folliculus

Folliculus is a Latin term meaning “little sac” and is used in anatomy to denote a small sac, pouch, or vesicle within an organism. In modern biology the term underpins several common structures, most prominently the hair follicle and the ovarian follicle, among other glandular or cavity-like formations.

A hair follicle is a complex structure embedded in the skin from which hair grows. It consists of the hair bulb at the base, the dermal papilla, and concentric epithelial sheaths. The associated pilosebaceous unit includes sebaceous glands and, in many places, an arrector pili muscle. The hair grows through cycles that alternate between growth (anagen), regression (catagen), and rest (telogen). Follicle activity is influenced by hormones, growth factors, and local signaling, and disruptions can contribute to hair loss or abnormal hair growth.

An ovarian follicle is a fluid-filled sac within the ovary that contains an immature oocyte (egg). Folliculogenesis begins with primordial follicles and progresses through primary, secondary, and eventually a Graafian (tertiary) follicle. The follicle produces hormones and follicular fluid; during ovulation, the mature follicle releases the oocyte, and the remaining follicle tissue forms the corpus luteum, which supports early pregnancy if fertilization occurs. Follicle development is regulated primarily by FSH and LH.

Beyond these, the term folliculus is used in histology to describe other small sac-like structures. Etymologically, it preserves the sense of a diminutive sac across anatomical contexts.