[Structure and pathogenic interpretation. Acquired dystrophies of the hair].

The size and form of a segment of a hair are two relatively constant parameters. Their homeostasis reflects precise regulation of the size and form of the matrix (therefore of the papilla), of the proportion of cells normally involved in the cycle of cell division and of the duration of this cycle and of the increase in the average volume of cells during differentiation. Any disturbance in this precise pattern causes dystrophies. Some of these acquired abnormalities in hair structure have been examined by scanning electron microscopy. Two malformations arising from indentation of the papilla were compared: pili gemini and pagothrix. Their cuticular organisation does not appear to be impaired and remains independent of the fusion and separation of the hair shafts. In addition to these dystrophies due to abnormal formation of the matrix, there are other malformations, bayonet hair and the Pohl-Beau line, which are secondary to temporary disturbances in other volumetric control parameters. Their pathogeny appears similar and their varying aspect is the result of graduated cellular response to the inductor. The structure of the cuticlar of these hairs varies according to their volume: a narrow segment shows a corresponding decrease in the free surface of the cuticular cells; on the other hand, an increase in the volume of the hair causes less overlapping of cuticle cells. The hair generally breaks where there is constriction. These fractures are transversal due to concentration of normal constraint and resemble trichoclasia or trichoschisis in normal or peladic hair. They never occur in the form of a trichoptilosis affecting normal or permed hair.