A comparative study of pre-alpha islands in the entorhinal cortex from selected primates and in lissencephaly.

The entorhinal cortex (EC) is the main interface between the sensory association areas of the neocortex and the hippocampus. It is crucial for the evaluation and processing of sensory data for long-term memory consolidation and shows damage in many brain diseases, for example, neurodegenerative diseases, such as Alzheimer's disease and developmental disorders. The pre-alpha layer of the EC in humans (layer II) displays a remarkable distribution of neurons in islands. These cellular islands give rise to a portion of the perforant path-the major reciprocal data stream for neocortical information into the hippocampal formation. However, the functional relevance of the morphological appearance of the pre-alpha layer in cellular islands and the precise timing of their initial appearance during primate evolution are largely unknown. Here, we conducted a comparative study of the EC from 38 nonhuman primates and Homo sapiens and found a strong relationship between gyrification index (GI) and the presence of the pre-alpha cellular islands. The formation of cellular islands also correlated with brain and body weight as well as neopallial volume. In the two human lissencephalic cases, the cellular islands in the pre-alpha layer were lacking. These findings emphasize the relationship between cortical folding and island formation in the EC from an evolutionary perspective and suggest a role in the pathomechanism of developmental brain disorders.