Fine structure of the lutein cell in the house musk shrew, Suncus murinus.

The ultrastructure of lutein cells during pregnancy and the post partum period was examined by transmission electron microscopy in the house musk shrew, Suncus murinus. Lutein cells on day 13 of pregnancy contained an extensive system of anastomosing tubules or cisternae of smooth ER and many enlarged mitochondria with numerous tubulovesicular cristae. From day 13 on, the number of small granules, possibly lysosomes, increased gradually. Between day 20 and 25, the loss of smooth ER began and elongated, or flattened mitochondria increased. Regressing lutein cells observed after parturition were characterized by abundant large dense bodies, bizarre mitochondria and a decrease in the amount of smooth ER. Unusual forms of mitochondria were always observed after day 5 of pregnancy. Two types could be distinguished; one, found frequently in the second third of pregnancy, was ring-, disc-, cup- or dumb-bell-shaped with tubulovesicular cristae, and the other, found exclusively in the last third of pregnancy and after parturition, was elongated, flattened and sometimes twisted. The paucity of lipid droplets was a characteristic feature of the lutein cells of this species. The significance of these ultrastructural changes of cellular organelles is discussed in relation to the ovarian and plasma levels of progesterone.