Testis. Light micrograph (LM) of a section through a testis showing a seminiferous tubule. The seminiferous tubules are the site of sperm production (spermatogenesis). Mature sperm cells can be seen closest to the tubule's lumen (white centre), with their tails protruding into the lumen. Moving away from the lumen are increasingly immature precursor cells; first spermatids, then spermatocytes and finally spermatogonia. The outermost cells are Sertoli cells that nourish the maturing sperm. Magnification: x200 when printed at 10cm wide.