Corpus luteum of ovary. Light micrograph of a section through steroid-secreting luteal cells in the corpus luteum of an ovary. The corpus luteum develops from an ovulated follicle. Its main functions are the secretion of oestrogen and progesterone to regulate pituitary gland secretion at the end of the menstrual cycle, and preparation of the growth of the uterine lining (endometrium) for implantation of a fertilised egg. Luteal cells are very large, around 50 micrometres in diameter, and contain much lipid (fat, green granules), mitochondria and smooth endoplasmic reticulum. If there is no pregnancy the corpus luteum entirely degenerates and is absorbed within the ovary. If pregnancy ensues, the corpus luteum is retained for several months until the