Cell dynamics of the ovarian cycle

Long-term effects of in vitro growth of mouse oocytes on their maturation and development

Since very few oocytes grow completely in vivo, in vitro growth (IVG) of ovarian oocytes may provide a new source of functional oocytes. The long-term effects of in vitro maturation (IVM) of oocytes and in vitro culture of fertilized eggs have been reported; however, the effects of IVG of oocytes are unknown. Here in, we report the long-term effects of IVG of oocytes. Ovaries from 1-day-old mice containing non-growing oocytes were cultured for 10 days; the isolated follicles were then cultured for 11 days. Secondary follicles from 10-day-old mice were also cultured for 11 days. The nuclei of oocytes collected from the IVG and Graafiais follicles of adult mice were transferred to enucleated oocytes grown in vivo, respectively. Developmental competence was examined following IVM of the reconstituted oocytes. Chronologically, oocytes of 1-day-old, 10-day-old and adult mice were cultured for 22, 12 and 1 day(s). The result showed that the reconstituted eggs developed into pups at high rates after nuclear transfer and in vitro fertilization (IVF) in all the experimental groups (29-45%). However, the pups from reconstituted eggs containing the nuclei of 22-day cultured oocytes were heavier than the control pups (P<0.05). We concluded that long-term culture of oocytes did not affect their nuclear ability to develop to term; however, fetal growth was affected by the culture duration or culture conditions during the initial phase of follicular growth.

Quantification of healthy follicles in the neonatal and adult mouse ovary: evidence for maintenance of primordial follicle supply

Proliferation and partial meiotic maturation of germ cells in fetal ovaries is believed to establish a finite, non-renewable pool of primordial follicles at birth. The supply of primordial follicles in postnatal life should be depleted during folliculogenesis, either undergoing atresia or surviving to ovulation. Recent studies of mouse ovaries propose that intra- and extraovarian germline stem cells replenish oocytes and form new primordial follicles. We quantified all healthy follicles in C57BL/6 mouse ovaries from day 1 to 200 using unbiased stereological methods, immunolabelling of oocyte meiosis (germ cell nuclear antigen (GCNA)) and ovarian cell proliferation (proliferating cell nuclear antigen (PCNA)) and electronmicroscopy. Day 1 ovaries contained 7924+/-1564 (s.e.m.) oocytes or primordial follicles, declining on day 7 to 1987+/-203, with 200-800 oocytes ejected from individual ovaries on that day and day 12. Discarded oocytes and those subjacent to the surface epithelium were GCNA-positive indicating their incomplete meiotic maturation. From day 7 to 100 mean numbers of primordial follicles per ovary were not significantly depleted but declined at 200 days to 254+/-71. Mean numbers of all healthy follicles per ovary were not significantly different from day 7 to 100 (range 2332+/-349-3007+/-322). Primordial follicle oocytes were PCNA-negative. Occasional unidentified cells were PCNA-positive with mitotic figures observed in the cortex of day 1 and 12 ovaries. Although we found no evidence for ovarian germline stem cells, our data support the hypothesis of postnatal follicle renewal in postnatal and adult ovaries of C57BL/6 mice.

Dynamic regulation of mouse ovarian stanniocalcin expression during gestation and lactation

Stanniocalcin is a glycoprotein hormone that appears to play a paracine/autocrine role in several mammalian tissues. Recently studies have shown that stanniocalcin is highly expressed in the ovaries of mice and humans and we have investigated its expression in the mouse ovary during several physiological states to identify potential functional relationships. During postnatal development the pattern of stanniocalcin (STC) gene expression begins to become thecal-restricted as early as day 5 and achieves the adult pattern of expression by two weeks of age. During postnatal development the primary sites of STC protein localization are the theca and oocytes and after maturation it is also strongly concentrated in the corpora lutea. Over the estrous cycle the pattern of both STC gene expression and protein localization do not show dramatic changes though STC immunoreactivity (STCir) staining appears to be greatest during metestrus I. In the superovulation model, however, we observed a significant increase in STC messenger RNA (mRNA) levels after treatment with hCG implying regulation by LH. During gestation the expression of ovarian STC increases 15-fold and is localized to the theca-interstitial cells with lower expression also being found in the corpora lutea. STC also becomes detectable in the serum for the first time suggesting an endocrine role for STC during gestation. Interestingly, the presence of a nursing litter appears to up-regulate STC gene expression in lactating mice suggesting a role for ovarian STC in lactation. Also striking is the intense STCir staining found in oocytes as they are devoid of STC mRNA, thus implying a role for STC in oocyte maturation. Stanniocalcin, to our knowledge, is unique because no other secreted proteins produced by the ovarian thecal-interstitial compartment are significantly induced during mouse pregnancy. In summary, our data provide evidence for the active regulation of STC expression in the ovary during gestation and lactation and therefore implies that STC is a new regulator of the gestational and nursing state.

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.

Follicular growth and kinetics during the estrous cycle, pregnancy and postpartum in the Indian mole rat (Bandicota bengalensis)

Follicular growth and kinetics were studied in detail in the ovaries of the Indian mole rat (Bandicota bengalensis) during various stages of the estrous cycle; days 7, 12, 15, 19, and 21 of pregnancy; and day 2 postpartum. The sizes of follicles, oocytes, nuclei, and nucleoli were measured. In all rats, regression coefficients, a, and intercepts, b, were calculated in oocyte/follicle, oocyte nucleus/follicle and oocyte nucleus/oocyte regressions. The oocyte reached its maximum size when the average follicle diameter was 117 microns in nonpregnant rats and 131 microns in pregnant rats. The oocyte nucleus attained maximum size when the follicle diameter was 110 microns during the estrous cycle and 111 microns during pregnancy and postpartum. Maximum values of the diameter of the largest antral follicle and average diameter of the four largest antral follicles were observed during proestrus (473 and 442 microns, respectively) and on day 21 of pregnancy (611 and 538 microns, respectively). Chi 2 analysis showed that distribution of various types of follicles was not independent of the stage of the estrous cycle and pregnancy. In estrus and metestrus most of the follicles were between stages I and V. However, by diestrus and proestrus, follicles of all size groups developed. The numbers of stage I and II follicles did not differ as pregnancy advanced. More stage V follicles were present on day 12 than on day 7 of pregnancy; however, their numbers decreased by day 15. Afterwards, progressive increase of stage V and (VI + VII) follicles was observed until day 21. This was accompanied by the shift of follicles from stage (III + IV) on days 19 and 21 of pregnancy and even of stage II on day 2 postpartum. Wherever possible, the results have been compared with previous observations in various rodent species.

Ovarian function and the immune system

A hypothesis is presented on the interaction between the immune system and ovary in the regulation of the reproductive system and in the origin of some of its disorders. It has been suggested that the beginning, duration and age dependent failure of ovarian ovulatory function depends among other things on the adequate relationship between the immune system and approriate ovarian target structures. The cyclicity of ovarian function is considered to be primarily dependent on the induction of a specific cyclic immune response to the ovary. Similarly, the selection of a species-specific number of ovulating follicles during sexual maturity is thought to be ensured by immune mechanisms. This hypothesis, on the role of the immune system in regulation of ovulatory ovarian function respects the physiological effect of gonadotropins and steroids on the ovarian structures. The interaction between the ovary and the hypothalamus-pituitary system appears to be modulated by the relationship between the ovary and the immune system.

An autoradiographic study of long-term follicular development in the cyclic hamster

The time required for ovarian follicular development in the cyclic hamster was determined by marking follicles with [3H]-thymidine on day 1 of the estrous cycle (day of ovulation) and by following their fate over the next three cycles by autoradiography. A model of follicular development was formulated, based on the total number of follicles and the changing percentage of labelled follicles with time. It was estimated that 20 days elapsed before a preantral follicle with two to three layers of granulosa cells (stage 1) was able to ovulate. Follicles spent eight days in stage 1 and an additional eight days in stage 2 (4-5 granulosa layers). Preantral follicles entering stage 3 (6-7 granulosa layers) and 4 (greater than 8 layers) on day 1 of the cycle ovulated four days later. Approximately 30 follicles per ovary entered and left stage 1 each cycle, representing a 50% turnover. Of the follicles leaving stage 1, about 50% were transformed into stage 2 follicles; the remaining ones underwent atresia. Stage 2 follicles also had a 50% turnover each cycle. Half of the stage 2 follicles underwent atresia; the remainder developed into stages 3 and 4 follicles. Fifty percent of the stages 3 and 4 follicles present on day 1 of the cycle ovulated at the next estrus with the remainder undergoing atresia.

DNA synthesis in the oocyte of the mature mouse: a radioautographic study

Mice were injected three times over an 8-hour period with a total of 48 muci 3H-T/gm of body weight and were sacrificed 1, 3, 5, 9 and 17 days afterwards. Radioautographs of the ovaries showed significantly higher grain counts in oocytes of follicles that are in the antrum formation stage. The radioautographic visualization of DNase digestible 3H-thymidine incorporation into the juxtanucleolar region in oocytes of mature mice occurs in association with ooctye growth in follicles that are in the antrum formation stage. The scheduled disappearance of this juxtanucleolar oocyte DNA and its label during later oocyte growth suggests a degradation or dispersion of this labeled DNA prior to ovulation.

An analytical model for ovarian follicle dynamics

Differential counts of follicles of different sizes in the ovaries of intact and hypophysectomized strain A mice of various ages have been analysed statistically. The influence of age on the number of follicles in each size group is described. The number of small follicles in the ovarian pool declined exponentially, whereas numbers of follicles in other groups displayed two peaks, one during immaturity and the other in adult life. These appeared sequentially in follicles in successive groups. The proportion of follicles growing and dying was estimated and age dependent fluctuations in the time spent by follicles in each group were demonstrated. Further calculations then determined the mean inflow and outflow of follicles to and from the various groups throughout the lifespan. Hypophysectomy during post-puberty decreased only slightly the utilization of small follicles from the pool and did not prevent their subsequent development to the two-layered stage. Many follicles then died at this stage of development, indicating that the initial main site of gonadotrophic action may be on medium sized follicles with two layers of granulosa cells.

Fertility and irradiation: a preconceptional investigation in teratology

The effects of preconceptional irradiation on female fertility were investigated in six groups of 35 nulliparous Swiss-Webster mice whole-body irradiated with single doses of 25, 50, 75, 100, 150, and 300 rads of 60Co. They were mated with nontreated males and killed on gestational day 18. The litters were recorded in the chronologic order of their occurrence within each group. The percentage of pregnancies decreased with the dose, while the prenatal loss increased. Up to 100 rads, the middle group of pregnancies was devoid of resorptions, fetal death, and congenital anomalies, thus giving rise to a "maximum viability enclave." Beyond this level those three adverse effects were more or less homogeneously distributed over the whole period for which pregnancies were recorded. Exencephaly, microcephaly, and eye defects were the most frequent malformations. Among other sequelae, hypertrophy of the higher pole occurred in the left kidney in the surviving infertile females killed 5 months after treatment.