Under the Microscope: Hair and Scalp Anatomy

Hair is much more complex than it looks. In this section, we describe how hair follicles grow and alter with time and other factors. Understanding how hair follicles function (and what makes them stop functioning) can help you maintain them, thereby slowing the hair loss process.

Going beneath the skin

Most people think of hair only in terms of what they see above the scalp, but hair is actually a rather complex organ that goes beneath the surface. Anatomically speaking, hair is part of your skin. But because hair is physically distinct from skin, it’s referred to as a skin appendage. (Other skin appendages include sweat glands, fingernails, and toenails.)

Skin is composed of three main layers:

The epidermis: This outer layer of the skin is less than 1 millimeter thick. It’s 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.

The dermis: This tough layer of connective tissue is about 2 to 3 millimeters thick on the scalp. This layer gives the skin its strength and contains both sebaceous (oil) glands and sweat glands. Sebaceous glands produce an oily substance, which creates a plug of wax (sebum) to cover the opening to the growing hair
follicle. As the hair grows upward from the skin surface, some of the waxy substance is taken up by the hair shaft as a lubricant, giving the hair a waxy sheen. Sweat glands help control body temperature, particularly when it’s hot. These glands produce a watery, salty sweat; as the sweat evaporates, body heat is lost.

Subcutaneous fat and connective tissue: This layer contains the larger sensory nerve branches and the blood vessels that nourish the skin. In the scalp, the lower portions of the hair follicles (called the bulbs) are found in the upper part of this
fatty layer.

Dissecting a hair follicle

The hair follicle must function properly in order to maintain a healthy head of hair. A hair follicle is a complex structure that measures about 4 to 6 millimeters in length. Each follicle produces one to four hair shafts, each about 0.1 millimeters in width (in other words, these are really, really tiny structures).

The layers of a hair follicle

Hair follicles have three layers surrounding the strand of hair,

The outer root sheath, or trichelemma: This is the outer layer, which surrounds the follicle in the dermis and then blends into the epidermis on the surface of the skin, forming the pore from which the hair grows.

The inner root sheath: This middle layer is composed of three parts, with the cuticle being the innermost portion that touches the strand of hair. The cuticle of the inner root sheath interlocks with the hair cuticle (described in “the layers of the hair shaft” section) to give it rigidity.

The bulb: This is the lower portion of each hair follicle. It contains the inner matrix cells, which produce bundles (also called spindles) of hair cells that look like fine wires in an electric cord. These bundles are actually made up of even smaller bundles, which literally twist as they’re made. The size of the bulb and the number of matrix cells determine the width of the fully grown hair. The layers of the hair shaft

The hair shaft is composed of three layers.

The cuticle: This layer forms the surface of the hair and is what you see as the hair shaft emerges from the follicle.

The cortex: This middle layer comprises the bulk of the hair shaft and is what gives hair its strength. It’s composed of an organic protein called keratin, the same material that comprises rhinoceros horns and deer antlers.

The medullas: This is the center, or core, of the hair shaft, and it’s only present in terminal hair follicles (a fully grown hair).

The dermal papillae

At the bottom of the hair follicle, there is a bulbous (bulb) portion which contains a small collection of specialized cells called the dermal papillae. Scientists believe these cells are at least partially responsible for determining. how long the hair will eventually grow, how thick it will be, and the character of the hair will be. For many years, scientists thought that hair actually grew from the dermal papillae. Recent evidence shows that the growth center is not totally controlled from the dermal papillae. Elements that control hair growth can be found all the way up to the region of the follicle where the sebaceous glands are attached. If the dermal papillae is removed (which sometimes happens during a hair transplant, the hair follicle may still be able to regenerate new hair, although the new hair may not be a characteristically healthy looking terminal hair. It may be shorter,
thinner, and more kinky or wavy.