Meiosis-activating sterols: background, discovery, and possible use.

Several years ago we discovered that spent media from cultured human and bull testes contain components that initiate meiosis in germ cells from fetal mouse testes which have been cultured for 6 days in the spent medium. The active substance(s) was termed meiosis-inducing substance. We later found that human follicular fluid harvested after stimulation with gonadotropins has a similar effect. These meiosis-activating substances have now been identified and characterized in extracts from bull testes and human preovulatory follicular fluid as naturally occurring sterols (meiosis-activating sterols, MAS). MAS are intermediates in the cholesterol biosynthetic pathway and are thus present in all cells which produce cholesterol de novo and from lanosterol. However, MAS accumulate only in the gonads. We discuss the possible physiological role of these sterols in initiating meiosis and in oocyte resumption of meiosis, and their potential use in promoting and preventing fertility.