(A) Standard depiction of the cycle of the seminiferous epithelium for the mouse testis. The variable distances between the stages of the cycle are proportional to the duration of each of these cellular associations. The red arrow indicates the time in the cycle when vitamin A is required for the commitment to meiosis. (B) Depiction of how the cycle is generated. Spermatogonia undergo mitotic expansion, and as a result of the action of vitamin A (in the form of RA) (red arrows), they initiate meiosis and ultimately become spermatozoa. The time required for this process from the time of the onset of meiosis to the formation of spermatozoa is particular to the species and the germ cells themselves (blue arrow). The periodic initiation of the differentiation process by vitamin A generates the cellular associations that define the cycle in A. In_m, intermediate (mitosis); B, B spermatogonia; Pl, preleptotene spermatocytes; L, leptotene spermatocytes; Z, zygotene spermatocytes; P, pachytene spermatocytes; D, diplotene spermatocytes; m2°m, secondary spermatocytes. Round and elongating spermatids are labeled as steps 2–3, 8, 12, 16.