Hair restoration has progressed rapidly since its introduction about 50 years ago. Innovative hair restoration surgeons steadily improved surgical techniques which led to refinement of the aesthetic aspects of hair transplant surgery.
Contributing to progress in hair transplantation is scientific research into the physical and chemical properties of hair. While biochemical research was discovering more about the hormonal and biomolecular signals that stimulate hair growth and hair loss (see About hair loss for more information on causes of hair loss), other scientists were learning more about the physical properties of hair.
By examining hair fibers with sophisticated chemical analysis and powerful microscopes, they discovered more about the complex structure of hair and more about the properties that differentiate hair into “ethnic” categories. As one investigator notes, it is important to recognize that modest differences in hair structure make all hair “ethnic”. The three major ethnic hair types that are generally recognized are African, Asian and Caucasian. The ethnic differences in hair characteristics are slight when viewed in the broader context of human hair versus hair from other species. The ethnic differences can, however, be important aesthetic considerations in hair transplantation. See Hair Restoration in African-Americans and Hair Restoration in East Asians for more discussion.
Under low-power microscopic examination, a human scalp hair is seen to have an outer layer of flat scales pointing outward from root to tip. These flat scales are called the cuticle; the scales are made of the same, tough keratin protein that is found in skin, fingernails and toenails. A major function of cuticle on the hair shaft is to protect the inner, softer tissue called the cortex. The cortex is where hair growth occurs. When hair has been abused by hard brushing or use of harsh chemicals such as bleaches, the cuticle is damaged and the flat scales are disrupted. Severe damage allows damage to occur to underlying cortex.
Some “ethnic” differences in hair shape are also apparent under low-power microscopic examination:
On cross-section, Asian hair is seen to circular in shape and is larger in diameter than the other ethnic types, African hair is more elliptical (oval) with a wide degree of variation in shape, and Caucasian hair tends to be intermediate in cross-section between Asian and African hair.
In longitudinal (lengthwise) shape, African hair shows irregular curling and twisting, Asian hair is straight with no irregularities, and Caucasian hair is generally straight with few irregularities in shape. The irregularities in longitudinal shape of African hair contributes to its tendency to break more easily than Asian or Caucasian hair. Irregularities in longitudinal shape of African hair also can have both “good and bad” implications for transplantation: because hair may emerge at a variety of angles from the scalp the harvesting of donor hair may be more difficult, but the curliness of African hair allows coverage of scalp areas with fewer number of follicles to achieve “full” coverage and thus can reduce the number of follicles needed for transplantation.
The cross-sectional and longitudinal shape of hair begins in the hair follicle where new hair is formed. Research over the past decade has indicated that the cross-sectional and longitudinal shape of hair is programmed by the lower half of the hair follicle. This finding confirms the observation of hair restoration surgeons that hair follicles transplanted from a donor site to a recipient site will continue to grow hair of the same type they produced at the donor site.
A Microscopic View Inside the Hair Shaft
The outer, cuticle layer of hair is firmly attached to the inner cortex—as firmly as tree bark is attached to the inner layer of a tree. The cuticle layer is thin, like tree bark, covering the cortical cells underneath. Cortical cells constitute the bulk of a hair, and it is the cortex that gives a hair fiber its shape, resilience, and the cosmetic attribute called “body”.
A look at the cortex with the powerful magnification of an electron microscope shows why the cortex gives hair its high tensile strength. A microscopic look at the cortex in longitudinal (lengthwise) section shows it is packed with cortical cells and fibers running from root to tip. When the cortex is examined in cross-section by the electron microscope, the cortical cells are seen to be packed with fine macro- and microfibrils. Looking at the cortex in cross-section is somewhat like looking at the cross-section of an electrical cable made up of hundreds of closely packed small wires. The way in which macro- and microfibrils are packed together contributes to the distinctive shapes of “ethnic” hair. Research has shown that the determination of shape probably begins in the lower half of the follicle, under control of hormonal and molecular signals that are still being characterized.
Wolfram LJ. Human hair: a unique physicochemical composite. J Amer Acad Dermatol 2003; 48:S106-S114.
Franbourg A, Hallegot P, Baltenneck F et al. Current research on ethnic hair. J Amer Acad Dermatol 2003; 48:S115-S119.
Lavker RM, Bertolini AP, Freedberg IM et al. Biology of hair follicles. In: Freedberg IM et al (eds.). Fitzpatrick’s Dermatology in General Medicine, 5th ed. New York: McGraw-Hill; 1999:230-238.