Hair follicle

Hair follicle is a complex, self-renewing mini-organ of the skin that produces hair through a cyclical process of growth, regression, and rest. It is a tubular invagination of the epidermis that extends deep into the dermis (and sometimes into the subcutaneous fat), enclosing the hair root and bulb.

Structural Components

The hair follicle consists of several key anatomical structures:

• Hair bulb: The expanded, rounded base of the follicle that houses the dermal papilla — a cluster of specialized mesenchymal cells that provides inductive signals for hair growth — and the hair matrix, a population of rapidly proliferating keratinocytes that give rise to the hair shaft.

• Inner root sheath (IRS): Composed of three concentric layers (Henle's layer, Huxley's layer, and the cuticle of the IRS), it molds and supports the growing hair shaft within the follicle.

• Outer root sheath (ORS): A continuation of the basal epidermis that surrounds the inner root sheath and is contiguous with the surface epithelium.

• Connective tissue sheath (dermal sheath): A layer of mesenchymal tissue surrounding the outer root sheath, continuous with the dermis.

• Bulge region: Located at the insertion point of the arrector pili muscle, this region harbors a reservoir of multipotent epithelial stem cells critical for follicle regeneration and wound healing.

Associated Structures

• Sebaceous gland: Opens into the upper follicular canal and produces sebum.

• Arrector pili muscle: A small smooth muscle that inserts at the bulge and, upon contraction, causes the hair to stand erect (piloerection).

• Sensory nerve endings: Surround the follicle and provide tactile sensation.