corpus luteum

The corpus luteum, Latin for yellow body, is a temporary hormone-producing gland that forms in the ovary from the follicle left behind after it releases its egg. It is a remarkable example of biological recycling: the very structure that grew and nurtured the egg is repurposed into a progesterone factory the moment ovulation is over.

Triggered by the LH surge, the emptied follicle's granulosa and theca cells transform — a process called luteinization — accumulating yellowish lipid and producing large amounts of progesterone along with estrogen. This progesterone dominates the luteal phase, maintaining the uterine lining in a secretory, implantation-ready state and preparing the body to support an early pregnancy.

The corpus luteum has a self-limiting lifespan. If no pregnancy occurs, it degenerates after about 14 days into a scar called the corpus albicans; progesterone falls and menstruation follows. If an embryo implants, its hCG rescues the corpus luteum so it keeps secreting progesterone for the first weeks of pregnancy until the placenta takes over. A caveat: this dependence is why supplemental progesterone is sometimes given after assisted reproduction, where the natural corpus luteum may be inadequate.