Testicular histology in prepubescent males. The cross-section of a seminiferous tubule contains both spermatogonial progenitor cells (blue) and supportive Sertoli cells (pink). The average number of spermatogonia per tubule (S/T ratio) is significantly less in boys with SCA (A), compared with a normal reference control (B). For boys with SCA, the S/T ratio is not different according to hydroxyurea exposure, indicating that spermatogonial depletion is related to the underlying disease itself, rather than hydroxyurea toxicity. Professional illustration by Patrick Lane, ScEYEnce Studios.