Corpus luteum, yellow hormone-secreting body in the female reproductive system. It is formed in an ovary at the site of a follicle, or sac, that has matured and released its ovum, or egg, in the process known as ovulation. The corpus luteum is made up of lutein cells (from the Latin luteus, meaning “saffron-yellow”), which develop immediately following ovulation, when yellow pigment and lipids accumulate within the granulosa cells lining the follicle. The size of the corpus luteum is highly variable. The corpus luteum secretes estrogens and progesterone. The latter hormone causes changes in the uterus that make it more suitable for implantation of the fertilized ovum and the nourishment of the embryo. If the egg is not fertilized, the corpus luteum becomes inactive after 10–14 days, and menstruation occurs.