Collapsed ovarian follicle. Coloured scanning electron micrograph (SEM) of an ovarian follicle just after ovulation (release of an egg). The central lumen is filled with red blood cells from a collapsed blood vessel. Surrounding the lumen are cuboidal follicular cells. The follicle will form into the corpus luteum, or yellow body. The function of the corpus luteum is to secrete the hormone progesterone to ready the uterus for the implantation of a fertilised egg. If the egg is not fertilised it stops secreting and degrades after roughly 12 days.