A mature ovarian follicle (Graafian follicle) consists of an oocyte and several surrounding groups of (mainly hormone-producing) cells. Specifically, the typical follicle consists of, from inside outwards:
- The oocyte (egg cell)
- The granulosa cells
- The theca cells (subdivided into an external and an internal layer)
At ovulation, the oocyte is ejected from the follicle, and the remaining cells undergo modifications to become the corpus luteum (literally, "yellow body"). The function of the corpus luteum is basically to secrete the sex hormones during the second half of the menstural cycle. Of particular importance is the progesterone it secretes, which maintains the uterus' endometrium in a state of readiness to house a possible pregnancy.
The fate of the corpus luteum is determined by whether or not the woman falls pregnant after ovulation. If no conception occurs, the progesterone is no longer needed, so the corpus luteum degenerates, eventually withering away to fundamentally become a scar. It then goes by the name, corpus albicans ("white body").
If the oocyte is fertilised by a sperm, however, the story goes a little differently. The embryo secretes beta human chorionic gonadotropin (BHCG), which effectively instructs the corpus luteum to continue its prostaglandin secretion (instead of degenerating). It persists in this function until the placenta is big enough to take over the role of prostaglandin synthesis. At this point, its duties fulfilled, the corpus luteum gradually degenerates into a corpus albicans.
Thus, the corpus luteum is a feature of the ovary for the duration of the second half of the menstrual cycle - from just after ovulation to just before menstruation.
(Incidentally, BHCG is the hormone that pregnancy tests use to diagnose pregnancy - the urine or the blood is tested for it.)