The cerebral neo cortex, unique to mammals, is regarded as the
prerequisite for higher cognitive function and is the structure most
closely associated with the idea of the "mind" . Expansion of mental
capa city between mammals is most typically associated with an
evolutionary increase in neocortical volume that culminates in the
intricately folded configuration of sulci and gyri so charac- teristic
of the primate cerebral cortex. Yet, the basic unit structure and funda-
mental connectivity of cortex appears to have been preserved from the
smooth cortex of the mouse or rat to the highly convoluted cortical
mantle of the human that, if stretched out as a sheet, would be large
enough to wrap the entire human brain multiple times. The basic
similarity in structure and func- tion has made it possible to conduct
studies in the relatively simple cortices of rat or mouse and have the
results pertain to the understanding of the primate, including human,
cortex. The neo cortex is an intriguing structure for the study of cell
differentiation. Its dozens of neuronal cell types and small handful of
different glial types have their origin in a pseudostratified germinal
epithelium lining the ventricular surface of the forebrain. In its
mature form, neocortex is a six-Iayered struc- ture; five of its layers
contain multiple different but characteristic neuronal types with the
sixth occupied by neuronal processes. Various glial cells are dis-
persed throughout all six layers.