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How does a follicle develop during the menstrual cycle?

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Front view anatomical diagrams of the male and female reproductive systems, with labeled organs including penis, testis, prostate gland, uterus, ovary, and vagina. — human reproductive system Organs and structures of the male and female reproductive systems.

During the menstrual cycle, follicular development occurs primarily during the follicular phase, which lasts approximately two weeks. The process involves the following stages:1

Initiation and Migration: At the beginning of the menstrual cycle, several immature follicles—hollow balls of cells containing eggs—begin to enlarge and migrate from the ovary's cortex toward its outer surface.1
Cellular Changes: The cells lining the follicle multiply to form a layer called the zona granulosa. Simultaneously, surrounding stromal and interstitial cells arrange themselves concentrically to create an enclosing sheath known as the theca.1
Antrum Formation: A fluid-filled cavity, or antrum, develops within the granulosa layer. Under the influence of follicle-stimulating hormone (FSH) and luteinizing hormone (LH), one or more follicles are selected for further maturation.12
Maturation: These selected follicles, known as Graafian follicles, grow significantly—reaching up to 30 mm in diameter—before they eventually rupture to release the egg during ovulation.1

Throughout this development, the theca and granulosa cells work together to produce estrogens, which are essential ovarian hormones.1