Male pattern hair loss retreats along a recognisable outline: the temples first, then the crown, with a band at the back and sides persisting. The consistency of that shape points to a regional property of the follicles themselves.
Follicles differ by scalp region
Hair follicles are not interchangeable. Those on the frontal scalp and crown differ biochemically from those along the occipital region at the back of the head.
The difference is set early in development and is largely genetic, which is why the pattern runs in families.
The hormone concerned circulates everywhere, so the pattern cannot come from uneven exposure. It comes from uneven response.
A converted hormone does the signalling
Testosterone is converted within tissue into dihydrotestosterone by an enzyme called 5-alpha reductase. The converted form binds androgen receptors more strongly.
Susceptible follicles carry receptors that respond to that signal by shortening their growth phase.
Follicles at the back of the scalp carry a different receptor and enzyme profile and are largely unaffected by the same circulating levels.
Miniaturisation is gradual, not sudden loss
Affected follicles do not die at once. With each growth cycle the active phase gets shorter and the follicle produces a finer, shorter, less pigmented hair.
Over successive cycles a terminal hair becomes something closer to the fine vellus hair found on the rest of the body.
Thinning therefore reads as reduced density long before any area is genuinely bare, and the process is slow enough to be missed for years.
Regional immunity explains transplantation
Because the resistance is a property of the follicle rather than of its location, follicles moved from the occipital region keep their behaviour in a new site.
This principle is the entire basis of hair transplantation. It also explains the donor limit, since the supply of resistant follicles at the back and sides is finite and cannot be expanded.
Follicles moved the other way, from a susceptible region, would carry their susceptibility with them, which is why the direction of transfer is never reversed.
Pattern is not the only cause of shedding
Diffuse thinning across the whole scalp, patchy circular loss, or shedding accompanied by scaling or scarring follow different mechanisms altogether.
Thyroid disorders, iron status, some medications and periods of significant physical stress can all shed hair without producing the androgenetic outline.
Distinguishing between them requires examination and sometimes blood work, which is a reason to see a doctor rather than infer a cause from the mirror.