Undifferentiated (A-type) spermatogonia at the base of the seminiferous epithelium divide mitotically until they enter the differentiation pathway to become A1 spermatogonia. A1 spermatogonia undergo division to A1-A4 and finally B spermatogonia. B spermatogonia divide to produce preleptotene (primary) spermatocytes that migrate away from the base of the seminiferous tubule to undergo meiosis. The first meiotic division produces secondary spermatocytes, and after the second meiotic division, spermatids (haploid cells) begin the differentiation process (spermiogenesis) to spermatozoa. In vitamin A deficiency, the transition from A to A1 spermatogonia is blocked