Follicular cells are a type of cell that forms part of a small, sac-like structure known as a follicle. While the term “follicular cell” broadly describes cells within such structures, it is most notably applied to distinct cell types performing unique roles in two different and important parts of the human body: the thyroid gland and the ovaries. This specialized cellular organization allows for the intricate processes of hormone production and reproductive development.
Follicular Cells of the Thyroid Gland
Within the thyroid gland, follicular cells are the primary cell type, forming spherical units known as thyroid follicles. These cells are arranged in a single layer, cuboidal in shape, surrounding a central cavity filled with a protein-rich fluid called colloid. Their apical surface faces the colloid-filled lumen, while the basal surface interacts with the bloodstream for substance uptake.
These cells synthesize and secrete thyroid hormones, specifically thyroxine (T4) and triiodothyronine (T3). This process involves absorbing iodine from the bloodstream and incorporating it into the thyroglobulin protein stored within the colloid. T4 contains four iodine atoms, while T3 contains three, with T3 being the more potent form.
These thyroid hormones are important for regulating the body’s metabolism, influencing functions such as energy conversion, heart rate, body temperature, and brain development. The activity of thyroid follicular cells is controlled by Thyroid-Stimulating Hormone (TSH) from the pituitary gland. When TSH binds to receptors on the follicular cells, it stimulates iodine uptake, thyroglobulin synthesis, and the release of T3 and T4 into the circulation.
Dysfunction of these cells can lead to thyroid disorders. For instance, hypothyroidism results from insufficient hormone production, while hyperthyroidism occurs due to excessive hormone release. These conditions highlight the impact thyroid follicular cells have on overall health.
Follicular Cells of the Ovaries
In the ovaries, follicular cells surround and support the developing egg, or oocyte, within ovarian follicles. The ovarian follicle undergoes a maturation process, with these cells providing support and producing hormones that regulate the reproductive cycle. These cells differentiate into two main types: granulosa cells and theca cells, working together for egg development and hormone synthesis.
Granulosa cells directly surround the oocyte, multiplying as the follicle matures. They are stimulated by Follicle-Stimulating Hormone (FSH) and produce estrogen, a reproductive hormone. Estrogen promotes the growth and development of the female reproductive system.
Theca cells form an outer layer of connective tissue around the granulosa cells and are stimulated by Luteinizing Hormone (LH). Their function is to synthesize androgens, which then diffuse into the adjacent granulosa cells. Within the granulosa cells, an enzyme called aromatase converts these androgens into estrogen.
The activity of these follicular cells influences the menstrual cycle. As the follicle grows in response to FSH and LH, estrogen levels rise, triggering an LH surge that leads to ovulation, the release of a mature egg. Following ovulation, the remaining follicular cells transform into the corpus luteum, which continues to produce hormones, including progesterone, to support a potential pregnancy. The health and number of these cells are important for female fertility.
How Follicular Cells Are Studied
Understanding follicular cells involves diagnostic techniques specific to their location. For thyroid follicular cells, medical professionals use imaging and biopsy procedures to evaluate their condition. Ultrasound imaging provides a non-invasive way to visualize the thyroid gland, allowing for the detection of nodules or abnormalities that may arise from these cells.
If an abnormality is identified, a fine-needle aspiration (FNA) biopsy may be performed. During this procedure, a thin, hollow needle is guided into the thyroid nodule, with ultrasound assistance, to collect a cell sample for microscopic examination. While FNA can indicate a “follicular neoplasm,” a definitive diagnosis of follicular thyroid cancer often requires surgical removal of the mass for full histopathological assessment, as benign and malignant follicular tumors can appear similar on initial biopsy.
In the context of ovarian follicular cells, assessment focuses on evaluating female fertility and reproductive potential. Hormone levels in the blood provide insights into follicular activity. For example, Anti-Müllerian Hormone (AMH), produced by small antral and pre-antral follicles, serves as an indicator of ovarian reserve, reflecting the number of remaining eggs.
Transvaginal ultrasounds are also routinely used to visualize and count antral follicles within the ovaries. This “antral follicle count” (AFC) provides a direct estimate of the number of developing follicles and is a widely used marker in fertility assessments. These methods, including hormone level measurements and ultrasound imaging, are widely used tools in fertility clinics for guiding treatments like in vitro fertilization (IVF), as they help predict ovarian response to stimulation and inform treatment strategies.