Introduction: The evolution of the structural organization of the mammalian neocortex can only be appreciated studying its ontogenetic development.
Development: We studied its evolution in embryos of different mammals (hamster, mouse, rat, cat and man) using the Golgi method. Development of the neocortex starts with the establishment of a primordial plexiform layer (PPL) in the telencephalus. This PPL represents a primitive cortical organization which is shared by amphibians, reptiles and mammals. From the PPL derived: the layer 1 with its elements including: the Cajal Retzius cells (CR) and primitive afferent fibres of a possibly origin in mesencephalic nuclei and the elements the interstitial cells and afferent fibres of the subplaca. The formation of the PPL is a prerequisite for the subsequent formation of the cortical plate (CP) from which the remaining layers of the neocortex derived. The ascending neuronal migration, the morphology of pyramidal neurons and the inside outside arrangement of neurons within the CP are evolutionary processes controlled by the CR cells. These neurons secrete a glycoprotein reelin which attracts the migrating neurons toward the first layer. All migrating neuroblasts, guided by the radial glia, must reach layer 1, establish contacts with the CR cells, develop an apical dendrite and become pyramidal cells. Without losing either their original contact with layer 1 or their cortical level, each neuron has to elongate its apical dendrite to accommodate the arrival of subsequent neurons, such that all develop a common initial pyramidal morphology. Older neurons have longer apical dendrites and deeper cortical level than the newer ones. The subsequent morphological and functional maturation of the CP neurons also follows an ascending progression and is under thalamic control. Two basic types of neurons develop in the neocortex following its ascending stratification. Some neurons maintain their original contact with the first layer to become the pyramidal cells. Others lose their original contact with layer 1 and become stellate interneurons. These neurons are free to develop specific size, spatial morphology and synaptic endings. The CP is a mammalian innovation and represents a biologically open and stratified nucleus which adds, during mammalian evolution, new pyramidal cell strata to those already present.
Conclusions: Based on the above observations, we propose a new cytoarchitectural theory and nomenclature for the structural evolution of the mammalian neocortex. The theory emphasizes the ascending structural and functional stratification of the mammalian neocortex and the increase in the number of pyramidal cell strata reflecting the motor needs of each species.