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The aim of this investigation was to determine the influence of epidermal growth factor (EGF) on follicular growth and steroidogenesis in mice. Follicles were cultured in medium containing human recombinant EGF at concentrations of 1–20 ng ml−1. Oestradiol production was assayed immunoenzymatically, and growth was measured by recording follicle diameter daily and by analysing the total DNA content of follicles. The effect of EGF on cumulus–oocyte complexes isolated from cultured follicles was also assessed. Results showed that EGF inhibited oestradiol production in a dose-dependent manner, but had no mitogenic effect. Despite almost complete inhibition of oestradiol production at concentrations of EGF ≥ 10 ng ml−1, follicles were still able to achieve preovulatory size and morphology, although the incidence of atresia was increased over controls. Conversely, at a concentration of only 1 ng EGF ml−1, a significantly greater number of follicles reached the Graafian stage compared with control follicles. Cumulus expansion and meiotic maturation by isolated cumulus–oocyte complexes from cultured follicles was dramatically stimulated in the presence of EGF and FSH, but not by FSH alone. These findings suggest that EGF may have a modulatory effect on oestradiol production in vivo, and that follicular growth and differentiation may be uncoupled from steroidogenesis. Finally, ovulatory changes in the cumulus–oocyte complex may require the presence of this factor.
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Analysis of chimaeric mouse ovaries using DNA in situ hybridization was undertaken to (i) investigate the morphogenesis of follicular cell clonal expansion, (ii) evaluate whether the different cell populations within the ovary are derived from the same or unrelated progenitor cells and (iii) estimate the number of progenitor cells giving rise to the different types of ovarian cell. Chimaeras were produced by aggregation of eight-cell morulae from normal mice and those transgenic for the β-globin gene. Chimaeric blastocysts were transferred to pseudopregnant hosts and the ovaries of resultant adult offspring were prepared for in situ hybridization using a digoxigenin-labelled cDNA probe to the β-globin gene. Results showed that follicles are constructed by the non-random, radial proliferation of granulosa cell clones, which form long, thin, unbranched columns across the follicle wall. Qualitative and quantitative studies revealed that both peripheral and central granulosa cells are derived from the same progenitor cells. Phenotypic differences may therefore be due to positional cues within the follicle rather than being cell lineage dependent. It is suggested that granulosa cells and germinal epithelium may be partly derived from the same progenitor cells and that theca externa is probably derived from interstitial tissue. However, results from this study did not support the contention that theca interna and theca externa–interstitial tissue have the same origin, and it is suggested that the former cell type may exist in an undifferentiated state from early stages of follicle development. Furthermore, granulosa and germinal epithelium appear to be derived from different progenitor cells from either theca interna or theca externa–interstitial cells. Evidently, all types of ovarian cell, including the somatic cells of individual follicles, are derived from more than one progenitor cell.
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A whole follicle culture system has been used to investigate the actions of gonadotrophic hormones, oestrogen and progesterone in the regulation of follicular development and steroidogenesis. Recombinant human FSH was required for the growth of preantral follicles and for Graafian morphogenesis, whereas recombinant LH was ineffective. While pure FSH was sufficient for growth and morphogenesis, production of oestrogen was greater when androstenedione or LH was present in combination with FSH, confirming that there is a two-cell mechanism for oestradiol production in the mouse follicle. When an antiserum to oestrogen or to progesterone or an oestrogen receptor antagonist were added to the culture medium, there was no significant effect on either follicular growth or oestradiol production. Thus, physiological concentrations of oestradiol are not needed for follicle development, although a role cannot be completely ruled out. In conclusion, the obligatory role of FSH was demonstrated. It appears to be sufficient for follicle development even in the absence of LH, and the paracrine or autocrine effects of oestradiol and progesterone, if any, appear to be minor in the mouse ovary.
