Dermal papilla

The dermal papilla is the small cluster of mesenchymal cells at the base of each hair follicle. It signals to the surrounding matrix cells whether to grow hair and what kind of hair to grow.

Two reasons it matters for hair loss specifically.

First, it is the site of greatest 5α-reductase activity in the follicle. Activity in the dermal papilla exceeds activity in other follicle compartments by a factor of at least 14. Local DHT concentration around the papilla is what determines whether a susceptible follicle continues to produce terminal hair.

Second, it is the cell population that responds to scalp massage, microneedling, and minoxidil signalling. Mechanical stretching of papilla cells alters gene expression in ways that may promote hair growth. Minoxidil’s effect on prolonging anagen is mediated partly through papilla-level changes.

Hair-transplanted follicles bring their dermal papilla with them, which is why they keep growing in their new location: the genetic instructions live in the papilla, not the recipient site.