Hair is a protein filament that grows from follicles deep in the skin. It is traditionally separated into two distinct parts: the hair follicle and the hair shaft. The follicle is the living structure beneath the skin, and the shaft is the non-living, visible fiber extending above the surface. These two parts work together to produce and maintain the hair.
The Hair Follicle (The Root Structure)
The hair follicle is a living organ within the dermis layer of the skin, functioning as the hair’s production factory. At the base of the follicle is the hair bulb, which surrounds the dermal papilla. The papilla contains blood vessels that deliver oxygen and nutrients to fuel hair growth.
The hair matrix, located above the papilla, contains rapidly dividing keratinocytes responsible for forming the hair shaft. These matrix cells also include melanocytes, which produce the melanin pigment that gives hair its color. Accessory structures are attached to the follicle. The sebaceous gland releases sebum, an oily substance that lubricates the hair and surrounding skin. The arrector pili muscle, a small band of smooth muscle, causes the hair to stand up, resulting in “goosebumps.”
The Hair Shaft (The Visible Structure)
The hair shaft extends beyond the skin’s surface and is considered non-living tissue. It is primarily composed of keratin, a tough, fibrous protein. The shaft consists of three distinct layers.
The innermost layer is the medulla, a soft, central core that is not always present, especially in finer hair types. Surrounding this is the cortex, the thickest layer, which provides the hair with strength, elasticity, and color. The cortex contains the melanin granules that determine hair shade. The outermost layer is the cuticle, which serves as the hair’s protective barrier. It is formed by flat, overlapping cells that resemble shingles on a roof. This arrangement protects the inner cortex and medulla from damage and influences the hair’s overall shine and smoothness.
How Hair Grows and Sheds
Hair production is a continuous, cyclical process that ensures the renewal of the hair fiber. This cycle is divided into three main phases: anagen, catagen, and telogen.
The anagen phase is the active growth period where matrix cells rapidly divide to form the hair shaft, lasting two to seven years for scalp hair. The catagen phase is a short transitional period lasting a few weeks. During this time, the hair follicle shrinks, and growth slows as it prepares for the resting stage. The telogen phase is the resting period, lasting a few months, where the old hair is shed to make way for a new hair beginning its anagen phase.