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Hair and Scalp Disorders
Medical, Surgical, and Cosmetic Treatments, Second Edition
Amy J. McMichael, Maria K. Hordinsky, Amy J. McMichael, Maria K. Hordinsky
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eBook - ePub
Hair and Scalp Disorders
Medical, Surgical, and Cosmetic Treatments, Second Edition
Amy J. McMichael, Maria K. Hordinsky, Amy J. McMichael, Maria K. Hordinsky
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A succinct and comprehensive guide examining the treatment of hair and scalp disorders, with a special emphasis on how hair type, morphology, and ethnicity can impact on hair disease and treatment. This updated and revised second edition builds on a successful first edition with additional chapters on dermoscopy, on how technology can best be integrated into clinical practice, and on aging hair.
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1Human Hair
John Gray
Introduction
Hair first arrived on the evolutionary scene some 310 million years ago on reptilian-like animals, preceded and survived the rise and extinction of the dinosaurs, and emerged as the dominant skin appendage of the class of mammals. As members of this class, modern humans have âinheritedâ skin which, although bearing several million hair follicles covering 95% of the body, have largely confined growing hair to the scalp.
Human Scalp Hair
Unlike our primate cousins, we, the fifth and ânakedâ ape, do not possess an all-encompassing, thick, and pigmented pelage. Once the body-wide intrauterine lanugo hairs are shed, visible follicular activity is confined to the scalp, with secondary sexual sites a poor second even after puberty.
Since the human head bears some 100,000â150,000 hair follicles, an individual adult with 30 months continuous, unstyled growth at 0.9 cm per month will carry some 30 kilometers of hair. This is a significant investment in protein and energy consumption.
Function of Human Hair
The function of human hair is, curiously, unresolved. Hypotheses vary: Is it a relic of an aquatic phase of human development where a pelage would be an impairment, or an integral adaptation for thermoregulation and ultraviolet protection? Is it a mere adornment, or the result of Fisherian runaway sexual selection? All of these theories can be disproved, not least by the tendency for humans of both sexes to bald. Sociological research has suggested that hair is often interpreted as a marker of age, health, nutrition, and fecundity. In its styled form, it is employed in all societies to express social status or cultural affiliation. Hair in most cultures is at its zenith on the wedding day. By contrast, sociological studies have revealed the full impact of so-called âbad hair days,â where subjective and objective negative assessment of hair may reduce self-esteem.
Modern hair care involving the use of many products is an (almost) ubiquitous human habit in the twenty-first century. These products are increasingly designed to repair and protect hair from environmental and self-inflicted damage while preparing it for styling.
Structure of Hair
Many other publications describe in detail the human hair follicle, its unparalleled metabolic activities, and its failings. It is the end-product of this activityâthe human hair shaftâwhich we discuss in this chapter.
There are three essential types of hair in humans, which are dependent to some extent on the size of the follicle.
Lanugo Hair
Lanugo hair is fine and non-medullated hair which appears on the fetus, and with rare exceptions is shed prior to or immediately after birth (Figure 1.1).
Vellus Hair
Vellus hair is fine, short, and non- or lightly pigmented (less than 40 microns in diameter), and is the most numerous of human hairs. It can be seen from the neonatal period onward covering all surfaces other than the palms of the hands and soles of the feet. At puberty, some vellus hairs enlarge to become terminal hairs, and develop sebaceous glands. Vellus hairs occur on the scalp but are far less numerous than terminal hairs.
In male and female pattern hair loss, terminal hairs miniaturize and return to the size of vellus hairs. This can be reversed with treatment.
Terminal Hair
Terminal hair is thick, long, and pigmented. It is some 50 to 150 microns thick. Terminal hairs are the dominant hairs on the scalp, eyebrows, lashes, axillae, and genital areas. In men, terminal hairs are variably found on the trunk and legs. There is great regional difference in terminal body hairs.
Cross-section of the terminal hair shaft reveals three major components:
- Cuticleâthe outer protective layer
- Cortexâthe core of the hair
- Medullaâa central soft protein core which is more common in thicker hair, and particularly so in white hairs
The main constituents of these structures are sulfur-rich proteins, lipids, water, melanin, and trace elements.
Cuticle
The cuticle is composed of specialized keratins and consists of six to eight layers of flattened overlapping cells with their free edges directed upward to the tip of the hair shaft. There are several layers to each cell. The innermost endocuticle is covered by the exocuticle, which lies closer to the external surface and is comprised of three parts: the b-layer, the a-layer, and the epicuticle. The b-layer and the a-layer are largely proteinaceous, while the epicuticle is a hydrophobic lipid layer of 18-methyleicosanoic acid (a main component of the outer layer of the epicuticle of human hair that endows hydrophobicity to the outer hair surface) attached via a covalent chemical bond to the surface of the fiber. This is commonly known as the f-layer.
The f-layer is of critical importance to hair health. The cuticleâs complex structure (Figure 1.2) allows it to slide as the hair swells, and the f-layer imbues a considerable degree of hydrophobicity. It is critical in protecting the hair and rendering it resistant to the influx and outflow of moisture.
The undamaged cuticle has a smooth appearance and feel. It is primarily responsible for the luster and texture of the hair (Figure 1.3).
The cuticle may be damaged by any of four major âinsultsââenvir...