Effect of follicular steroids on the maturation and fertilization of mammalian oocytes.

Pronuclear development was used to measure the effects on ovine oocytes of altering follicular steroidogenesis during maturation in vitro. Follicular steroid secretion was altered using enzyme inhibitors and exogenous steroid supplementation. Abnormalities induced during maturation were measured 24 h after tranfer of oocytes to the oviducts of inseminated hosts. The presence throughout maturation of aminoglutethimide, an inhibitor of the conversion of cholesterol to pregnenolone, reduced steroid secretion to 7% of that in controls and decreased from 77% to 33% the number of normal oocytes. Abnormalities were substantially reduced by the addition of aminoglutethimide during the final 8 h of maturation only. The inhibition of 17 alpha-hydroxylase enzymes with SU10603 reduced oestrogen and testosterone secretion to about 10% of control levels but had no effect on progestin secretion. Only 13% of oocytes matured in the continual presence of SU10603 underwent normal fertilization. The number of oocytes undergoing normal fertilization was increased to about 50% by (i) delaying the addition of SU10603 until the last 8 h of the maturation period or (ii) adding exogenous steroids to follicles cultured with inhibitor from explantation. It is concluded that oocytes require a specific intra-follicular steroid environment for the completion of the full maturation process. Alterations to the steroid profile during maturation induce changes in the oocyte which are expressed as gross abnormalities at fertilization.