CELL-TYPES AND HORMONAL MECHANISMS ASSOCIATED WITH MID-CYCLE CORPUS-LUTEUM FUNCTION

This article reviews literature on the highly differentiated nature of the luteal cell types and their regulation in the mid-cycle corpus luteum of ruminants. The blood vessels of the corpus luteum are composed primarily of endothelial cells with few intraluteal arterioles or arteries. Blood flow to the corpus luteum does not seem to be regulated by tropic hormones, metabolic activity, or autonomic nerves, but it is dependent on maintenance of high blood pressure. The density of luteal capillaries and lack of luteal arterioles allows an elevated, relatively unregulated luteal blood flow. Steroidogenesis in the ruminant corpus luteum manifests both dependence on and independence from the actions of LH. Initial luteinization, growth, and development of the corpus luteum is dependent on LH action, but progesterone production by the mid-cycle corpus luteum of ruminants is relatively independent of acute LH stimulation. This apparent enigma may be due to distinct functional properties of the steroidogenic cell types in the corpus luteum. Progesterone production by the large luteal cells is relatively independent of LH action, but the small luteal cells respond to LH stimulation. Much of the progesterone produced by the mid-cycle corpus luteum is produced by the large luteal cells and may not be acutely dependent on LH stimulation. Progesterone production by both cell types is dependent on lipoproteins delivered by the abundant luteal blood supply. Thus, distinct cellular features of the mid-cycle corpus luteum produce considerable steroidogenic capacity.