Interlaminar astroglia of the cerebral cortex: a marker of the primate brain

Evidence for "cable-like" processes stemming from astroglial cells in the supragranular cerebral cortex has been recently presented. In addition to what could be called the "general mammalian-like" astroglial architecture (the so-called "panglial syncytium") of the cerebral cortex, composed of typical stellate astrocytes (intralaminar astrocytes), the anthropoid species, mostly catarrhines, show a manifest vertical, radial distribution of long (interlaminar) astroglial processes. It can be tentatively proposed that evolutionary pressures resulted in the progressive appearance, in primates, of a new type of glial cell. Its soma has a superficial location and unusually long cellular processes that invade, in a predominant radial fashion, the supragranular region of the cerebral cortex. Their existence has been ignored for more than a century. On the neuronal side, modular (columnar) organization of the cerebral cortex may represent an evolutionary acquisition that could optimize communication and information processing, with the least volume compromise in terms of wiring. Yet, for such columns to be functionally operative, adequate isolation from neighboring units would be required. A "mass" operation of the astroglial architecture would tend to compromise spatial definition and the degrees of freedom of such columnar modules. It is proposed that the presence of a "palisade" of interlaminar glial processes represents a relatively recent evolutionary event, instrumental for the optimization of the modular (columnar) organization of the cerebral cortex. It is interesting that the supragranular cortical region has undergone the largest growth among mammalian species during brain evolution, and has been associated with a crucial role in cortico-cortical interactions.