The HPG axis: The three-phase hormonal sequence that triggers puberty: As you can see here, in response to various genetic and environmental influences, the hypothalamus releases a hormone that stimulates the pituitary gland to produce its own hormones, which cause the ovaries in girls and the testes in boys to grow and secrete estrogens and testosterone, producing the physical changes of puberty.

Adapted from Tanner, 1978, p. 103.