Distinct domanial and lamellar distribution of clustered lipofuscin granules in microglia in the main olfactory bulb of young mice

Lipofuscin granules are generally considered as age-pigment. However, we encountered numerous large irregular clusters of lipofuscin granules in the olfactory nerve layer and glomerular layer of the main olfactory bulb (MOB) of young adult and even juvenile mice of C57BL/6J strain. Those numerous autofluorescent irregular lipofuscin granules were contained in the cytoplasm of microglial cells. Importantly they showed a prominent pattern of distribution; that is, they were rather restricted to the OCAM positive ventro-lateral domain (V-domain) of the MOB but few in the OCAM negative dorso-medial domain (D-domain), even when microglia distributed rather homogeneously in both OCAM positive V-domain and OCAM negative D-domain. Those lipofuscin granules were not seen in MOBs of 10 days and 2w old C57BL mice, but usually encountered in the MOBs of 3w old mice. Similar clusters of lipofuscin granules in the olfactory nerve layer and glomerular layer were also encountered in BALB/c strain, and, although less prominent, in ICR and ddY strains. However, they were not encountered in young adult rats of three strains, Wistar, Sprague–Dawley and Long-Evans, indicating one of prominent species differences between mice and rats.