Light microscopy of luteal cells of the corpus luteum of the ovary. The corpus luteum develops from a ruptured follicle that has released an oocyte at ovulation. Filled with lipid (fat) droplets (dark blue stain), mitochondria (purple dots) and smooth endoplasmic reticulum, the luteal cells secrete progesterone, estrogen and other products that prepare the lining of the uterus for pregnancy. The corpus luteum survives for about 6 weeks during pregnancy but regresses and degenerates if the oocyte is not fertilized. Light blue-stained structures between cells are blood in capillaries. Magnification x400 when printed at 10cm.