Several years ago we discovered that spent media from cultured human a
nd bull testes contain components that initiate meiosis in germ cells
from fetal mouse which have been cultured for 6 days in the spent medi
um. The active substance(s) was termed meiosis-inducing substance. We
later found that human follicular fluid harvested after stimulation wi
th gonadotropins has a similar effect. These meiosis-activating substa
nces 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 th
e cholesterol biosynthetic pathway and are thus present in all cells w
hich produce cholesterol de novo and from lanosterol. However, MAS acc
umulate only in the gonads. We discuss the possible physiological role
of these sterols in initiating meiosis and in oocyte resumption of me
iosis, and their potential use in promoting and preventing fertility.