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In a nonfertile cycle, the corpus luteum of the ewe secretes progesterone for about 2 weeks, and then regresses and is resorbed within the ovary. Luteolysis does not occur, however, if fertilization takes place. In fact, luteolysis must be prevented for pregnancy to be successful. The most important function of the corpus luteum appears to be the secretion of progesterone, since removal of the corpus luteum in early gestation causes abortion, but abortion is prevented by progesterone therapy (Amoroso & Perry, 1977). Since progesterone plays a crucial role in the establishment and maintenance of early preg¬ nancy, elucidation of the factors which regulate progesterone secretion is important for under¬ standing the mechanisms involved in the maternal recognition of pregnancy. Perhaps the most important factors involved in the regulation of progesterone secretion are luteinizing hormone (LH) and the prostaglandins. LH stimulates progesterone secretion by luteal tissue and can maintain luteal function in hypophysectomized animals (reviewed by Hansel, Concannon & Lukaszewska, 1973; Niswender, Schwall, Fitz, Farin & Sawyer, 1985). Prostaglandins (PG) F-2a and E-2 appear to have opposing effects. Injection of PGF-2a reduces serum concentrations of pro¬ gesterone and this agent is thought to be responsible for regression of the corpus luteum at the end of the nonfertile cycle (Horton & Poyser, 1976). In contrast, PGE-2 stimulates progesterone secre¬ tion (Speroff & Ramwell, 1970; Marsh & LeMaire, 1974), delays the onset of normal luteolysis (Pratt, Butcher & Inskeep, 1979), and counteracts the luteolytic effects of PGF-2a (Henderson, Scaramuzzi & Baird, 1977). The regulation of progesterone secretion is a complex process. Not only are several hormones involved, but corpora lutea of many animals contain at least two types of steroidogenic luteal cells. Examples of animals in which two types of luteal cells have been identified are listed in Table 1. Although the two types of luteal cells have been referred to by a variety of names (see Table 1), the terms small and large luteal cell are becoming generally accepted because the most obvious differ¬

Differential regulation by LH and prostaglandins of steroidogenesis in small and large luteal cells of the ewe