On areas of transition between entorhinal allocortex and temporal isocortex in the human brain. Normal morphology and lamina-specific pathology in Alzheimer's disease.

The allocortical entorhinal region does not gradually transform into the temporal isocortex. Instead, there is an extended stretch of "transentorhinal" cortex with interdigitation of allocortical and isocortical laminae. The main feature of this transition zone is that the superficial layer of large multipolar nerve cells (Pre-alpha) of the entorhinal region gradually sweeps downward and follows an oblique course through the outer layers. During this course the star-shaped nerve cells of Pre-alpha are transformed into pyramidal cells. The layer Pre-alpha projection cells are particularly prone to the development of neurofibrillary changes of the Alzheimer type. In cases of presenile and senile dementia almost all of the layer Pre-alpha projection neurons are changed pathologically. The isocortical pyramidal cells of layers II to IV are far less inclined to develop neurofibrillary changes. In the transentorhinal cortex, the tangle-bearing neurons follow an oblique course through the superficial laminae and are finally located between the isocortical layers III and IV, findings that confirm the assumption that these neurons are constituents of the allocortical layer Pre-alpha. Layer-specific pathology of the profound stratum as well confirms the transentorhinal region as being formed by interdigitating allocortical and isocortical layers.