), N-19; mouse anti-actin (Chemicon International, Temecula, CA), 1:1000; anti-Trail (Santa Cruz Biotechnology), H-257, 1:200; and rabbit anti-connexin-43 (Zymed Laboratories, San Francisco, CA), 1:50] for 1 h at room temperature. The CEOs were then washed with PBS/BSA and incubated in the appropriate secondary antibody, goat antirabbit Alexa Fluor 488 antibody or goat antimouse Alexa Fluor 488 antibody, for approximately 30 min. The slides were washed, and nuclear staining was performed in 4 µM To-Pro-3-iodide. After extensive washing in PBS/BSA, confocal immunofluorescent microscopy (Nikon C1 laser scanning microscope) was then used to detect fluorescence as described above.
Expression of DR5 (KILLER) receptor and connexin-43 by Western analysis
CEOs were collected at specific time points (0, 2, and 6 h) for DR5 and (t = 0 h, 6 h) for anti-connexin 43 in equivalent groups (n = 70). The pooled samples were washed in several different washes of human tubal fluid and were frozen in human tubal fluid. (HTF) (Irvine Scientific, Irvine, CA). HeLa cell extracts were used as a positive control for the DR5 receptor. The CEOs were placed in Laemmli buffer with ß-mercaptoethanol. All samples were boiled for at least 5 min at 100 C. The samples were subjected to 10% SDS-PAGE and transferred to nitrocellulose. Mouse DR5 or KILLER protein was then detected with a rabbit polyclonal DR5 antibody (Santa Cruz Biotechnology; 1:500) as the primary antibody and a horseradish peroxidase (HRP)-labeled goat antirabbit antibody (Santa Cruz Biotechnology) as the secondary antibody. Connexin-43 protein was detected by a rabbit polyclonal antibody (Zymed Laboratories; 1:1000), and an HRP-labeled donkey antirabbit antibody (Pierce; 1:20,000) was used as the secondary antibody. HRP-labeled bands were quantified using NIH Image (version 1.6). All experiments were performed in triplicate. Mouse anti-actin (Chemicon International; 1:2000) was used as the loading control for all blots.
Ovarian sections
Ovaries were obtained at specific time points and submerged in Bouin’s fixation solution (Sigma). They were then paraffin embedded, cut, and stained with hematoxylin and eosin at the histology core at Washington University in St. Louis Medical School. The sections were then examined and evaluated by a blinded animal pathologist.
Statistics
Differences between the two groups with protein expression and percent TUNEL staining were analyzed using Student’s t test. If multiple groups were being compared, ANOVA was used with Fisher post hoc test. Results are expressed as means ± SE of at least three separate experiments. For each set of oocyte experiments, at least five mice were used for each group, and approximately 15 oocytes were obtained from each animal. All experiments were performed at least two times, and the majority of the experiments were performed three times. The numbers of ovaries sectioned are described below.
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Results |
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Increased apoptosis was visualized in the hyperglycemic streptozotocin-induced model
At time 0, in animals that had received ovarian priming with pregnant mare serum gonadotropin, but no hCG, there was no significant difference in the number of TUNEL-positive nuclei in the control vs. the diabetic CEOs. The percentage of TUNEL-positive nuclei or apoptotic nuclei was fairly low in both groups with the nondiabetic group at 6.24% and the diabetic group at 9.36%. This difference was not significant. However, at 6 h after hCG injection, there was a trend of higher levels of apoptosis in the diabetic group as predicted. In the diabetic group, 13.81% of the nuclei were TUNEL positive, whereas in the nondiabetic CEOs, approximately 5.2% of the nuclei were TUNEL positive (P = 0.0517). Each study group had approximately 12 animals.
Ovaries from diabetic (n = 4) and nondiabetic mice (n = 4) were also sectioned to assess in situ apoptosis. After TUNEL-stained ovarian sections were examined with confocal microscopy, each of the follicles was examined, and the percentage of TUNEL-positive follicles was counted. At 6 h after hCG, 68.3% of the diabetic follicles were TUNEL positive, whereas only 15.1% of the nondiabetic follicles had TUNEL staining. Similar to what had been visualized in the CEOs, more of the diabetic follicles were TUNEL positive. Representative examples of nondiabetic and diabetic sections are shown in Fig. 2, A and B, respectively. The diabetic follicles had remarkably more TUNEL-positive follicles when compared with the nondiabetic group. These data are demonstrated in bar graph form in Fig. 2C. This difference was statistically significant with a P value of 0.012.
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Increased apoptosis was visualized in the chronic hyperglycemic Akita model
Akita mice harbor an autosomal dominant mutation in the insulin 2 gene that causes them to have chronic insulinopenia and hyperglycemia because of a conformational problem in insulin secretion (19, 20). As a result, these mice have chronic hyperglycemia and hypoinsulinemia with glucose levels in the range of 300 mg/dl and above at the age of 6–9 wk. Ovarian sections from Akita mice (n = 3) and control C57BL/6 mice (n = 3) were obtained 6 h after hCG injection, and TUNEL staining was performed. Approximately 64.07% of the mutant hyperglycemic mice follicles were found to be TUNEL positive, and 36.7% of the control mice follicles demonstrated TUNEL-positive staining. This difference was significant with a P value of 0.035. Examples of control vs. Akita ovarian sections are displayed in Fig. 3, E and D, respectively. This increase in follicle apoptosis confirmed the increased apoptosis seen in the short-term streptozotocin-induced hyperglycemia model.
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Histology revealed smaller follicles and increased apoptosis in the Akita mice
Paraffin-embedded ovarian sections from Akita mice were visualized 6 h after hCG by a blinded pathologist. Sixteen sections from each ovary (n = 4) were examined. The five most representative sections were recorded, and an average of these five was reported. Comparing the population of follicles from the Akita with those of the control mice, which were strain-matched B6 nondiabetic mice, fewer large follicles were seen in the ovaries from the Akita ovaries. On average, the control mice had seven large follicles per ovary, whereas the Akita mice had 2.5 large follicles per ovary. The average largest diameter of an Akita follicle was 0.44 µm, which was markedly smaller than the average that was seen in a control mouse, 0.62 µm. In addition, there were fewer antral follicles as well as medium and small follicles. An example of the larger, more developed follicles in the control vs. the smaller, less mature follicles from the Akita mice is displayed in Fig. 3, C and B, respectively. Finally, the pathologist noted that the number of apoptotic foci per field was higher in the Akita follicles compared with the strain-matched control ovaries. Again, this was confirmed as seen in the TUNEL experiments on the ovarian sections, showing increased TUNEL-positive, nonatretic follicles in the Akita mice (Fig. 3D) compared with nondiabetic controls (Fig. 3E).
Increased TRAIL and KILLER expression was visualized in the streptozotocin-induced diabetic model
To test the hypothesis that TRAIL/KILLER interaction was involved in diabetes-induced follicular apoptosis, CEOs from the streptozotocin-induced diabetic mice were examined for TRAIL protein expression. The protein appeared to be localized to the plasma membrane as well as the cytoplasm. Higher levels of expression were seen in the diabetic CEOs (n = 4 mice) at 2 and 6 h after hCG, compared with the nondiabetic control CEOs (n = 4 mice) at both time points. A representative example of the increased expression in the diabetic CEO 6 h after hCG administration is displayed in Fig. 4, A and B.
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, D and C. To confirm this finding, protein expression was also quantified by Western immunoblotting using CEOs from diabetic and nondiabetic mice. Approximately 70 CEOs from each different test group were used as samples. The experiment was performed in triplicate at 0, 2, and 6 h. At time 0, no significant difference in the expression levels of KILLER was discerned with a P value of 0.336; however, significant differences were seen at 2 h as displayed in Fig. 5. The values are expressed as X-fold difference over the control value. The difference by ANOVA with the Fisher post hoc test at 2 h was significant with a P value < 0.01; however, at 6 h there was a trend toward significance with a P value of 0.07 by ANOVA with the Fisher post hoc test. As with other TRAIL/KILLER apoptotic signaling, it is possible that this peak in expression at 2 h and decrease at 6 h reflects the initiation and progression of the apoptotic cascade.
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with a CEO from a nondiabetic control mouse compared with the immunopositivity seen in Fig. 6B of a CEO from a diabetic mouse. The quantity of immunopositivity was verified using Western blots in triplicate. At 6 h, significantly more connexin-43 protein was present in equal numbers of pooled CEOs from nondiabetic mice compared with the diabetic mice (P = 0.0085). An example of a Western blot is shown in Fig. 6C.
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Discussion |
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The development of ovarian follicles requires a complex set of cell-cell interactions, and the communication between somatic and germ cells involves endocrine, autocrine, paracrine, and gap junction pathways. In order for both the oocyte and granulosa cell components of the follicle to develop successfully, bidirectional communication is essential (21). Colton et al. (9) demonstrated a loss of metabolic communication between granulosa cells and oocytes in CEOs from diabetic mice. It is possible that a decrease in gap junction communication between granulosa cells leads to changes in paracrine communication between oocyte and granulosa cell that is related to the increase in apoptosis. High-glucose conditions have been found to decrease expression of connexin-43 in human retinal pericytes (11) and simultaneously increase apoptosis (22, 23). In addition, studies in rodent myocytes have demonstrated that interrupted cell-cell communication by down-regulation of connexin-43 is associated with accelerated apoptosis (24). Similarly, we have found that hyperglycemia is associated with a decrease in connexin-43 protein expression, which may impact granulosa cell intercellular communication and accelerate granulosa cell apoptosis. A similar inverse correlation between apoptosis and connexin-43 expression has recently been reported in avian granulosa cells (12). It is difficult to discern whether the deficient communication causes differences in cell signaling that then result in higher levels of apoptosis or whether apoptosis acts as a precursor for downstream problems in communication. In either case, this study links the diabetic condition with apoptosis, a decrease in connexin-43 expression, and an up-regulation of the TRAIL-KILLER cell-death pathway, providing some evidence that a relationship between these phenomena may exist in cell-cell communication in the CEO complex.
Delays in oocyte maturation are seen in the oocytes from both types of diabetic mice. We hypothesize that an insult or a preprogramming event may occur at the oocyte stage secondary to maternal hyperglycemia that permanently alters the course of normal development, and this manifests first as a maturational delay. Several prior studies from our laboratory and others support this hypothesis (5, 7, 8, 9). Embryos recovered in vivo at 48 h after fertilization from chemically induced diabetic mice experience an in vivo developmental delay. Only approximately 70% of the diabetic embryos had reached a two-cell stage, whereas 90% of the control embryos had already reached this maturational stage. Similarly, this delay in development persisted in vitro when these same embryos were cultured in control media for 72 h and assessed at each 24-h interval. A significant impairment in development was seen in the embryos from diabetic mice in their rates of progression to blastocyst compared with nondiabetic mice, despite the fact that both were cultured in identical medium conditions (7, 18). We postulate that this later effect on embryo development is related in part to the oocyte maturation delays described and that these changes are also associated with the increase in apoptosis in the CEOs. Similar conclusions have been drawn in experiments involving exposure to certain toxic agents during the preorganogenic period, from the time of sperm entry and zygote development to late blastocyst stage. Generoso and colleagues (25, 26, 27, 28, 29) published a series of papers demonstrating that brief exposure to ethylene oxide resulted in an increased incidence of fetal death and certain types of fetal malformations, such as craniofacial abnormalities, abdominal wall defects, limb defects, and stillbirths. These effects were not associated with induced chromosomal abnormalities or gene mutations (29). Therefore, there is evidence that oocyte-directed insults may be the result of poorly controlled diabetes during folliculogenesis in animal models as well as humans and that these insults may result in later reproductive failures, such as a higher incidence of malformation and miscarriages in this population of patients
In both the streptozotocin-induced and Akita diabetic mice, antral follicle oocyte size is significantly smaller compared with nondiabetic controls. In addition, in the Akita mice, the follicles at all stages of development are smaller compared with the follicles from the control animals. This smaller follicle and oocyte size may reflect abnormal cell growth and survival, which would correlate with the increased apoptosis seen in the CEOs from the diabetic mice. Similar findings of apoptosis and smaller cell size have been described in Drosophila as well as hematopoietic cell lines as a result of decreased Akt expression and activity (30, 31). Akt determines cell size and survival by modulating mammalian target of rapamycin activity and protein synthesis in the leukocytic cell line FL5.12 (31). TRAIL/KILLER interaction has also been linked to decreased Akt/phosphatidylinositol 3-kinase activity (32, 33). Therefore, it is possible that TRAIL/KILLER up-regulation in the CEOs may be modulating the activity of Akt, leading to decreased cell size and apoptosis. Dysregulation of the Akt pathway would also have downstream effects on protein synthesis and transcriptional regulation of genes perhaps involved in further embryonic development. Using this paradigm, up-regulation of TRAIL/KILLER, increased apoptosis, and decreased cell size may all be contributing to the poor oocyte quality and developmental potential of the resulting embryos.
Separating the physiological effects of high serum glucose levels from a lack of insulin is difficult in both of these models of type 1 diabetes, as would be expected in an animal model of human disease. Destruction of the ß-cells of the pancreas by streptozotocin or prevention of insulin secretion by the Ins2 mutation in the Akita mice causes significantly lower insulin and IGF-I levels as well as hyperglycemia. Previous studies from Kezele et al. (34) have shown that insulin acts as a paracrine factor to facilitate transition from primordial to primary follicle at the level of the oocyte. This group has suggested that abnormally low insulin levels as seen in a type 1 diabetic state may inhibit or retard primordial to primary follicle transition. In other studies, Demeestere et al. (35) have shown that IGF-I has a stimulatory effect on follicular steroidogenesis without any effect on oocyte maturation. They did report, however, improved embryo development as a result of exposure to IGF-I during follicular maturation, suggesting that IGF-I improved the quality of the oocyte. Other studies have suggested that IGF-I and insulin enhance granulosa cell proliferation and increase follicle diameter (36). Thus, it is possible that the effects of maternal diabetes seen in this study, specifically granulosa cell apoptosis and decreased oocyte and follicle size, as well as delayed oocyte maturation might be a combined effect of hyperglycemia and hypoinsulinemia. Colton et al. (8) found that the delay in FSH-stimulated oocyte maturation experienced by the streptozotocin-induced diabetic mice could be only partially recreated in vitro with high-glucose conditions, suggesting other factors may be involved. Although the presence of both simultaneous conditions in the animal models may be considered a limitation of this study, these physiological models may more accurately reflect the true follicular milieu caused by this disease, rather than the in vitro experiments examining the two conditions in isolation.
In both the acute chemically induced diabetic mouse model and in the chronically hyperglycemic Akita mouse, detrimental effects are seen in oocyte development and the degree of apoptosis in the surrounding cumulus cells. For these reasons, women with type I diabetes mellitus may be placing their oocytes and offspring at risk. It is known from human studies on patients undergoing in vitro fertilization that increased cumulus cell apoptosis correlates with poorer-quality oocytes and poor pregnancy outcome including increased rates of miscarriage. Similarly, several studies in human oocytes have correlated small oocyte size with poorer developmental potential and pregnancy rates (37, 38, 39, 40, 41). As a result of these findings, we speculate that diabetic women who experience uncontrolled or poorly controlled diabetes during ovulation and fertilization may suffer detrimental effects on oocytes/CEOS secondary to hyperglycemia and hypoinsulinemia including developmental delay, up-regulation of cell-death effector pathways, and increased granulosa cell apoptosis. These early insults may then lead to an increased rate of miscarriage and congenital anomalies depending on the signaling and cell-death pathways involved.
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Acknowledgments |
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Footnotes |
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First Published Online February 17, 2005
Abbreviations: CEO, Cumulus-enclosed oocyte; GVBD, germinal vesicle breakdown; HRP, horseradish peroxidase; TRAIL, TNF-related apoptosis-inducing ligand; TUNEL, terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick end labeling.
Received November 12, 2004.
Accepted for publication February 8, 2005.
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B induced by diabetes and high glucose regulates a proapoptotic program in retinal pericytes. Diabetes 51:2241–2248This article has been cited by other articles:
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  A. M. Ratchford, A. S. Chang, M. M.-Y. Chi, R. Sheridan, and K. H. Moley Maternal diabetes adversely affects AMP-activated protein kinase activity and cellular metabolism in murine oocytes Am J Physiol Endocrinol Metab, November 1, 2007; 293(5): E1198 - E1206. [Abstract] [Full Text] [PDF]
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 K. Kim, Chung Hoon Kim, K. H. Moley, and Y.-P. Cheon Disordered Meiotic Regulation of Oocytes by Duration of Diabetes Mellitus in BBdp Rat Reproductive Sciences, July 1, 2007; 14(5): 467 - 474. [Abstract] [PDF]
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 M. L. Sutton-McDowall, M. Mitchell, P. Cetica, G. Dalvit, M. Pantaleon, M. Lane, R. B. Gilchrist, and J. G. Thompson Glucosamine Supplementation During In Vitro Maturation Inhibits Subsequent Embryo Development: Possible Role of the Hexosamine Pathway as a Regulator of Developmental Competence Biol Reprod, May 1, 2006; 74(5): 881 - 888. [Abstract] [Full Text] [PDF]
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 A A Fouladi-Nashta and K H S Campbell Dissociation of oocyte nuclear and cytoplasmic maturation by the addition of insulin in cultured bovine antral follicles. Reproduction, March 1, 2006; 131(3): 449 - 460. [Abstract] [Full Text] [PDF]
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