The hair we see on the surface of our skin, known as the hair shaft, is a non-living protein structure. This visible portion only represents a fraction of the entire biological unit. The true manufacturing center is hidden beneath the skin’s surface, where the living, dynamic hair root is located. Understanding this anatomy reveals how hair is produced, nourished, and grows.
Pinpointing the Hair Root
The hair root is securely encased within the hair follicle, a tunnel-like sheath. The follicle is an invagination of the epidermis, the skin’s outermost layer, which projects downward into the underlying tissue. For fine, downy hair across the body, the root is usually situated within the dermis, the layer directly beneath the epidermis.
For longer, thicker terminal hairs, such as those on the scalp, the root extends much deeper. These larger follicles can penetrate through the entire dermis and often reach into the subcutaneous layer, the fatty tissue below the skin. The follicle acts as a protective anchor, holding the developing hair firmly in place. The base of this deeply embedded structure is where the biological engine of hair growth resides.
The Engine of Hair Growth
The deepest part of the hair root is the hair bulb, an enlarged, rounded structure. This bulb is a highly active biological zone responsible for all hair production. Cupped within the base of the hair bulb is the dermal papilla, a small, cone-shaped formation.
The dermal papilla is rich in capillaries, delivering the blood supply, oxygen, and nutrients necessary to fuel growth. It also contains nerve endings and sends signaling molecules to regulate hair production. Surrounding the dermal papilla is the hair matrix, a layer of rapidly dividing cells. These matrix cells continuously multiply, pushing older cells upward to form the hair shaft.
How the Root Creates the Hair Shaft
The activity of the matrix cells is regulated by the hair growth cycle, a predictable, recurring pattern. This cycle determines when the root is actively producing hair and when it is resting. The first and longest phase is Anagen, the active growth stage, which can last anywhere from two to seven years. During Anagen, the matrix cells manufacture the hair fiber at a rate of approximately half an inch per month.
Once the Anagen phase concludes, the root enters the Catagen phase, a short transitional period lasting only a few weeks. In Catagen, the hair follicle shrinks, mitotic activity ceases, and the hair detaches from the dermal papilla. This results in the formation of a club hair, marking the end of active growth.
The final stage is the Telogen, or resting phase, which typically lasts two to four months. During this period, the club hair is held in the follicle, but no growth occurs. Eventually, the follicle begins a new Anagen phase, and the emerging hair pushes the old club hair out, leading to natural shedding.