A hair follicle is a tiny living factory
A hair is not a living strand that keeps growing from its tip. Its shaft is made mostly of keratin, a tough protein, and consists of cells that have died as they were pushed upward. The living work happens below the skin, inside the follicle: cells there divide, take on specialized roles, and build the shaft.
A follicle also cycles through phases. During its active growth phase, it lengthens a hair; later, growth stops, the hair is released, and the follicle rests before beginning another cycle. That is why shedding a hair does not usually mean losing the follicle that produced it. The same follicle can make another shaft.
Why hair can thin without new follicles appearing
Hair coverage depends on more than the number of follicles. A follicle can produce a thicker or finer shaft, and its growth phase can change. In common pattern hair loss, for example, susceptible follicles gradually shrink and make finer, shorter hairs. The follicle may still be present even when its hair becomes difficult to see.
Follicles can also be damaged or destroyed, including by some scarring conditions or injury. In those cases, a replacement hair may not grow from that spot. The key distinction is between losing a hair and losing the follicle that makes it: ordinary shedding is part of the cycle, while lasting loss of the follicle changes what can grow there.
