In addition to spermatogonia, a number of other cells are associated with the testes. Sertoli cells, for instance, are found in both the testes of young boys and adult males. These specialized cells are responsible for supporting and protecting the spermatogonia. Also Leydig cells, which comprise the interstitial tissue between the seminiferous tubules, are believed to secrete androgens that are important for the maturation of the male reproductive apparatus as well as the development of secondary sex characteristics. The number and physical characteristics of Leydig cells vary to a significant extent in different species, but are always influenced by the action of the pituitary gland.