Incorporation of (3H)uridine by the chromatoid body during rat spermatogenesis

The in vitro incorporation of tritiated uridine into RNA by the spermatogenic cells of the rat has been analyzed by high-resolution autoradiography. Special attention has been focused on the unique cytoplasmic organelle, the chromatoid body. After a short labeling time (2 h), this organelle remains unlabeled in the vast majority of the early spermatids although the nuclei are labeled. When the 2-h incubation with (3H)uridine is followed by a 14-h chase, the chromatoid body is seen distinctly labeled in all spermatids during early spermiogenesis from step 1 to step 8. Very few grains are seen elsewhere in the cytoplasm of these cells. When RNA synthesis in the spermatid ceases, the chromatoid body also remains unlabeled. It is likely that the chromatoid body contains RNA which is synthesized in the nuclei of the spermatids. The function of this RNA as a stable messenger RNA needed for the regulation of late spermiogenesis is discussed.