Hormones are responsible for the changes occurring during adolescence. A hormone released by pituitary gland stimulates ovaries in girls to produce female sex hormones, oestrogen and progesterone. Oestrogen controls ova production and development of secondary sexual characters in girls. In boys, a hormone secreted by pituitary gland stimulates the production of male sex hormone testosterone from the testes. Testosterone controls sperm cell production and development of secondary sexual characters in boys.