An ovarian follicle is a small, fluid-filled sac within the ovaries, each containing an immature egg (oocyte). These structures are important to the female reproductive system, playing a central role in hormone production and egg release for potential fertilization. Follicles secrete hormones that influence various stages of the menstrual cycle.
The Journey of an Ovarian Follicle
The journey of an ovarian follicle begins with primordial follicles, microscopic structures present in the ovaries from birth. At puberty, approximately 300,000 to 400,000 primordial follicles remain, each holding an oocyte arrested in an early stage of development. Throughout a woman’s reproductive life, cohorts of these follicles are continuously recruited to begin growth, though only a small fraction will reach full maturity.
Once activated, a primordial follicle transitions into a primary follicle. Here, the oocyte begins to grow, and surrounding granulosa cells multiply into a single layer. This progresses to the secondary follicle stage, characterized by multiple granulosa cell layers and an outer layer of cells called the theca. Follicle-Stimulating Hormone (FSH), released by the pituitary gland, stimulates granulosa cells to proliferate and the follicle to grow larger during these stages.
The next phase is the antral follicle stage, marked by a fluid-filled cavity, or antrum. Accumulation of follicular fluid and continued granulosa cell proliferation significantly increase the follicle’s size. From the cohort of growing antral follicles, typically one is selected to become the dominant, or Graafian, follicle, which will ultimately release an egg. Most follicles that begin this journey do not complete it; instead, they undergo atresia, a degenerative process that can occur at any stage and accounts for the loss of over 99% of follicles.
Ovulation and the Corpus Luteum
The culmination of the follicular journey is ovulation, the process by which a mature egg is released from the ovary. This event is triggered by a sharp increase in Luteinizing Hormone (LH), known as the LH surge, which typically occurs around the midpoint of the menstrual cycle. The LH surge signals the dominant Graafian follicle to rupture, expelling the mature oocyte into the fallopian tube for potential fertilization. This release usually happens about 24 to 36 hours after the LH surge begins.
Following egg release, the ruptured follicle transforms within the ovary. It reorganizes and develops into a temporary endocrine structure called the corpus luteum. Its primary function is to produce significant amounts of progesterone, and to a lesser extent, estrogen. Progesterone prepares the uterine lining (endometrium), making it receptive for a fertilized embryo’s implantation. If pregnancy does not occur, the corpus luteum degenerates, leading to a drop in progesterone levels and the onset of menstruation, initiating a new menstrual cycle.
Measuring Follicles and Ovarian Reserve
Doctors assess follicles to understand a woman’s ovarian reserve, which refers to the quantity of eggs remaining in the ovaries. A common method is the Antral Follicle Count (AFC), performed using a transvaginal ultrasound. During an AFC, a healthcare provider counts the small, fluid-filled antral follicles visible in both ovaries.
The number of antral follicles observed estimates the pool of microscopic primordial follicles available for future development. An AFC indicates how a woman might respond to ovarian stimulation medications, especially in fertility treatments like in vitro fertilization (IVF). While a typical AFC varies, a range of 3 to 30 follicles is often considered normal, though this can differ based on age and individual factors. This diagnostic tool helps evaluate fertility potential and guide treatment decisions.
Common Follicular Disorders
When follicular development or function deviates from the typical pattern, specific conditions can arise. Polycystic Ovary Syndrome (PCOS) is a common hormonal disorder characterized by numerous small follicles, often 12 or more, on the ovaries that fail to mature and release an egg. This arrest in follicle development at the small antral stage contributes to irregular or absent ovulation, a hallmark of PCOS. People with PCOS often have higher levels of anti-Müllerian hormone (AMH) due to these increased numbers of small, undeveloped follicles.
Diminished Ovarian Reserve (DOR) is another condition where the number of functional follicles in the ovaries is lower than expected for a person’s age. This reduction means fewer eggs are available for ovulation, which can affect fertility. While a low antral follicle count indicates DOR, it does not necessarily mean infertility, as conception can still occur, sometimes with assisted reproductive technologies. Both PCOS and DOR involve abnormalities in the quantity or maturation process of ovarian follicles, affecting reproductive health.