The replicating phase of the sperm cell in the testis, equivalent to the oogonium in the ovaries, but, unlike the oogonia, spermatogonia normally persist until old age. Located among the supporting cells (Sertoli cells) in the testicular tubules. Divides by the process of mitosis until it begins to undergo meiosis by changing into the primary spermatocyte, the first step in the process of sperm cell formation, or spermatogenesis. Plural: spermatogonia.