Light microscopy of a growing ovarian follicle, classified as a secondary follicle due to multiple layers of follicular (granulosa) cells surrounding the central oocyte (egg). Follicular cells multiply by mitosis and are stimulated by follicle-stimulating hormone (FSH) produced by the pituitary gland. Granulosa cells secrete estrogen. The oocyte is enclosed by a protein barrier called the zona pellucid that ensures only one sperm can merge with the oocyte during fertilization. Magnification x520 when printed at 10cm.