follicles

Follicle is a general term for a small sac or cavity in a tissue that contains a secretory product or a developing structure. In vertebrate anatomy, the term is used for several distinct structures, including hair follicles, ovarian follicles, lymphoid follicles, thyroid follicles, and dental follicles. Each type serves different biological functions but shares the basic concept of a compartment enclosing a developing or secretory component.

Hair follicle: In the skin, the hair follicle is a tubular invagination of the epidermis that houses the hair shaft and is associated with sebaceous and sometimes sweat glands. It extends from the surface to the dermis and undergoes growth cycles: anagen (growth), catagen (regression), and telogen (rest). The follicle interacts with dermal papillae and immune cells to regulate hair formation and cycling.

Ovarian follicles: In the ovary, follicles are fluid-filled sacs containing a developing oocyte. They progress from primordial to primary, secondary, and antral stages, with granulosa and theca cells producing sex steroids. Typically, one dominant follicle ovulates during each cycle, leaving behind the corpus luteum, which secretes progesterone.

Lymphoid follicles: Lymphoid follicles are dense B-cell–rich regions within lymph nodes and mucosa-associated lymphoid tissue, often featuring germinal centers. They support adaptive immune responses through clonal expansion, somatic hypermutation, and differentiation into plasma cells and memory B cells.

Thyroid follicles: Thyroid follicles are spherical units lined by follicular cells and filled with colloid containing thyroglobulin. They synthesize thyroid hormones T3 and T4, which are released in response to thyroid-stimulating hormone.

Dental follicles: The dental follicle surrounds a developing tooth and contributes to the formation of supporting structures, including the periodontal ligament, cementum, and alveolar bone.

Dysfunction of follicles can be involved in conditions such as folliculitis, ovarian cysts, or thyroid disorders.