The ovary is the primary reproductive organ in the female body, responsible for producing sex hormones and female gametes. Its function centers entirely on the ovarian follicle, the basic functional unit of the ovary. This follicle is a complex, fluid-filled sac containing an immature egg cell, known as an oocyte. The follicular ovary is defined by the continuous presence and development of these specialized structures.
Anatomy of the Follicular Ovary
The ovary is structurally divided into two main regions: the inner medulla and the outer cortex. The cortex is the functional zone where all ovarian follicles reside and develop. The medulla contains the blood vessels, nerves, and lymphatic tissue supplying the organ. Females are born with a finite reserve of these structures, all initially in a dormant stage.
Resting follicles are called primordial follicles. Each consists of a small, immature oocyte surrounded by a single layer of flattened support cells, known as pre-granulosa cells. The vast majority of follicles remain dormant for years, awaiting a signal to begin maturation.
As a follicle begins to grow, the surrounding tissue organizes into two distinct, hormone-producing layers. The granulosa cells are the inner layer that directly surrounds the oocyte, separated from the outer layer by a membrane. The theca cells form the outer envelope, differentiating from ovarian connective tissue to provide structural support and blood supply.
The Follicle Maturation Process
The transformation of a primordial follicle into a mature structure is a gradual process called folliculogenesis. This journey takes many months, though only the final two weeks are linked to the monthly cycle. The process begins when the flattened pre-granulosa cells transform into cuboidal granulosa cells, marking the transition to a primary follicle.
The oocyte begins to enlarge, and a specialized glycoprotein shell, the zona pellucida, forms around it, separating it from the granulosa cells. As the granulosa cells proliferate, forming multiple layers, the structure becomes a secondary follicle. The outer theca layer fully differentiates into the theca interna and theca externa.
The next stage, the tertiary or antral follicle, is defined by the formation of a fluid-filled cavity called the antrum. This cavity expands as the granulosa cells secrete follicular fluid, pushing the oocyte to one side. The expansion of the antrum creates the large, fluid-filled Graafian follicle, the final mature stage. Although a cohort of follicles starts growth each month, hormonal selection ensures that only one, the dominant follicle, completes the final phase and reaches pre-ovulatory size.
Ovulation and the Corpus Luteum
The culmination of follicular maturation is ovulation, which occurs when the mature Graafian follicle ruptures. Driven by a precise hormonal signal, the follicle swells and releases the oocyte, surrounded by a cluster of granulosa cells, into the abdominal cavity near the fallopian tube. This expulsion of the egg marks the end of the follicular phase of the reproductive cycle.
Following the rupture, the remaining follicular tissue transforms into a temporary endocrine gland. This new structure is known as the corpus luteum, or “yellow body,” named for the yellowish, lipid-rich material accumulating within its cells. The corpus luteum takes over the primary role of hormone production in the second half of the cycle.
The primary function of this temporary gland is to secrete large amounts of progesterone and some estrogen. Progesterone acts on the lining of the uterus, causing it to become thick and highly vascularized in preparation for the implantation of a fertilized egg. If fertilization does not occur, the corpus luteum begins to degenerate after ten to fourteen days.
Hormonal Control of Follicular Development
Follicular growth and function are precisely regulated by a signaling pathway involving the brain and the ovaries. The hypothalamus releases Gonadotropin-Releasing Hormone, which stimulates the pituitary gland to secrete two key messengers: Follicle-Stimulating Hormone and Luteinizing Hormone.
Follicle-Stimulating Hormone, or FSH, initiates the growth of the initial cohort of follicles each cycle. It promotes the proliferation of granulosa cells and stimulates them to produce estrogen. Rising estrogen levels then feed back to the pituitary, causing a decrease in FSH and effectively selecting the single dominant follicle that can thrive with less hormonal support.
Luteinizing Hormone (LH) plays a different but equally significant role, particularly in the later stages of development. LH stimulates the theca cells to produce precursor molecules that the granulosa cells convert into estrogen. The surge of estrogen produced by the mature follicle triggers a spike in LH release from the pituitary gland. This LH surge is the direct trigger for the final maturation of the oocyte and the rupture of the follicle wall, resulting in ovulation.