In addition to these changes, growth velocity is increased in relation to the growth and maturation of bone cartilage. Simultaneous psychological and behavioral changes are observed during this period, also called adolescence. Both act on the gonads so that testosterone is secreted and spermatogenesis develops in boys, and estradiol is secreted simultaneously with the development of ovarian follicles in girls. Biological clock genes, about which little is known, are probably important.
Many neurotransmitters are important factors in the regulation of GnRH neuron secretion.
NeuropeptideY markedly increases appetite, and leptin inhibits neuropeptide Y secretion in the hypothalamus. Adrenarche is expressed by a rise in plasma dehydroepiandrosterone, its sulfate, and in androstenedione. The main characteristic of stage II in boys is an increase in testicular size, with or without pubic hair. Both methods indicate that a testicular size of less than 4 is prepubertal. An index of testicular volume above 4 signifies pubertal development.