the hypothalamus
Puberty in girls starts when the hypothalamus tells the pituitary gland to secrete hormones that target the ovaries. Two pituitary hormones are involved: luteinizing hormone LH and follicle-stimulating hormone FSH . These hormones stimulate the ovary to produce estrogen . Estrogen, in turn, promotes growth and other physical changes of puberty. It stimulates growth and development of the internal reproductive organs, breasts, and pubic hair see Figure below . You can watch an animation of these and other changes that girls experience during puberty at this link: http://www. bbc. co. uk/science/humanbody/body/interactives/lifecycle/teenagers/ .