Vellus transformation
Peach fuzz becomes beard hair on a hormone switch. Same follicle, different program.
Grounded in the research on Vellus hair (en.wikipedia.org/wiki/Vellus_hair)
The default: peach fuzz
Vellus hair is the short, thin, pale, barely-visible hair covering almost your whole body since childhood. Strands run under 2cm and the follicle has no sebaceous gland attached. It replaces lanugo (fetal hair) around 36-40 weeks gestation. It is everywhere except lips, palms, soles, navel, parts of the genitals, and scar tissue. Its job is boring but real: thermal regulation, wicking sweat to cool you.
The trigger: androgens hit at puberty
At puberty androgen hormones flip a switch on specific follicles. Testosterone gets converted to DHT (dihydrotestosterone) by 5-alpha-reductase inside the follicle, and DHT binds androgen receptors in the dermal papilla. Genetically androgen-sensitive follicles get the signal; others ignore it. Same hormone, different follicle wiring is why the same person grows a beard but not a hairy forehead.