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 IGF's, 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 β-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 β-carotene reduced the amount of cross-linking. We conclude that the high levels of β-carotene in corpora lutea are to protect against damage due to oxygen free radicals generated in the course of progesterone synthesis.