The testis is composed of many tubes called "seminiferous tubules." The seminiferous tubule on the left is from a testis that was not treated with ultrasound while the tubule on the right is from a testis that was treated with ultrasound. Note that the tubule from the control testis has many darkly stained germ cell nuclei. Most germ cell nuclei are round; the long, thin nuclei closest to the center of the tubule belong to germ cells called spermatids and they will soon be released as testicular sperm. In contrast, the ultrasound-treated tubule is completely lacking testicular sperm and has lost almost all immature germ cells, decreasing its overall diameter while greatly increasing the amount of "empty" space in the center of the tubule.
Image courtesy of James Tsuruta, Ph.D., and Paul Dayton, Ph.D.