Hair Analysis Panel Discussion: Section 2.4
2.4 General Physiology of Hair—An Overview
Robert Baratz, M.D., Ph.D., D.D.S.
To provide a foundation for subsequent discussions, Dr. Baratz described the general characteristics of hair and the underlying skin (e.g., structure, composition, growth patterns, growth cycles). Understanding the characteristics of hair, the temporal and spatial patterns of hair growth, and the factors that affect hair growth, for example, is important when collecting and interpreting hair analysis data. Dr. Baratz's presentation is summarized below.
Anatomy of hair. Hair is encompassed in the follicle located below the skin surface in the dermis, the fiber-rich layer that makes up the bulk of the skin. The follicle has a connective tissue component (muscles) and glandular component (sebaceous glands). The muscles elevate the hair and the glands lubricate the hair.
The primary components of the hair follicle are the dermal
papilla and the follicle cells. The dermal papilla is the
"generative zone" of hair (it contains blood vessels, nerves,
and pigment-forming cells). The follicle cells generate
the hair shaft; the hair shaft is composed of essentially
dead cells, which are the outermost layers of the epithelium
and form a solid cylinder in the dermis. Mitotic activity
at the base of the hair follicle generates different layers
that will "keratinize" (see below).
Keratinization of hair. Hair is composed of hard keratin (a family of proteins ranging in size from 20,000 to 70,000 Daltons) and is chemically denser than other forms of keratin (e.g., calluses, dander flakes). Keratinized cells contain more than 85% protein. Where the hair shaft separates from the follicle it undergoes "disjunctive" keratizination, which involves the splitting of layers and exposing surfaces not previously exposed.
Keratinized cells have a very distinctive appearance,
and have tiny pores littering their surfaces. The cells
are flattened and tightly bound to their neighbors in a
very complex array. When they begin to split apart (by an
unknown process), large "nooks and crannies" are formed.
These types of anatomical features allow external environmental
agents to be easily trapped in the outer surface of the
Elements found in hair. Because so many elements
are ubiquitous in the environment and therefore found in
the human body, merely finding a particular element in the
hair does not prove that it got there via a specific route/source,
or that finding it has clinical significance.
Growth rates. Hair growth varies depending on body region. For example, average eyelash/brow growth rates have been reported at 0.16 millimeters (mm) per day, scalp hair at 0.34 to 0.36 mm/day, and beard hair at 0.38 mm/day. Growth rates also are affected by age, gender, hair color, and ethnicity. For example, scalp hair in a prepubescent, adolescent, adult, and older adult have been reported at 0.41, 0.30, 0.34, and 0.32 mm/day, respectively (Myers and Hamilton 1951).
Interindividual variability also occurs. Scalp hair grows
at an average rate of 1 centimeter (cm) per month, but can
range from 0.6 to 3.36 cm/month (Harkey 1993). Thus, 12
cm can represent 3½ to 20 months of hair growth.
Growth cycles. Hair grows in phases (see Figure 2-2). Usually, more than 90% of the hair is in the growing (or anagen) phase. The length of anagen varies from 2 to 6 years. The longer the hair, generally the longer the phases. For example, long hair tends to grow more slowly. Through apoptosis, the hair will begin to enter the relatively short catagen phase, during which the follicle will begin to regress and move toward the surface (the papilli will essentially disappear). During the next phase, telogen, the hair will actually fall out. If the cycle is complete, a resting phase will follow and then the follicle will resume the anagen phase. However, hair can "exit" the cycle and cease being a terminal hair. For example, it can become a vellus hair (non-pigmented "peach-fuzz" hair) or the hair follicle may permanently disappear, as is the case with male-pattern baldness.
Events known to affect the hair follicle and its cycle
include local signaling events (e.g, cytokines, hormones,
adhesion molecules). However, no firm theory of cycle control
exists. Hypotheses include the presence of (1) a morphogenesis
clock, (2) a cycling inducer, (3) a desynchronizer, and
(4) an actual cycle clock, but none of these are specifically
Generation, cycling, and "patterning" of hair. The hair growth cycle changes throughout life and varies based on species and body location. Patterning of hair is important to the generation and cycling of hair, and to how it relates to its neighbors (e.g., signaling goes on in various regions to space follicles in even arrays). Because of similarities in hair growth patterns, studying sheep hair growth has been useful in understanding human hair growth patterns. Rodent hair growth models, on the other hand, may not be applicable to humans because rodents have regional variation in hair growth; the hair cycles, but in waves across the body.
Substances affecting hair growth. A great number of substances can affect hair growth. For example, some drugs, such as alkalating agents, are cytotoxic and can make hair fall out (e.g., cancer chemotherapeutic agents). Other agents drive hair into telogen (e.g., heparin, Vitamin A, ß-blockers, L-dopa, lithium, and some of the non-steroidals). Drugs that inhibit hair growth include parathyroid hormone (PTH) and PTH-related proteins. Variable agents also exist, such as Vitamin D. At low concentrations, Vitamin D may simulate hair growth, but at high concentrations hair growth is inhibited.
Substances such as testosterone, danazol, adrenocorticotropin hormone, metyrapone, anabolic steroids, glucocorticoids, retinoids, and insulin can lead to hirsutism (growth of hair where it does not normally occur). Cyclosporin, minoxidil, diazoxide, and chromakalin increase the growth rate and size of hair (hypertrichosis). However, some regional variation may occur. For example, steroids will decrease the rate of growth of eyebrows, lashes, and hair on the extremities, but estrogen and testosterone will generally stimulate the growth of pubic and axillary hair.
Other factors can potentiate or inhibit hair growth by affecting the growth of the dermal papillae, hair, and follicle (see Table 2-1).