Actions of hormones and other factors upon oocyte maturation

Oocyte maturation is controlled by a combination of hormonal and local follicular factors. Osmolarity, pH, and perhaps Ca2+ concentration of the surrounding medium are also important. Follicular fluid contains a low molecular weight OMI which acts to keep the oocyte from maturing. Luteinizing hormone added to cultured cumulus enclosed porcine oocytes can reverse the inhibitory action of OMI. The level of OMI in the follicular fluid appears to decrease as the follicle matures. Addition of FSH and prolactin to cultured granulosa cells stimulates OMI secretin whereas addition of testosterone or dihydrotesterone brings about a decrease in OMI secretion. Elevated LH in vivo may bring about oocyte maturation before ovulation by (a) an antagonist action on OMI; (b) stimulating the synthesis of testosterone by theca cells and thus inhibiting the synthesis of OMI by granulosa cells; and (c) action on the granulosa cells to promote luteinization which may also cause a decrease in OMI synthesis. The hastened oocyte maturation associated with follicular atresia could be due to a decline in OMI due to granulosa cell death and/or elevated follicular androgens.