Structural and functional properties of the corpus luteum of pregnancy.

In domestic ruminants the parenchyma of the corpus luteum consists of two subpopulations of steroidogenic cells commonly referred to as small and large luteal cells. These cells differ not only in size and structural characteristics, but also in functional properties. During the mid-luteal phase of the oestrous cycle approximately 60% of the corpus luteum is occupied by steroidogenic cells. Although the steroidogenic capacity of these cells declines as pregnancy advances, the cells persist as distinct populations throughout pregnancy and for several days following parturition. In general, structural changes typically observed at the end of the oestrous cycle also occur after parturition, but over a more extended period. These include deletion of endothelial cells and occlusion of capillary lumina with cellular debris and apoptotic bodies, an infiltration of eosinophils and macrophages, and fragmentation and lysis of parenchymal cells. However, not all parenchymal cells undergo lysis, nor are they rapidly phagocytosed by macrophages. Instead, many fuse to form what appear to be large syncytia that contain numerous lipid droplets, tightly packed mitochondria and multiple nuclei with condensed chromatin. Fusion of parenchymal cells to form syncytial profiles begins 2-3 days after parturition and the syncytia persist for at least 22 days post partum. By day 35 post partum transformation of the corpus luteum into a corpus albicans is essentially complete.