Corpus luteum. Transmission electron micrograph (TEM) of a section through the corpus luteum of an ovary, showing several luteal cells. The corpus luteum is the tissue formed from a mature ovarian follicle after the follicle has released its oocyte during ovulation. It secretes progesterone, androgens, oestrogens and inhibin, which together serve to inhibit follicle development while preparing the uterus lining for pregnancy. Magnification: x5600, when printed 10 centimetres wide.