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Summary The size and shape of a segment of a hair are two fairly constant parameters. Their homeostasis reflects a precise regulation of the size and shape of the matrix (therefore of the papilla), of the proportion of cells normally engaged in the cycle of cellular division, of the duration of this cycle, of the increase in the average volume of cells during differentiation, and of their speed of migration. Any disturbance in this precise pattern causes dystrophies that are acquired or genetically determined. These abnormalities in hair structure can be observed and recognised by optical and by scanning electron microscopy. The metabolic pathology acting through the matrix cells has to be differentiated from other alterations induced on the hair shaft by physical or chemical agents. These latter alterations can be present all along the hair shaft after its emergence from the skin or rise in tiers. These injuries are cumulative, irreversible and increase fragility and brittleness of the hair shaft. |
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