Granule cell number, cell death and cell proliferation in the dentate gyrus of wild-living rodents

Adult neurogenesis in the dentate gyrus occurs at species-specific levels. Wood mice (Apodemus flavicollis) show higher proliferation rates than laboratory mice and voles (Clethrionomys glareolus, Microtus subterraneus). We compare rates of cell death and proliferation and investigate if cell proliferation leads to the long-term recruitment of granule cells. Granule and pyknotic cell numbers were estimated in wild-living rodents in different age classes and compared with laboratory mice of mixed genetic background. All species differ significantly in their number of granule cells, except for the comparison of laboratory mice with European pine voles. Granule cell number is significantly higher in old bank voles and wood mice as compared to adults (23 and 37%, respectively). The number of pyknotic cells is highest in wood mice and lowest in laboratory mice. Across all species, the numbers of proliferating and pyknotic cells correlate. Despite differences in cell proliferation and cell death, the ratio of proliferating to pyknotic cells does not differ between adults of the wild-living species, but in laboratory mice a significantly lower proportion of cells die compared with the other species. In addition, the ratio of proliferating to pyknotic cells was significantly higher in old wood mice than in adults. We conclude (i) that cell proliferation can lead to an increase in granule cell number in wild-living rodents and (ii) that species- and age-specific changes of the ratio between proliferating and pyknotic cells occur as deviations from a close correlation of these two numbers across all species and age groups.