Chapter 2 – Hair and Its Functions
The condition and appearance of one’s hair is an important indicator of age and the body’s general state of health. Other similar indicators, such as skin condition, muscular coordination, brightness of the eye and alertness of manner, are often more subtle or may be masked by clothing. Hair, however, is in plain sight. There are many associations and social reactions that are elicited when a person observes someone with graying, or little or no hair. It has been thought that such reactions were based on primal judgments, such as whether the person is fit for warfare, reproduction, or active labor. A full, glossy head of hair is a signal that one is youthful, vigorous, and desirable.
What is hair anyway? Hair is composed of a complex protein called keratin. Of the human body’s three basic compounds; proteins, fats, and carbohydrates; the synthesis of protein requires the greatest investment of energy. When a person becomes ill or malnourished, his/her hair stops growing. When illness or malnutrition is severe or prolonged, the hair may temporarily fall out (the medical term for this shedding is telogen effluvium). The resumption of hair growth is a sign that recovery has begun. Science continues to explore why hair grows or fails to grow, and why it disappears permanently in some people, but not in others.
Mammals share three main characteristics. Most mammals bear live offspring (as opposed to laying eggs) and nurture their young with milk made in special glands on the female’s body. Mammals are warm-blooded, that is, they maintain constant body temperature independent of the outside temperature. A third feature, shared by all mammals, is the presence of hair. Like many mammals, man’s skin is covered with hair. Human skin has more hair follicles per unit of surface area than the skin of most other primates. This is surprising since most primates appear to be much hairier than humans. This impression is caused by the greater length and coarseness of the individual hair shafts in primates such as monkeys and apes. In contrast, the majority of human body hair consists of a very fine, almost invisible, type of hair called vellus hair.
Human hair is classified into two main types: fine, vellus hair; and the coarser, more visible terminal hair. Except for the palms of the hands and the soles of the feet, most of the human body’s areas of seemingly bare skin are actually covered with very fine vellus hairs that may be almost invisible except under very close or microscopic inspection. There are several distinct subtypes of terminal hair. For example, eyelash hairs, called cilia, are different from head and body hair. Pubic (groin) and axillary (armpit) hairs are also different from terminal hairs on the head and are associated with different types of glands in the skin. Even scalp hairs have several different sub-groupings. For example, there is a fringe of very fine hair surrounding the circumference of the head. This hair undergoes a gradual change in thickness from the bare skin appearance of the vellus hair to the dense, thick hair of the crown. Similarly, the hair above the ears or at the base of the neck is not as coarse as that of the crown or the top of the head.
Functions of Hair
The reasons we have hair, and the functions of its growth patterns, are not completely understood. Our pre-historic ancestors were much hairier than we are today; the reason for the decreased hairiness of modern man is unknown, although it is reasonable to assume that it parallels the use of clothing for warmth and protection from the sun and physical trauma. Hair serves as insulation from the cold; however, this does not explain why different human groups have distinct patterns of hair growth. Most people of Asian descent have very sparse body and facial hair, but some of these peoples such as the Inuit, Tibetans and Mongols, inhabit some of the coldest regions on earth.
Hair has the additional function of extending the sensory capability of the skin beyond its surface. Although human hair lacks the wealth of sensory nerve fibers found at the root of whiskers of some animals, each hair has a nerve fiber going to the bulb of the hair follicle. Mechanical displacement of each hair causes a sensation that translates into an awareness movement on the skin’s surface. For example, when an ant or fly walks on one’s arm, one feels the displacement of hairs caused by the insect.
Hair also plays a role in the defense mechanisms of most fur-bearing animals. When an animal confronts a potential enemy, its fur bristles; standing on end to make the animal appear to be larger and more threatening. In dogs, this response is most visible in the neck area where the neck hairs, called hackles, rise. In cats, the most visible response is in the tail. An extreme example of the use of hair for self-defense occurs in porcupines: their quills, which are modified hairs, stand out from the body when the animal feels threatened. Porcupines have converted a reflex (that in most animals is purely defensive) into a formidable weapon. In modern man, with relatively sparse body hair, only vestigial traces of these reactions remain. A tiny muscle, called the erector pili, connects the lower portion of each hair shaft with the underside of the skin. When you are frightened, cold or angry, these small muscles contract, causing your skin texture to change and your hair to stand on end; but since the hairs are so fine, all you actually see are “goose bumps” on your skin.
Each hair shaft also contains a small gland called the sebaceous gland, located next to the hair shaft. Sebaceous glands make a yellow, fatty substance called sebum that lubricates the hair. Each time the erector pili muscle contracts, the gland is squeezed, and a small amount of lubricant is applied to the surface of the hair.
Hair, along with skin pigmentation, is the major natural protection that we have against the sun’s harmful ultra-violet rays. Scalp hair also plays an important role in preventing trauma to the skull. Hair acts to protect areas were the skin rubs together, such as under the arms and in the groin, and it serves to disperse pheromones, the body secretions that are involved in sexual attraction.
Hair is integral to our body image and can have a profound influence on our self-esteem and self-confidence. There is no other part of the human anatomy that can be changed or manipulated so easily. Hair can be groomed, styled, waved, straightened, dyed, braided, or cut, and, unlike tattoos or body piercing, changes made to our hair can be completely reversed. Hair serves as an important means of self-expression, and the loss of this form of self-expression in those going bald may account, at least in part, for the despair that they may experience.
In spite of its simple appearance, hair is a complex organ. Although we usually think of hair only in terms of the visible portion of the hair shafts, hair is an intricate structure comprised of muscles, sebaceous glands, blood vessels and nerves – that must all be working in harmony to maintain its health.
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, which is the long, visible part.
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 hair transplant surgery), 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.
Hair Growth and the Hair Cycle
The normal human scalp contains between 100,000 to 150,000 follicles that produce thick terminal hair. These hairs do not emerge individually from the scalp, but are arranged in follicular units, small groups of 1 to 4 hairs each. There are approximately 50,000 to 65,000 follicular units on the human scalp. For comparison, the human body has approximately 5 million follicles that produce the fine, vellus hair.
At any given time, about 90% of terminal hairs on one’s head are actively growing. This phase, called anagen, can last from 2 to 7 years, though the average is about three years. In catagen, which is the shortest phase lasting about 2-3 weeks, growth stops, the middle of the follicle constricts and the lower portion expands to form the “club.” The other remaining 10% of scalp hairs are in a resting state called telogen that, in a normal scalp, lasts about 3 to 4 months.
If an average hair grows about 1/2 inch a month, then after three years it will attain a length of about 18 inches. With seven years of growth, the hair would be about 3 and one half feet long. In rare cases, hair doesn’t cycle, but keeps growing – a condition referred to as “angora.”
When a hair enters its resting phase, growth stops and the bulb detaches from the papilla. The old hair is pushed out as the new shaft starts to grow. When an old hair is shed, a small, white swelling is found at the bottom of the hair shaft. Most people assume that this is the growth center of the hair, but it is just the clubbed, detached lower end of the hair shaft. The dermal papillae and the growth center of the hair remain in the scalp. Scalp hair grows at a rate of about 0.44 mm/day (or 1/2 inch per month). Each hair follicle goes through the hair cycle 10-20 times in a lifetime.
Humans normally lose about 100 hairs per day; everyone has a few hairs stuck to the comb each time they comb their hair. The presence of a large number of hairs on the comb, in the sink, or in the tub, can be a sign of hair loss caused by disease or medications. Common genetic balding, however, is not caused by excessive hair loss, but rather by the successive replacement of hair that is normally lost with smaller, finer hair – a process called “miniaturization” that will be discussed in greater detail in other parts of this hair transplant surgery book.
Continue reading: Chapter 3 – Causes of Hair Loss »»
Table of Contents
|Chapter 1||Brief History of Hair|
|Chapter 2||Hair and Its Functions|
|Chapter 3||Causes of Hair Loss|
|Chapter 4||Hereditary Baldness|
|Chapter 5||Psychology of Hair Loss|
|Chapter 6||Hair Loss Medications|
|Chapter 7||Hair Transplant Basics|
|Chapter 8||Follicular Unit Transplantation|
|Chapter 9||Follicular Unit Extraction|
|Chapter 10||Master Plan for Restoring Hair|
|Chapter 11||Goals and Expectations|
|Chapter 12||Numbers of Grafts Needed|
|Chapter 13||Hair Transplant Repair|
|Chapter 14||Hair Loss in Women|
|Chapter 15||Hair Systems and Camouflage|
|Chapter 16||Preparing for a Hair Transplant|
|Chapter 17||The Hair Restoration Procedure|
|Chapter 18||What to Expect Following Surgery|
|Chapter 19||Hair Transplant Fallacies|
|Chapter 20||Choosing Your Doctor|
|A Final Note|
|About the Author|