Physiology of puberty.

Adolescence represents the period of important somatic changes which lead to sexual maturation, pubertal growth and active functions of reproduction. Mean ages of onset of puberty are 10.9 and 11.2 in girls and boys respectively. Menarche occurs at a mean age of 13.4 yr and may be related to a critical weight. In boys, testicular growth above 4 cm2 or 4 ml is the first clinical sign of gonadal pubertal maturation. In girls, the first sign is the budding of the breast. At onset of puberty, the hypothalamus after a "quiescent" period resumes a marked pulsatile secretion of gonadotrophin-releasing hormone, leading to an increased secretion of pituitary gonadotropins which in turn stimulate the gonadal functions, i.e. the secretion of testosterone or estradiol and maturation of the spermatogenesis or the ovarian follicle. Neuroendocrine factors which probably control the onset of puberty are numerous: cerebral adrenergic and/or dopamine neurotransmitters, endogenous opioids, melatonin from the pineal gland. Gonadal maturation (gonadarche) is preceded in the infant by a post-natal surge of luteinizing hormone and at age 7 to 8 yr by an adrenal maturation (adrenarche).