THE JOURNAL OF COMPARATIVE NEUROLOGY 361:602-616 (1995) Morphology and Distribution of Microglial Cells in the Young and Adult Mouse Cerebellum JOSE MIGUEL VELA, ISHAR DALMAU, BERTA GONZ~EZ, AND BERNARD0 CASTELLANO Unit of Histology, Department of Cell Biology and Physiology, Faculty of Medicine, Autonomous University of Barcelona, Bellaterra 08193, Barcelona, Spain ABSTRACT The morphology and distribution of microglial cells were studied in the normal cerebellum of young and adult mice using the histochemical demonstration of nucleoside diphosphatase as a specific microglial marker. Our results showed that microglial cells were present in all cerebellar lobules of both young and adult mice, but their distribution and morphology were not homogeneous throughout the cerebellum. Heterogeneity in microglial cell distribution was exclusively related to their location in the different histological layers, and no significant differences were found either between the different cerebellar lobules or between young and adult mice. Microglial density was higher in the cerebellar nuclei than in the cortex; within the cortex, the molecular layer was less densely populated by microglial cells than the granular layer and the white matter. The morphological study revealed that microglial cells were ramified in all cerebellar lobules of both young and adult mice but showed different sizes and ramification patterns as a function of their specific location in the different histological layers. Several typologies of microglial cells were described on the basis of observations in both horizontal and coronal sections. The specific layer-related pattern of microglial distribution and morphology in mouse cerebellum strongly suggests a physical and functional adaptation of these cells to the characteristics of their microenvironment. Indexing terms: neuroglia, purine nueleosides, nucleoside diphosphatase, enzyme histochemistry, CNS o 1995 Wiley-Liss, Inc. The mammalian cerebellum is one of the better known parts of the central nervous system (CNS) because of the relative simplicity of its basic cytoarchitectonic organiza- tion. The cerebellar cortex displays a geometrical and uniform three-layered pattern that has facilitated both morphological and topographic distribution studies of the different cell types (Cajal, 1911; Eccles et al., 1967; Sotelo, 1967; Palay and Chan-Palay, 1974; Ghandour et al., 1980; Wilkin and Levi, 1986).The histological organization of the cerebellar nuclei has also been well characterized (Cajal, 1911; Sotelo and Angaut, 1973; Chan-Palay, 1977; Beitz and Chan-Palay, 1979a,b). Although cytological features of the different types of neurons and macroglial cells in the cerebellum are well established, very little is known about the characteristics of the microglial cell population. The general lack of knowl- edge of microglial cells in the cerebellum, like other loca- tions of the CNS, may be attributed to the absence of reliable techniques allowing specific visualization of this cell type. In the last two decades, the development of new and reliable specific markers for microglia has substantially contributed to the increase in interest in this glial cell type. At present, it is mainly accepted that microglial cells are a resident, specialized macrophage population derived from hematogenous monocytes invading the developing CNS during the late embryonic and early postnatal period (for review, see Perry and Gordon, 1988, 1991). These precur- sors of microglial cells, the so-called ameboid microglial cells, have been reported in different areas of the CNS (Ling, 1979; Ferrer and Sarmiento, 1980; Ling et al., 1980; Murabe and Sano, 1982b; Innocenti et al., 19831,including the mouse cerebellum (Ashwell, 19901, and differentiate during the perinatal period into ramified microglial cells. Ramified microglial cells that are widely distributed through- out the white and gray matter of the adult apparently remain as “quiescent” cells in the nervous parenchyma until they are confronted with pathologically or experimen- tally induced changes in conditions that compromise the normal integrity of the CNS. In these circumstances, Accepted March 17,1995 Address reprint requests to Jose Miguel Vela, Departamento de Biologfa Celular y Fisiologfa, Histologfa, Facultad de Medicina, Torre M-5, Universi- dad Autonoma de Barcelona, Bellaterra 08193, Barcelona, Spain. o 1995 WILEY-LISS, INC.