Ovaries and Ovarian Follicles

Ovarian Follicles

An ovarian follicle is a structure found within the ovariesThe female gonads that produce eggs and hormones., consisting of an immature egg (oocyte) surrounded by supporting cellsThe basic structural and functional units of life.. These supporting cells are important for the growthAn increase in size and number of cells. and maturation of the egg, as well as hormone production. Here’s a breakdown using some key terms:

An oocyte is what you might call an egg cell.  This isn’t a great word to use because “egg” could refer to a fertilized or an unfertilized egg.  We use the term ovumA mature egg cell, which is singular, for an unfertilized egg. The plural of ovum is ova. We use the term zygote for a fertilized ovum.  Ova go through cellular divisions of meiosis in a process called oogenesisThe process of egg cell formation..  At the same time, there are cells surrounding the ovum, called granulosa, that also grow.  Together these granulosa cellsCells in the follicle that support the oocyte and produce estrogen. and the ovum form a follicle.  These follicle grow along with the stages of oogenesis.  The growth of a follicle, any follicle is called folliculogenesis.  At any given time in an ovary, you can find folliclesStructures in the ovaries that contain developing oocytes. at different stages of developmentThe process of growth and differentiation., as shown here. 

Structure

The female has two lumpy Oval shaped structures called ovaries each measuring about 5 centimeters in length. The surface of the ovary is covered by a simple cuboidal epithelial layer called the germinal epithelium. Blood is brought to the ovary by an ovarian artery. An ovarian vein drains the blood. They are in the center or medulla of the ovary. Production of gametes occurs in the cortex. Ideally the ovaries in a sexually mature female alternate to produce 1 ovum each month. the specific process for developing an ovum is called oogenesis. This is a part of the ovarian cycleThe cycle of follicle development, ovulation, and corpus luteum formation.. It encompasses all of the processes that occur in the ovary during this monthly event.

Primordial Follicles

Primordial follicles are the most numerous and earliest stage of follicular development in the ovary. They make up the majority of oocytes in the ovary. Each contains a primary oocyte arrested in Meiosis I, a state it can remain in for years, even decades. The oocyte is surrounded by a single layer of simple squamous granulosa cells. These cells provide minimal support and protection while the oocyte is dormant. These follicles are located in the outer cortex of the ovary and are not hormonally active.

Each month, in response to subtle intraovarian signals, a small group of these primordial folliclesThe earliest stage of follicle development. are activated. They are recruited or “awakened” to enter the growth phase. This recruitment is independent of gonadotropinsHormones (FSH, LH) that regulate reproductive organs and is driven by local paracrineLocal hormone-like molecules affecting nearby cells. growth factors. Once recruited, the granulosa cells become cuboidal and the follicle transitions into a primary follicleAn early stage of follicle development..  This monthly activation ensures the ovary maintains a steady supply of developing follicles throughout the reproductive lifespan.

Primary Follicles

A primordial follicle is recruited into the growth phase during the early part of the ovarian cycle. This process forms a primary follicle. It contains an oocyte that has been recruited from the resting pool in the ovarian cortex. This oocyte remains a primary oocyte arrested in prophaseThe first stage of mitosis where chromatin condenses into visible chromosomes. I of meiosis. The hallmark of the primary follicle is the transformation of its surrounding follicular cellsCells surrounding the oocyte that support its development. from flat squamous cells. They transform into a single layer of cuboidal granulosa cells. These cells begin to support the oocyte’s growth. The zona pellucidaA glycoprotein layer surrounding the oocyte., a protective glycoproteinA protein with a carbohydrate chain attached, important for cell signaling. coat around the oocyte, also begins to form at this stage.

At the primary follicle stage, the follicle is just responding to local growth factors produced within the ovary. It is not responding to circulating gonadotropins like FSH. These local signals stimulate granulosa cell proliferation and oocyte development. The primary follicle marks a critical transition. It moves from a dormant to an actively growing follicle. This stage lays the groundwork for more advanced stages of follicular development if selected for further maturation.

Secondary Follicles

A primary follicle develops into a secondary follicleA developing follicle with multiple layers of cells. during the follicular phaseThe first phase of the ovarian cycle when follicles grow and estrogen levels rise. of the ovarian cycle. This secondary follicle is found in the cortex of the ovary. At this stage, the follicle still contains a primary oocyte arrested in prophase I of meiosis. As the follicle matures, it develops two or more layers of granulosa cells. These cells proliferate and begin producing small amounts of follicular fluidThe fluid inside the antrum that nourishes the developing oocyte.. This fluid is a precursor to the antrum seen in the next stage of development. These granulosa cells also begin to express FSH (follicle-stimulating hormone) receptorsProteins located on the surface or inside cells that bind specific molecules (e.g., neurotransmitter. This expression allows the follicle to respond to hormonal signals. These signals support continued growth and estrogen production.

Outside the granulosa layers, the follicle becomes surrounded by developing thecal cells. These cells organize into two layers: the thecaA layer of cells surrounding the follicle that produces hormones. interna and theca externa. The theca interna is hormonally active and produces androgensMale sex hormones, including testosterone and DHEA.. Granulosa cells then convert these androgens into estrogensFemale sex hormones responsible for secondary sex characteristics and regulation of the menstrual cy. The theca externa is more fibrous and provides structural support. This coordinated development between the granulosa and thecal layers is under hormonal control. It ensures the ovary is ready to release a viable oocyte if fertilization is possible.

Tertiary Follicles

A tertiary ovarian follicle has other names. It is also known as a Graafian follicle or mature follicle. This is the final stage of follicular development before ovulationThe release of a mature oocyte from the ovary.. The follicle houses an oocyte. It has completed the first meiotic division just prior to ovulation. The oocyte is arrested in metaphaseA stage of mitosis where chromosomes align at the metaphase plate. II. A small cluster of granulosa cells called the cumulus oophorus, anchors the oocyte to the inner wall of the follicle.

A thick glycoprotein coat surrounds the oocyte. This coat is called the zona pellucida. It plays a key role in sperm binding during fertilization. Immediately outside the zona pellucida is a single layer of specialized granulosa cells. This layer is known as the corona radiataThe layer of follicular cells that surrounds a developing oocyte.. It remains with the oocyte at ovulation. The antrum is a fluid-filled cavity. It develops and expands greatly in the tertiary follicleA late-stage developing follicle.. This expansion pushes the oocyte and its surrounding cells to one side. This fluid, called follicular fluid, contains hormones and growth factors.

Ovarian Scars

After ovulation, the ruptured tertiary follicle transforms into a temporary endocrine structure called the corpus luteum. This yellowish body forms from the remaining granulosa and theca cells. These cells luteinize and begin secreting large amounts of progesteroneA hormone that supports pregnancy and regulates the menstrual cycle.. They also secrete some estrogen. Progesterone plays a critical role in maintaining the uterine lining (endometriumThe inner lining of the uterus that thickens during the menstrual cycle to support a potential pregn). It makes it thick and vascular in preparation for possible pregnancy. If fertilization occurs, the embryo releases human chorionic gonadotropin (hCG). This hormone signals the corpus luteum to persist and continue hormone production. This supports the early stages of pregnancy until the placentaThe organ that facilitates nutrient and waste exchange between the mother and fetus. takes over.

If pregnancy does not occur, the corpus luteum degenerates after about 10–14 days. It then becomes the corpus albicansA scar-like structure that forms when the corpus luteum degenerates., a small, whitish, fibrous scar tissue remnant in the ovary. The drop in progesterone and estrogen levels leads to the breakdown and shedding of the uterine lining during menstruation. Each ovary contains follicles at various stages of development. There are also past remnants like the corpus albicans. These remnants accumulate over a woman’s reproductive years.

Ovaries

PCOS is a hormonal disorder. It causes enlarged ovaries with multiple small cysts. This condition often leads to irregular menstrual cycles, infertility, acne, and excess hair growth. It is associated with insulin resistanceThe opposition to airflow in the respiratory tract, influenced by airway diameter. and increased risk of type 2 diabetes and cardiovascular disease.

Ovarian cysts are fluid-filled sacs that develop on or inside an ovary. They often form during ovulation and typically resolve on their own. Some cysts can cause pelvic pain, bloating, or complications like rupture or ovarian torsion if they become large.

Ovarian cancer starts in the cells of the ovary. It is often detected late because of vague early symptomsSubjective experiences reported by the patient (e.g., nausea, fatigue). like bloating and pelvic discomfort. It is one of the most lethal gynecological cancers due to its tendency to spread before diagnosis.

HPV is a common sexually transmitted infection that can cause genital warts and is the leading cause of cervical cancer. Most infections clear on their own, but persistent high-risk strains may lead to cancerous changes in cervical cells.

Oophorectomy is the surgical removal of one or both ovaries. It is often performed to treat ovarian cancer or cysts. It is also done to reduce the risk of cancer in high-risk individuals. Removing the ovaries can induce early menopauseThe natural cessation of menstruation and ovarian function. and affect hormone levels, fertility, and overall health.