The biology and technology of follicular oocyte development in vitro

Isolation and culture of chicken primordial follicles

The establishment of a primordial follicle culture system is important for the study of follicular development. Hence, the objective of this study was to isolate chicken primordial follicles and establish culture methods. Ovaries from 2-wk-old chickens were treated with trypsin-EDTA, collagenase II, or collagenase type IA, along with a mechanical isolation technique. Isolated follicles were cultured under different conditions. Results showed a significant difference in the follicular recovery and survival rates among different enzymes and methods used. The maximal follicular yield was obtained by trypsin+EDTA and collagenase II digestion, followed by collagenase type IA digestion. However, the highest follicular viability rate was observed in groups of collagenase type IA digestion and the mechanical isolation method. Enzymatic treatment resulted in higher misshapen oocytes or follicles, though the diameters of the follicles were not significantly changed. In addition, our follicle culture results for different conditions showed maximal survival rates of primordial follicles in alginate hydrogel beads after 12 d of culture. Thus, we successfully established methods for isolating and culturing chicken primordial follicles. The present method will greatly facilitate investigation of the regulation of follicular development.

A critical evaluation of ovarian tissue cryopreservation and grafting as a strategy for preserving the human female germline

Ovarian tissue freezing has been used successfully in animals and it has recently begun to be offered clinically to young women who have medical conditions with a high risk of sterility. Although no frozen human ovarian grafts have yet been returned to the donor and resulted in a pregnancy, there are many indications that this procedure should be feasible. Although live young have been derived from frozen grafts in several species, research should aim to make further improvements to the cryopreservation and grafting procedures to optimize follicle survival, and hence minimize the amount of tissue that needs to be collected, stored and returned. Ovarian tissue freezing, particular if used in combination with egg and embryo freezing, should allow a patient to safeguard their chance of becoming a parent later (Table 1). In cases where the patient has a systemic cancer or infection and malignant cells or viruses may be present in the systemic circulation and the gonadal tissue, ovarian tissue could be collected and frozen, but grafting is not currently recommended (Table 3).

Co-culture of buffalo preantral follicles with different somatic cells

The effect of co-culture of buffalo preantral follicles (PFs) with different somatic cells, i.e, cumulus, granulosa, ovarian mesenchymal and oviductal epithelial cells was studied. Large PFs (250-450 microm) were isolated by microdissecting the trypsin (1%) digested ovarian cortical slices. Cumulus cells were isolated by repeated pipetting of oocytes, granulosa cells were isolated by aspirating from punctured PFs and ovarian mesenchymal cells were isolated from ovarian cortex by scraping the cortical slices and passing through 20 microm filter. Preantral follicles were cultured in standard culture medium without somatic cells or co-cultured with cumulus cells, granulosa cells, ovarian mesenchymal cells and oviductal epithelial cells for 80 days. The growth rate (microm/day) of the PFs was monitored by measuring follicular diameter on day 0, 30, 60 and 80 days of culture. The viability of PFs was evaluated by trypan blue staining. The results indicated that PFs co-cultured with cumulus, granulosa and ovarian mesenchymal cells had a better development and survivality compared with control and those co-culture with oviductal epithelial cells. Maximum growth and survivality of PFs were achieved when cultured with cumulus cells. It is concluded that inclusion of somatic cells in PF culture media had beneficial effect on the growth of PFs and cumulus cells supported maximum growth and survivality of PFs in vitro of all somatic cells tested.

Mouse oocyte differentiation during antral follicle development

During antral follicle development mouse oocytes undergo rearrangement of granulosa cell interactions and the oocytes released from follicles at the beginning or at the end of antral development are either devoid of denuded oocytes (DO) or strictly associated with cumulus-intact (CI) cumulus cells. In this study, these two oocyte classes were analyzed before germinal vesicle (GV) and after in vitro maturation (IVM) to evaluate (a) the ultrastructural aspect of oolemma microvilli by scanning electron microscopy analysis and (b) specific morphological markers of differentiation (chromatin organization, mitochondria, cortical granules, microfilaments, and spindle of metaphase II- MII-). At GV-stage, CI oocytes exhibited remarkable differences (a) in the oolemma microvillar ultrastructure and distribution with respect to DO and (b) in the chromatin organization that was typical of meiotically competent germ cells. By contrast, homogeneous patterns of distribution of mitochondria, cortical granules, and microfilaments characterized both the oocyte classes. At the end of culture, CI oocytes, even when matured without cumulus cells, reached more efficiently the MII stage and acquired an ultrastructural microvillous configuration different from DO. In addition, MII-arrested DO had a higher percentage of meiotic spindles with abnormal morphology in comparison with preovulatory oocytes, while cortical granule and microfilament patterns revealed no appreciable differences between the groups. With regard to mitochondria, a polarized distribution of these organelles was found in 82% of DO and in 97% of CI oocytes. These observations suggested that the achievement of the full antral follicle development is a condition for the acquisition of specific qualitative properties that are essential for the production of fertilizable oocytes, both in in vivo and in vitro models as well.

Impact of various endocrine and paracrine factors on in vitro culture of preantral follicles in rodents

Folliculogenesis is a complex process regulated by various paracrine and autocrine factors. In vitro growth systems of primordial and preantral follicles have been developed for future use of immature oocytes, as sources of fertilizable oocytes and for studying follicular growth and oocyte maturation mechanisms. Rodents were often chosen for in vitro follicular culture research and a lot of factors implicated in folliculogenesis have been identified using this model. To date, the mouse is the only species in which the whole process of follicular growth, oocyte maturation, fertilization and embryo transfer into recipient females was successfully performed. However, the efficiency of in vitro culture systems must still be considerably improved. Within the follicle, numerous events affect cell proliferation and the acquisition of oocyte developmental competency in vitro, including interactions between the follicular cells and the oocyte, and the composition of the culture medium. Effects of the acting factors depend on the stage of follicle development, the culture system used and the species. This paper reviews the action of endocrine, paracrine factors and other components of culture medium on in vitro growth of preantral follicles in rodents.

Ficoll density gradient method for recovery of isolated human ovarian primordial follicles

OBJECTIVE

To develop a simple and efficient technique to allow rapid recovery of a maximum number of good quality isolated follicles.

DESIGN

Prospective experimental study.

SETTING

Academic research unit of the department of gynecology in a university hospital.

PATIENT(S)

Biopsies were obtained from five women (between 26 and 31 years of age).

INTERVENTION(S)

Biopsies were cut with a tissue sectioner. Enzymatic digestion was performed in a collagenase solution for 90 min at 37 degrees C. The follicles were recovered using a discontinuous Ficoll density gradient method.

MAIN OUTCOME MEASURE(S)

The number of follicles present in the interface layers of Ficoll gradient was quantified. Follicular viability of these recovered follicles was assessed with live-dead stains, using calcein-AM and ethidium homodimer-I.

RESULT(S)

Out of a total of 6,811 recovered follicles, we found 63% (n = 4,201) at the medium-1.06 Ficoll interface and 36.9% (n = 2,590) at the 1.06-1.09 Ficoll interface, which represents 99.9% of total recovered follicles. Analysis by vital fluorescent staining showed that 95.8% of the follicles treated with Ficoll were totally viable.

CONCLUSION(S)

The Ficoll density gradient method allows us to maximize the recovery of isolated human ovarian follicles and minimize the manipulation time while maintaining high follicular viability.

Growth, Antrum Formation, and Estradiol Production of Bovine Preantral Follicles Cultured in a Serum-Free Medium1

The objective of this study was to identify factors that would allow the establishment of a serum-free culture system that could support follicular and oocyte growth, antrum formation, and estradiol-17beta (E(2)) production in preantral follicles of bovine ovaries. Large preantral follicles (145-170 micro m in diameter) were microsurgically dissected from ovaries, embedded in 0.15% type I collagen gels, and maintained in a serum-free medium for up to 13 days at 38.5 degrees C in 5% CO(2) in air. This culture environment allowed most preantral follicles to maintain a three-dimensional structure with the presence of a thecal layer and basement membrane surrounding the granulosa cells throughout the entire culture period. The effects of insulin, insulin-like growth factor (IGF)-I, IGF-II, FSH, and LH on preantral follicle growth were investigated in serum-free medium. Follicular diameters were significantly larger in the presence of insulin, IGF-I, IGF-II, or FSH after 13 days in culture. Oocyte diameters were also significantly larger in the presence of all hormones tested. The single addition of insulin, IGF-I, or FSH induced antrum formation between Days 7 and 13 of culture. Insulin progressively induced E(2) secretion by follicles after antrum formation, but IGF-I and FSH had no apparent effect. FSH and LH caused an increase in oocyte diameter in the presence of insulin. The addition of three hormones (insulin, FSH, and LH) initiated antrum formation and E(2) production earlier than insulin-containing medium alone. Furthermore, maximal E(2) secretion was maintained steadily between 7 and 13 days in this culture condition. From these results, we have demonstrated that insulin, FSH, and LH play substantial roles in the growth and development of bovine large preantral follicles in a serum-free medium.

In-vitro growth of murine pre-antral follicles after isolation from cryopreserved ovarian tissue

The low temperature storage of ovarian tissue allows patients at risk of premature menopause to preserve their fecundity. One strategy for harvesting mature oocytes may be to isolate small follicles from the stroma for in-vitro culture. The aim of this study was to assess the survival and growth of murine follicles during in-vitro culture after isolation from tissue frozen-thawed in various cryoprotective agents. The effect of different seeding and thawing temperatures was also investigated. Pre-antral follicles 100--135 microm in diameter were isolated from fresh and frozen-thawed tissue and cultured in vitro for 8 days. In the fresh control group 79 +/- 3% of follicles survived in-vitro culture and grew to antral stages. Fewer follicles survived after isolation from tissue cryopreserved in dimethyl sulphoxide (43 +/- 5%) or propylene glycol (24 +/- 2%) and none survived freeze-thawing in glycerol. Lowering the seeding temperature from -5 degrees to -7 degrees or -9 degrees C reduced follicle survival rates from 49 +/- 4% to 26 +/- 1% and 28 +/- 3% respectively. If thawing was carried out at 27 degrees C follicle survival rate was higher (38 +/- 7%) than at 37 degrees C (26 +/- 2%) or 47 degrees C (20 +/- 6%). Follicles surviving 8 days of in-vitro culture were stimulated with human chorionic gonadotrophin. The number of mature oocytes released did not differ between any experimental group. The results indicate that follicles isolated from frozen-thawed tissue can be grown to antral sizes and produce mature oocytes. The in-vitro culture system also proved a sensitive method for testing variations in the freeze-thaw protocol.

In vitro follicle growth: achievements in mammalian species

The exact mechanisms regulating in vivo folliculogenesis in mammalians have only been partly unravelled. Some processes, such as the initiation of growth of primordial follicles are still poorly understood. This increases the difficulty to culture follicles in vitro as the primordial follicles will be the ultimate starting material for culture. There are important species differences in regulation and timing of maturation, which makes it difficult to transpose techniques. Only in the mouse model, live pups were born when primordial or early preantral follicles were cultured entirely in vitro. Although no systems are as yet permitting complete in vitro culture of early follicle stages in large animals or humans, parts of folliculogenesis have been successfully reproduced in vitro. This review summarizes achievements of the last years in follicle culturing starting off at several stages of development. Future applications of in vitro follicle culture include fertility preservation for humans, preservation of rare animal species and creation of oocyte banks for research.

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.

Efficient isolation and long-term viability of bovine small preantral follicles in vitro

A comparison of isolation techniques for small preantral follicles (30-70 microm) from bovine ovaries using a mechanical method with a grating device or collagenase treatment was performed. The mean number (157.0) of intact follicles per ovary isolated by the mechanical method was significantly greater (P < 0.05) than that (26.0) of follicles isolated by the enzymatic method. Isolated morphologically normal follicles (MNF) were cultured for up to 30 d either in control cultures (non-coculture) or in coculture with bovine ovary mesenchymal cells (BOM), fetal bovine skin fibroblasts (FBF), and/or bovine granulosa cells (BGC). In control cultures, most of the follicles degenerated and only a few MNF (1.2%) were present after 30 d in culture. In contrast, the cocultures with BOM, FBF, and BGC resulted in 50.7, 46.6, and 21.4% viable MNF, respectively. Trypan blue and Hoechst 33258 staining were used for a quick and sensitive assessment of oocyte and granulosa cell viability during follicle isolation and culture in vitro. After 30 d, percentages of viable follicles in coculture with BOM (18.6%) and FBF (17.1%) were significantly greater than those of follicles in the control cultures (0%) or in coculture with BGC (10.0%). There was a gradual increase in the average diameter of the MNF during culture. The mean diameter of the follicles increased by 15.4 and 30.0% in coculture with BOM and FBF, respectively, by day 30. In conclusion, small bovine preantral follicles were efficiently isolated using a mechanical method that utilizes a grating device, and could be maintained for up to 30 d in the presence of mesenchymal cell cocultures such as BOM and FBF. This in vitro culture system that supports long-term survival of bovine preantral follicles should be beneficial for studying follicle growth and development.

Gonadal cryopreservation in the young patient with gynaecological malignancy

For patients who are planning to have chemotherapy, radiotherapy or to undergo bilateral oophorectomy, the loss of ovarian function will result in premature ovarian menopause and loss of fertility. Embryo preservation is not an option for single women or married women because delaying treatment for at least 2 months of in-vitro fertilization cycles is inappropriate and may be life-threatening. This study reports on the indications for ovarian tissue cryobanking and the state of the art of this method in preserving fertility in women with iatrogenic premature menopause.

In vitro culture of rat pre-antral follicles with emphasis on follicular interactions

The present study was designed to determine whether rat pre-antral follicles can grow under in-vitro conditions. Emphasis is on whether follicular interaction is involved in in-vitro follicle culture, and furthermore its role in follicular development has been assessed. Pre-antral follicles were isolated mechanically from 10-day old rat ovaries. They were divided into small (50 microm < diameter < 100 microm) and large (120 microm < diameter < 200 microm) pre-antral follicles and cultured individually or in groups for 6 days in medium with or without fetal calf serum (FCS). Based on morphological criteria, large pre-antral follicles cultured in groups in serum-free medium had significantly higher survival rates than those cultured individually. In the presence of FCS, no significant difference was detected with respect to the survival. However, the large pre-antral follicles cultured in groups had a significantly greater increase in diameter than those cultured individually. Furthermore, follicles cultured in groups in FCS-containing medium exhibited significantly more follicular cell proliferation than those in serum-free medium, based on DNA measurement. The present culture system (with or without FCS) proved to be insufficient for small pre-antral follicles to stimulate growth comparable to that of large pre-antral follicles. The transmission electron microscopical (TEM) study revealed the ultrastructural differences between follicles cultured in FCS-containing and serum-free media. Taken together, the results suggest that interfollicular factors are involved in follicle development in vitro, which especially at the early folliculogenesis stage plays a positive role in terms of follicular growth as well as survival. The present culture model allows further investigation of factors that regulate early folliculogenesis.

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

Mechanically isolated early preantral mouse follicles were cultured singly for 16 d and fully grown oocytes were obtained from these follicles. We then compared in vitro and in vivo follicle growth by trypsinising the follicles and counting their cell numbers in a Neubauer-counting chamber and recording the diameter and meiotic status of oocytes under an inverted microscope. As long as the granulosa cells were within the basal membrane, proliferation was slow. From Day 6, when granulosa cells had broken through the basal membrane, the proliferation rate progressed up to Day 10 and decreased thereafter to approximately 12,000 cells per culture droplet. Incorporation of BrdU revealed that proliferating cells were evenly distributed throughout the follicle until antrum formation. As granulosa cell differentiation progressed, proliferation of mural-granulosa cells ceased, while cells around the oocytes continued dividing. Oocyte diameter increased discontinuously in relation to follicle remodelling. During the first growth phase, diameters increased from 56.5 (+/- 4.4 microns) to 67 (+/- 4.1 microns) until the onset of antral-like cavity formation. The last growth phase started after Day 10, and by Day 14 oocyte diameters were not significantly different from those of 26-d-old in vivo control oocytes. The potential to resume meiosis after mechanical removal of granulosa cells was first reached on Day 8; thereafter, removal of the corona showed that all oocytes cultured with FSH remained arrested at the GV stage up to Day 16. After Day 8, approximately 70% of all oocytes underwent GVBD as a result of granulosa-cell removal, but only 23% of these reached MII after 24 h. The in vivo controls reached a comparable GVBD rate (66%) when the granulosa was removed, but most of the oocytes (82%) underwent first polar body extrusion 24 h later. These results suggest that although oocyte diameters after IVM are not different from those of the controls, culture conditions are not yet adequate to support complete meiotic maturation.

Mechanical isolation and in vitro growth of preantral and small antral human follicles

OBJECTIVE

To develop a procedure for isolating small human follicles and to determine their growth requirements.

DESIGN

Preantral and early antral follicles were isolated manually, allocated randomly to experimental groups, and cultured for a few weeks.

SETTING

Patients giving informed consent in hospitals.

PATIENT(S)

Women undergoing laparotomy or oophorectomy.

INTERVENTION(S)

None.

MAIN OUTCOME MEASURE(S)

Follicular size, E2, histology.

RESULT(S)

Human FSH (at a dose of 1.5 U/mL) induced antral growth of follicles, and the addition of human LH (2.5 ng/mL) to human FSH stimulated growth and antral development. Histologic studies showed that most of the early antral follicles did not contain an oocyte and already had begun to undergo atresia before culturing. Levels of E2 increased in the incubation medium as the follicles increased in size, but those levels were significantly greater when the follicles contained oocytes.

CONCLUSION(S)

It is possible to grow small human follicles after they have been isolated manually. To develop successfully, they require a low concentration of human LH in addition to human FSH. The rate of atresia between the preantral and early antral stages in vivo is very high; therefore, it is worthwhile to develop techniques for isolating and culturing the follicles before the antral stages.

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.