Ribosomal RNA synthesis in spermatogonia and Sertoli cells of the mouse testis

The effects of boric acid (BA) on testicular cells in culture

High-dose boric acid (BA) exposure produces testicular lesions in adult rats characterized by inhibited spermiation that may progress to nonrecoverable atrophy. The mechanism for the testicular toxicity of BA is unknown. To examine possible direct effects, the present study evaluated selected aspects of various testicular cell culture systems after in vitro BA exposure. Specifically, the hallmarks of the BA testicular toxicity were addressed: the mild hormone effect, the initial inhibition of spermiation, and atrophy. No effect of BA on the steroidogenic function of isolated Leydig cells was observed, supporting the contention of a CNS-mediated rather than a direct hormone effect. Since increased testicular cyclic AMP (cAMP) produces inhibited spermiation, and a role for the serine proteases plasminogen activators (PAs) in spermiation has been proposed, we evaluated both Sertoli cell cAMP accumulation in Sertoli-germ cell cocultures and the stage-specific secretion of PA activity in cultured seminiferous tubules after in vitro BA exposure, respectively. The results showed that the inhibited spermiation is not due to BA effects on either process. To address the atrophy, we evaluated BA effects in Sertoli-germ cell cocultures on 1) morphology/germ cell attachment, which might identify a target cell; 2) Sertoli cell energy metabolism, because lactate, secreted by Sertoli cells, is a preferred energy source for germ cells; and 3) DNA/RNA synthesis, because germ cells synthesize DNA/RNA and BA impairs nucleic acid synthesis in liver and may do so in testis. Despite the absence of overt morphologic changes and germ cell loss, the most sensitive in vitro endpoint was DNA synthesis of mitotic/meiotic germ cells, with energy metabolism in Sertoli or germ cells affected to a lesser extent. A re-evaluation of testis sections from rats exposed to BA revealed a decrease in the early germ cell/Sertoli cell ratio prior to atrophy. Thus, although the mechanism for the inhibited spermiation is still undefined and is the subject of future work, these combined studies revealed some changes offering a plausible explanation for the atrophy aspect of the BA testicular lesion.

Association of centromeric heterochromatin with the nucleolus in mouse Sertoli cells

In adult Sertoli cells of most strains of mice, all the centromeric heterochromatin is condensed in two chromocenters, one on each side of a single, large nucleolus. In a random-bred Swiss OF-1 strain, however, the nucleus has a different structural organization. Part of the heterochromatin Is seen as chromocenters in contact with the nucleolus; the rest of it is dispersed in granules of unequal size in the nucleoplasm. Such an unusual spatial arrangement of heterochromatin in interphase nucleus cannot be explained either by a difference in the nucleolar organizing regions or by a polymorphism of the C-banding of metaphase chromosomes.

In vitro synthesis of RNA by Xenopus spermatogenic cells I. Evidence for polyadenylated and non-polyadenylated RNA synthesis in different cell populations

Premeiotic and postmeiotic (haploid) gene expression during spermatogenesis in the anuran, Xenopus laevis, was studied by analyzing the accumulation of radioactively labelled cytoplasmic polyadenylated [poly (A +)] and non-polyadenylated [poly (A -)] RNAs. Dissociated spermatogenic cells were labelled and maintained in an in vitro system capable of supporting cell differentiation. Labelled cells were separated by density gradient centrifugation into subpopulations enriched for individual spermatogenic stages. RNA was extracted and purified from each cell fraction, and separated into poly (A +) and poly (A -) species. Comparison of poly (A +) to non-poly (A) radioactivity in cells labelled with tritiated uridine or adenosine demonstrated that (1) all cell fractions produced significant quantities of polyadenylated RNA relative to total RNA synthesis; and (2) that a cell fraction enriched for pachytene spermatocyte RNA contained up to 15% of total cytoplasmic and 35% of total polysomal RNA labelled as poly (A +) containing species. RNA was also characterized by sucrose density gradient centrifugation and polyacrylamide gel electrophoresis. All cell types showed typical poly (A -) peaks of 4S, 18S and 28S, corresponding to tRNA (4S) and rRNAs (18, 28S) respectively. Spermatids and spermatozoa had additional absorbance peaks at 13 and 21S which cosedimented with Xenopus oocyte mitochondrial rRNA. Patterns of incorporation of uridine and adenosine into poly (A +) RNA in all germ cell fractions tested were complex. In all cases, major areas of radioactivity were found in a broad band sedimenting between 6-17S. Spermatid fractions showed a prominent peak of incorporation at 6-8S, while pachytene cells also showed heavier poly (A +) peaks in the 17-25S region. A non-polyadenylated RNA species sedimenting at 6-8S with a relatively rapid rate of turnover was also observed in spermatids. From these results it is concluded that synthesis of transfer, ribosomal, and putative messenger RNA species continues in spermatogenic cells throughout all but the very last stages of spermatogenesis in Xenopus.

RNA synthesis in different stages of rat seminiferous epithelial cycle

In vitro RNA synthesis has been analyzed using autoradiographic and electrophoretic methods in isolated 1-2 mm segments of rat seminiferous tubules. The cellular composition of each segment was accurately identified using microscopic analysis of the transillumination pattern of the freshly isolated, unstained, seminiferous tubules combined with phase-contrast microscopy of the living spermatogenic cells. The RNA synthesized in the seminiferous tubules was found to be mostly heterogenous nuclear RNA (HnRNA), which appeared to have a long lifetime. It was most actively formed in the stages which contain mid-pachytene spermatocytes. Formation of rRNA was slow in all stages and it was first observed when a 2-h pulse with [3H]uridine was followed by a 6-h chase. A very low RNA synthetic rate was observed in the stage containing the meiotic reduction divisions. The function of the meiotic RNA in regulation of spermiogenesis is discussed.

Structural and transcriptional features of the mouse spermatid genome

A whole-mount electron microscope technique has allowed direct visualization of the transcription process in mouse spermatids. Thes observations have been supported by light and electron microscope autoradiographic techniques that employ [3H]uridine and [3H]arginine in attempts to clarify mechanisms of RNA synthesis and their relationship to nuclear histone changes throughout spermiogenesis. Early spermatid genomes are dispersed almost completely, whereas in later spermiogenic steps the posterior or flagellar nuclear region is readily dispersed and the anterior or subacrosomal nuclear region remains compact. Display of genome segments permits identification of regions where transcription complexes, presumably heterogeneous nuclear RNA species, are seen related to chromatin. These complexes appear as ribonucleoprotein chains, some of them of considerable length, decreasing progressively in number in late spermiogenic steps. This decrease coincides with diminishing rates of [3H]uridine incorporation. Two distinct patterns of chromatin have been identified: a beaded chromatin type associated with transcription complexes encounterd in early spermatids; and a smooth chromatin type not involved in transcriptive activity observed in advanced spermiogenic genomes. Protein particles staining densely with phosphotungstic acid become apparent in nuclei of spermatids after [3H]arginine incorporation becomes significant. There is no structural or autoradiographic evidence for the presence of nucleoli during spermiogenesis. From these data and from previous experimental findings, we conclude that: (a) spermatogonia, spermatocytes and Sertoli cells are transcriptionally expressed into heterogeneous nuclear RNA and preribosomal RNA species whereas transcription in spermatids is predominantly heterogeneous nuclear RNA; and (b) the modification of the chromatin patterns in late spermiogenic steps indicates a stabilized genome that restricts transcriptive functions.