Anatomically, hair is a distinct part of the skin referred to as an appendage. Other skin appendages include sweat glands, fingernails and toenails. Skin is composed of three main layers. The outer layer of skin is the epidermis. This layer is less than a millimeter in thickness and is composed of dead cells that are in a constant state of sloughing and replacement. As dead cells are lost, new ones from the growing layer below replace them.
Beneath the epidermis is the dermis, a tough layer of connective tissue (collagen) that is about 2 to 3 mm thick on the scalp. This layer gives the skin its strength, and contains both sebaceous glands and sweat glands.
Beneath the dermis is a layer of subcutaneous fat and connective tissue. The larger sensory nerve branches and the blood vessels that nourish the skin run deep in this layer. In the scalp, the lower portions of the hair follicles (the bulbs) are found in the upper part of this fatty layer.
An interesting characteristic of hair is that, in contrast to the commonly held notion that it grows as individual strands, it actually emerges from the scalp in groups of one to four (and sometimes even five or six). The reason for this is that hair follicles are not solitary structures, but are arranged in the skin in naturally occurring groups called follicular units. Although skin pathologists recognized this fact in the early 1980’s, its profound importance in hair transplantation was not appreciated until the mid-1990’s. The use of grafts composed of naturally occurring, individual follicular units, rather than an arbitrary number of hairs, has revolutionized hair transplant surgery.
Each hair follicle measures about 3-4 mm in length and produces a hair shaft about 0.1 mm in width. The hair follicle has five main parts. Starting from the bottom of the follicle, they are; the dermal papillae, matrix, outer root sheath (ORS), inner root sheath (IRS), and the hair shaft.
The dermal papillae contains specialized cells called fibroblasts that regulate the hair cycle and hair growth. The dermal papillae contains androgen receptors sensitive to DHT. For many years, scientists thought that hair growth originated from the dermal papillae. Recent evidence has shown that the growth center extends from the dermal papillae all the way up to the region of the follicle where the sebaceous glands are attached. It is now believed that the primary function of the dermal papillae is to regulate follicular growth and differentiation. If the dermal papillae is removed (this sometimes happens during a hair transplant), the hair follicle is often able to regenerate a new one, although the growth of the new hair will be delayed.
The matrix sits over the dermal papillae and contains actively dividing, immunologically privileged cells. Together, the dermal papillae and the matrix are referred to as the hair bulb. The size of the bulb and the number of matrix cells will determine the width of the fully-grown hair. The cells of the matrix differentiate into the three main components of the hair follicle: ORS, IRS and hair shaft.
The outer root sheath or trichelemma (Greek for coating sac), surrounds the hair follicle in the dermis and then blends into the epidermis on the surface of the skin, forming the structure commonly referred to as the pore (from which the hair emerges).
The inner root sheath essentially forms a mold for the developing hair shaft. It is composed of three parts (Henley layer, Huxley layer, and cuticle), with the cuticle being the innermost portion that touches the hair shaft. The cuticle of the IRS is formed by a layer of overlapping cells that interlock with the cuticle of the hair shaft. This overlapping mechanism holds the hair shaft securely in place, but also allows it to grow in length.
The cells of the IRS keratinize giving it rigidity and strength. Racial variations are felt to be due to the asymmetric formation of the IRS. If you look at the cross section of the IRS, the shape is oval in Europeans, flat in Africans, and round in Asians.
The hair shaft is the only part of the hair follicle to exit the epidermis (the surface of the skin). The hair shaft itself is also composed of three layers. The cuticle, the outer layer that interlocks with the internal root sheath, forms the surface of the hair and is what we see as the hair shaft emerges from the follicle. The middle layer, the cortex comprises the bulk of the hair shaft and is what gives hair its strength. It is composed of an organic protein called keratin, the same material that comprises rhinoceros horns and deer antlers. The center, or core, of the hair shaft, is the medulla, and is only present in terminal hair follicles.