Ovarian follicle. Light micrograph of a section through a secondary ovarian follicle showing signs of maturation into the antral follicle class, as indicated by small fluid-filled cavities among the layers of follicle cells. At centre is the large oocyte (egg cell) surrounded by a distinct coat of proteins called the zona pellucida, important for the penetration of the oocyte by a sperm during fertilisation. External to the follicle cells is the thecal cell layer, which produces androgen-type steroids that in turn are converted to oestrogens by the follicle cells. Oestrogen production by follicles is necessary for continuation of follicle maturation, preparation of the lining of the uterus and coordination of hormonal stimulation during the menstrual