Light microscopy of the testis. Seminiferous tubules are shown in cross-section. Tubules contain the seminiferous epithelium where germ cells multiply and develop into sperm. Sperm heads and tails stain bright blue. Cells showing DNA synthesis (spermatogonia, early primary spermatocytes) have been selectively stained brown with an immunostaining technique for proliferating cell nuclear antigen (PCNA). This stain demonstrates that DNA replication is restricted to mitotic spermatogonia and early meiotic spermatocytes. All other germ cells are not engaged in DNA synthesis. Magnification x150 when printed at 10 cm.