The different faces of pili bifurcati. A review.

Pili bifurcati is an uncommon hair shaft dysplasia characterized by bifurcation of the hair shaft. The two characteristics that define the dysplasia are: 1. Each bifurcation produces two separate parallel branches which fuse again to form a single shaft. 2. Each branch of the successive bifurcations is covered with its own cuticle. Currently, there is confusion between the terms "pili bifurcati" and "pili gemini". The name "pili gemini" is used to define a kinetic papilla that splits at the upper end from single to double-tipped during the anagen phase and consequently the same follicular matrix produces two different-sized hair shafts having separate cuticles that emerge through a single pilary canal. Pili gemini maintains the double tipped papilla and consequently the hair shaft does not fuse again. Papillar tips that divide into several tips will produce several hair shafts, that characteristically do not fuse again. When the same papilla changes its shape repeatedly during the anagen phase, it can produce hair shafts with bifurcations at irregular intervals: pili multibifurcati. And, as it is also possible for one of the new papillae to split again in two, the hair shaft may be doubly bifurcated: pili bi, bifurcati. Pili bifurcati should be distinguished from acquired splitting of hair shafts that do not represent true bifurcations because the two split parts are never surrounded by a complete cuticle. Such acquired splitting may be called "central trichoptilosis".