Ribosomal RNA in mouse spermatocytes

Abstract

Several earlier autoradiographic observations have reported the absence or the very low rate of nuclear incorporation of RNA precursors in male gametocytes. The problem of ribosomal RNA synthesis in mouse spermatocytes was approached biochemically by using ionizing radiation which at low doses selectively destroys spermatogonia without affecting the other germ cells and Sertoli cells. With this method two testicular populations were obtained, one composed by spermatocytes, spermatids and Sertoli cells (group B) and the other only by Sertoli cells and late spermatids (group C). The control and irradiated animals were injected intraperitoneally with 32PO 4 and at various intervals thereafter the RNA was extracted from the seminiferous tubules and analysed by sucrose gradient sedimentation. It was possible to rule out effects of radiation on the rate of incorporation or of breakdown of radioactive RNA within the dose range used. Since spermatids are inactive in RNA synthesis the difference in the rate of incorporation into rRNA between the control and group B animals and between group B and group C animals should indicate the activity of spermatogonia and spermatocytes, respectively. The results indicate that the radioactivity incorporated into 18 and 28S ribosomal RNA is practically the same in group B and group C testes 3 and 6 h after labelling, but increases at a faster rate in the former than in the latter at long intervals after 32PO 4 administration. Three hypotheses are discussed to explain these results. One is that during meiosis there is a deceleration of rRNA transcription so that long periods of labelling are needed to detect the activity of the spermatocytes. This hypothesis seems unlikely on the basis of actinomycin D experiments. Another interpretation is that the rate of maturation of ribosomal precursors is greatly reduced during male meiosis, thus leading to an accumulation of ribosomal precursors in the nucleolus and to a delayed appearance in the cytoplasm of mature ribosomal molecules. Finally, a third hypothesis is that spermatocytes do not synthesize appreciable amounts of rRNA during meiotic prophase and that there is a transfer of rRNA from the Sertoli cell into spermatocytes.