Hair follicle

A skin organ that surrounds the hair root and produces hair through specialized cells. Its growth cycle alternates between active production, regression, and rest.

Hair bulb: The enlarged base of a hair follicle, where dividing cells form the hair shaft. Its matrix cells produce the hair at the follicle’s base.
Hair shaft: The visible, keratinized portion of a hair that extends above the skin surface. It is the structure produced by follicular cells and emerging from the follicle.
Androgenetic alopecia: A patterned form of hair loss caused by genetically influenced sensitivity to androgens. Sensitive scalp follicles progressively miniaturize, producing shorter and finer hairs.
Epidermis: The outermost layer of skin, forming a protective barrier at the body surface. The follicular epithelium extends downward from this surface layer.
Dermal papilla: A small connective-tissue structure at the base of a hair follicle that supplies signals and nutrients. Signals from this structure help regulate follicle growth and hair production.
Hair root: The portion of a hair enclosed within the follicle beneath the skin surface. The follicle surrounds this embedded part of the hair.
Alopecia areata: An autoimmune disorder in which immune activity causes nonscarring hair loss. Immune attack disrupts the growth of follicles, often producing sharply defined bald patches.
Pilosebaceous unit: A skin structure comprising a hair follicle, its hair, and an associated sebaceous gland and arrector pili muscle. This larger unit places the follicle among its closely connected structures.
Hair matrix: A layer of rapidly dividing cells in the hair bulb that generates the hair shaft and inner root sheath. These proliferating cells differentiate into the structures the follicle produces.
Outer root sheath: The outer epithelial layer of a hair follicle, continuous with the epidermis. It forms the follicle’s epithelial boundary and contains cells with regenerative capacity.

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