Textbook of Plastic and Reconstructive Surgery

Edited by Deepak M. Kalaskar, Peter E. Butler & Shadi Ghali

Aesthetic Surgery - The Evolution of Hair Transplant Surgery

2.1. Anatomy of the hair follicle

Anatomically, hair is broadly divided into an upper, middle and lower segment. The upper segment contains the infundibulum and isthmus, the middle segment contains the bulge which stores stem cells and the inferior segment contains the bulb (Figure 20.1) (Schneider et al., 2009).

Figure 20.1.
The human hair follicle: structure of the shaft.

The hair follicle is a part of the epidermis that extends down into the dermis. The hair follicular unit (FU) is the grouping of terminal hairs, which can be seen under a microscope and contains approximately one to four hairs (Bernstein et al., 1998).

The infundibulum is the superficial part of the follicle. The isthmus is the part between the sebaceous gland duct opening and the bulge. The bulb is the deepest portion of the follicle and houses the stem cells that form matrix cells. The matrix cells keratinise to form the hair cortex and divide to form the remaining hair layers (Kahle et al., 1993). The bulb also surrounds the dermal papilla, a structure which is responsible for the embryonic generation of a hair follicle and, in the fully formed follicle, contains capillaries, nerves and melanocytes (Blume-Peytavi et al., 2008).

The follicle is surrounded by many layers. From innermost to outermost are the layers of the hair shaft which can be seen above the skin: the medulla, cortex and outer cuticle. In the dermis, this is further enveloped by the inner root sheath, outer root sheath and connective tissue sheath (Figure 20.1).

2.1.1. Medulla

The medulla is in the centre of the shaft. It is sometimes absent in hair; when present (usually in coarse hair), it is often discontinuous along the length of hair (Krstic, 1991).

2.1.2. Cortex

The cortex represents 90% of the total weight of the shaft. It is made of spindle-shaped cortical cells filled with keratin. Young cortical cells contain melanin granules which determine the hair colour. These cells become elongated as they progress from the matrix. They are held within intercellular cement composed of keratin and lipids (Krstic, 1991; Kahle et al., 1993).

2.1.3. Cuticle

The cuticle is the outermost layer of the shaft and is made of keratin. It protects the inside of the hair shaft from damage. The cells forming the cuticle are also held within intercellular cement which is rich in lipids. The cells overlap one another and are said to resemble the slates on a roof (Krstic, 1991).

2.1.4. Inner root sheath

The inner root sheath extends from the isthmus to the base of the bulb. It consists of three layers: an inner cuticle, the Huxley layer and the outer Henle layer. The cells in these layers are identifiable by the presence of a structural protein called trichohyalin, which is a marker for hair follicular differentiation (Kahle et al., 1993).

2.1.5. Outer root sheath

The outer root sheath is found at the periphery and continues within the epidermis. It contains Golgi complexes, mitochondria and glycogen. A ‘bulge’ can be seen on the outer sheath where the erector pili muscle inserts. This bulge sits below the sebaceous gland and causes the gland to secrete sebum when it contracts (Kahle et al., 1993)