Testosterone is a steroid hormone, so it can readily cross the cell membrane to combine with specific receptors and enter the nucleus, where it facilitates the transcription of certain genes. In addition to the testes, the adrenal cortex also produces testosterone as well as other androgens. The collective action of adrenal androgens is not strong enough to substitute for testosterone secretion by the testes. Testosterone secretion by the Leydig cells of the testes (acting as a paracrine hormone since the activity is local) is required for spermatogenesis, as is also FSH acting on Sertoli cells. Testosterone is also required for the development of male secondary sex characteristics.