An OOCYTE-containing structure in the cortex of the ovary. The oocyte is enclosed by a layer of granulosa cells providing a nourishing microenvironment (follicular fluid). The number and size of follicles vary depending on the age and reproductive state of the female. The growing follicles are divided into five stages: primary, secondary, tertiary, Graafian, and atretic. Follicular growth and steroidogenesis depend on the presence of gonadotropins.