The transentorhinal cortex of the African green monkey: a combined light- and electron-microscopic study of calcium-binding protein containing neurons

The transentorhinal cortex (TEC) is a primate-specific transition zone betw een the entorhinal allocortex and the temporal isocortex. Neurons in the la mina pre-alpha of TEC an known to be the first to develop intraneuronal cha nges in the course of Alzheimer's disease. In order to shed light on this i mportant feature, we studied as yet unknown morphological and neurochemical characteristics of the TEC of the African green monkey (Cercopithecus aeth iops sabaeus). Using light- and electron-microscopic immunocytochemistry, t he distribution and morphology of neurons containing calcium-binding protei ns were described and compared with those in the adjacent cortices. Light-m icroscopic analysis revealed that parvalbumin-containing neurons were distr ibuted in all cortical layers. Calbindin-containing cells were fewer but al so present in each layer. Calretinin-containing neurons were largely confin ed to the upper layers of the TEC. All three types of neuron showed pyramid al-like, multipolar and bipolar shapes; their dendrites were smooth or bead ed. Ultrastructural studies revealed immunopositive somata with infolded nu clei and large amounts of cytoplasm. The somata were only sparsely innervat ed by symmetric synapses. Immunopositive dendrites were almost exclusively covered with immunonegative axon terminals establishing symmetric and asymm etric synapses. Immunopositive terminals established symmetric contacts wit h immunonegative dendrites and somata. Only occasionally, could synaptic co ntacts between immunopositive pre- and postsynaptic structures be observed. The comparison of neurons in the TEC and adjacent cortices revealed no str iking differences. In summary, the morphological and neurochemical characte ristics of TEC neurons as analyzed in our study do not provide an explanati on for the early onset of neurodegenerative changes in the TEC.