Follikkelit

Follikkelit (follikkel in singular) are small sac‑like structures found in many tissues of the body. They are typically lined by epithelial cells and surrounded by connective tissue; their main role is to enclose, protect, or support developing cells or secretions. The specific form and function of a follikkel vary by organ.

In the skin, a hair follikkel extends from the surface epidermis into the dermis and anchors a hair shaft. It includes structures such as the hair matrix, sebaceous glands, and the arrector pili muscle. Hair follikkelit go through a cyclical process with growth (anagen), regression (catagen), and rest (telogen).

In the ovaries, ovarian follikkelit surround and nourish developing oocytes. They progress from primordial to primary and secondary stages, culminating in a mature Graafian follicle before ovulation. Their development is regulated by hormones, notably follicle-stimulating hormone (FSH) and luteinizing hormone (LH), which coordinate growth, ovulation, and the formation of the corpus luteum.

Other organs contain follicular structures as well, such as thyroid follikkelit, which are lined by epithelium and surround colloid to produce thyroid hormones, and lymphoid follikkelit within lymph nodes and the spleen that organize B cells. Histologically, follicles are often described by the type of lining and the contents within their lumen.

Clinically, disturbances in follicle development or function can contribute to conditions such as polycystic ovary syndrome, hair‑loss disorders involving hair follikkelit, or thyroid diseases affecting follicular activity. The term derives from Latin and is used across multiple specialties to describe these basic architectural units.