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How does a follicle develop into a mature Graafian follicle?

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Diagram of the menstrual cycle showing hormone levels, follicle development, and endometrial changes across 28 days. It highlights menstruation, ovulation, and the follicular and luteal phases, with labeled curves for estrogen, progesterone, FSH, and LH. — menstruation Cyclical changes during a woman's normal ovulatory menstrual cycle.

During the early follicular phase of the menstrual cycle, several immature follicles begin to enlarge and migrate from the ovary's cortex toward its outer surface. This process is regulated by follicle-stimulating hormone and luteinizing hormone secreted by the pituitary gland.12

As development progresses, the cells lining the follicle multiply to form a layer called the zona granulosa, and a fluid-filled cavity forms within this zone. Simultaneously, surrounding stromal and interstitial cells arrange themselves concentrically to create an enclosing sheath known as the theca.1

One or more of these follicles are selected for further maturation, eventually becoming a mature Graafian follicle. These mature follicles can reach up to 30 mm in diameter before they rupture to release an egg. Throughout this maturation, the theca and granulosa cells work together to produce estrogens and other steroid hormones that bathe the oocyte in the follicular fluid.1