folliklar

Folliklar, commonly translated as follicles, are small sac-like structures found in various tissues and organs. They typically consist of an epithelial lining that encloses a lumen or cavity and participate in growth, secretion, or maturation processes. The term is used for several anatomically distinct structures, each with specialized cells and regulatory signals.

In the skin, hair follicles are downward invaginations of the epidermis that produce hair. Each follicle contains a hair bulb, the growing hair shaft, and a sebaceous gland. Hair follicles undergo a cycle with phases called anagen (growth), catagen (regression), and telogen (rest). They contribute to temperature regulation, protection, and sensory function and are influenced by hormones and signaling pathways.

In the ovaries, ovarian follicles are functional units that contain immature oocytes surrounded by granulosa and theca cells. They progress from primordial to primary, secondary, and finally mature Graafian follicles. Folliculogenesis is regulated by follicle-stimulating hormone (FSH) and luteinizing hormone (LH). A mature follicle releases an oocyte during ovulation and produces estrogens; many follicles undergo atresia if not selected.

In the thyroid gland, follicles are spherical units lined by follicular cells and filled with colloid containing thyroglobulin. They synthesize and secrete thyroid hormones (T3 and T4) in response to thyrotropin (TSH). The thyroid follicle is a key element of endocrine regulation, linking gland architecture to hormone production.

Clinical notes: follicle-related conditions include folliculitis (inflammation of hair follicles), ovarian follicular cysts, and certain thyroid disorders that affect follicle function. Understanding follicle structure helps explain patterns of growth, secretion, and maturation across tissues.