In vitro models for oocyte development

The mammalian ovary has a large store of primordial follicles, which are a potential source of oocytes for in vitro production of embryos. Several culture systems have been developed to support the growth and development of oocytes from rodent primordial and preantral follicles and progress is slowly being made in modifying these techniques to support the in vitro growth of porcine and bovine follicles. Oocytes from porcine preantral follicles can acquire competence to resume meiosis and proceed to Metaphase II after in vitro growth (IVG) but fertilisation has yet to be demonstrated. This paper presents the current status of technology for the in vitro growth and development of immature mammalian oocytes. Culture systems used successfully to grow immature rodent oocytes are compared and adaptations of these methods to support porcine and bovine oocyte growth discussed.

Bovine cumulus cell expansion does not depend on the presence of an oocyte secreted factor

Communication between the oocyte and its somatic cells has been shown to be important in oocyte development. Here we examined how the oocyte may be involved in bovine cumulus cell expansion. Intact bovine cumulus oocyte complexes (COC) were obtained by puncturing antral follicles. From the intact COC, oocytectomised complexes (OOX) were produced by micro surgical removal of the oocyte. Clumps of cumulus cells (CC) were obtained by micro-dissection. Intact or OOX complexes or CC were matured in the presence of fetal calf serum and hFSH (6 mlU/ml) for 24 hr and the degree of expansion measured. The presence of the oocyte is not essential to allow bovine cumulus expansion to occur as expansion occurred in all groups. Murine OOX complexes from eCG primed 35-40-day-old C57BL6/CBA F1 hybrids (known to require the presence of an oocyte secreted factor for cumulus expansion) were cultured with or without denuded bovine oocytes (1 oocyte/microliter). Murine OOX complexes expanded only in the presence of denuded bovine oocytes. Thus some factor produced by bovine oocytes enabled expansion of murine OOX complexes. To determine whether the factor is secreted by bovine oocytes, murine OOX were cultured with or without media conditioned by bovine oocytes (1 oocyte/microliter for 4 hr). Significant expansion of murine OOX occurred in media conditioned by bovine oocytes. This shows that the cumulus expansion enabling effect of bovine oocytes is released into the surrounding media. Media conditioned by bovine oocytes and then frozen for up to 1 month showed that the activity by the factor can withstand freezing.

Production of cumulus expansion enabling factor by mouse oocytes grown in vitro: preliminary characterization of the factor

The objective of this study was to determine whether fully grown oocytes, obtained after isolation from preantral follicles and growth in vitro, secrete paracrine factors affecting granulosa cell development and function. If so, the relative ease in producing oocytes in this way could facilitate the identification and characterization of the factors. As a test of this idea, the ability of in vitro grown oocytes to produce a paracrine factor that is known to enable the isolated cumulus oophorus to undergo expansion in response to follicle stimulating hormone (FSH) was determined. Initial experiments compared culture systems, which differed in the orientation of the oocyte-granulosa cell complexes from preantral follicles to an extracellular matrix, for their ability to support oocyte growth and the acquisition of competence to resume meiosis. The systems for culture on the surface of the matrix produced larger oocytes and the highest percentage of oocytes having competence to resume meiosis. Oocytes grown using this system secreted active cumulus expansion enabling factor, albeit at levels about half that of oocytes grown in vivo. A preliminary characterization of the cumulus expansion enabling factor secreted by the oocytes grown in vitro showed that activity was lost upon treatment with either heat (65 degrees C for 15 min) or proteinase K. Activity did not pass through a membrane having a nominal molecular weight limit (NMWL) of 100 kd but did pass through a membrane having a NMWL of 300 kd. It is concluded that cumulus expansion enabling factor is secreted by oocytes grown in vitro. This factor is probably a protein or depends upon a protein for its activity.(ABSTRACT TRUNCATED AT 250 WORDS)

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

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

Impact of exogenous progesterone on ovarian follicular dynamics and function in mice

Raising progesterone concentrations in young adult mice by subcutaneous implants resulted in ovulation being blocked and the cessation of oestrous cycles. The effect of this treatment on the numbers and dynamics of preantral follicles during 36 days of treatment was studied using a compartmental model to analyse differential follicle counts. Changes in growth and/or death rates were detected at all stages of follicular development. An increased rate of growth through preantral stages was predicted in the treatment group when compared with the controls, but most of these follicles did not reach the antral stage of development as an increased death rate was observed at large preantral stages (stage IV). Antral follicles were formed in the treatment group, but all succumbed to atresia. Increased atresia in the antral population of follicles in the treatment group was observed directly.

Extra-ovarian production of mature viable mouse oocytes from frozen primary follicles

Isolated primary mouse follicles can be frozen successfully and thawed in the presence of 1.5 m-DMSO. Similar proportions of freshly collected and frozen-thawed primary follicles undergo folliculogenesis in the absence of other ovarian tissue. Some of the mature oocytes recovered from these follicles were fertilized in vitro and, after transfer to pseudopregnant recipients at the 2-cell stage, developed into live young. Cryopreservation and extra-ovarian development of immature follicles provide a unique opportunity to store large numbers of female gametes.

Morphological study of cultured preantral ovarian follicles of mice after transplantation under the kidney capsule

Isolated ovarian follicles taken from 10-day-old mice and cultured in collagen gel for 5 days, in the presence or absence of serum, were transplanted under the kidney capsule of ovariectomized mice. Hosts showed vaginal opening within 5 days and cornified vaginal smears by 9 days. Follicles proceeded to Graafian stages and luteinization occurred. Ovulation was not observed and oocytes degenerated within the luteinized follicle. Theca formation was preceded by the appearance of blood vessels within the graft. In-vitro fertilisation of harvested oocytes resulted in embryos.

Quantitative study of the development of isolated mouse pre-antral follicles in collagen gel culture

Follicles were isolated from the ovaries of 10-day-old C57BL6/CBA F1 hybrids by mechanical and enzymic treatment, embedded in a collagen-gel matrix to maintain the 3 dimensional integrity of the follicle and cultured for up to 14 days. Gels were removed at various times during the culture period and prepared for histology. Follicles grew from unilaminar to multilaminar stages within 6 days of the culture period. A more detailed assessment of growth by counting follicles at different stages and measuring oocyte and follicle diameters showed that follicle growth was maintained for up to 14 days in culture. Initially the proportion of unhealthy follicles was high but this declined after 6 days in culture.

Ovarian cyclicity and follicular recruitment in unilaterally ovariectomized mice

Halving the numbers of follicles in young adult mice by unilateral ovariectomy resulted in compensatory Graafian follicle growth with a reduction by about 25% of the expected number of oestrous cycles. The impact of the operation on the numbers and dynamics of preantral follicles during the first 2 months after ovariectomy was studied using a compartmental mathematical model to analyse differential follicle counts. There were changes in growth and/or death rates at all stages of follicle development, and the patterns emerging were time-dependent. The rate of follicle survival from the pool of unilaminar stages was paradoxically reduced, but those forming two granulosa cell layers continued to develop towards Graafian size. As the frequency of follicle death declined, the numbers of healthy large preantral and antral stages in unpaired ovaries rose to approach those in pairs of age-matched control ovaries, suggesting that follicles otherwise undergoing atresia were being rescued. In the long-term, follicle dynamics after unilateral ovariectomy at young ages did not appear to compromise fecundity seriously.

The number of clonal precursors of the follicular epithelium in the mouse ovary

The number of clonal precursors of granulosa cells in mouse ovarian follicles has been estimated using a technique based on the phenomenon of random X-chromosome inactivation of somatic cells and the use of an X-linked alloenzyme variant of the glycolytic enzyme PGK-1. The granulosa cells of follicles were oligoclonal in origin and founded by a small number of cells (about 5) which was consistent with histological observations. When the analysis was extended to two subcompartments of the follicle, the mural and cumulus granulosa cells, the results indicated that the cumulus and mural granulosa cells had a common origin.

The kinetics of pre-antral follicle development in ovaries of CBA/Ca mice during the first 14 weeks of life

The kinetics of ovarian follicle growth and death have been estimated in virgin inbred mice using a compartmental model and data obtained from differential follicle counts in histologically sectioned ovaries. The results showed that both growth and death rates are dependent on stage of development, defined by the compartments, and age, indicated in the model by step functions with transitions at 20 and 60 days of age. During the initial phase of postnatal ovarian development, large numbers of follicles disappeared from the non-growing reserve as a result of the combined effects of follicle death and recruitment into the growing population. The reduced death rate after 20 days led to a secondary peak in the numbers of follicles at intermediate stages. In contrast to these fluctuations, the number of large follicles, including pre-ovulatory types, were remarkably constant after this age and the rate of outflow stabilized at two to three follicles per day after an initially high value. This rate is sufficient for the normal ovulation rate in a 4-day oestrous cycle with a small surplus of follicles undergoing atresia. The rates of migration through successive stages of development decreased during ageing as large follicles began to emerge at the approach of puberty: this result may indicate that the recruitment of small growing follicles is influenced by a feedback effect.

A quantitative cytological study of polyovular follicles in mammalian ovaries with particular reference to the domestic bitch (Canis familiaris)

The incidence of polyovular types in the growing follicle population was estimated using quantitative cytology. Of 15 species studied, polyovular follicles were recorded in the following species and in ascending order of abundance: rabbits, rhesus monkeys, humans, cats, dogs. The incidence in bitches was 14% in animals aged 1-2 years but only 5% at 7-11 years old. The frequency of the various types of polyovular preantral follicle varied inversely with the numbers of oocytes per follicle and the probability of finding a follicle with more than 5 oocytes was remote. In young ovaries the frequency was constant in the early stages of growth but decreased in the largest preantral stage. The pattern in ageing ovaries was, by contrast, one of declining frequency such that few if any polyovular types completed development. The ovary of the ageing bitch was also characterized by a higher incidence of degenerating follicles and a much smaller pool of primordial stages. Polyovular follicles were larger than uniovular types at comparable stages which were defined by the number of granulosa cell layers. Their oocytes were smaller but the overall ooplasmic mass was increased with a corresponding increase in the mass of granulosa cells.