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 smal l and large luteal cells. These cells differ not only in size and stru ctural characteristics, but also in functional properties. During the mid-luteal phase oi 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 per sist as distinct populations throughout pregnancy and for several days following parturition. In general, structural changes typically obser ved at the end of the oestrous cycle also occur after parturition, but over a more extended period. These include deletion of endothelial ce lls and occlusion of capillary lumina with cellular debris and apoptot ic bodies, an infiltration of eosinophils and macrophages, and fragmen tation and lysis of parenchymal cells. However, not all parenchymal ce lls undergo lysis, nor are they rapidly phagocytosed by macrophages. I nstead, many fuse to form what appear to be large syncytia that contai n numerous lipid droplets, tightly packed mitochondria and multiple nu clei with condensed chromatin. Fusion of parenchymal cells to form syn cytial profiles begins 2-3 days after parturition and the syncytia per sist for at least 22 days post partum. By day 35 post partum transform ation of the corpus luteum into a corpus albicans is essentially compl ete.