1. 1. Ribonucleic acid and protein synthesis in germ cells and Sertoli cells of the mouse testis was studied using tritium autoradiography with 3H-uridine and 3H-amino acids, and cytochemical methods. 2. 2. Spermatogonia.—The rate of RNA and protein syntheses is much higher in the immature type A spermatogonia than in the mature type B spermatogonia. This difference in the synthetic rates is possibly related to a difference in the degree of DNA condensation between the two types of cells. Nuclear and cytoplasmic protein synthesis occurs at all stages of the division cycle, whereas chromosomal RNA synthesis stops during metaphase and anaphase. 3. 3. Meiosis.—In the autosomes, RNA synthesis ceases or falls to an insignificant level during two periods of the meiotic cycle, early prophase (leptotene to early pachytene) and late diakinesis to metaphase and anaphase I and II. Between these two minima it undergoes a stepwise increase during middle prophase to a peak in middle pachytene, followed by a drop during late pachytene to early diakinesis. In contrast to the autosomes, the heteropycnotic XY bivalent is invariably unlabeled with 3H-uridine throughout meiotic prophase. Protein synthesis, on the contrary, continues during the periods of arrest or depression of RNA synthesis, and is also present in the sex chromosomes. 4. 4. Spermiogenesis.—RNA synthesis stops very soon after second meiotic division, in very early spermiogenesis. The ribonucleic acids synthesized during meiosis and early spermiogenesis are completely lost from the nucleus both by breakdown and by transfer to the cytoplasm during the course of spermiogenesis, so that middle and late spermatids (from stage 9 to stage 15) do not show any detectable amount of RNA within the nucleus, whereas the cytoplasm exhibits fair amounts of RNA that had been synthesized at least a week earlier during meiotic prophase. The protein synthesis seen to occur in the cytoplasm of spermatid stages 9 to 15 is, then, probably sustained by “templates” produced during meiosis and conserved in the cell to become available during spermiogenesis. 5. 5. Histone transition during spermiogenesis.—Protein labeling over the nucleus is either absent or very low in spermatids, except in late, elongated, spermatids (stages 11 to 14) which show a marked nuclear labeling as early as 15 min after injection of 3H-arginine. With other amino acids, the nuclear labeling in elongated spermatids is either low or absent. The nuclear incorporation of 3H-arginine is accompanied by changes in the staining properties of nuclear histone (persistance of alkaline fast green stainability after acetylation) which indicate that elongated spermatids synthesize a new nuclear histone very rich in arginine that replaces the previous histone which is a typical histone rich in lysine. A number of evidences suggest that this “arginine-rich” spermatid histone is effectively synthesized within the nucleus, although the alternative interpretation of a cytoplasmic synthesis followed by transfer of the newly synthesized histone into the nucleus cannot be actually rejected. 6. 6. Sertoli cells.—Nucleoplasm and nucleolus of the supporting Sertoli cells incorporate very rapidly 3H-uridine and 3H-amino acids. The satellite karyosomes (or heteropycnotic bodies) flanking the nucleolus are, however, unlabeled with 3H-uridine and scarcely labeled with 3H-amino acids. 7. 7. The evidence that the heteropycnotic sex chromosomes during male meiosis and the heteropycnotic karyosomes in Sertoli cells do not incorporate RNA precursors is interpreted to indicate that only dispersed chromatin is active in ribonucleic acid synthesis.
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