The hair of Monilethrix is characterized by a noded appearance with the internode being very susceptible to breakage resulting in hair loss at the scalp. Freshly epilated hairs from a moderately affected male Caucasian having light brown hair were used.

Histological examination of the bulb indicates the presence of vacuoles in the cortical prekeratinized area of the zone of cellular differentiation. Fig. 1 shows a morphological defect in the cells of the precortical region of the germinal matrix. The nuclei of several of these cells have become elongated and in some cases, string-like, with the nuclei becoming pyknotic. It is possible that the lack of a functioning nucleus early in cell growth results in large open areas or vacuoles. Gross structural analysis indicates that the internode region of the shaft has less cortical area than does the node.