(C57Bl/Cne X C3H/Cne)F1 male mice were irradiated with single acute doses of 0.4 MeV neutrons ranging from 0.05 to 2 Gy, and testis cell suspensions were prepared for cytometric analysis of the DNA content 2-70 days after irradiation. Various cell subpopulations could be identified in the control histogram including mature and immature spermatids, diploid spermatogonia and spermatocytes, tetraploid cells and cells in the S-phase. Variations in the relative proportions of different cell types were detected at each dose and time, reflecting lethal damage induced on specific spermatogenetic stages. The reduction of the number of elongated spermatids 28 days after irradiation was shown to be a particularly sensitive parameter for the cytometrical assessment of the radiosensitivity of differentiating gonia. A D0 value of 0.13 Gy was calculated and compared with data obtained after X-irradiation, using the same experimental protocol. In the latter case a biphasic curve was obtained over the dose range from 0.25 to 10 Gy, possibly reflecting the existence of some cell population heterogeneity. RBE values were estimated at different neutron doses relative to the radiosensitive component of the X-ray curve, and ranged from 3.3 to 4, in agreement with data in the literature. Genotoxic effects were monitored 7 days after irradiation by a dose-dependent increase of the coefficient of variation (CV) values of the round spermatid peak, reflecting the induction of numerical and structural chromosome aberrations, and 14 or 21 days after irradiation by the detection of diploid elongated spermatids, probably arising from a radiation-induced complete failure of the first or second meiotic division.

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A cytogenetic approach to estimate in vivo mitotic nondisjunction is proposed, based on chromosomal counting of mouse bone marrow metaphases differentially stained by bromodeoxyuridine (BrdU) incorporation. The method allows the simultaneous assessment of cell cycle delay, aneuploidy and SCE induction. Male mice, implanted with agar-coated BrdU tablets, were injected i.p. with diethylstilboestrol-diphosphate (DES-dp) in the dose range 10-300 mg/kg and killed 18 or 24 h later. To investigate possible sex differences a group of female mice of the same strain and age was injected with 100 mg/kg. As positive controls six males were injected with 1.8 mg/kg of vinblastine (VBL) sulphate. The induction of cell cycle delay was estimated by the relative frequency of first, second and third mitoses after treatment. In spite of a large biological variability, a dose-dependent delay of cellular proliferation kinetics was observed in DES-treated male mice. Treatment with VBL strongly delayed cell cycle progression, according to its antimitotic activity. Hyperploidy was assessed by chromosome counting of second generation metaphases only. After VBL injection, 10.2% of second mitoses were hyperploid, which is a frequency significantly higher than the 0.2% seen in control mice. No significant effect was detected at any DES dose. SCE induction was estimated in the same cells. A significant increase over the control frequency was observed after 200 and 300 mg/kg of DES. By analysis of variance (MANOVA) the dose--effect relationship was fitted by a quadratic model. A sex difference was observed only for spontaneous frequency of SCE with females showing higher levels probably due to their lower weight and relatively higher BrdU in vivo concentration.