thegenenearby.And,iftheentirecelldies,othercells
can fill in.
Nonetheless,asthedefectsinacomplexsystemincrease,
thetimecomeswhenjustonemoredefectisenoughto
impair the whole, resulting in thecondition knownas
frailty. It happens to power plants, cars, and large
organizations.Andithappenstous:eventually,onetoo
manyjointsaredamaged,onetoomanyarteriescalcify.
Therearenomorebackups.Weweardownuntilwecan’t
wear down anymore.
It happensin abewildering arrayofways.Hair grows
gray, for instance, simply because we run out of the
pigment cells that give hairits color.The natural life
cycleofthescalp’spigmentcellsisjustafewyears.We
relyon stemcellsunder thesurfaceto migratein and
replacethem.Gradually,however,thestem-cellreservoir
isusedup.By theageoffifty,asa result,halfofthe
average person’s hairs have gone gray.
Inside skin cells, themechanisms thatclear out waste
products slowly breakdown and theresidue coalesces
into a clot of gooey yellow-brown pigment known as
lipofuscin.Thesearetheagespotsweseeinskin.When
lipofuscinaccumulatesinsweatglands,thesweatglands
cannotfunction,whichhelpsexplainwhywebecomeso
susceptible to heat stroke and heat exhaustion in old age.
Theeyesgofordifferent reasons.Thelensismadeof
crystallinproteinsthataretremendouslydurable,butthey
changechemicallyinwaysthatdiminishtheirelasticity
over time—hence the farsightedness that most people
develop beginning in their fourth decade. Theprocess