Early preantral mouse follicle in vitro maturation: oocyte growth, meiotic maturation and granulosa-cell proliferation

Nutrition and maternal metabolic health in relation to oocyte and embryo quality: critical views on what we learned from the dairy cow model

Although fragmented and sometimes inconsistent, the proof of a vital link between the importance of the physiological status of the mother and her subsequent reproductive success is building up. High-yielding dairy cows are suffering from a substantial decline in fertility outcome over past decades. For many years, this decrease in reproductive output has correctly been considered multifactorial, with factors including farm management, feed ratios, breed and genetics and, last, but not least, ever-rising milk production. Because the problem is complex and requires a multidisciplinary approach, it is hard to formulate straightforward conclusions leading to improvements on the 'work floor'. However, based on remarkable similarities on the preimplantation reproductive side between cattle and humans, there is a growing tendency to consider the dairy cow's negative energy balance and accompanying fat mobilisation as an interesting model to study the impact of maternal metabolic disorders on human fertility and, more specifically, on oocyte and preimplantation embryo quality. Considering the mutual interest of human and animal scientists studying common reproductive problems, this review has several aims. First, we briefly introduce the 'dairy cow case' by describing the state of the art of research into metabolic imbalances and their possible effects on dairy cow reproduction. Second, we try to define relevant in vitro models that can clarify certain mechanisms by which aberrant metabolite levels may influence embryonic health. We report on recent advances in the assessment of embryo metabolism and meantime critically elaborate on advantages and major limitations of in vitro models used so far. Finally, we discuss hurdles to be overcome to successfully translate the scientific data to the field.

Effects of reaggregated granulosa cells and oocytes derived from early antral follicles on the properties of oocytes grown in vitro

In this study, we examined the effects of reconstructed oocyte-granulosa cell complexes (OGCs) on the development of porcine oocytes derived from early antral follicles (EAFs; 0.5-0.7 mm in diameter). When denuded oocytes were cocultured with granulosa cells derived from other EAFs, the oocytes and granulosa cells aggregated to form OGCs after 2 days of culture. After 14 days of culture, we compared cell number, oocyte diameter, and oocyte chromatin configuration in unmanipulated (natural) OGCs, reconstructed OGCs, and OGCs collected from antral follicles (AFs, 3.0-6.0 mm in diameter). The diameters of oocytes from reconstructed OGCs grown in vitro were not different from those of oocytes from natural OGCs, although they were significantly smaller than those of oocytes from antral follicle (AF) OGCs. Oocyte chromatin configuration did not differ among the 3 OGC groups, but the oocyte nuclear maturation rate was lower in the reconstructed OGCs and higher in the AF OGCs. However, when the in vitro culture period for the reconstructed OGCs was extended by 2 days, the nuclear maturation rate of oocytes from reconstructed OGCs was similar to that of oocytes from natural OGCs. In addition, blastocysts were successfully obtained from oocytes from reconstructed OGCs. In conclusion, we established an innovative culture method that allows oocytes and granulosa cells from EAFs to reaggregate as reconstructed OGCs, which yield oocytes with the ability to develop to the blastocyst stage.

Live births from isolated primary/early secondary follicles following a multistep culture without organ culture in mice

Although the ovary has a large store of germ cells, most of them do not reach mature stages. If a culture system could be developed from early growing follicles to mature oocytes, it would be useful for biological research as well as for reproductive medicine. This study was conducted to establish a multistep culture system from isolated early growing follicles to mature oocytes using a mouse model. Early growing follicles with diameters of 60-95 μm corresponding to primary and early secondary follicles were isolated from 6-day-old mice and classified into three groups by diameter. These follicles contained oocytes with diameters of ~45 μm and one or a few layered granulosa cells on the basal lamina. Embedding in collagen gel was followed by first-step culture. After 9-day culture, the growing follicles were transferred onto collagen-coated membrane in the second step. At day 17 of the culture series, the oocyte-granulosa cell complexes were subjected to in vitro maturation. Around 90% of the oocytes in follicles surviving at day 17 resumed second meiosis (metaphase II oocytes: 49.0-58.7%), regardless of the size when the follicle culture started. To assess developmental competence to live birth, the eggs were used for IVF and implantation in pseudopregnant mice. We successfully obtained two live offspring that produced next generations after puberty. We thus conclude that the culture system reported here was able to induce the growth of small follicles and the resultant mature oocytes were able to develop into normal mice.

Ultrastructure of isolated mouse ovarian follicles cultured in vitro

BACKGROUND

In vitro maturation of ovarian follicles, in combination with cryopreservation, might be a valuable method for preserving and/or restoring fertility in mammals with impaired reproductive function. Several culture systems capable of sustaining mammalian follicle growth in vitro have been developed and many studies exist on factors influencing the development of in vitro grown oocytes. However, a very few reports concern the ultrastructural morphology of in vitro grown follicles.

METHODS

The present study was designed to evaluate, by transmission and scanning electron microscopy, the ultrastructural features of isolated mouse preantral follicles cultured in vitro for 6 days in a standard medium containing fetal calf serum (FCS). The culture was supplemented or not with FSH.

RESULTS

The follicles cultured in FCS alone, without FSH supplementation (FCS follicles), did not form the antral cavity. They displayed low differentiation (juxta-nuclear aggregates of organelles in the ooplasm, a variable amount of microvilli on the oolemma, numerous granulosa cell-oolemma contacts, signs of degeneration in granulosa cell compartment). Eighty (80)% of FSH-treated follicles formed the antral cavity (FSH antral follicles). These follicles showed various ultrastructural markers of maturity (spreading of organelles in ooplasm, abundant microvilli on the oolemma, scarce granulosa cell-oolemma contacts, granulosa cell proliferation). Areas of detachment of the innermost granulosa cell layer from the oocyte were also found, along with a diffuse granulosa cell loosening compatible with the antral formation. Theca cells showed an immature morphology for the stage reached. Twenty (20)% of FSH-treated follicles did not develop the antral cavity (FSH non-antral follicles) and displayed morphological differentiation features intermediate between those shown by FCS and FSH antral follicles (spreading of organelles in the ooplasm, variable amount of microvilli, scattered granulosa cell-oolemma contacts, signs of degeneration in granulosa cell compartment).

CONCLUSIONS

It is concluded that FSH supports the in vitro growth of follicles, but the presence of a diffuse structural granulosa cell-oocyte uncoupling and the absence of theca development unveil the incomplete efficiency of the system. The present study contributes to explain, from a morphological point of view, the effects of culture conditions on the development of mouse in vitro grown follicles and to highlight the necessity of maintaining efficient intercellular communications to obtain large numbers of fully-grown mature germ cells.

Different follicle-stimulating hormone exposure regimens during antral follicle growth alter gene expression in the cumulus-oocyte complex in mice

Follicle-stimulating hormone (FSH) and oocyte-secreted factors influence granulosa cell differentiation and follicle development. Whereas FSH stimulates the expression of mural cell transcripts, oocyte-secreted factors regulate specific cumulus cell genes and suppress the appearance of mural cell transcripts. This study addresses the extent to which clinically relevant changes in FSH doses applied during antral follicle development in vitro could alter the expression of oocyte and cumulus cell transcripts. A 12-day culture system in which mouse ovarian preantral follicles can grow to preovulatory follicles was used. The following three FSH regimens were considered: 1) continuous exposure to an FSH level of 10 mIU/ml (control), 2) decreasing concentrations of FSH (low FSH), and 3) an FSH level of 25 mIU/ml (high FSH) as soon as the antrum is formed. Transcripts in oocytes (Gdf9, Bmp15, and Fgf8) and in cumulus cells (Amh, Lhcgr, Ar, and Pfkp) were quantified by real-time PCR. Under high FSH, the three oocyte transcripts were upregulated, while in cumulus cells a shutdown of the Amh signal and substantial increases in Lhcgr and Ar expression were measured. In contrast, low FSH tended to reduce Lhcgr to levels comparable to those in vivo. Levels of Pfkp were not affected by FSH doses. These results demonstrate that a 2.5-fold increase in FSH changes both oocyte and cumulus cell transcript levels. Conversely, a decrease in FSH does not affect transcript levels but seems to limit inappropriate Lhcgr expression. Modulating FSH within physiological ranges during the antral phase of culture alters cumulus cell differentiation.

Establishment of a basic method for manipulating preantral follicles: effects of retrieval method on in vitro growth of preantral follicles and intrafollicular oocytes

The aim of this study was to establish a basic manipulation protocol of preantral follicles for deriving developmentally competent oocytes. Primary, early and late secondary follicles retrieved from the ovaries of 14-day-old F1 (C57BL/6 × DBA2) female mice mechanically or enzymatically were cultured singly and in vitro growth of the follicles and maturation of intrafollicular oocytes were subsequently monitored. A mechanical method retrieved more (p < 0.0001) follicles (339 ± 48 vs. 202 ± 28) than an enzymatic method. However, the enzymatic method collected more singly isolated follicles that could be provided for subsequent culture (102 ± 26 vs. 202 ± 28). When an enzymatic method was employed, early and late secondary follicles required 9 and 6 days for reaching the maximal incidence of the pseudoantral stage. However, primary follicles were not possible to develop into the pseudoantral stage. The optimal duration of oocyte maturation from the onset of follicle culture was 7 days and 5–7 days for early and late secondary follicles, respectively. A general decrease in oocyte diameter (65.2–65.53 μm vs. 75 μm) and zona thickness (5.41–5.74 μm vs. 7.76 μm) was detected in in vitro-derived compared with in vivo-derived matured oocytes. Pronuclear formation was detected in 86–94% of mature oocytes after parthenogenetic activation and no significant difference was detected among groups. These results showed that preantral follicles retrieved by an enzymatic method underwent step-by-step growth in vitro, which could yield mature oocytes.

Distribution of extracellular matrix proteins type I collagen, type IV collagen, fibronectin, and laminin in mouse folliculogenesis

The extracellular matrix (ECM) plays a prominent role in ovarian function by participating in processes such as cell migration, proliferation, growth, and development. Although some of these signaling processes have been characterized in the mouse, the relative quantity and distribution of ECM proteins within developing follicles of the ovary have not been characterized. This study uses immunohistochemistry and real-time PCR to characterize the ECM components type I collagen, type IV collagen, fibronectin, and laminin in the mouse ovary according to follicle stage and cellular compartment. Collagen I was present throughout the ovary, with higher concentrations in the ovarian surface epithelium and follicular compartments. Collagen IV was abundant in the theca cell compartment with low-level expression in the stroma and granulosa cells. The distribution of collagen was consistent throughout follicle maturation. Fibronectin staining in the stroma and theca cell compartment increased throughout follicle development, while staining in the granulosa cell compartment decreased. Heavy staining was also observed in the follicular fluid of antral follicles. Laminin was localized primarily to the theca cell compartment, with a defined ring at the exterior of the follicular granulosa cells marking the basement membrane. Low levels of laminin were also apparent in the stroma and granulosa cell compartment. Taken together, the ECM content of the mouse ovary changes during follicular development and reveals a distinct spatial and temporal pattern. This understanding of ECM composition and distribution can be used in the basic studies of ECM function during follicle development, and could aid in the development of in vitro systems for follicle growth.

Effects of epidermal growth factor and hormones on granulosa expansion and nuclear maturation of dog oocytes in vitro

Gonadotropins, steroids and growth factors stimulate or inhibit cumulus expansion, nuclear maturation, or both, of most mammalian oocytes in vitro. The objective was to evaluate the effects of epidermal growth factor (EGF) and various hormone combinations on in vitro granulosa/cumulus (G-C) expansion and nuclear maturation of domestic dog oocytes derived from advanced preantral and early antral follicles. Follicles were collected after enzymatic digestion of ovarian tissue and cultured for 66 h in F-12/DME with 20% fetal bovine serum, 2mM glutamine and 1% antibiotic-antimycotic (Control). Treatments comprised the following groups; each was cultured both with and without EGF (5 ng/mL): Control, FSH (0.5 microg/mL), LH (5 microg/mL), estradiol-17beta (E2, 1 microg/mL), FSH+LH, and FSH+LH+E2. Granulosa/cumulus expansion was scored on a scale of 0 (no expansion) to +3 (maximum expansion). The interaction between EGF and hormone treatment affected (P=0.011) maximum G-C expansion. With the exception of the E2 group, EGF increased (P<0.05) the proportion of oocytes exhibiting +3 expansion. The synergism of E2 with FSH+LH enhanced maximum G-C expansion; compared to all other treatments, the greatest expansion was observed in the FSH+LH+E2+EGF group (83.5+/-3.5%). When cultured in EGF alone, oocytes failed to reach metaphase I-II (MI-MII) stages. The interaction between EGF and hormone treatment tended (P=0.089) to increase the proportion of oocytes resuming or completing nuclear maturation (GVBD-MII). In addition, supplementing culture media with hormones increased (P=0.010) the GVBD-MII rate. Therefore, EGF in combination with FSH and LH enhanced G-C expansion of cultured canine oocytes, with no significant effect on the proportion of oocytes derived from advanced preantral and early antral follicles that reached MI-MII.

Ovarian follicle bioassay reveals adverse effects of diazepam exposure upon follicle development and oocyte quality

A mouse ovarian follicle bioassay was used to study folliculogenesis and oocyte quality in vitro. Diazepam (DZ) was chosen as test compound to evaluate the system for its ability to detect possible effects of chemicals on reproduction. The bioassay is suitable to analyze the influence of DZ on each of the follicular components at any stage of development. A dose finding study revealed that follicle growth, differentiation and steroidogenesis were significantly disturbed by > or =5 microg/ml DZ. A transient exposure procedure was used to examine stage-specific sensitivities of oogenesis to DZ. The oocyte appeared to be most vulnerable during its growth process within the follicle. Resumption of meiosis was disturbed dose-dependently with reduced oocyte quality after chronic exposure to > or =2.5 microg/ml DZ. The bioassay is a highly efficient and informative tool to assess the hazards of chemical compounds for female fertility and to elucidate their mechanisms of action.

Cytogenetic studies in mouse oocytes irradiated in vitro at different stages of maturation, by use of an early preantral follicle culture system

In vivo studies on X-irradiated mice have shown that structural chromosome aberrations can be induced in female germ cells and that the radiation-induced chromosomal damage strongly depends on the stage of maturation reached by the oocytes at the time of irradiation. In the present study, the sensitivity of oocytes to induction of chromosome damage by radiation was evaluated at two different stages, by use of a recently developed method of in vitro culture covering a crucial period of follicle/oocyte growth and maturation. A key feature of this system is that growth and development of all follicles is perfectly synchronized, due to the selection of a narrow class of follicles in the start-off culture. This allows irradiation of well-characterized and homogenous populations of follicles, in contrast to the situation prevailing in vivo. Follicles were X-irradiated with either 2 or 4 Gy, on day 0 of culture (early preantral follicles with one to two cell layers) or on day 12, 3h after hormonal stimulation of ovulation (antral Graafian follicles). Ovulated oocytes, blocked in metaphase I (MI) by colchicine, were fixed 16 h after hormonal stimulation and analyzed for chromosome aberrations. The results confirm the high radiosensitivity of oocytes at 2 weeks prior to ovulation and the even higher radiosensitivity of those irradiated a few hours before ovulation, underlining the suitability of the in vitro system for further studies on the genetic effects of ionising radiation in female mammals.

Localization of Mitotic Arrest Deficient 1 (MAD1) in Mouse Oocytes During the First Meiosis and Its Functions as a Spindle Checkpoint Protein1

The present study was designed to investigate the localization of mitotic arrest deficient 1 (MAD1) in mouse oocytes during meiotic maturation and its relationship with kinetochores, chromosomes, and microtubules. Oocytes at various stages during the first meiosis were fixed and immunostained for MAD1, kinetochores, microtubules, and chromosomes. The stained oocytes were examined by confocal microscopy. Some oocytes were treated with nocodazole or Taxol before examination. The anti-MAD1 antibody was injected into the oocytes at the germinal vesicle (GV) stage for examination of chromosome alignment and spindle formation. It was found that MAD1 was present in the oocytes from the GV to prometaphase I stages around the nuclei. When the oocytes reached the metaphase I (M-I) to metaphase II (M-II) stages, MAD1 was mainly localized at the spindle poles. However, MAD1 relocated to the vicinity of the chromosomes when spindles were disassembled by nocodazole or cooling, and the relocated MAD1 moved back to the spindle poles during spindle recovery. Taxol treatment did not affect the MAD1 localization. Although anti-MAD1 antibody injection did not affect nuclear maturation, significantly higher proportions of injected oocytes had misaligned chromosomes when the oocytes reached the M-I to M-II stages. The results of the present study indicate that MAD1 is present in mouse oocytes at all stages during the first meiosis and that it participates in spindle checkpoint during meiosis. However, MAD1 could not check misaligned chromosomes during spindle recovery after the spindles were destroyed by drug or cooling, which caused some chromosomes to scatter in the oocytes.

Survival of frozen-thawed human ovarian fetal follicles in long-term organ culture

Ovarian follicles obtained from second and third-trimester human fetuses survived 4 weeks in organ culture and secreted 17-beta estradiol (E(2)).

In vitro growth of preantral follicles isolated from cryopreserved ovine ovarian tissue

In the present study, we compared the in vitro development of sheep preantral follicles obtained from unfrozen or frozen ovarian cortex. After thawing, follicles stored by a slow-freezing protocol with dimethyl sulfoxide (DMSO) or ethylene glycol (EG) were mechanically isolated and cultured for 10 days. After 1 day, approximately 50% and 34% of the DMSO and EG follicles, respectively, showed overt signs of degeneration, as confirmed by histological analysis. Follicles that survived thawing grew and formed antral-like cavities, without significant differences among experimental groups. However, the percentages of healthy oocyte-cumulus cell complexes (OCCs) retrieved from in vitro-grown follicles, as well as estradiol, were lower in DMSO than in EG or unfrozen follicles. Although cryopreservation did not cause appreciable differences in follicle morphological aspects, frozen OCCs showed lower metabolic cooperativity levels, as determined by [3H]uridine uptake. During culture, oocytes increased in diameter, but the percentage of germinal vesicle stage-arrested oocytes showing a rimmed chromatin configuration was significantly lower in the frozen groups. Our results indicate that cryopreserved sheep preantral follicles underwent growth in vitro but that freezing/thawing specifically affected gap junctional permeability and impaired the progression of regulative processes, such as the acquisition of a specific oocyte chromatin configuration. Moreover, because the cryoprotectant toxicity test excluded the occurrence of direct cellular damage, this method allowed us to discriminate the effects exerted by different cryoprotectants during the cryopreservation procedure on whole-follicular development.

In vitroculture of mouse GV oocytes and preantral follicles isolated from ovarian tissues cryopreserved by vitrification

Cryopreservation of ovarian tissues containing many immature oocytes occurs in both gamete/embryo research and clinical medicine. Using vitrification, we studied factors related to meiosis after cryopreservation using the COCs (cumulus oocyte complexes) and preantral follicles obtained from cryopreserved ovarian tissues. COCs were isolated and cultured for 17 approximately 19 hr. Thereafter, Metaphase II stage (MII stage) oocytes and fertilized oocytes after IVF were observed at a rate of 76.5% and 60.0%, respectively. Preantral follicles (100 approximately 130 microm in diameter) were isolated and cultured in alpha MEM containing hFSH, ITS, and FBS. HCG and EGF were added to the media to stimulate ovulation on the 12th day of culture. The survival rates of the follicles obtained from the frozen/thawed ovaries were 66.4%. After 12 days of culture, the diameter of the follicles isolated from fresh (620.2 +/- 11.3 microm) and frozen/thawed ovaries (518.7 +/- 15.1 microm) differed as did the estradiol concentrations (3474.2 +/- 159 pg/ml vs. 1508.2 +/- 134 pg/ml). After in vitro ovulation, MII stage oocytes were observed in 84.5% of the fresh group and 60.5% of the frozen/thawed group while the fertilization rate was 74.2% and 53.5%, respectively. These studies demonstrate that cryopreservation of mouse ovarian tissues by vitrification did not affect the oocyte's ability to undergo meiosis. Thus, this technique may become a powerful tool for the preservation of the female gamete.

In vitro culture and in vitro maturation of mouse preantral follicles with recombinant gonadotropins

OBJECTIVE

To develop an effective method for in vitro maturation of preantral follicles isolated from mice ovarian tissue.

DESIGN

Isolated preantral follicles were randomly allocated to designed experimental groups for study.

SETTING

University-based research lab.

PATIENT(S)

Healthy, normal mice.

INTERVENTION(S)

Superovulation with pregnant mare serum gonadotropin and hCG.

MAIN OUTCOME MEASURE(S)

Morphological changes and E(2) production were assessed.

RESULT(S)

To obtain competent oocytes, preantral follicles must be cultured with medium containing insulin and recombinant gonadotropins (i.e., recombinant FSH and recombinant LH), with a change of medium daily. A high initial recombinant LH or recombinant FSH facilitates E(2) secretion, enhances granulosa cell outgrowth, and has earlier antral formation. However, prolonged culture in high-recombinant LH or recombinant FSH triggers early differentiation and luteinization of granulosa cells, which results in low metaphase II oocyte and blastocyst formation.

CONCLUSION(S)

We have developed a culture system that allows the successful maturation of preantral follicles in vitro. The matured follicles are a physiologically functional unit that not only secrete E(2) but also generate competent oocytes. In a special condition, 90% of the cultured follicles survived, 53.5% of them produced MII oocytes, and 50% of the derived MII oocytes were fertilized and reached the blastocyst stage after culture in vitro.

Timed analysis of the nuclear maturation of oocytes in early preantral mouse follicle culture supplemented with recombinant gonadotropin

OBJECTIVE

To analyze the effects of combined FSH and variable doses of LH on the nuclear maturity and capacity to resume meiosis in oocytes from preantral follicles from prepubertal mice.

DESIGN

Prospective, randomized, and controlled in vitro laboratory experiment.

SETTING

Academic research environment.

INTERVENTION(S)

Meiosis was studied after somatic cell removal or after stimulation with hCG plus epidermal growth factor in three culture conditions: maturation medium with FSH alone and with two different doses of LH.

MAIN OUTCOME MEASURE(S)

The nuclear maturation of the oocytes and the E2, progesterone, and alpha-specific inhibin content of the conditioned medium.

RESULT(S)

Somatic cell removal and hormonal stimulation were equally effective in inducing germinal vesicle breakdown, but the hormonal stimulus was essential for the completion of meiosis, which was maximal (70%) on day 13 of culture. Continuous addition of LH to FSH during the oocytes' growth made them more prone to spontaneous resumption of meiosis I but resulted in a higher proportion of oocytes reaching the completion of meiosis. Estradiol and progesterone measurements demonstrated that the presence of LH influences luteinization.

CONCLUSION(S)

In contrast to oocytes grown in vivo, cumulus cell removal by itself is an insufficient stimulus for oocytes cultured in vitro to complete meiosis. Timed stimulation with hCG and epidermal growth factor increases nuclear maturation rates. A maximum number of metaphase II oocytes are obtained after a 13-day in vitro growth period when LH is added to the maturation medium.

Effects of recombinant activin A on in vitro culture of mouse preantral follicles

Activins are part of the intragonadal factors that can modulate the actions of gonadotropins and regulate cellular functions during preantral or early antral stages of folliculogenesis in vivo. In a mouse early preantral follicle culture system, activin A production was measured and recombinant bovine activin A (r-ACT A) was added (10 or 50 ng/ml) to recombinant follicle-stimulating hormone (r-FSH)-supplemented (10 or 100 mIU/ml) medium for a 12-day culture period. Specificity of activin A action was ascertained by addition of recombinant human follistatin (r-FA; 20 or 100 ng/ml). Immunoreactive activin A concentrations in mouse follicle-conditioned medium increased by a factor of 20-50, reaching concentrations from 2 to 5 ng/ml at end of culture. In the initial days of culture, additions of r-ACT A to r-FSH-supplemented medium provoked a dramatic volumetric increase and earlier attachment of the follicle. A dose of 100 ng/ml r-FS was able to block the actions of 10 ng/ml but not those caused by 50 ng/ml r-ACT A. In follicle cultures supplemented with 10 mIU/ml r-FSH, additions of r-ACT induced a dose-dependent inhibin (INH) and estradiol (E2) increase. Basal and human chorionic gonadotropin (HCG)-induced progesterone (P) production were not influenced by r-ACT A or r-FS additions. Addition of r-ACT A decreased (P = 0.017) the intact follicle survival rate and had no influence on final oocyte diameter. In cultures supplemented by 10 mIU/ml r-FSH, additions of r-ACT A did not influence progression and resumption of meiosis I. Use of a higher r-FSH supplementation dose (100 mIU/ml) tended to affect meiosis I adversely (P = 0.052), and r-ACT A addition amplified this effect significantly (P = 0.007). These in vitro experiments demonstrate pronounced effects from r-ACT on r-FSH-mediated follicle survival, growth, and estrogen biosynthesis.