Morphological Hallmarks of White Matter Astrocytes Across Development.

While the mechanisms underlying protoplasmic, or gray matter (GM), astrocyte morphogenesis have been extensively studied, far less is known about the development of fibrous, or white matter (WM), astrocytes. In particular, no study has comprehensively characterized the morphological development of WM astrocytes. To address this gap, we used an intracortical adeno-associated virus injection technique to sparsely label individual astrocytes in the mouse corpus callosum for morphological analysis across postnatal development. We found that astrocytes in the corpus callosum follow a distinct trajectory of morphological development from their GM cousins. They establish their branching complexity in the first two postnatal weeks and significantly increase in volume into adulthood. We further show that tiling is conserved in WM astrocytes and that a subset of mature astrocytes in the corpus callosum lack endfeet. These findings provide the first developmental framework for the morphogenesis of callosal astrocytes and establish a foundation for future studies investigating the molecular mechanisms regulating the development of WM astrocytes.