Cell proliferation, cell death and total granule cell-number in the dentate gyrus in the hippocampus of the four-striped mouse (Rhabdomys pumilio) and common mole-rat (Cryptomys hottentotus)

Abstract

This study investigated a wild and a captive-bred wild animal models to provide evidence that adult neurogenesis occurs in the brain of the four-striped mouse (FSM) (Rhabdomys pumilio) and common mole-rat (CMR) (Cryptomys hottentotus) and also estimate the total granule cell number in the dentate gyrus (DG) of the hippocampus as a baseline for comparing cell proliferation and cell death in the two species. Adult male four-striped mouse (n=6) and common mole-rat (n=7) were used to investigate for cell proliferation, cell death and immature neurons. The animals were anaesthetized and transcardially perfused with saline followed by paraformaldehyde fixative (4% paraformaldehyde in 0.1 M phosphate buffer), pH 7.4. Brains were removed and post fixed in the same fixative overnight. Following equilibration in 30% sucrose in PB, the left hemisphere was cut at 50 μm thickness, frozen serial sagittal sections. Ki-67 immunohistochemistry was carried out to identify proliferative cells and Doublecortin (DCX) staining for immature neurons. The right hemispheres of the brains were plastic embedded and sections cut at 20 μm, were subjected to Giemsa staining for the estimation of total granule and pyknotic cell numbers in the dentate gyrus. Ki-67 immunostaining confirmed adult cell proliferation in the dentate gyrus (DG) of the hippocampus of the four-striped mouse and common mole-rat. DCX immunopositive cells confirmed presence of immature neurons in the DG of the hippocampus of four-striped mouse and common mole-rat. There was no difference in the category of DCX positive cells observed in the DG of four-striped mouse and common mole-rat as both were predominantly in the postmitotic stage. The mean Ki-67 immunopositive cell number, an indication of cell proliferation was 993.33 ± 683.4 in the four-striped mouse and 190.83 ± 209.51 in the common mole-rat. The mean pyknotic cell number, an indication of cell death was 199.17 ± 92.8 in the four-striped mouse and 227.5 ± 108.77 in the common mole-rat. The estimated total granule cell number in the dentate gyrus was between 1.3 x 106 and 2.0 x 106 in the four- striped mouse and 0.6 x 106 and 1.1 x 106 in the common mole-rat. The Gundersen coefficient of error was between 0.05 - 0.06 in the four-striped mouse and 0.05 and 0.078 in the common mole-rat. The estimated total granule cell number in the four-striped mouse and common mole-rat represents over a 50 times increase compared to the values reported from laboratory rodents. Despite the larger brain size and body weight in the common mole-rat, the four-striped mouse had about five times the number of cell proliferation rate. A statistically significant higher cell proliferation rates and granule cell numbers were seen in the four-striped mouse compared to common mole-rat (p < 0.03 and 0.00). There was no significant difference in the immature neuron and pyknotic cell (p=0.43 and 0.70) respectively. The rate of immature neurons and cell death was higher in the common mole-rat than in the four-striped mouse. There was no significant correlation between cell proliferation, cell death and Total granule cell number in both animals. Regression analysis did not yield any significant relationship between these variables except between cell proliferation and cell death in common mole-rat. In conclusion, despite the complex burrowing system in the common mole-rat with associated higher cognitive, learning and memory function, similar adult neurogenesis was found in both four-striped mouse and common mole-rat. The cell proliferation and cell death does not have any effect on the total granule cell number in the four-striped mouse and common mole-rat.