Luteinizing Hormone-Independent Luteinization and Ovulation in the Hypophysectomized Rat: A Possible Role for the Oocyte?1

Abstract

Immature hypophysectomized, estrogen-treated rats were used to study the regulation of luteinization. Particular attention was focused on the potential role of the oocyte in this process. Rats were injected for 2 days with follicle-stimulating hormone (FSH) to stimulate follicular development. Within 48 h following FSH treatment, many follicles became luteinized, as determined by morphometric analysis. This lutenization occurred in the absence of detectable levels of luteinizing hormone (LH). The number of follicles undergoing luteinization was dependent on the FSH dose. In addition, ovulation occurred in some of the animals receiving the highest doses of FSH (3-.mu.g or 5-.mu.g injections). The majority of follicles undergoing luteinization or ovulation were greater than 400 .mu.m in diameter. Luteinized follicles exhibited positive reactivity for cholesterol side-chain cleavage enzyme, 3.beta.-hydroxysteroid dehydrogenase, lipid, and alkaline phosphatase, which was similar to that found in corpora lutea of the cycle. Serum progesterone (PO) and 20.alpha.-hydroxypregn-4-en-one levels were elevated in animals with luteinized follicles, especially in those animals that also underwent ovulation. Morphological evaluation of oocytes showed that the majority of luteinized follicles contained a degenerating oocyte. Oocyte degeneration was highly correlated (r = 0.94) to luteinization. These results demonstrate that luteinization and ovulation can occur in the FSH/estrogen-primed hypophysectomized rats in the absence of detectable serum LH. Furthermore, LH-independent luteinization was strongly correlated to degenerative changes in the oocyte. These results provide new evidence to support the concept that the oocyte may be an intraovarian regulator of luteinization.