Abstract: Objective Agerelated morphological changes of glial cells, S100and glial fibrillary acid protein (GFAP)-immunoreactive cells in the medulla of primary somatosensory area (SI) were comparatively investigated between 4 young cats and 4 old cats, the causation and the effect for these changes were also discussed. Methods Modified Holzer staining was applied to show the total glial cells and the adult animal. Golgi method was used to display the cells’ morphology; Immunohistochemical method was employed to exhibit S100-immunoreactive (S100-IR) and GFAP-immunoreactive (GFAPIR) cells. Under microscope, the cell morphology was comparatively observed; The numbers of the glial cells, S100-IR and GFAP-IR cells were counted; the average optical density and the area ratio of the immunoreactive cells were calculated with the software of BI-2000. Results The density of the glial cells, S100-IR and GFAP-IR cells showed a significant increase in the medulla SI of old cats (EM>P/EM><0.01), accompanied with a notable increased ratio of S100-IR and GFAP-IR cells to the total glial cells. The average absorbance and the area ratio of the positive cells showed a remarkable increase in the old cats than those in the young ones (EM>P/EM><0.01). In addition，the GFAP-IR astrocytes exhibited larger soma and denser processes in the old animals. Conclusion The medulla of SI exists reactive gliosis during senescence, especially a more sensitivity of the astrocytes, which indicates that an increased glial activity might exert a protective function for aging nervous fibres and m

Key words: Primary somatosensory area, Reactive gliosis, Astrocyte, S100 protein, Immunohistochemistry, Cat