Ovarian follicle size or growth rate can both be determinants of ovulatory follicle selection in mice†

The endocrinology regulating ovulation of the desired number of oocytes in the ovarian cycle is well described, particularly in mono-ovulatory species. Less is known about the characteristics that make one follicle suitable for ovulation while most other follicles die by atresia. Bromodeoxyuridine (BrdU) injection was used to characterize granulosa cell proliferation rates in developing ovarian follicles in the estrous cycle of mice. This methodology allowed identification of follicle diameters of secondary (80-130 μm), follicle-stimulating hormone (FSH)-sensitive (130-170 μm), FSH-dependent (170-350 μm), and preovulatory (>350 μm) follicles. Few preovulatory-sized follicles were present in the ovaries of mice at estrus, the beginning of the cycle. Progressive increases were seen at metestrus and diestrus, when full accumulation of the preovulatory cohort (~10 follicles) occurred. BrdU pulse-chase studies determined granulosa cell proliferation rates in the 24-48 h before the follicle reached the preovulatory stage. This showed that slow-growing follicles were not able to survive to the preovulatory stage. Mathematical modeling of follicle growth rates determined that the largest follicles at the beginning of the cycle had the greatest chance of becoming preovulatory. However, smaller follicles could enter the preovulatory follicle pool if low numbers of large antral follicles were present at the beginning of the cycle. In this instance, rapidly growing follicles had a clear selection advantage. The developing follicle pool displays heterogeneity in granulosa cell proliferation rates, even among follicles at the same stage of development. This parameter appears to influence whether a follicle can ovulate or become atretic.

Temperature gradients in the mammalian ovary and genital tract: A clinical perspective

Temperature within mammalian reproductive tissues is noted to be a key component of fertility, and significant gradients in temperature can be demonstrated deep within the abdomen shortly before ovulation. Indeed, in the absence of such gradients in the ovary and genital tract, the processes of ovulation and fertilisation are severely compromised. This review aims to assess literature produced during the last five decades regarding temperature gradients in the mammalian ovary and genital tract. A large body of observations derived from rabbits, women, pigs and cattle is summarised in tabular form, ovarian follicular values being as much as 2.5 °C or more cooler than neighbouring ovarian tissues or deep rectal temperature. We highlight recent works demonstrating a positive correlation between pre-ovulatory follicular cooling and pregnancy. Understanding the significance of follicular cooling should help us (a) explain why so many potential pregnancies fail in vivo as in vitro and (b) inspire ways for improving the processes of fertilisation and establishment of a full-term pregnancy. Based on our findings in domestic animals, and most recently in cows whose Graafian follicles are comparable in size and timing of response to the LH peak with human follicles, we wish to encourage IVF and fertility preservation clinics to take advantage of this work. By so doing, the incidence of full-term pregnancies in women should be improved.

Ovarian follicular development in the unilateral ovariectomized rat

Background and Aims:  Following unilateral ovariectomy, the contralateral ovary underwent hypertrophy and the number of ovulations remained unchanged. The aim of the present study was to elucidate the mechanism, which occurred by this compensation, through examination of ovarian follicular development in operated rats. Methods:  Rats were examined 1-8 weeks post unilateral operation. The ovaries were serially sectioned. All sections were examined and the size of each follicle was measured. Results:  Large follicles soon recovered after surgery, but it took 5 weeks for small and medium-sized follicles to recover. The rate of atresia at the 'critical point' was 31.1% in unilateral ovariectomized animals and 73.9% in the sham-operated rats. Conclusions:  These findings showed that in operated rats, the recovery of the number of follicles varied depending on the size of the follicle. Compared with the control animals, approximately twice as many large healthy follicles were saved from atresia at the 'critical point', producing a compensatory increase in the ovulation number (P < 0.01). (Reprod Med Biol 2005; 4: 89-93).

Ovarian follicular development in the unilateral ovariectomized rat

Background and Aims:  Following unilateral ovariectomy, the contralateral ovary underwent hypertrophy and the number of ovulations remained unchanged. The aim of the present study was to elucidate the mechanism, which occurred by this compensation, through examination of ovarian follicular development in operated rats. Methods:  Rats were examined 1-8 weeks post unilateral operation. The ovaries were serially sectioned. All sections were examined and the size of each follicle was measured. Results:  Large follicles soon recovered after surgery, but it took 5 weeks for small and medium-sized follicles to recover. The rate of atresia at the 'critical point' was 31.1% in unilateral ovariectomized animals and 73.9% in the sham-operated rats. Conclusions:  These findings showed that in operated rats, the recovery of the number of follicles varied depending on the size of the follicle. Compared with the control animals, approximately twice as many large healthy follicles were saved from atresia at the 'critical point', producing a compensatory increase in the ovulation number (P < 0.01). (Reprod Med Biol 2005; 4: 89-93).

Histological analysis of the 'critical point' in follicular development in mice

Aim:  To histologically examine ovarian follicular development in cycling mice. Methods:   Mice were observed at 13:00 h at each stage of the estrous cycle. The ovaries were cut into complete serial sections. All sections were observed, and the size of each follicle was measured by using a micrometer. Follicles with advanced atresia were excluded and non-atretic healthy follicles were differentiated from atretic follicles. Results:  The measurement of the number of follicles in each stage of the estrous cycle, with reference to their size, showed that in each mouse, the number of small healthy follicles (100-249 µm in average diameter) was approximately 100 for all stages and the number of medium-sized healthy follicles (250-349 µm) was close to 20 in only the metestrus and diestrus stages. In contrast, large healthy follicles (≥350 µm) showed marked changes throughout the estrous stage. Many healthy large follicles were observed in the proestrus stage, but they disappeared in the estrus stage, which suggests that they have ovulated. This was supported by observations of oocytes resuming meiosis in large healthy follicles. Conclusion:  As follicular atresia was frequently observed in follicles of 250-399 µm diameter, this size range may be a 'critical point' for atresia. The results suggest that there is a 'critical point' in follicular development, and that only follicles that pass this point will ovulate, while those that do not will become atretic follicles. (Reprod Medical Biol 2004; 3: 141-145).