Light microscopy of an oocyte in the centre of a pre-ovulatory ovarian follicle. The oocyte has a large nucleus and dense nucleolus, and is surrounded by a thick protein-rich layer called the zona pellucida. It ensures only one sperm can reach the oocyte during fertilization. The cells around the oocyte belong to the follicular (or granulosa) cells of the follicle and are termed cumulus cells. The empty looking periphery is follicular fluid. Magnification x350 when printed at 10 cm.