Anne N. Hirshfield

Development of Follicles in the Mammalian Ovary

Publisher Summary This chapter discusses the development of follicles in the mammalian ovary. The unresolved issues in follicular development are focused. Folliculogenesis culminates in the production of fully ripe, preovulatory follicles visible to the naked eye as large bulges on the surface of the ovary. Each ripe follicle contains thousands of highly differentiated cells. This complex, functional miniature organ arises from the handful of cells that constitute a simple primordial follicle, a structure so small that it is invisible at the lower magnifications of a light microscope. All regulatory influences can only permit or prevent cells from completing the full maturation process; they cannot change the course of differentiation. A plethora of modulating influences act as permissive inducers, impeding or propelling the committed follicular cells through the process of clonal expansion. As each follicle progresses through its program of limited clonal expansion and maturation, its cells proliferate more and more rapidly. With every passing generation, the proliferative potential of the granulosa and theca cells continues to diminish, while the state of maturation increases.

Effect of Bcl-2 on the Primordial Follicle Endowment in the Mouse Ovary

Abstract Little is known about the embryonic factors that regulate the size of the primordial follicle endowment at birth. A few studies suggest that members of the B-cell lymphoma/leukemia-2 (bcl-2) family of protooncogenes may be important determinants. Thus, the purpose of this study was to test whether bcl-2 regulates the size of the primordial follicle pool at birth. To test this hypothesis, three lines of transgenic mice (c-kit/bcl-2 mice) were generated that overexpress human bcl-2 in an effort to reduce prenatal oocyte loss. The overexpression was targeted to the ovary and appropriate embryonic time period with the use of a 4.8-kilobase c-kit promoter. This promoter provided two to three times more expression of bcl-2 in the ovaries with minimal or no overexpression in most nongonadal tissues. On Postnatal Days 8–60, ovaries were collected from homozygous c-kit/bcl-2 and nontransgenic littermates (controls) and processed for histological evaluation of follicle numbers. All lines of c-kit/bcl-2 mice were born with significantly more primordial follicles than control mice (P ≤ 0.05). By Postnatal Days 30–60, however, there were no significant differences in follicle numbers between c-kit/bcl-2 and control mice. These results indicate that bcl-2 overexpression increases the number of primordial follicles at birth, but that the surfeit of primordial follicles is not maintained in postnatal life. These data suggest that it is possible that the ovary may contain a census mechanism by which excess numbers of primordial follicles at birth are detected and removed from the ovary by adulthood.

Overview of ovarian follicular development: considerations for the toxicologist.

Folliculogenesis is the lengthy process that results in the production of a species‐specific, highly consistent number of follicles, which ripen during each reproductive cycle at precisely the appropriate time for ovulation. Certain features of folliculogenesis may have special implications for toxicologists studying effects of environmental mutagens on oocytes. Such features include the constantly changing geometry of the ovarian follicle, the great excess of developing follicles (most of which will degenerate rather than ovulate), the exponential nature of follicular growth, the acceleration of cell proliferation as follicular size increases, and the location of the principal feedback regulatory step at the penultimate stage of the developmental process. Because the ovary can respond quickly and completely to loss of homeostasis over the short term, damage from toxic insult may not be readily apparent. However, long‐range fertility may nevertheless be impaired. The finite size of the follicular pool and the absence of feedback regulatory steps during the early stages of follicular growth render the ovary incapable of restoring the status quo among small and medium‐sized follicles. This will eventually result in loss of fine control over the number of follicles that ripen and the regularity of the reproductive cycles and could reduce the overall duration of the fertile life span. Environ. Mol. Mutagen. 29:10–15, 1997

Ovarian volume and antral follicle counts as indicators of menopausal status.

ObjectiveRecent studies suggest that ovarian volume and antral follicle numbers may be sensitive, specific, and early indicators of menopausal status. The accuracy of these markers, however, has not been compared directly to more traditional markers [age and follicle-stimulating hormone (FSH) levels]. Thus, the purpose of this study was to test whether ovarian volume and antral follicle counts are more sensitive and specific markers of menopausal status than age or FSH levels. DesignPremenopausal (n = 34) and postmenopausal (n = 25) women between 40 and 54 years old received a transvaginal ultrasound for determination of ovarian volume and antral follicle numbers, provided blood for measurement of FSH levels, and completed a questionnaire. FSH levels, age, ovarian volume, and antral follicle numbers were compared using t tests. Receiver operating characteristic curves were generated to evaluate the sensitivity and specificity of each marker. ResultsPostmenopausal women had significantly higher FSH levels (p ≤ 0.0001), smaller ovarian volumes (p ≤ 0.002), and fewer antral follicles (p ≤ 0.002) than premenopausal women. Ovarian volume and antral follicle numbers had similar sensitivity (27.3–100%) and specificity (3.4–92.9%) in indicating postmenopausal status as FSH levels and age. ConclusionThese data suggest that ovarian volume and antral follicle numbers may be useful indicators of menopausal status.

Ovarian volume and menopausal status.

Objective: The purposes of this study were to (1) examine whether ovarian volume differs by age and menopausal status in healthy women; (2) evaluate whether ovarian volume could be a sensitive and specific predictor of menopausal status; and (3) assess whether ovarian volume is affected by cigarette smoke, oral contraceptives (OCs), and hormone replacement therapy (HRT). Design: Each participant (527 women) completed an extensive in‐home interview that assessed age, menopausal status, smoking history, OC use, and HRT use. Each participant also received a transvaginal ultrasound that measured ovarian volume. Geometric means for ovarian volume were compared between premenopausal and postmenopausal women using t tests. Tests for trends were conducted using linear regression analyses. Results: Ovarian volume declined with age (p ≤ 0.0001) and also differed by menopausal status; postmenopausal women had smaller ovarian volumes than premenopausal women of the same age (p ≤ 0.0001). Ovarian volume was not associated with smoking history or HRT use. However, it was significantly smaller in current users of OCs compared with past users of or those who never used OCs (p ≤ 0.0001). Ovarian volume was a sensitive and specific predictor of postmenopausal status. Conclusions: The data suggest that age, menopausal status, and OC use may be determinants of ovarian volume. They also suggest that ovarian volume may be useful for predicting menopausal status in women. (Menopause 2000;7:53‐61.

Using women's health research to develop women leaders in academic health sciences: the National Centers of Excellence in Women's Health.

While the number of women entering U.S. medical schools has risen substantially in the past 25 years, the number of women in leadership positions in academic medicine is disproportionately small. The traditional pathway to academic leadership is through research. Womens health research is an ideal venue to fill the pipeline with talented women physicians and scientists who may become academic leaders in positions where they can promote positive change in womens health as well as mentor other women. The Office on Womens Health (OWH) in the U.S. Department of Health and Human Services has contracted with 18 academic medical centers to develop National Centers of Excellence in Womens Health. Emphasizing the integral link between womens health and women leaders, each of the Centers of Excellence must develop a leadership plan for women in academic medicine as part of the contract requirements. This paper describes the training programs in womens health research that have developed at five of the academic medical centers: the University of Wisconsin, Magee Womens Hospital, the University of Maryland, Medical College of Pennsylvania Hahnemann University, and the University of Illinois at Chicago. We discuss some of the challenges faced for both initiation and future viability of these programs as well as criteria by which these programs will be evaluated for success.

A Role for Neurotrophic Factors in Ovarian Development

In recent years it has become increasingly clear that several growth factors involved in the differentiation of neural cells are similar or identical to those that control the developmental fate of nonneural cells. One of the most striking examples of this regulatory linkage is provided by stem cell factor (SCF), the gene product of the mouse steel (Sl) locus that is required for the differentiation and proliferation of three entirely different cell lineages: neural crest-derived melanocytes, hematopoietic cells, and gonadal germ cells (reviewed in 1). Although SCF induces neither proliferation nor migration of gonadal germ cells, it appears to be essential for their survival (2). Thus, mutations at either the Sl locus (that disrupt the synthesis of SCF) or at the white spotting (W) locus that encodes the SCF receptor (1) severely deplete the number of germ cells able to colonize the genital ridge (2, 3). Strong evidence exists that the SCF receptor is the product of the c-kit protooncogene (4), a transmembrane tyrosine kinase receptor encoded by W (5).

Histological assessment of follicular development and its applicability to risk assessment.

The objective of this article is to describe histological methods currently used to study ovarian physiology that may have applicability to reproductive toxicology. Histologic techniques are, for the most part, inexpensive and easy to perform. Many of these techniques require little equipment and can be readily implemented in small laboratories. The resulting histological preparations are permanent, can be used over again for several different types of analyses, and provide objective, quantifiable endpoints. These techniques have been extremely useful for studying the ovary.

Correlation of age-associated increases in follicle stimulating hormone secretion with decreases in antral follicles: Failure of progesterone-induced acyclicity to prevent these changes

It is well known that the number of follicles in the mammalian ovary decreases with age. In light of previous data from this laboratory showing age-related alterations in the secretion and production of follicle-stimulating hormone (FSH) in rats by 5-6 months of age, one objective of the present study was to determine if alterations in FSH secretion were accompanied by changes in the number of antral follicles. A second objective of this study was to determine whether or not interruption of cyclic activity by continuous progesterone (P) treatment could decelerate age-associated changes in FSH secretion possibly by retarding the depletion of follicles through ovulation. For this study, one group of 4-day cycling, 7-week-old rats received one empty Silastic implant while another group received 3-40 mm implants containing 30 mm crystalline P. Implants were replaced every 2 weeks until the animals were 5 months old. Progesterone-implanted rats were acyclic during treatment exhibiting predominantly leukocytic vaginal smears. Regular 4-day cycles resumed when P implants were withdrawn (rats approximately 5-6-months-old). A group of 2-3-month-old untreated rats were used for comparison. As expected from our previous results, serum FSH levels at 1600 h on estrus were significantly higher in 5-6-month-old rats receiving empty capsules than in younger rats. Serum FSH concentrations measured in P-treated rats at this time also were significantly higher than levels of this gonadotropin measured in younger rats. Ovaries of older control and P-treated rats contained significantly fewer medium and large antral follicles (greater than 250 microns) than the ovaries of younger rats despite the curtailment of estrous cyclicity and ovulation by continuous P treatment. Interestingly, P treatment prevented the age-associated decrease in thymus weight. Taken together, the present observations suggest that a decrease in the number of growing follicles may be a factor contributing to early age-related alterations in FSH secretion. Furthermore, the prevention (at least temporarily) of age-related thymic involution by P treatment may be indicative of an interrelationship between thymic and reproductive aging.

50 – Environmental Exposures and Women's Reproductive Health

Environmental chemicals have the potential to alter female reproductive tissues and thus affect the ability of women to conceive healthy offspring. During the twentieth century, industrial and technological advances lead to the production and use of many new chemicals. Chemical exposures can interfere with the structure and function of the ovaries, uterus, breasts, cervix, and the hypothalamic pituitary unit. Chemicals also may affect reproductive health by altering reproductive processes such as menarche, menstrual cycle, pregnancy, libido, and menopause. The incidence of many reproductive disorders is unknown. However, data from Global Health Statistics show that the incidence of infertility for women of all ages varies by country. The lowest incidence is reported in China and the highest incidence is found in Africa. Relatively few studies have examined associations between chemical exposures and adverse reproductive outcomes in women. Observations in wildlife have suggested the specific possibility that human exposure to environmental compounds with endocrine-disrupting capability may have adverse effects on reproduction.