Vellus hairs are very fine hairs that are normally present on many areas of the body. In androgenetic alopecia, terminal scalp hairs gradually miniaturise and become more vellus-like. This is why a balding area may still contain hair, but the hairs are too fine and short to provide visible coverage. Trichoscopy and phototrichogram analysis can quantify the proportion of vellus-like hairs, helping distinguish miniaturisation from simple shedding.
DermNet male pattern hair loss: https://dermnetnz.org/topics/male-pattern-hair-loss ; DermNet trichoscopy of generalised non-cicatricial hair loss: https://dermnetnz.org/topics/trichoscopy-of-generalised-noncicatricial-hair-loss
