The ovaries are reproductive organs located on either side of the uterus in the lower abdomen. They are responsible for two primary functions: releasing eggs during ovulation and producing reproductive hormones like estrogen and progesterone. Ovarian follicles, tiny fluid-filled sacs within the ovaries, contain immature eggs and are central to both these functions, playing a fundamental role in overall reproductive health.
The Ovary’s Follicular Structure
The ovaries are almond-shaped organs, roughly 2.5 to 5 centimeters long, nestled within the pelvic cavity. Ovarian follicles are the basic structural and functional units inside. Each follicle is a small, fluid-filled sac that houses an immature egg, also known as an oocyte. At birth, ovaries contain a large reserve of primordial follicles, estimated at 1 to 2 million.
Follicles undergo several stages of development. They begin as primordial follicles, which are microscopic. These then progress to primary follicles, where the egg is surrounded by a single layer of cuboidal cells. Next, they become secondary follicles, characterized by multiple layers of cells around the egg. The tertiary, or antral, follicle stage is when a fluid-filled cavity, called the antrum, forms within the follicle, making it visible on ultrasound scans. Only a small percentage ultimately reach maturity.
The Journey of an Egg: Follicle Development and Function
Folliculogenesis describes the process of follicular maturation, where primordial follicles develop towards ovulation. This journey takes approximately a year for a primordial follicle to reach the ovulatory stage. At the start of each menstrual cycle, a group of primordial follicles begins to grow, stimulated by follicle-stimulating hormone (FSH) from the pituitary gland. As these follicles develop, they produce increasing amounts of estrogen.
Typically, one follicle becomes dominant, growing while others cease development and degenerate through atresia. The dominant follicle can reach a diameter of up to 20 millimeters. The rising estrogen from the dominant follicle triggers a surge in luteinizing hormone (LH) from the pituitary gland. This LH surge signals ovulation, prompting the mature egg’s release, usually around day 14 of a typical 28-day cycle.
Follicles also produce hormones. As they grow, granulosa cells within the follicle produce estrogen, which thickens the uterine lining for potential pregnancy. After ovulation, the ruptured follicle transforms into the corpus luteum, a temporary structure producing progesterone. Progesterone further prepares and maintains the uterine lining for implantation. If pregnancy does not occur, the corpus luteum degenerates, leading to a drop in hormone levels and menstruation.
Common Conditions Affecting Follicular Ovaries
Several conditions can disrupt ovarian follicle development and function. Polycystic Ovary Syndrome (PCOS) is a common hormonal disorder with an imbalance in follicular development. In PCOS, numerous small follicles (typically less than 10 millimeters) develop along the ovary’s outer edge but often fail to mature and release an egg. This leads to irregular periods or anovulation (lack of ovulation) and can include elevated androgens, often called “male” hormones.
Ovarian cysts are common, many being functional cysts related to the follicular process. A follicular cyst forms when a follicle does not rupture, continuing to fill with fluid. These are generally harmless and often resolve within one or two menstrual cycles. A corpus luteum cyst can form if the corpus luteum, which develops after ovulation, fills with fluid instead of dissolving.
Anovulation (absence of ovulation) is a direct consequence of follicular dysfunction and can be a symptom of conditions like PCOS. It results from disruptions in hormone interplay that regulate follicle maturation and egg release. Premature ovarian insufficiency (POI), also called premature ovarian failure, involves the loss of normal ovarian function, including a decreased number of primordial follicles, before age 40. This leads to irregular or absent menstruation and menopause-like symptoms.
Understanding and Managing Ovarian Health
Identifying issues related to follicular ovaries often involves diagnostic tools that provide insight into ovarian structure and hormone levels. Pelvic ultrasounds are commonly used to visualize the ovaries and detect the presence, size, and nature of follicles or cysts. An antral follicle count, performed via ultrasound, can assess the number of visible tertiary follicles, which provides an indication of ovarian reserve. Blood tests are also important for measuring hormone levels, such as FSH, LH, estrogen, progesterone, androgens, to identify hormonal imbalances.
Management varies by condition and severity. For functional ovarian cysts, “watchful waiting” is often adopted as most resolve spontaneously. Lifestyle adjustments, such as diet and exercise, can benefit conditions like PCOS, where weight management improves hormonal balance. Medications may be prescribed to regulate hormone levels, induce ovulation, or manage symptoms. Consulting with healthcare professionals for accurate diagnosis and a personalized treatment plan is always recommended, as this information serves as a general overview.