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.