Cortical interlaminar astrocytes across the therian mammal radiation

Interlaminar astrocytes (ILA) in the cerebral cortex possess a soma in layer I and extend an interlaminar process that runs perpendicular to the pia into deeper cortical layers. We examined cerebral cortex from 46 species that encompassed most orders of therian mammalians, including 22 primate species. We described two distinct cell types with interlaminar processes that have been referred to as ILA, that we termed pial ILA and supial ILA. ILA subtypes differ in somatic morphology, position in layer I, and presence across species. We further described rudimentary ILA that have short GFAP⁺ processes that do not exit layer I, and "typical" ILA with longer GFAP⁺ processes that exit layer I. Pial ILA were present in all mammalian species analyzed, with typical ILA observed in Primates, Scandentia, Chiroptera, Carnivora, Artiodactyla, Hyracoidea, and Proboscidea. Subpial ILA were absent in Marsupialia, and typical subpial ILA were only found in Primate. We focused on the properties of pial ILA by investigating the molecular properties of pial ILA and confirming their astrocytic nature. We found that while the density of pial ILA somata only varied slightly, the complexity of ILA processes varied greatly across species. Primates, specifically bonobo, chimpanzee, orangutan, and human, exhibited pial ILA with the highest complexity. We showed that interlaminar processes contact neurons, pia, and capillaries, suggesting a potential role for ILA in the blood-brain barrier and facilitating communication among cortical neurons, astrocytes, capillaries, meninges, and cerebrospinal fluid.