Light micrograph of a vertical section through the testis, showing seminiferous tubules. This is the site of sperm production (spermatogenesis). Sperm cells are made up of a head, containing genetic material and fertilization enzymes, and a tail. These cells are the result of a process of maturation. Mature sperm cells can be seen closest to the tube's lumen (white), with elongated nuclei (black). Moving further away from the lumen behind the sperm are increasingly more immature precursor cells called spermatids (round brown nuclei), spermatocytes and spermatogonia respectively. The outermost cells are sertoli cells (pale brown), which nourish the maturing sperm. The leydig cells produce the hormones (androgens) needed for sperm development.