Morphology of spiny neurons in the human entorhinal cortex: intracellular filling with lucifer yellow.

The present study was designed to investigate the morphology of spiny neurons in the human entorhinal cortex. Coronal entorhinal slices (n = 67; 200 microm thick) were obtained from autopsies of three subjects. Spiny neurons (n = 132) filled with Lucifer Yellow were analysed in different subfields and layers of the entorhinal cortex. Based on the shape of the somata and primary dendritic trees, spiny neurons were divided into four morphological categories; (i) classical pyramidal, (ii) stellate, (iii) modified stellate, and (iv) horizontal tripolar cells. The morphology of filled neurons varied more in different layers than in the different subfields of the entorhinal cortex. In layer II, the majority (81%) of spiny neurons had stellate or modified stellate morphology, but in the rostromedial subfields (olfactory subfield and rostral subfield) there were also horizontal tripolar neurons. Dendritic branches of layer II neurons extended to layer I (94%) and to layer III (83%). Unlike in layer II, most (74%) of the filled neurons in layers III, V and VI were classical pyramidal cells. The majority of pyramidal cells in the superficial portion of layer III had dendrites that extended up to layer II, occupying the space between the neuronal clusters. Some dendrites reached down to the deep portion of layer III. Apical dendrites of layer V and VI pyramidal cells traveled up to the deep portion of layer III.Our data indicate that the morphology of spiny neurons in different layers of the human entorhinal cortex is variable. Vertical extension of dendritic branches to adjacent layers supports the idea that inputs terminating in a specific lamina influence target cells located in various entorhinal layers. There appears to be more overlap in the dendritic fields between superficial layers II and III than between the superficial (II/III) and deep (V/VI) layers, thus supporting the idea of segregation of information flow targeted to the superficial or deep layers in the human entorhinal cortex.