Genotoxic effects of ethyl methanesulfonate and X-rays at different stages of rat spermatogenesis, studied by inhibition of DNA synthesis and induction of DNA repair in vitro

Stage-specific DNA synthesis of rat spermatogenesis as an indicator of genotoxic effects of vinblastine, mitomycin C and ionizing radiation on rat spermatogonia and spermatocytes

We have studied the effects of three known mutagens: vinblastine sulphate, mitomycin C and local irradiation of testes on the stage-specific DNA synthesis in the rat testis by using transillumination assisted microdissection of rat seminiferous tubules. It enables us to investigate the sensitivity of different types of spermatogonia and preleptotene spermatocytes to the genotoxic effects of these agents. According to our results, spermatogonia and preleptotene spermatocytes are quite resistant to the action of vinblastine at the treatment times and the doses used. After treatment with mitomycin C, type A2, A3 and A4 spermatogonia seem to be the first cell types affected, which shows itself as a reduction in the DNA synthesis at stages I, II-III, XIII-XIV of the epithelial cycle two and/or three days after the treatment. It also seems that they are mostly affected during the S-phase of their cell cycles. In addition, preleptotene spermatocytes are also sensitive to the action of mitomycin C when they are treated in the G1 phase of the cell cycle. The local irradiation of 3 Gy has severe effects on the spermatogonia of rat testis which can be seen already 18 h after the treatment and becomes more evident 42 and 66 h after the treatment as a reduction of DNA synthesis at stages XII-V. Type A spermatogonia (A1-A4) seem to be the most sensitive cell types to the action of irradiation. This study indicates that the novel method of stage-specific DNA synthesis in rat spermatogenesis allows detailed studies of sensitivities in differentiating spermatogonia to genotoxic agents.

Effects of etoposide on stage-specific DNA synthesis during rat spermatogenesis

The stage-specific effect of etoposide on spermatogenic DNA synthesis was measured 1, 3 and 18 days after a single intraperitoneal injection of etoposide. Etoposide inhibited premitotic DNA synthesis most effectively at stages II-III and IV-V of the seminiferous epithelial cycle in which DNA synthesis of late spermatogonia takes place. Compared with control levels, DNA synthesis at stages II-III was maximally inhibited 43% and 57% at doses of 5 and 10 mg/kg, respectively, and at stages IV-V the maximal inhibition was 67% and 62%, at doses of 5 and 10 mg/kg respectively. Premeiotic DNA synthesis was not as vulnerable to the etoposide action as premitotic DNA synthesis, the maximal inhibition of premeiotic DNA synthesis was 39% and 41% compared with control at doses of 5 and 10 mg/kg, respectively. Induction of most probably repair-type DNA synthesis was demonstrated in stages I-III, VIIa-b and XII of the cycle. All the effects of etoposide were most apparent 1 and 3 days after treatment but had not totally disappeared 18 days after the treatment.

Culture of intact Sertoli/germ cell units and isolated Sertoli cells from Squalus testis: I. Evidence of stage-related functions in vitro

As part of an ongoing program of research using the testis of the dogfish shark (Squalus acanthias) to characterize morphologic and functional changes during spermatogenesis, we have developed procedures for culturing intact spermatocysts (germ cell/Sertoli cell clones) and isolated Sertoli cells from premeiotic, meiotic, and postmeiotic stages of development. Phase contrast and light microscopy confirmed the stage and cellular composition of spermatocysts and showed that they retained their closed, spherical configuration for at least 15 d in culture. Stage-related variations in [3H]thymidine incorporation (premeiotic much greater than meiotic = postmeiotic) were observed, a pattern that was the same quantitatively and qualitatively after one or seven days of culture. [3H]Leucine-labeled protein synthesis was twofold greater in cultures with premeiotic spermatocysts than in cultures with more mature stages, whether medium or cysts were analyzed. Sertoli cells isolated from spermatocysts of different stages differed in size, shape, cytological appearance, ability to form flattened monolayers, and rate of DNA synthesis. One day after seeding, [3H]thymidine labeling of Sertoli cells corresponded to the pattern obtained with intact spermatocysts (premeiotic much greater than meiotic = postmeiotic); however, 7 days in culture effected a 40- to 200-fold increase in this parameter and altered the stage-dependent pattern (premeiotic = meiotic greater than postmeiotic). Also, when [3H]leucine-labeled macromolecules secreted by Sertoli cells from premeiotic versus meiotic stages were analyzed by polyacrylamide gel electrophoresis (PAGE), banding patterns differed. Initial results demonstrate the feasibility and potential of this in vitro system for studying qualitative and quantitative changes during spermatogenesis.

Attenuation of 2-methoxyethanol-induced testicular toxicity in the rat by simple physiological compounds

2-Methoxyethanol (2-ME) is an industrial solvent which is toxic to both male and female reproductive systems of laboratory animals. Earlier data have demonstrated that the developmental toxicity of 2-ME can be attenuated by simple physiological compounds such as serine, acetate, sarcosine, glycine, and D-glucose. The present experiments were designed to evaluate the same compounds for their ability to ameliorate the testicular toxicity that occurs in rats after 2-ME exposure. The extent of testicular damage was assessed by quantitating daily sperm production (DSP) on Day 24 following a single dose of 2-ME (6.6 mmol/kg, 500 mg/kg). Serine completely eliminated 2-ME-induced decreases in DSP, while glucose was without effect. Acetate, sarcosine, and glycine were of similar efficacy resulting in DSP that was significantly greater than that observed in rats which received 2-ME alone. Histopathological studies revealed that 2-ME treatment resulted in stage-specific degeneration of late stage pachytene spermatocytes 24 hr after treatment. No apparent degenerative changes occurred after concurrent treatment with serine. Similarly, serine also prevented the decreased number of spermatids in the lumina of the seminiferous tubules on Day 24 after 2-ME exposure alone. All of the compounds utilized in this study are linked to oxidation pathways involving tetrahydrofolic acid as a catalyst for one-carbon moiety transfer into purine and pyrimidine bases which are necessary precursors for DNA and RNA synthesis. The ability of these compounds to attenuate the testicular toxicity of 2-ME may result from their ability to donate one-carbon units which can be used in purine base biosynthesis. Reduced availability of bases would be expected to affect late stage pachytene spermatocytes which are known to be undergoing rapid RNA synthesis.

Mechanistic approaches in the study of testicular toxicity: agents that directly affect the testis

Toxicants which affect the male reproductive system can act indirectly via alteration of the complex hormonal control of the testes, or directly by virtue of their chemical reactivity. Both categories of agents may require metabolic biotransformation to attain activity, and the site of activation may play a significant role in the modulation of testicular toxicity. The modes of action of 2 well known direct-acting male reproductive toxicants are discussed here. The use of a combination of in vivo and in vitro systems to distinguish the site and mechanism of action of these chemicals, ethylene glycol monomethyl ether and dinitrobenzene, is described. Because of the complexity of the reproductive process and of the interaction between gonadal and extragonadal metabolism, the use of multiple endpoints and multiple test systems can lend a greater degree of precision and confidence to our conclusions.

Inhibition of stage-specific DNA synthesis in rat spermatogenic cells by polycyclic aromatic hydrocarbons

Changes in the rate of DNA synthesis in spermatogenic cells after treatment of segments of rat seminiferous tubule at defined stages of epithelial cycle with benzo[a]pyrene (BP) or 7,12-methylbenz[a]anthracene (DMBA) were studied. The incorporation of labeled thymidine into DNA was used as a measure of the rate of DNA synthesis. Very little or no inhibition of DNA synthesis at stages V and VIII of the cycle was observed at BP and DMBA concentrations lower than 100 microM. In contrast, in the presence of added mitochondria and/or microsomes from whole rat testis, 20 microM BP or DMBA inhibited DNA synthesis 5% and 80%, respectively. This inhibition of DNA synthesis was prevented by inhibitors of the cytochrome P-450 system and by free radical scavengers. These results suggest that polycyclic aromatic hydrocarbons (PAH) require metabolic activation in order to inhibit DNA replication in seminiferous tubules. The first step of this biotransformation is cytochrome P-450-dependent and occurs in Leydig cells. However, the metabolites produced in this step may be further metabolized to reactive metabolites by peroxidative pathways in the seminiferous tubules; these latter products may affect DNA replication.

Use of seminiferous tubule segments to study stage specificity of unscheduled DNA synthesis in rat spermatogenic cells

DNA repair in spermatogenic cells at various stages of maturity was determined by quantitation of unscheduled DNA synthesis (UDS). Male F-344 rats were exposed (i.p.) to methyl methanesulfonate (MMS, 35 mg/kg); 1 hr later, segments of seminiferous tubules corresponding to spermatogenesis stages II, IV-V, VI, VII, VIII, IX-X, XII, and XIV were isolated with the transillumination pattern of the tubules as a guide. Intact tubule segments were cultured 24 hr in the presence of [3H]thymidine, and UDS was quantitated by autoradiography as net grains/nucleus (NG). In primary spermatocytes from treated rats, NG count increased with increasing maturity from leptotene primary spermatocytes (3.5 NG) up through stage VIII and IX-X pachytene spermatocytes (22 NG), after which NG decreased in stage-XII pachytene and diplotene spermatocytes (to 16 NG and 8 NG, respectively). Round spermatids of steps 2-8 of spermiogenesis all exhibited approximately the same UDS response (8 NG). Elongating spermatids as mature as step 14 underwent UDS after exposure to MMS, but step-15 and later-step spermatids did not. The DNA repair response of pachytene spermatocytes cultured within segments of seminiferous tubule corresponding to stages VIII and IX-X was 4 to 25 times greater, depending on the dose of MMS, than pachytene spermatocytes isolated by enzymatic digestion and cultured in suspension [Bentley and Working, Mutat Res 203:135-142, 1988]. Thus, the use of segments of seminiferous tubule both increased the sensitivity of UDS as an indicator of DNA damage in rat germ cells and enabled the study of UDS in spermatogenic cells at different stages of maturity.

Concomitant mutagenicity of ethanol and x-ray irradiation in the mouse male germ cells

The mutagenic effect of ethanol and x-ray irradiation on the male germ cells was investigated in the mouse. The meiotic micronuclei in the testis were recorded and the abnormalities in the epididymal sperm heads were counted. The number of micronuclei in early spermatids did not differ in the ethanol-exposed groups from that of their control groups at the end of 5 week ethanol diets. However, in the mice irradiated with a dose of 0.5 Gy, there was a statistically significant (p less than 0.01) difference in micronucleus induction between the groups of different ethanol exposure levels. No statistically significant differences were found in the numbers of abnormal sperm heads immediately after the diet or 25 days, 5 weeks and 10 weeks after the cessation of the diet. However, sperm abnormality frequencies were elevated at 5 weeks after 4 and 6% (v/v) ethanol diets and at 25 days after the 6% diet. The results suggest that ethanol is co-mutagenic with x-rays in the mouse male germ cells. Ethanol alone showed also a tendency to be mutagenic in the sperm abnormality test. The stage and mechanism of action of ethanol is discussed.

Trends in Scandinavian Cell Toxicology

Over a period of two years the Scandinavian Society for Cell Toxicology* has met three times (on 21 October 1983 and 6–7 June 1984 in Uppsala, Sweden, and on 6–7 September 1985 in Roskilde, Denmark), to present research advances in cell toxicology and to discuss the effects of xenobiotics in isolated and cultured cells. The first part of this report represents a summary of these discussions. In the second part, some of the individual research reports presented by the participating members are summarised. The purpose is to give a review of problems currently dealt with in Scandinavian laboratories associated with the Society for Cell Toxicology.

Toxic and mutagenic influences on spermatogenesis

Cells at various stages of spermatogenesis show remarkable differences in their sensitivities to toxic and mutagenic agents. For accurate analyses of toxicity and mutagenicity, the most sensitive cell types should be obtained for studies soon after treatment before development of disturbing secondary alterations. The transillumination phase contrast microscopic technique has proved to be useful for this purpose. It allows recognition of rat and mouse seminiferous tubules in living unstained conditions for accurate selection of the desired stages of the cycle of the seminiferous epithelium for morphological and biochemical studies. Specific cell damage can be frequently recognized by transillumination only, whereas in most cases phase contrast microscopy is useful in screening the early stage-specific toxic alterations. A summary is presented of the results obtained by treatment with heat and with anticancer drugs. A meiotic micronucleus method has been developed for direct estimation of the severity of mutagenic insult on rat spermatogenesis. The segment that contains the second meiotic division and the early postmeiotic cells (stages XIV and I) is selected for study. Chromosome damage induced by mutagens may result in the formation of acentric fragments that after meiotic divisions are seen in early spermatids as separate micronuclei. They can be quantitated in squash preparations; the sensitivity of this method is comparable with that of the meiotic metaphase scoring. Recently, an in vitro technique has been developed for stage and cell specific measurements of replicative and repair syntheses of DNA during rat spermatogenesis after mutagen treatments.