Anatomical mapping of GFAP-immunoreactive astrocytes in the tree shrew brain

Astrocytes are associated with varying brain size between rodents and primates. As a close evolutionary relative of primates, the tree shrew (Tupaia belangeri) provides a valuable comparative model for investigating glial architecture. However, the anatomical distribution and morphological characteristics of astrocytes in the tree shrew brain remain poorly characterized. In this study, glial fibrillary acidic protein (GFAP) immunofluorescence was employed to systematically examine the spatial distribution and morphology of astrocytes in the whole brain of tree shrews. Notably, GFAP-immunoreactive (ir) astrocytes were detected throughout the telencephalon, diencephalon, mesencephalon, metencephalon, and myelencephalon. Distinct laminar distribution was evident in regions such as the main olfactory bulb and hippocampus. Semi-quantitative comparisons revealed significant regional differences in astrocyte density between tree shrews and mice, encompassing the main olfactory bulb, accessory olfactory bulb, olfactory tubercle, cortex, hippocampus, cortical amygdaloid nucleus, hypothalamus, thalamus, superior colliculus, interpeduncular nucleus, median raphe nucleus, and parabrachial nucleus. Compared to mice, tree shrews exhibited higher astrocyte density with increased morphological complexity in the posterior hypothalamic nucleus, dorsomedial hypothalamic nucleus, ventromedial hypothalamic nucleus, and periaqueductal gray, but lower density with greater morphological complexity in the hippocampus and substantia nigra. In the paraventricular hypothalamic nucleus and lateral hypothalamic area, GFAP-ir astrocytes displayed comparable densities between tree shrews and mice but exhibited region-specific differences in morphological complexity. This study provides the first brain-wide mapping of GFAP-ir astrocytes in tree shrews, revealing marked interspecies differences in their distribution and morphology, and establishing a neuroanatomical framework for understanding astrocyte involvement in diverse physiological and behavioral functions.