Genomic imprinting is the parent-of-origin specific gene expression which is a vital mechanism through both development and adult life. One of the key elements of the imprinting mechanism is DNA methylation, controlled by DNA methyltransferase enzymes. Germ cells undergo reprogramming to ensure that sex-specific genomic imprinting is initiated, thus allowing normal embryo development to progress after fertilisation. In some cases, errors in genomic imprinting are embryo lethal while in others they lead to developmental disorders and disease. Recent studies have suggested a link between the use of assisted reproductive techniques and an increase in normally rare imprinting disorders. A greater understanding of the mechanisms of genomic imprinting and the factors that influence them are important in assessing the safety of these techniques.
A K E Swales and N Spears
N. Spears, J. P. de Bruin and R. G. Gosden
An in vitro model of dominant and subordinate ovarian follicles was developed to allow a closer investigation of the phenomenon of follicular dominance. Preantral mouse ovarian follicles were cultured either alone or in pairs. Pairs of follicles were either in direct contact or in shared medium, but without physical contact. The experiments showed that where contact was allowed to develop between follicles one follicle invariably became dominant, while the other would grow and develop little during the culture period. In contrast, there was no effect of co-culture on follicle development in the absence of contact between the follicles. There was, therefore, no evidence of secretion of a diffusible factor by a dominant follicle that could affect the development of neighbouring follicles. After 6 days of co-culture with contact, histological examination of the subordinate follicle showed that it was healthy, in spite of remarkably little growth during culture. In a further experiment, the subordinate follicle was separated from the dominant one after 2 days of co-culture (when a significant difference in size had already developed), and cultured alone. These 'released' follicles exhibited a spurt of growth during the remaining culture period, attaining a size and appearance indistinguishable from those of controls by the end of culture. This confirms that the dominant follicle, while depressing the growth of its neighbour, is not inducing irreversible atresia in the subordinate follicle in this model. The in vitro model will allow a more detailed study of direct influences of dominant–subordinate follicle interactions, and should increase our knowledge of a poorly understood phenomenon.
A. A. Murray, R. G. Gosden, V. Allison and N. Spears
The effects of androgens on ovarian follicular development have been investigated using a whole follicle culture system. Follicles obtained from mouse ovaries and cultured in the presence of anti-androgen serum grew more slowly than control follicles. This effect was reversed by the addition of androstenedione to the medium. A similar effect was obtained when receptor-mediated effects of androgens were blocked using an androgen receptor antagonist. When follicles were grown in concentrations of FSH that are marginal for follicle development, they developed faster in the presence of a non-aromatizable androgen, dihydroxytestosterone. The results indicate that androgens exert a direct, stimulatory role on the growth and development of mouse antral follicles, in vitro.
N. Spears, A. A. Murray, V. Allison, N. I. Boland and R. G. Gosden
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.
FH Thomas, R Leask, V Srsen, SC Riley, N Spears and EE Telfer
During ovarian folliculogenesis, ascorbic acid may be involved in collagen biosynthesis, steroidogenesis and apoptosis. The aims of this study were to determine the effects of ascorbic acid on bovine follicle development in vitro. Preantral follicles were cultured for 12 days in serum-free medium containing ascorbic acid (50 microg ml(-1)). Half of the medium was replaced every 2 days, and conditioned medium was analysed for oestradiol and matrix metalloproteinase 2 (MMP-2) and MMP-9 secretion. On day 12, cell death was assessed by TdT-mediated dUTP-biotin nick end labelling (TUNEL). In the absence of serum, there was significant (P < 0.05) follicle growth and oestradiol secretion over the 12 day culture period. Ascorbic acid had no effect on these parameters. The addition of serum from day 0 stimulated follicle growth (P < 0.05), but compromised follicle integrity. By day 12 of culture, a higher proportion of follicles remained intact in the presence of ascorbic acid in serum-free conditions (P < 0.05), and significantly (P < 0.01) less granulosa and theca cell death was observed in these follicles than in control follicles. Moreover, ascorbic acid significantly (P < 0.05) increased production of MMP-9, an enzyme involved in basement membrane remodelling. In conclusion, this culture system was capable of supporting follicle differentiation over the 12 day culture period. Furthermore, ascorbic acid maintains bovine follicle health and basement membrane remodelling in vitro.
AA Murray, MD Molinek, SJ Baker, FN Kojima, MF Smith, SG Hillier and N Spears
Ascorbic acid has three known functions: it is necessary for collagen synthesis, promotes steroidogenesis and acts as an antioxidant. Within the ovary, most studies have concentrated on the role of ascorbic acid in luteal formation and regression and little is known about the function of this vitamin in follicular growth and development. Follicular growth and development were investigated in this study using an individual follicle culture system that allows the growth of follicles from the late preantral stage to Graafian morphology. Follicles were isolated from prepubertal mice and cultured for 6 days. Control media contained serum and human recombinant FSH. Further groups of follicles were cultured in the same media but with the addition of ascorbic acid at concentrations of either 28 or 280 micromol l(-1). Addition of ascorbic acid at the higher concentration significantly increased the percentage of follicles that maintained basement membrane integrity throughout culture (P < 0.001). Ascorbic acid had no effect on the growth of the follicles or on oestradiol production. Metalloproteinase 2 activity tended to increase at the higher concentration of ascorbic acid and there was a significant concomitant increase in the activity of tissue inhibitor of metalloproteinase 1 (P < 0.01). Follicles cultured without the addition of serum but with FSH and selenium in the culture media underwent apoptosis. Addition of ascorbic acid to follicles cultured under serum-free conditions significantly reduced apoptosis (P < 0.05). From these data it is concluded that ascorbic acid is necessary for remodelling the basement membrane during follicular growth and that the ability of follicles to uptake ascorbic acid confers an advantage in terms of granulosa cell survival.