The Hair Follicle’s Complex Environment: How It Contributes to the Success of Hair Transplantation

The Hair Follicle’s Complex Environment: How It Contributes to the Success of Hair Transplantation

May, 2008

Every part of your body has unique anatomical features, and the scalp is no exception. Your scalp is anatomically complex—far more than skin stretched over your skull with some hair growing out of it, although this may be what you seem to see at a cursory glance in the mirror.

Your scalp also plays important roles in the success of hair transplantation, if or when you may choose to have this surgical hair restoration procedure. The anatomic features discussed below all contribute to a successful outcome of hair transplantation.

You probably seldom take notice of your scalp unless it gives you problems (such as dandruff or annoying itch), or if you begin to see too much of it because of thinning hair.

If you do give some thought to your scalp, one way to think about it is in relation to your hair: your scalp is a complex anatomical environment for the complex biochemical factory encased inside the hair follicle. The anatomy of the scalp supports the hair follicle’s activity of producing hair and cycling hair through its anagen-catagen-telogen phases (see About Your Hair Loss) Scalp anatomy provides a dynamic environment for the transplanted follicles to thrive: physical support for the transplanted follicles, nourishment for the transplanted follicles, and a nerve supply to conduct the signaling that passes among all cells of the body.

Anatomy of the Scalp

In order for hair follicles to produce, maintain and cycle hair, they need (1) a solid foundation of supportive tissue, (2) a copious blood supply to furnish oxygen, nutrients, hormones and various biochemical signaling molecules to the follicle, and (3) a network of nerves to carry the subtle neuronal signals necessary to all of the body’s activities.

Anatomic Layers of the Scalp

The scalp that may appear to be no more than skin stretched over bone is actually a structure with five distinct layers. From top to bottom, these layers are:

• Skin—the outermost layer, itself divided into two layers called epidermis or outer layer, and dermis or deep layer. The hair follicle passes through both the epidermis and dermis and rests in the next subcutaneous layer. In the dermis are the tiny arrector pili muscles that pull hair into the "standing on end position in response to exposure to externally experienced cold air, or internally experienced fear.
• Subcutaneous layer, beneath the skin, consists of fatty tissue, fibrous tissues that connect this layer to the skin above, and the blood and nerve supply for hair follicles that are encased in this layer.
• Galea aponeurotica is a tendon-like layer that connects with scalp muscles and provides tensile strength to the scalp.
• Subgalea fascia is a thin layer that contributes to the pliability of the scalp.
• Pericranium is the layer that attaches to the bones of the skull.

At the microscopic level, scalp tissue also provides a highly specialized microenvironment for the hair follicle which fosters the maintenance of stem cells that retain the ability to regenerate skin and follicle tissue. (see Hair Follicle Origin)

Blood Supply of the Scalp

The scalp has a rich arterial and venous blood supply. This accounts for the copious bleeding that occurs with even minor lacerating injury to the scalp. Arteries that supply blood to the scalp derive from the carotid arteries that pass through the neck and, through other branches, also supply blood to the brain. The rich blood supply is essential to the functioning of hair follicles in manufacturing, maintaining and cycling hair. Maintenance of an adequate blood supply can be compromised by any factor that tends to interfere with blood supply. A major factor of this nature is tobacco smoking. Smoking reduces blood flow in all parts of the body, including the scalp. Thus, one of the preoperative recommendations of a physician hair restoration specialist may be for the patient to stop smoking before and for a period of time after hair transplantation.

Nerve Supply of the Scalp

Nerves of the scalp serve both sensory and motor functions. Like its blood supply, the scalp’s nerve supply is very rich. The sensory nerves serve the entire scalp area, including the ophthalmologic (eye), auricular (ear) and facial, carrying nerve signals in both directions. Nerves with motor function are essential for muscle movements such as those involved in facial expressions. A network of sensory nerves surrounds hair follicles, carrying signals to and from the follicle. This sensory network contributes to the fine perception of heat, cold and movement across the surface of the skin.

Hair Follicle Patterning

The anatomic positioning of hair on the scalp is less like trees in a forest and more like islands in the sea. At first glance, it appears that scalp hair grows like trees in a forest—individual trees (or hairs) spaced rather evenly apart. Closer examination under a magnifying glass makes it clear that hairs grow in clumped structures called follicular units. Read more about follicular units and their uses in hair transplantation by clicking on Follicular Unit Transplantation and Extraction: and Hair Transplants That Don’t Look Like Transplants: Follicular Unit Transplantation Led the Way:

The typical follicular unit consists of one to four or five follicles that produce mature hair, two follicles that produce vellus-like ("peach fuzz) hair, sebaceous (oil) glands, blood vessels, nerve network, arrector muscles, and supportive tissue. The grouping of follicles is believed to have developmental advantages over a "tree farm type of follicular distribution.

Follicular unit transplantation is widely used today by physician hair restoration specialists. Depending on the needs and scalp characteristics of the individual patient, follicular unit transplantation may be used wholly or in part to achieve an outcome that meets the patient’s expectations.

Problems of the Scalp, Hair Follicle and Hair

Scalp and hair problems fall into three general categories:

• Follicle problems—all problems of hair loss, hair malformation and hair mal-coloration are associated with the hair follicle, and are usually under some form of genetic control. While hair loss problems such as androgenetic alopecia (see About Your Hair Loss:) can often be medically or surgically treated, they cannot be prevented.
• Hair damage—physical, chemical or thermal damage to hair is almost always preventable because it is under our control to prevent it. Common causes of hair damage are weathering, bleaching, straightening with chemicals and/or heat, and styling with harsh chemicals.
• Scalp problems—most scalp problems occur in the outer (epidermal and dermal) layers of skin, and range from dandruff due to excessive scalp oiliness to inflammatory conditions such as psoriasis