Development of the ovarian surface and associated germ cells in the human fetus. A correlated study by scanning and transmission electron microscopy

Becoming female: Ovarian differentiation from an evolutionary perspective

Differentiation of the bipotential gonadal primordium into ovaries and testes is a common process among vertebrate species. While vertebrate ovaries eventually share the same functions of producing oocytes and estrogens, ovarian differentiation relies on different morphogenetic, cellular, and molecular cues depending on species. The aim of this review is to highlight the conserved and divergent features of ovarian differentiation through an evolutionary perspective. From teleosts to mammals, each clade or species has a different story to tell. For this purpose, this review focuses on three specific aspects of ovarian differentiation: ovarian morphogenesis, the evolution of the role of estrogens on ovarian differentiation and the molecular pathways involved in granulosa cell determination and maintenance.

Six Decades of Research on Human Fetal Gonadal Steroids

Human fetal gonads acquire endocrine steroidogenic capabilities early during their differentiation. Genetic studies show that this endocrine function plays a central role in the sexually dimorphic development of the external genitalia during fetal development. When this endocrine function is dysregulated, congenital malformations and pathologies are the result. In this review, we explain how the current knowledge of steroidogenesis in human fetal gonads has benefited from both the technological advances in steroid measurements and the assembly of detailed knowledge of steroidogenesis machinery and its expression in human fetal gonads. We summarise how the conversion of radiolabelled steroid precursors, antibody-based assays, mass spectrometry, ultrastructural studies, and the in situ labelling of proteins and mRNA have all provided complementary information. In this review, our discussion goes beyond the debate on recommendations concerning the best choice between the different available technologies, and their degrees of reproducibility and sensitivity. The available technologies and techniques can be used for different purposes and, as long as all quality controls are rigorously employed, the question is how to maximise the generation of robust, reproducible data on steroid hormones and their crucial roles in human fetal development and subsequent functions.

Formation of the Bovine Ovarian Surface Epithelium during Fetal Development

When first formed, the ovary only has an established epithelium at its base or hilum. Later, an epithelium is established around the rest of the ovary. To examine this further, we conducted scanning electron microscopy of the surface of bovine fetal ovaries and immunohistochemistry of ovarian cross-sections. From the earliest time point, the cells on the surface of the base or hilum of the ovary were cuboidal. On the remainder of the ovary, the surface was more irregular. By mid-development, the surface was covered completely with either a stratified or simple epithelium of cuboidal cells. Clefts were observed in the surface and appeared to form due to the expansion of stroma surrounding each open ovigerous cord, elevating the areas surrounding each cord, while leaving the opening of the cord to form the base of each cleft. The continued expansion of the surrounding stroma below the surface appeared not only to close the ovigerous cords from the surface but to compress the clefts into the shape of a groove. Later, most of the ovarian surface was covered with a simple cuboidal epithelium. The changes to the ovarian surface during fetal development coincide with the remodeling of the stroma and cords below.

Hyperplasia and Metaplasia of Ovarian Surface Epithelium in Women with Endometrial Carcinoma. Suggestion for a Hormonal Influence in Ovarian Carcinogenesis

Surface ovarian epithelium and that of related inclusion cysts were comparatively studied in two groups of patients: one group of 50 non-pregnant women without myometrial, endometrial, or cervical hyperplasia and/or neoplasia; a second group of 50 women surgically treated for endometrial cancer and without evidence of ovarian pathology. Papillomatosis, hyperplasia and tubaric, squamoid, endometrioid and mucinous metaplasia were more frequently present in ovarian surface epithelium or in related inclusion cysts in patients with endometrial carcinoma. These findings may be correlated with a hormonal oncogenic stimulus involved in the genesis of endometrial cancer and suggests a new « hormonal » hypothesis in the pathogenesis of common epithelial tumors of the ovary.

Ovarian regeneration: The potential for stem cell contribution in the postnatal ovary to sustained endocrine function

The endocrine function of the ovary is dependent upon the ovarian follicle, which on a cellular basis consists of an oocyte surrounded by adjacent somatic cells responsible for generating sex steroid hormones and maintenance of hormonal stasis with the hypothalamic-pituitary axis. As females age, both fertility and the endocrine function of the ovary decline due to waning follicle numbers as well as aging-related cellular dysfunction. Although there is currently no cure for ovarian failure and endocrine disruption, recent advances in ovarian biology centered on ovarian stem cell and progenitor cell populations have brought the prospects of cell- or tissue-based therapeutic strategies closer to fruition. Herein, we review the relative contributions of ovarian stem cells to ovarian function during the reproductive lifespan, and postulate steps toward the development of ovarian stem cell-based approaches to advance fertility treatments, and also importantly to provide a physiological long-term means of endocrine support.

Ultrastructural Aspects of the Prenatal Bovine Ovary Differentiation with a Special Focus on the Interstitial Cells

The aim of this investigation was to study the ultrastructural features during the development of fetal bovine ovaries (crown rump length ranging from 11.4 to 94.0 cm). An interesting observation was the occurrence of big elongated cells containing a variety of electron dense granules and light homogenous vacuoles/bodies. They were located between the stroma cells surrounding the germ cell cord ends, adjacent to the first formed primordial follicles, typically situated near blood vessels. ER alpha and ER beta receptor positive cells could be detected in the same regions by means of immunohistochemistry. Intercellular bridges linked the germ cells nests oogonia. Germ cell cords consisted of centrally located, large, pale oogonia, surrounded by elongated somatic cells with very long cytoplasm extensions. Primordial follicles with flat pale follicular cells could be observed on the inner end of the cords. Extrusions of the outer nuclear membrane could often been recognised in voluminous oocytes.

Defining the neighborhoods that escort the oocyte through its early life events and into a functional follicle

The ovary functions to chaperone the most precious cargo for female individuals, the oocyte, thereby allowing the passage of genetic material to subsequent generations. Within the ovary, single oocytes are surrounded by a legion of granulosa cells inside each follicle. These two cell types depend upon one another to support follicle formation and oocyte survival. The infrastructure and events that work together to ultimately form these functional follicles within the ovary are unprecedented, given that the oocyte originates as a cell like all other neighboring cells within the embryo prior to gastrulation. This review discusses the journey of the germ cell in the context of the developing female mouse embryo, with a focus on specific signaling events and cell-cell interactions that escort the primordial germ cell as it is specified into the germ cell fate, migrates through the hindgut into the gonad, differentiates into an oocyte, and culminates upon formation of the primordial and then primary follicle.

Seeking the origin of female germline stem cells in the mammalian ovary

The function of female germline stem cells (FGSCs, also called oogonial stem cells) in the adult mammalian ovary is currently debated in the scientific community. As the evidence to support or discard the possible crucial role of this new class of germ cells in mammals has been extensively discussed, in this review, we wonder which could be their origin. We will assume that FGSCs are present in the post-natal ovaries and speculate as to what origin and characteristics such cells could have. We believe that the definition of these features might shed light on future experimental approaches that could clarify the ongoing debate.

The role of ovarian surface epithelium in folliculogenesis during fetal development of the bovine ovary: a histological and immunohistochemical study

Although many aspects of ovarian differentiation have been established, comparatively little is known about prenatal follicle formation and differentiation of bovine ovaries. The objective of this investigation was to study the role of the surface epithelium during the development of germ cell nests, germ cell cords and follicle formation in the fetal bovine ovary. Associated important proliferation and apoptotic features were further investigated. Additionally, the expression pattern of the S100 protein was detected. A strong increase of mitotic figures was detected in the surface epithelium, germ cell nests and germ cell cords of ovaries with a crown-rump length (CRL) of 13.0-58.0 cm. Oocytes were positively stained with S100 in bovine ovaries from fetuses with a CRL of 21.0 cm. The staining intensity enhanced parallel to increasing oocyte and follicle sizes during the ovary development. In later stages, a strong staining for S100 was observed in healthy oocytes in contradistinction to atretic oocytes where no expression of the S100 protein could be found. In conclusion, increasing mitosis index of surface epithelium cells, as well as oogonia directly beneath the surface epithelium, in combination with open surface connection during stages from a CRL of 11.0-94.0 cm of bovine fetal ovaries could play an important role in the period of time of ongoing folliculogenesis and derivation of granulosa cells. Additionally, S100-positive oocytes in primordial and later follicle stages joined by a high rate of Ki67-positive index in surrounding granulosa cells indicate that in the oocytes the S100 protein can perhaps be a useful marker for intact oocytes in bovine ovaries.

Germline cells in ovarian surface epithelium of mammalians: a promising notion

It is a long held doctrine in reproductive biology that women are born with a finite number of oocytes and there is no oogenesis during the postnatal period. However, recent evidence challenges this by showing the presence of germ line stem cells in the human ovarian surface epithelium (OSE), which can serve as a source of germ cells, and differentiate into oocyte like structures. Postnatal renewal of oocytes may have enormous therapeutic potential especially in women facing the risk of premature ovarian failure idiopathically or iatrogenically after exposure to gonadotoxic chemotherapy and radiation for cancer therapy.This article reviews current knowledge on germ line stem cells in human OSE.

Immunoregulation of follicular renewal, selection, POF, and menopause in vivo, vs. neo-oogenesis in vitro, POF and ovarian infertility treatment, and a clinical trial

The immune system plays an important role in the regulation of tissue homeostasis ("tissue immune physiology"). Function of distinct tissues during adulthood, including the ovary, requires (1) Renewal from stem cells, (2) Preservation of tissue-specific cells in a proper differentiated state, which differs among distinct tissues, and (3) Regulation of tissue quantity. Such morphostasis can be executed by the tissue control system, consisting of immune system-related components, vascular pericytes, and autonomic innervation. Morphostasis is established epigenetically, during morphogenetic (developmental) immune adaptation, i.e., during the critical developmental period. Subsequently, the tissues are maintained in a state of differentiation reached during the adaptation by a "stop effect" of resident and self renewing monocyte-derived cells. The later normal tissue is programmed to emerge (e.g., late emergence of ovarian granulosa cells), the earlier its function ceases. Alteration of certain tissue differentiation during the critical developmental period causes persistent alteration of that tissue function, including premature ovarian failure (POF) and primary amenorrhea. In fetal and adult human ovaries the ovarian surface epithelium cells called ovarian stem cells (OSC) are bipotent stem cells for the formation of ovarian germ and granulosa cells. Recently termed oogonial stem cells are, in reality, not stem but already germ cells which have the ability to divide. Immune system-related cells and molecules accompany asymmetric division of OSC resulting in the emergence of secondary germ cells, symmetric division, and migration of secondary germ cells, formation of new granulosa cells and fetal and adult primordial follicles (follicular renewal), and selection and growth of primary/preantral, and dominant follicles. The number of selected follicles during each ovarian cycle is determined by autonomic innervation. Morphostasis is altered with advancing age, due to degenerative changes of the immune system. This causes cessation of oocyte and follicular renewal at 38 +/-2 years of age due to the lack of formation of new granulosa cells. Oocytes in primordial follicles persisting after the end of the prime reproductive period accumulate genetic alterations resulting in an exponentially growing incidence of fetal trisomies and other genetic abnormalities with advanced maternal age. The secondary germ cells also develop in the OSC cultures derived from POF and aging ovaries. In vitro conditions are free of immune mechanisms, which prevent neo-oogenesis in vivo. Such germ cells are capable of differentiating in vitro into functional oocytes. This may provide fresh oocytes and genetically related children to women lacking the ability to produce their own follicular oocytes. Further study of "immune physiology" may help us to better understand ovarian physiology and pathology, including ovarian infertility caused by POF or by a lack of ovarian follicles with functional oocytes in aging ovaries. The observations indicating involvement of immunoregulation in physiological neo-oogenesis and follicular renewal from OSC during the fetal and prime reproductive periods are reviewed as well as immune system and age-independent neo-oogenesis and oocyte maturation in OSC cultures, perimenopausal alteration of homeostasis causing disorders of many tissues, and the first OSC culture clinical trial.

Ovarian stem cells and aging

OBJECTIVE

To review successes to date in the field of ovarian stem cell research and discuss the evidence supporting their potential to rejuvenate the follicular pool during adult life; to present factors that may contribute to their competence; and to address the question of why menopause is an inevitable outcome of advanced age if ovarian stem cells exist.

METHOD

In a review of the literature, relevant articles were identified through a PubMed literature search from inception to July 2010.

RESULTS

The current concept that mammalian ovaries possess a static ovarian reserve is at odds with the experimental results discussed in this review. Ovarian stem cells are likely to be the source of germline stem cells during fetal and adult life, due to their potential to differentiate into competent oocytes given a suitable environment.

CONCLUSIONS

Stem cells in different compartments share properties such as pluripotency, self-renewal, and diminished regenerative potential in old age. Our model of ovarian stem cell aging suggests that menopause is driven by an age-related decline in ovarian stem cell function rather than depletion of a non-renewable follicular reserve. Understanding how ovarian stem cells interact with their surrounding environment moves us a step closer to controlling the female biological clock when it might be clinically desirable.

Serous papillary adenocarcinoma possibly related to the presence of primitive oocyte-like cells in the adult ovarian surface epithelium: a case report

INTRODUCTION

The presence of oocytes in the ovarian surface epithelium has already been confirmed in the fetal ovaries. We report the presence of SSEA-4, SOX-2, VASA and ZP2-positive primitive oocyte-like cells in the adult ovarian surface epithelium of a patient with serous papillary adenocarcinoma.

CASE PRESENTATION

Ovarian tissue was surgically retrieved from a 67-year old patient. Histological analysis revealed serous papillary adenocarcinoma. A proportion of ovarian cortex sections was deparaffinized and immunohistochemically stained for the expression of markers of pluripotency SSEA-4 and SOX-2 and oocyte-specific markers VASA and ZP2. The analysis confirmed the presence of round, SSEA-4, SOX-2, VASA and ZP2-positive primitive oocyte-like cells in the ovarian surface epithelium. These cells were possibly related to the necrotic malignant tissue.

CONCLUSION

Primitive oocyte-like cells present in the adult ovarian surface epithelium persisting probably from the fetal period of life or developed from putative stem cells are a pathological condition which is not observed in healthy adult ovaries, and might be related to serous papillary adenocarcinoma manifestation in the adult ovarian surface epithelium. This observation needs attention to be further investigated.

Ovarian surface epithelium in patients with severe ovarian infertility: a potential source of cells expressing markers of pluripotent/multipotent stem cells

The aim of this study was to confirm the presence of stem cells in the ovarian surface epithelium of patients with premature ovarian failure and no mature follicles and oocytes. In these patients, small round cells of unknown origin expressing SOX-2 marker of pluripotency were observed among the epithelial cells just after the ovarian surface epithelium scraping. These cells were an integral part of the ovarian surface epithelium. When the scraped cells were cultured in a medium with added follicular fluid to provide some ovarian niche, primitive oocyte-like cells and typical round-shaped cell clusters positively stained on alkaline phosphatase, and markers of pluripotency, such as SOX-2 and SSEA-4, were developed. These markers were expressed early and also later in the culture. Single oocyte-like cells expressed genes OCT4A, SOX-2, NANOG, NANOS, STELLA, CD9, LIN28, KLF4, GDF3, and MYC, characteristic for pluripotent stem cells. The results of this study confirmed the presence of putative stem cells in the ovarian surface epithelium of these patients and provided some basis to create a stem cell line in the future.

Estrogen regulation of placental angiogenesis and fetal ovarian development during primate pregnancy

During human and nonhuman primate pregnancy, an extensive blood vessel network is established within the villous placenta to support fetal growth and follicles develop within the fetal ovary to provide a pool of oocytes for reproductive function in adulthood. These two important developmental events occur in association with a progressive increase in placental estrogen production and levels. This review will describe the developmental processes required for placental vascularization and fetal follicular maturation and recent studies which show that estrogen has an important role in regulating these events.

Building pathways for ovary organogenesis in the mouse embryo

Despite its significant role in oocyte generation and hormone production in adulthood, the ovary, with regard to its formation, has received little attention compared to its male counterpart, the testis. With the exception of germ cells, which undergo a female-specific pattern of meiosis, morphological changes in the fetal ovary are subtle. Over the past 40 years, a number of hypotheses have been proposed for the organogenesis of the mammalian ovary. It was not until the turn of the millennium, thanks to the advancement of genetic and genomic approaches, that pathways for ovary organogenesis that consist of positive and negative regulators have started to emerge. Through the action of secreted factors (R-spondin1, WNT4, and follistatin) and transcription regulators (beta-catenin and FOXL2), the developmental fate of the somatic cells is directed toward ovarian, while testicular components are suppressed. In this chapter, we review the history of studying ovary organogenesis in mammals and present the most recent discoveries using the mouse as the model organism.

Ovarian germ cells

Surface cells in adult ovaries represent germ line-competent embryonic stem cells. They are a novel type of totipotent progenitors for distinct cell types including female germ cells/oocytes, with the potential for use in the autologous treatment of ovarian infertility and stem cell therapy. Ovarian infertility and stem cell therapy are complex scientific, therapeutic, and socioeconomic issues, which are accompanied by legal restrictions in many developed countries. We have described the differentiation of distinct cell types and the production of new eggs in cultures derived from adult human ovaries. The possibility of producing new eggs from ovarian surface epithelium representing totipotent stem cells supports new opportunities for the treatment of premature ovarian failure, whether idiopathic or after cytostatic chemotherapy treatment, as well as infertility associated with aged primary follicles, and infertility after natural menopause. The stem cells derived from adult human ovaries can also be used for stem cell research and to direct autologous stem cell therapy. This chapter describes general considerations regarding the egg origin from somatic progenitor cells, oogenesis and follicle formation in fetal and adult human ovaries (follicular renewal), including the promotional role of the immune system-related cells in vivo, and possible causes of ovarian infertility. It then provides detailed protocols for the separation and cultivation of adult ovarian stem cells.

Cell commitment by asymmetric division and immune system involvement

Asymmetric division is a fundamental means of generating cell diversity and may involve extrinsic or intrinsic factors. Here we review observations on symmetric and asymmetric expression of estrogen receptor alpha (ERA) and beta (ERB) during regeneration of trophoblast cells in human placenta and possibly other estrogen-responsive cell types. This is a type of differentiation from committed progenitor cells. Asymmetric segregation of ERA in dividing villous cytotrophoblast cells, accompanied by appearance of ERB in differentiating daughter cells and resulting syncytiotrophoblast, suggests a unique role of estrogen receptors in asymmetric division of estrogen responsive cells. We also review observations on asymmetric division of ovarian surface epithelium (OSE) stem cells resulting in formation of germ cells differentiating into oocytes in fetal and adult human ovaries. Besides germ cells, the OSE stem cells also give rise to primitive ovarian granulosa (follicular) cells, which are required for the formation of new primary follicles and preservation and differentiation of oocytes. This dual potential of OSE stem cells (germ or granulosa cells) is a type of differentiation from uncommitted and possibly totipotent adult stem cells. A possible role of immune system related cells (monocyte-derived cells and T lymphocytes-cellular signaling) and hormones in the stimulation of OSE differentiation toward germ cells by asymmetric division, and in the continuation of ovarian follicular renewal during prime reproductive period in human females is also reviewed. Follicular renewal ceases after prime reproductive period, possibly due to the diminution of cellular signaling required for asymmetric division of OSE stem cells into the germ cells. The primary follicles persisting in premenopausal ovaries appear to accumulate genetic alterations, a cause of exponentially growing chromosomal abnormalities in the progeny of mothers between 38 years of age and menopause.

Immune system involvement in the regulation of ovarian function and augmentation of cancer

Increasing evidence indicates a role for the immune system and mesenchymal-epithelial interactions in the regulation of ovarian function. Cytokines produced by mesenchymal cells can stimulate development and regression of ovarian structures. We report here that mesenchymal cells releasing surface molecules among epithelial cells--namely vascular pericytes and monocyte-derived cells (MDC)--and intraepithelial T lymphocytes are associated with oogenesis and formation of new primary follicles in both fetal and adult human ovaries. These activated mesenchymal cells interact with the ovarian surface epithelium, which appears to be a source of secondary germ cells and granulosa cells. Activated pericytes and MDC are also associated with stimulation of thecal development during selection of growing secondary follicles from the cohort of primary follicles. However, survival of the dominant follicle during mid-follicular phase selection is associated with a lack of activity of mesenchymal cells and retardation of thecal development, since immature granulosa cells lacking aromatase are unable to resist high levels of thecal androgens. Once the selected follicle matures (late follicular phase), it shows enhanced activity of thecal mesenchymal cells and advanced thecal development. Corpus luteum (CL) development is accompanied by a high activity of vascular pericytes and MDC. In mature CL and CL of pregnancy, luteal MDC and pericytes show a stable (inactive) state. Regression of the CL is associated with regression of pericytes, transformation of MDC into dendritic cells, infiltration by T lymphocytes, and binding of immunoglobulin G to the luteal cells. The immunoglobulin M (IgM) binds to young but not mature luteal cells. In the CL of pregnancy, IgM binds to luteal vessels, but not to luteal cells. Regressing CL shows IgM binding to both luteal cells and vessels. In ovarian cancers, highly activated MDC and sometimes activated pericytes (poorly differentiated carcinomas) are present. IgM binding is similar to that seen in the CL of pregnancy. These data indicate that vascular pericytes, MDC, T cells, and immunoglobulins may play an important role in the regulation of ovarian physiology and contribute to the augmentation of ovarian cancer growth.

Primordial follicle activation and follicular development in the juvenile rabbit ovary

Of all the stages of mammalian folliculogenesis, the primordial to primary follicle transition is the least understood. In order to gain new insights into this process, we have conducted a comprehensive morphological, morphometric and molecular study of ovarian organisation and early follicle development in the rabbit. The structure of ovaries collected from rabbits aged from 2-12 weeks (a period encompassing primordial follicle formation, activation and the first wave of folliculogenesis in this species) has been analysed by light microscopy and the follicles present have been measured and scored for their developmental stage. To establish useful molecular markers of activation, we have further classified follicles according to their expression of the proliferative marker, proliferating cell nuclear antigen, and the zona pellucida protein, ZPB. The activation of primordial follicles is initiated immediately following their formation in the rabbit ovary and is characterised by oocyte growth, granulosa cell morphogenesis and increased granulosa cell mitosis. Enhanced ZPB protein expression at the oolemma is also associated with follicle activation and development. Few primordial follicles in the juvenile rabbit ovary are lost by atresia, as assessed by the TUNEL assay. The appearance of apoptotic granulosa cells is however coincident with the development of antral follicles. This study thus describes the temporal and spatial regulation of early follicular development in the post-natal rabbit ovary and, for the first time, shows that the primordial to primary transition in the juvenile rabbit is a highly ordered process occurring within quantifiable parameters.

Potential new strategies for the treatment of ovarian infertility and degenerative diseases with autologous ovarian stem cells

The 50-year-old and currently prevailing view that all oocytes in adult human ovaries persist from the fetal period of life is controversial as it clashes with Darwinian evolutionary theory. Studies of oogenesis and follicular renewal in adult human ovaries, and of the role of hormonal signals and third-party cells (tissue macrophages and T cells), could all be helpful in providing better understanding of the causes of ovarian infertility, its prevention and potential therapy. In addition, the authors recently reported differentiation of distinct cell types and the production of new eggs in cultures derived from premenopausal and postmenopausal human ovaries. It is possible that fertilisation of such eggs will open up new opportunities for providing genetically related children to infertile women for whom conventional in vitro fertilisation has failed. As ovarian stem cells appear to represent a new type of totipotent adult stem cell, they could also be utilised for autologous stem cell therapy of degenerative diseases, without any involvement of allogeneic embryonic stem cells and somatic cell nuclear transfer.

Regulation of baboon fetal ovarian folliculogenesis by estrogen

Although it is well established that formation of the pool of follicles available for ovarian function and fertility in adulthood in human and non human primates occurs in utero, our understanding of the regulation of fetal ovarian development is incomplete. Our laboratories have been instrumental in establishing the baboon as a model for the study of human reproductive endocrinology and showed that estrogen plays a central integrative role in regulating fetal-placental development. Therefore, we adapted our baboon model to study the role of estrogen on fetal ovarian development. Estrogen receptors alpha and beta were expressed in pregranulosa cells and interfollicular nests of the baboon fetal ovary. In baboons in which estrogen levels had been suppressed by administration of an aromatase inhibitor throughout the second half of gestation, fetal ovarian follicle numbers were reduced by 50%, whereas the number of interfollicular nests comprised of oocytes and pregranulosa cells was increased. The decrease in follicles in estrogen-deprived animals was associated with a marked upregulation of expression of alpha-inhibin, but not activins or activin receptors and signaling molecules. Moreover, the majority of the follicles formed in ovaries of estrogen-depleted fetuses appeared unhealthy and contained oocytes with a marked reduction/depletion in microvilli, structures essential for uptake of substrates from surrounding granulosa cells. We propose that estrogen regulates fetal ovarian folliculogenesis and formation of healthy oocytes by controlling the intraovarian activin:inhibin ratio and the development of oocyte microvilli. These findings demonstrate a need for translational research studies of the impact of impairment of estrogen action/availability on reproductive function in adulthood.

Origin of germ cells and formation of new primary follicles in adult human ovaries

Recent reports indicate that functional mouse oocytes and sperm can be derived in vitro from somatic cell lines. We hypothesize that in adult human ovaries, mesenchymal cells in the tunica albuginea (TA) are bipotent progenitors with a commitment for both primitive granulosa and germ cells. We investigated ovaries of twelve adult women (mean age 32.8 +/- 4.1 SD, range 27-38 years) by single, double, and triple color immunohistochemistry. We show that cytokeratin (CK)+ mesenchymal cells in ovarian TA differentiate into surface epithelium (SE) cells by a mesenchymal-epithelial transition. Segments of SE directly associated with ovarian cortex are overgrown by TA, forming solid epithelial cords, which fragment into small (20 micron) epithelial nests descending into the lower ovarian cortex, before assembling with zona pellucida (ZP)+ oocytes. Germ cells can originate from SE cells which cover the TA. Small (10 micron) germ-like cells showing PS1 meiotically expressed oocyte carbohydrate protein are derived from SE cells via asymmetric division. They show nuclear MAPK immunoexpression, subsequently divide symmetrically, and enter adjacent cortical vessels. During vascular transport, the putative germ cells increase to oocyte size, and are picked-up by epithelial nests associated with the vessels. During follicle formation, extensions of granulosa cells enter the oocyte cytoplasm, forming a single paranuclear CK+ Balbiani body supplying all the mitochondria of the oocyte. In the ovarian medulla, occasional vessels show an accumulation of ZP+ oocytes (25-30 microns) or their remnants, suggesting that some oocytes degenerate. In contrast to males, adult human female gonads do not preserve germline type stem cells. This study expands our previous observations on the formation of germ cells in adult human ovaries. Differentiation of primitive granulosa and germ cells from the bipotent mesenchymal cell precursors of TA in adult human ovaries represents a most sophisticated adaptive mechanism created during the evolution of female reproduction. Our data indicate that the pool of primary follicles in adult human ovaries does not represent a static but a dynamic population of differentiating and regressing structures. An essential mission of such follicular turnover might be elimination of spontaneous or environmentally induced genetic alterations of oocytes in resting primary follicles.

Morphodynamics of the follicular-luteal complex during early ovarian development and reproductive life

Female reproductive activity depends upon cyclic morphofunctional changes of the ovarian tissue during the female's fertile period, but the primum movens of an active gonadal rearrangement can be found from early phases of embryo development. To offer a basic account of the main steps of ovarian dynamics, we review the morphofunctional behavior of the follicular-luteal complex in an integrated study using light microscopy and transmission and scanning electron microscopy as well as through the use of numerous drawings. Particular emphasis is given to some reproductive aspects including (1) germ-somatic cell relationships and onset of folliculogenesis during early gonadal development; (2) follicular development and oocyte-follicle cell associations through adult folliculogenesis, finally leading to ovulation; (3) morphodynamics of corpus luteum formation, development, and regression, and (4) degenerative processes involving germ and somatic cells. The results reported, many of which originated in our laboratory, arise from some experiments on laboratory mammals but mostly from a large selection of human specimens. The data obtained are integrated and correlated with classic reports as well as with current views. Crucial biochemical, histophysiological, and clinical aspects are also emphasized.

Origins of follicular cells and ontogeny of steroidogenesis in ovine fetal ovaries

Using fetal sheep as the experimental model, we have elucidated some of the key events that culminate in the formation of primordial follicles. A special effort was made to determine the source of the somatic cells that ultimately become granulosa cells of primordial follicles. Between gestational days 38-100: (1) light and electron microscopy was used to characterize changes in ovarian histoarchitecture; (2) incorporation of BrdU was used to identify populations of proliferating cells within fetal ovaries before, during and after, follicular formation; and (3) in situ hybridisation was used to determine the cell-specific and temporal patterns of expression of mRNAs encoding for selected steroidogenic enzymes. At day 38 somatic (pregranulosa) cells were in contact with oogonia and easily distinguished from endothelial and mesenchymal cells. Between days 38 and 45, pregranulosa cell-oogonia complexes progressively coalesced to form 'tube-like' structures referred to as ovigerous cords. These cords consisted of pregranulosa cells and oogonia arranged such that pregranulosa cells formed the outer wall of the cords. Ovigerous cords were avascular, enveloped in a prominent basal lamina, open-ended where they interfaced with the ovarian surface epithelium, and formed a separate compartment whereby oogonia/oocytes were segregated from the surrounding stroma and vasculature until the time of follicular formation. The structural integrity of ovigerous cords was maintained through day 75, at which time primordial follicles (type 1 and type 1a) first emerged from the cords at the interface of the cortex and medulla. On the basis of the sequential structural changes that occurred during the differentiation and development of fetal ovaries and location of proliferating cells identified by the incorporation of BrdU, we conclude that the majority of the granulosa cells in primordial follicles are derived from mesothelial cells originating from the ovarian surface epithelium. In addition, from the cell-specific distribution and temporal pattern of expression of mRNAs for key steroidogenic enzymes we hypothesize that steroid hormones may play a pivotal paracrine/autocrine role in the formation and/or function of ovigerous cords as well as the development of the ovarian vascular network.

Natural history of the female germ cell from its origin to full maturation through prenatal ovarian development

This paper contains a number of sketches concerning the main morphological ultrastructural features of the human female germ cell during the prenatal period. The morphodynamic outline of primordial germ cells has been traced, both in their extraembryonic site of origin and during their migration towards the developing ovary. After gonadal settlement, the intraovarian differentiation of the germ cells into primary oocytes through the stage of oogonia, as well as the dramatic fall in the number of germ cells before birth, is described. The presence of morphofunctionally relevant interactions between the differentiating female gamete and the surrounding somatic microenvironment has also been evaluated and discussed.

Elimination of germ cells during differentiation of the human ovary: an electron microscopic study

Observations by scanning and transmission electron microscopy during ovarian development (from 7 wk until term of gestation) showed numerous germ cells within the superficial epithelial layer covering the ovary. In early stages of differentiation (between 7 and 11 wk), germ cells appeared to be actively migrating to the surface of the ovary by ameboid-like movements. Later, the transfer of the germ cells to the surface epithelium was also occurring passively, as a consequence of an intense proliferation of germ and somatic cells - arranged in clusters and cords - migrating towards the most superficial areas of the ovary. The present evidence suggests that a number of primordial germ cells - as well as oogonia and oocytes - which lie in the superficial epithelium can leave the ovary and ultimately rest freely upon its surface. Elimination of germ cells during differentiation of the ovary in humans was always paralleled by necrosis of oogonia and oocytes and atretic alteration of primitive follicles. The significance of these processes is discussed in relation to the reduction of the pool of oocytes at birth.

Human ovarian surface epithelium in primary culture

The ovarian surface epithelium (OSE) represents a minute fraction of the cell mass of the ovary but gives rise to over 80% of human ovarian carcinomas. No experimental models for the study of human OSE exist. To characterize OSE cells in culture, explants of ovarian surface from normal ovary of premenopausal women were grown on plastic, glass, and collagen gel in 25% fetal bovine serum/Waymouth's medium 752/1. About 25% of explants produced epithelial outgrowths. Morphologically, these outgrowths resembled OSE in vivo and endothelial and mesothelial cells in culture, but they differed from cultured ovarian stromal, granulosa, and luteal cells. Only OSE among ovarian cell types were intensely keratin positive by immunofluorescence. Keratin also distinguished OSE cells from the keratin-negative endothelial cells. Most but not all OSE colonies tested showed 17 beta-hydroxysteroid dehydrogenase (HSD) activity, which was absent in peritoneal mesothelial cells. Colonies from most patients were limited to a few millimetres and became stationary within a few weeks. Changes that accompanied cessation of growth included senescence, increased keratin content, or the formation of multicellular papillary aggregates. With time, OSE cells tended to assume a fibroblast-like morphology but remained keratin positive and continued to resemble OSE by scanning electron microscopy (SEM). Subcultured OSE cells persisted in a stationary keratin-positive form for many weeks. Throughout this study, all pavementlike epithelial outgrowths that were contiguous with an explant stained for keratin; thus, such colonies can be assumed to be OSE. Conversely, fibroblast-shaped cells may represent OSE as indicated by keratin content and SEM appearance. The methods presented here permit culture of normal human OSE under conditions in which the cells exhibit morphologic plasticity, variable 17 beta-HSD activity, and presence of keratin.