Hair shaft

Hair bleaching removes the pigment melanin from the hair shaft by oxidative destmction. Alkaline hydrogen peroxide is the agent of choice. 

The surface of a strand of hair is covered with overlapping sheets, somewhat like the scales on a fish or the shingles on a house. This surface is called the cuticle. Alkaline solutions raise these scales, so they stand up. This makes the hair shafts stick together, which in turn makes the hair look and feel rougher and duller. 

Most shampoos are made slightly acidic, to keep the cuticle smooth and lying flat on the hair shaft. Ingredients like citric acid are added to acidify the shampoo. 

Conditioners are surfactants that neutralize the electrical charge on the surface of the hair, and smooth down protein scales on the hair shafts. These effects make the hair easier to style and less prone to static electricity. 

In an alkaline solution, the cuticle—the outermost layer of a strand of hair—swells up, softens, and becomes rougher. The cuticle is made up of translucent, flattened cells that line the hair shaft like shingles on a roof. The cuticle gets rougher when the cells do not lay flat. When the raised cuticle cells of one piece of hair get stuck on the raised cuticle cells of another piece of hair, the hair tangles. The raised cells also reflect light differently than smooth, flat cells, making the hair appear dull. 

Takizawa T, Arai S, Kizawa K, Uchiwa H, Sasaki I, Inoue T. 1999. Ultrastmctural localization of S100A3, a cysteine-rich, calcium binding protein, inhuman scalp hair shafts revealed by rapid-freezing immunocytochemistry. J Histochem Cytochem 47(4) 525—532. 

A hair shaft contains little or no DNA. The major source of DNA from hair is the hair root pulled from the scalp. The hair that sheds by itself or comes out easily on pulling from the scalp is most likely from the resting phase of the hair follicle and is not a good source of DNA, as it contains mostly cellular debris at its root. However, it is possible to isolate DNA from pulled hair roots [24]. 

The hair shaft (Figure 6.1) comprises three main structures (1) the outer cuticle responsible for the main optical and frictional properties of the fiber (2) the cortex, responsible for the bulk fiber mechanical properties such as strength and flexibility and (3) the porous medulla, which is more prominent in gray hair, but otherwise may or may not be present. Cuticle thickness varies markedly between species. Though much of our understanding of hair structural biology is derived from the study of wool, this homologous... 

Figure 6.4 Cuspate lesions to the hair shaft characteristic of insect damage to hair (bar equals 20 pm).

Bertolino, A. P. and O Guin, W. M. (1994). Differentiation of the hair shaft, in Disorders of Hair Growth Diagnosis and Treatment (E. A. Olsen, Ed.). London McGraw-Hill, 21-37. 

Gray, J., Dawber, R., and Whiting, D. (Eds.) (1997). The Hair Shaft—Aesthetics, Disease and Damage. London Royal Society of Medicine Press. 

Nozawa, H., Yamamoto, T., Uchihi, R., Yoshimoto, T., Tamaki, K., Hayashi, S., et al. (1999). Purification of nuclear DNA from single hair shafts for DNA analysis in forensic sciences. Legal Med. 1, 61-67. 

Puccinelli, V. A., Caputo, R., and Ceccarelli, B. (1967). The structure of human hair follicle and hair shaft An electron microscope study. G. Ital. Dermatol. Minerva Dermatol. 108,453-497. 

Wilson, A. S., Dodson, H. I., Janaway, R. C., Pollard, A. M., and Tobin, D. J. Selective biodegradation in hair shafts derived from archaeological, forensic and experimental contexts. Brit. J. Dermatol. 157(3) 450-457. 

Wilson, M. R., Polanskey, D., Butler, J., DiZinno, J. A., Replogle, J., and Budowle, B. (1995b). Extraction, PCR amplification and sequencing of mitochondrial DNA from human hair shafts. Biotechniques 18, 662-669. 

Hair dyes are of two types permanent and semipermanent. Permanent dyeing is achieved with a synthetic dye, applied with hydrogen peroxide that first bleaches the natural pigment melanin. Semipermanent dyes are generally made with vegetable extracts, such as henna, that coat rather than penetrate the hair shaft. There are a number of questions about the safety of synthetic hair dyes, since some of the products they contain cause cancer. 

A dye is considered permanent if it penetrates into the hollow hair shaft. Coloring of hair starts with a treatment of substances such as hydrogen peroxide and ammonia. The ammonia causes hair shafts to swell and open, allowing dye intermediates and couplers to penetrate. Dyes applied during the second step of coloring react with the precursors to form pigments that remain in the hair. 

Lightening or removal of hair color without structurally damaging the hair shaft is a difficult process. Oxidizing agents, including hydrogen peroxide,... 

Hair dyes are of three kinds temporary, semi-permanent, and permanent. The first kind are easily washed out of the hair, the second will survive several washings but eventually fade, and the third are locked fast within the hair shaft and disappear only as the hair grows... 

Wool fibers are hair that is covered with tiny, lustrous scales. Oils are secreted along the hairs from glands adjacent to the hair follicles. These oils travel up the hair shaft and give the fleece its natural waterproofing. In sheep, this wool grease is lanolin. Wool hairs are soft, wavy and flexible (Table 6.4). Beard hairs are longer, more flexible, and are found only in the outside coat. 

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