Tina C. Lavranos

Dynamics of the membrana granulosa during expansion of the ovarian follicular antrum

As an endocrine organ, the ovary has some unique characteristics. The formation, the maturation and the regression of the hormone producing cells really determine the timing, the amount and the type of hormone secreted. Here, we focus on the granulosa cells of ovarian follicles which express 17beta-hydroxysteroid dehydrogenase type 1 and cytochrome P450 aromatase. Follicles only produce estradiol late in follicular development before either ovulation or atresia ensues. We discuss the evidence that the membrana granulosa has many characteristics in common with other epithelia, including that it arises from stem cells. The corollary of this is that individual cells within the membrana granulosa are of different ages or stages of specialization. This is evident as regional differences across the membrana granulosa in terms of cell ages, shapes, gene expression, and even behaviour on cell death. We discuss theoretical considerations of the effects of antrum formation on the behavior of the membrana granulosa, and show evidence for differences between follicles in cell shapes, basal lamina phenotypes and location of younger cells, which we speculate is due to different rates of antrum expansion. Clearly, the membrana granulosa is dynamic, and this could explain much about the differences in the behaviors of cells from within the membrana granulosa, and between ovarian follicles.

Development of the ovarian follicular epithelium.

A lot is known about the endocrine control of the development of ovarian follicles, but a key question now facing researchers is which molecular and cellular processes take part in control of follicular growth and development. The growth and development of ovarian follicles occurs postnatally and throughout adult life. In this review, we focus on the follicular epithelium (membrana granulosa) and its basal lamina. We discuss a model of how granulosa cells arise from a population of stem cells and then enter different lineages before differentiation. The structure of the epithelium at the antral stage of development is presented, and the effects that follicle growth has on the behavior of the granulosa cells are discussed. Finally, we discuss the evidence that during follicle development the follicular basal lamina changes in composition. This would be expected if the behavior of the granulosa cells changes, or if the permeability of the basal lamina changes. It will be evident that the follicular epithelium has similarities to other epithelia in the body, but that it is more dynamic, as gross changes occur during the course of follicle development. This basic information will be important for the development of future reproductive technologies in both humans and animals, and possibly for understanding polycystic ovarian syndrome in women.

Basal lamina and other extracellular matrix produced by bovine granulosa cells in anchorage-independent culture

Bovine granulosa cells from 3–7 mm follicles were cultured without anchorage in soft agar/methylcellulose solution for 14 days, with or without 50 ng/ml basic fibroblast growth factor. The granulosa cells divided to form colonies of cells. These were analysed by light and electron microscopy, immunohistochemistry and Western immunoblotting. In approximately 20% of the colonies extracellular matrix was clearly visible at the light-microscope level. Ultrastructurally the matrix resembled a basal lamina 30–100 nm thick and was composed of tangled fibres or cords. Unidentified spherical structures of less than 50 nm diameter were sometimes present and attached to this basal lamina. The basal lamina of follicles had similar features, except that the basal lamina produced in vitro was a large aggregate of many convoluted layers. The cells produced collagen type IV and the cellular form of fibronectin. Intercellular areas not associated with basal lamina were identified. Ruthenium red staining revealed these areas to be rich in proteoglycan granules. Free granules were clustered near the cell surface, and the lumina of these areas were rich in fibres decorated with ruthenium red. This material did not resemble follicular fluid of antral follicles. Thus, granulosa cells in anchorage-independent cultures have a follicular cell morphology and secrete two distinct extracellular matrices, one similar to the follicular basal lamina.

The physiology of the ovary: Maturation of ovarian granulosa cells and a novel role for antioxidants in the corpus luteum

During folliculogenesis the granulosa cells divide whilst in contact with each other, and so exhibit some of the characteristics of stem cells. In vitro we have shown that bovine granulosa cells from 3-7 mm follicles, like stem cells, divide without the need for a substratum, and produce colonies of cells. Growth factors, bFGF and IGFs, stimulate their division. These cells secrete and assemble a basal lamina, suggesting that the follicular basal lamina is produced by the granulosa cells. They have the morphological characteristics of follicular granulosa cells. Thus this system is ideal for studying the functions of immature granulosa cells because the cells do not spontaneously differentiate or luteinize into luteal cells, as occurs in culture on a substratum. On differentiation into luteal cells in vivo the cells express the steroidogenic enzymes for progesterone production and accumulate beta-carotene. During culture of bovine luteal cells we observed that a proportion of the steroidogenic enzyme cholesterol side-chain cleavage cytochrome P450 enzyme became chemically cross-linked to its electron donor, adrenodoxin. P450 enzymes produce oxygen free radicals and oxygen free radicals can cause cross-linking between proteins in close proximity. Cell protect against this damage by the use of antioxidant vitamins. Repleting the cultured luteal cells with beta-carotene reduced the amount of cross-linking. We conclude that the high levels of beta-carotene in corpora lutea are to protect against damage due to oxygen free radicals generated in the course of progesterone synthesis.

Development of Human Granulosa Cell Lines

Ovarian granulosa cells play an essential role in the maturation of the developing ovum and in the synthesis of progesterone and estradiol. Granulosa cells of human origin are, however, difficult to obtain in sufficient numbers to accomplish detailed studies on the regulation of genes involved in granulosa cell division and hormone synthesis. Moreover, the granulosa cells that are readily available from women undergoing in vitro fertilization are in the process of differentiating into luteal cells. A culture system in which large numbers of functional cells can be propagated is a prerequisite for studying the molecular mechanisms controlling the expression of these proteins. In primary cultures of human granulosa (HG) cells it has proven particularly difficult to conduct molecular research due to the availability of only limited numbers of cells.