Hair Anatomy

Hair has two separate structures: the hair shaft (the part we see above the skin) and the follicle (the part that is below the surface of the skin). The hair shaft is composed of strong structural protein called keratin. This is the same protein that makes up the nails and the outer layer of the skin (Fig. 1.1).

Below the surface of the skin is the hair “root” or hair follicle comprised of several structures: dermal papilla, hair bulb, hair shaft, sebaceous gland, and a tiny muscle.

• The medulla is the innermost layer found in large and thick hair.
• The cortex is the middle layer, which provides strength as well as imparts color and texture to the hair.
• The cuticle is the outermost layer made up of tightly packed scales that form an overlapping structure similar to the roof shingles and function as a protective coat over the cortex.

Most hair-conditioning products attempt to affect the cuticle by making these scales lie flat, thereby imparting a silky feel to the hair.

Hair Growth Cycles

Unlike other mammals that shed or grow hair according to the season, in humans hair growth and loss are random and not strictly but somewhat seasonal. At any given time, in humans a random number of hairs will be in various stages of growth and shedding.

The three phases of hair growth cycle are as follows: anagen,catagen, and telogen.

• Anagen is the active growth phase during which the cells in the matrix are dividing rapidly, adding to the hair shaft. During this phase, the hair grows approximately 1–2 cm (i.e., 1/2–1 inch) per month.
• The catagen stage is a short transition phase that occurs at the end of the anagen phase during which the hair follicle receives a signal to stop growing. Shortly after the hair growth stops, the dermal papilla contracts and releases the hair shaft from the follicle allowing hair to shed.
• The telogen phase describes the resting time for the hair follicle. During this phase, a secondary hair germ forms from a band of epithelial cells that move upward, often seen during graft dissection as a short, smaller version of the hair bulb.

Each hair follicle goes through 10–20 cycles in a lifetime, while each cycle lasts a different time: anagen lasts 3–10 years, catagen 2–3 weeks, and telogen 3–4 months. About 25–100 hairs are shed normally each day (Fig. 1.2).

Some people have difficulty growing their hair beyond a certain length because they have a short active phase of growth. However, people with very long hair have a long active phase of growth.