Pili annulati, or ringed hair, characteristically shows light and dark bands when viewed by reflected light. The light bands are due to the periodic occurrence along the hair shaft of clusters of abnormal, air-filled cavities which scatter the light. Light-microscopic examination of intact and cross-sectioned hair shows that the holes range in size from less than 1μ to 10μ or more in diameter, and that they are distributed randomly throughout the cortex of the hair, both centrally and peripherally. Some of the cavities contain birefringent crystals, but these may be contaminants which were leached into the hair; various liquids can readily penetrate into and evaporate from the cavities. Electron microscopic examination shows that the smaller cavities are within cortical cells and that they occur between Macrofibrils; the latter are seemingly normal, and have the usual microfibrillar structure. The larger cavities probably represent the absence of entire cortical cells within the hair shaft. Medullary cells do not seem to be involved in formation of the cavities. Abnormally large spaces between macrofibrils are already present in the cells of the hair follicle, indicating that pili annulati results from an inherent error of growth. The air-filled cavities evidently arise in the mature hair shaft by dehydration of these abnormal, fluid-filled spaces.
Price VH, Thomas RS, Jones FT. Pili AnnulatiOptical and Electron Microscopic Studies. Arch Dermatol. 1968;98(6):640-647. doi:10.1001/archderm.1968.01610180084013