Luteal function: the estrous cycle and early pregnancy.

A number of morphological and biochemical changes occur as the cells of the recently ovulated follicle luteinize and develop into a functional CL. There are two distinct steroidogenic luteal cell types that appear to differentiate from thecal and granulosal cells in the follicle. The control of progesterone secretion is quite different in the two cell types. Prostaglandin F2 alpha (PGF2 alpha) is the primary luteolytic hormone in most mammals. PGF2 alpha appears to exert its antisteroidogenic actions via activation of the protein kinase C system, while its cytotoxic effects appear to be mediated via a dramatic increase in intracellular levels of free calcium. The mechanisms involved in maternal recognition of pregnancy are very diverse between species and may involve direct luteotropic stimulation of the CL, reduced uterine secretion of PGF2 alpha, and/or inhibition of actions of PGF2 alpha at the level of the CL.