The axis that sleeps and wakes
Surprisingly, the reproductive control system is fully built and briefly active in infancy — then it goes quiet for most of childhood. Puberty is not the system being created; it is the system being switched back on. The master switch is the hypothalamus releasing GnRH (gonadotropin-releasing hormone) in regular pulses.
When GnRH pulses return — first at night, then around the clock — the pituitary answers with LH and FSH, which travel to the gonads. The whole chain from brain to gonad is the hypothalamic-pituitary-gonadal axis. Its reawakening, which sets the gonads producing sex hormones, is called gonadarche.
Adrenarche: a separate, earlier switch
There is a second, independent maturation that confuses many beginners. A year or two before the gonads wake, the adrenal glands start making more weak androgens such as DHEA. This is adrenarche, and it is driven by the adrenal cortex, not by the GnRH/LH/FSH axis. Adrenarche is responsible for the first whiffs of body odor, oily skin, and the earliest pubic and underarm hair.
Keeping these two apart is clinically vital. A young child with only pubic hair and body odor but no breast or testicular development usually has isolated, benign adrenarche — the gonadal axis is still asleep. True central puberty means gonadarche has begun: breasts budding in girls, testicles enlarging in boys.
What tells the body it's time?
Why puberty starts when it does is still partly mysterious, but one clear input is energy stores. Fat cells release leptin, and the brain reads it as a signal that the body is fed well enough to afford reproduction. A minimum level of leptin appears to be permissive for the GnRH pulse generator to restart — which is one reason puberty can be delayed in severe undernutrition or extreme athletic thinness.
CHILDHOOD PUBERTY ONSET FULL PUBERTY GnRH pulses ──off──► night-time pulses ──► day & night pulses │ │ │ LH/FSH low LH rises (esp. night) LH/FSH adult pattern │ │ │ gonads quiet gonads begin sex steroids full sex-steroid output (Adrenarche runs on a SEPARATE track, 1–2 yr earlier: adrenal DHEA ↑)