Follicle cells are specialized cells that organize into a small, sac-like structure known as a follicle, a term derived from the Latin word for “small bag.” These cellular arrangements are found in several locations throughout the body, where they perform distinct functions based on their environment. The structure and purpose of a follicle are dictated by the needs of that particular biological system.
Follicle Cells in Hair Growth
Deep within the skin, hair follicle cells produce the hair shaft. The hair follicle is an organ where, at its base, the dermal papilla provides nutrients for matrix cells. These cells divide rapidly to form the keratinized cells that make up the hair fiber.
The activity of these cells is governed by a cycle with three phases. The anagen (growth) phase can last from two to seven years. The catagen phase is a short transition where growth stops and the follicle shrinks. The cycle concludes with the telogen (resting) phase, after which the hair is shed and the process restarts.
The health of these cells affects hair’s appearance and density. In androgenetic alopecia (pattern baldness), hormonal changes cause follicles to shrink, producing finer hairs until growth ceases. Graying occurs when the follicle’s pigment-producing cells, melanocytes, are lost, resulting in colorless hair.
Follicle Cells in Ovarian Function
Within the ovaries, follicle cells are centered on reproduction. Ovarian follicles each contain a single developing egg (oocyte) surrounded by two primary types of specialized cells that facilitate its growth: theca cells and granulosa cells.
Theca and granulosa cells have complementary roles in hormone production. Theca cells form the follicle’s outer layer and produce androgens. These androgens are then transferred to the inner granulosa cells, which use an enzyme to convert them into estrogen.
This hormone production drives the menstrual cycle. Rising estrogen promotes follicle development, a process called folliculogenesis. As the follicle grows, it nurtures the oocyte until a hormone surge leads to ovulation, releasing the egg. The remaining follicle cells then transform into the corpus luteum, which produces progesterone to prepare for a potential pregnancy.
Dysfunction of these cells can impact reproductive health. In Polycystic Ovary Syndrome (PCOS), theca cells may produce excess androgens, disrupting follicle development and ovulation. Premature ovarian insufficiency involves a depletion or dysfunction of ovarian follicles before age 40.
Follicle Cells of the Thyroid Gland
A third population of follicle cells, known as thyrocytes, resides in the thyroid gland. Arranged in spherical follicles, their function is to produce the thyroid hormones thyroxine (T4) and triiodothyronine (T3). These hormones are responsible for managing the body’s overall metabolism.
Thyroid follicular cells are adapted to absorb iodide from the bloodstream. They use a specialized protein to transport iodide into the cell, where it is attached to a large protein called thyroglobulin. This combination is stored as a substance called colloid in the center of the follicle.
When the body needs thyroid hormones, follicular cells reabsorb the thyroglobulin from the colloid. Enzymes then cleave T3 and T4 from the thyroglobulin, and the finished hormones are released into the bloodstream to regulate metabolic rate, growth, and development.
The function of these cells is necessary for metabolic balance. When follicular cells underproduce hormones, the result is hypothyroidism, leading to fatigue and weight gain. Conversely, overactive cells that produce excess hormones cause hyperthyroidism, which can lead to weight loss and a rapid heart rate.