Recovery of a low mutant frequency after ionizing radiation-induced mutagenesis during spermatogenesis

Abstract

Humans are exposed to ionizing radiation (IR) under various circumstances, e.g. cosmic radiation, diagnostic X-rays and radiotherapy for cancer. It has been shown that IR can impair spermatogenesis and can cause mutations in germ cells. However, the mutagenic responses of germ cells exposed to IR at different stages of testicular maturation have not been examined by directly assessing the mutant frequency in defined spermatogenic cell types. This study was performed to address whether preadult exposure to IR can increase mutations in adult germ cells that could in turn have a major impact on adult reproductive function and the health of ensuing offspring. Male Lac I transgenic mice were irradiated with a single dose of 2.5 Gy of gamma-ray at different ages before adulthood, reflecting different stages of testicular maturation, and then mutant frequency (MF) was determined directly in spermatogenic cell types emanating from the irradiated precursor cells. The results showed that (1) preadult exposure to IR did not significantly increase MF in adult epididymal spermatozoa; (2) spermatogenic stages immediately following the irradiated stage(s) displayed an elevated mutant frequency; but (3) the mutant frequency was restored to unirradiated levels in later stages of spermatogenesis. These findings provide evidence that there is a mechanism(s) to prevent spermatogenic cells with elevated mutant frequencies from progressing through spermatogenesis.

Figure 1

Experimental design. Mice were irradiated at different ages and then sacrificed at later ages. For example, mice were sacrificed at 8-d/o, 18-d/o, 30-d/o or 60-d/o after irradiation at 6-d/o. The predominant spermatogenic cell types present at irradiation are shown above the ages of irradiation. The cell types collected are shown below the collection ages. A and B, type A and type B spermatogonia; P-L, pre-leptotene; L/Z, leptotene/zygotene; P, pachytene spermatocytes; RS, round spermatids; SP, spermatozoa.

Figure 2

Mutant frequencies in spermatogenic cell types collected from mice irradiated at 6-d/o. The numbers following the cell type indicate the age of mice when the cells were collected. Data are presented as means ± SEM. P values represent significant differences between the IR-treated group and the control group, same cell type and same age. A and B, type A and B spermatogonia; P-L, pre-leptotene spermatocytes; L/Z, leptotene plus zygotene spermatocytes; P, pachytene spermatocytes; RS, round spermatids; SP, epididymal spermatozoa.

Figure 3

Mutant frequencies in spermatogenic cell types collected from mice irradiated at 8-d/o. The numbers following the cell type indicate the age of mice when the cells were collected. Data are presented as means ± SEM. P values represent significant differences between the IR-treated group and the control group, same cell type and same age. A and B, type A and B spermatogonia; P-L, pre-leptotene spermatocytes; L/Z, leptotene plus zygotene spermatocytes; P, pachytene spermatocytes; RS, round spermatids; SP, epididymal spermatozoa.

Figure 4

Mutant frequencies in spermatogenic cell types collected from mice irradiated at 18-d/o. The numbers following the cell type indicate the age of mice when the cells were collected. Data are presented as means ± SEM. P values represent significant differences between the IR-treated group and the control group, same cell type and same age. A and B, type A and B spermatogonia; P-L, pre-leptotene spermatocytes; L/Z, leptotene plus zygotene spermatocytes; P, pachytene spermatocytes; RS, round spermatids; SP, epididymal spermatozoa.

Figure 5

Mutant frequencies in spermatogenic cell types collected from mice irradiated at 25-d/o. The numbers following the cell type indicate the age of mice when the cells were collected. Data are presented as means ± SEM. P values represent significant differences between the IR-treated group and the control group, same cell type and same age. A and B, type A and B spermatogonia; P-L, pre-leptotene spermatocytes; L/Z, leptotene plus zygotene spermatocytes; P, pachytene spermatocytes; RS, round spermatids; SP, epididymal spermatozoa.