Folliculi

Folliculi are the plural of folliculus, a Latin term used in biology to denote small sacs, vesicles, or gland-like structures. The word appears in several organ systems to describe saclike units with various functions.

Hair follicles (folliculi pilorum) are epidermal invaginations from which hair grows. They consist of an inner epithelial root sheath surrounding the hair shaft, an outer root sheath, and a dermal papilla at the base; they are associated with sebaceous glands and arrector pili muscles and undergo cyclical phases of growth (anagen), regression (catagen), and rest (telogen).

Ovarian follicles (folliculi ovarii) are fluid-filled sacs located in the ovaries that encase immature oocytes. They progress from primordial to primary, secondary, and mature Graafian (antral) stages, producing estrogen and preparing the oocyte for ovulation.

Lymphoid follicles (folliculi lymphatici) are discrete nodules in lymph nodes and mucosa-associated lymphoid tissue. They feature a germinal center rich in proliferating B cells and a surrounding mantle zone, and participate in humoral immune responses.

Dental follicle (folliculus dentis) surrounds the developing tooth, contributing to the formation of the periodontal ligament and to eruption into the oral cavity.

Thyroid follicles (folliculi thyroideae) are spherical units lined by cuboidal epithelium and filled with colloid that stores thyroglobulin. They synthesize and secrete the thyroid hormones triiodothyronine (T3) and thyroxine (T4).