Describe the changes at puberty in boys and girls and the mechanism of its onset.

Definition

Sequence of Events

• Girls — thelarche → pubarche (pubic hair) → growth spurt → menarche. Early cycles are often anovulatory
• Boys — testicular growth → pubarche → penile growth → growth spurt → voice change → facial hair
• Staged clinically by the Tanner scale, stages 1 to 5

Mechanism of Onset

Childhood: GnRH pulse generator suppressed → Rising body fat → leptin signals adequate energy stores → Kisspeptin neurones activated → Pulsatile GnRH resumes, first at night → ↑ FSH and LH → ↑ Gonadal steroids → puberty

• Adrenarche — adrenal androgens rise from 6–8 years, independently of gonadarche; responsible for pubic and axillary hair
• A critical body weight and fat mass is required — hence delayed menarche in athletes, dancers and malnutrition

Growth at Puberty

• Peak height velocity — 9 cm/year in girls, 10.3 cm/year in boys
• Driven by growth hormone and sex steroids together
• Oestrogen closes the epiphyses in both sexes — in boys it acts after aromatisation of testosterone
• Boys end up taller because their growth spurt starts later, giving two extra years of pre-pubertal growth

Applied Aspects

• Precocious puberty — before 8 years in girls, 9 in boys. Central (GnRH dependent — treated with GnRH agonists, which down-regulate the receptor) or peripheral (gonadal or adrenal tumour, CAH). Causes short final height from early epiphyseal closure
• Delayed puberty — no signs by 13 in girls, 14 in boys. Commonest cause is constitutional delay; also Turner, Klinefelter, Kallmann syndrome (GnRH deficiency with anosmia)
• Turner syndrome (45, XO) — short stature, webbed neck, streak ovaries, primary amenorrhoea
• Secular trend — the age of menarche has fallen over the past century with better nutrition, and is now rising again in some populations with obesity
• Bone age (X-ray of the wrist) helps distinguish constitutional delay (bone age delayed, final height normal) from pathological causes