Differential morphology and distribution of GFAP astrocytes in vocal brain circuit in a songbird Southern house wren and humans.

Speech and song exhibit notable parallels between humans and birds. In humans, speech involves the Laryngeal Motor Cortex (LMC), Sensorimotor cortex (SMC), Broca's and Wernicke's areas, and the basal ganglia (striatum), which show convergent gene expression with avian song-control regions (RA, LMAN, HVC) and basal ganglia (Area X and medial striatum). While astrocyte morphology has been implicated in human speech, its role in song remains unknown. To compare astrocytes involved in speech and song, we evaluated cell density, astrocyte types, and their distribution in healthy humans and Southern house wrens using Nissl staining, GFAP and GS immunostaining, and 3D confocal imaging. The basal ganglia, human striatum and avian medial striatum, showed the highest cell density in both species. Human astrocyte distribution followed established cortical patterns, with enrichment in layers I-III and white matter (WM). In contrast, Southern house wrens exhibited restricted GFAP-positive astrocytes in vocal nuclei, with expression instead concentrated in telencephalic borders, vascular regions, and basal ganglia WM. Astrocyte morphology varied regionally in both species; basal ganglia astrocytes were especially complex, yet Southern house wrens exhibited reduced branching even after normalizing for brain volume/body weight ratio, indicating species-specific differences in complexity. GS-positive astrocytes were abundant and homogeneous throughout the pallium, including all vocal nuclei, unlike the more restricted GFAP-positive subset. Cross-species analysis of public songbird datasets confirmed minimal