Hair density is the number of hairs growing in a given area of scalp, usually counted per square centimetre. It declines through a process called follicular miniaturisation, in which individual follicles produce progressively finer and shorter shafts over successive growth cycles.
Each follicle moves through a repeating cycle: a growth phase (anagen), a brief regression (catagen), and a rest (telogen) before the cycle restarts. Density depends on how many follicles are in the growth phase at any one time and on how long that phase lasts. When the growth phase shortens cycle after cycle, the shaft it produces gets thinner, and the follicle contributes less to overall density.
This process is governed by signaling between the dermal papilla, a small cluster of cells at the base of the follicle, and the surrounding epithelial cells. The dermal papilla releases growth factors that regulate entry into the growth phase and how long it is sustained. Changes in that signaling environment are associated with the shift toward shorter cycles and finer shafts.
Miniaturisation is gradual and measurable. Because it is a change in the calibre and cycling of existing follicles rather than their sudden absence, it can be tracked over time with imaging that records both the count and the diameter of shafts in a fixed area. Note 08 documents that method.