Parvalbumin-immunoreactive structures in the entorhinal and transentorhinal region of the adult human brain were studied using the avidin-biotin-peroxidase technique. Parvalbumin-immunoreactive neurons and fibers (axons) were present in all layers (layer nomenclature according to Rose, 1927). The density of fibers was high in the islands of the superficial cell layer pre-alpha and in layer pre-beta and still heavier in pre-gamma. In the subjacent lamina dissecans it diminished abruptly and remained low in all layers of the internal principal stratum (layers pri-alpha, -beta, -gamma). This low density of fibers facilitated recognition of axon cartridges in layers pri-alpha and pri-gamma. Axon cartridges were also present within layers pre-beta and pre-gamma but were obscured by the dense fiber network there. Parvalbumin immunoreactivity was observed in the nerve cell soma and throughout the dendritic tree allowing the distinction of numerous nerve cell types. All parvalbumin-immunoreactive neurons belonged to the class of nonpyramidal neurons. Their lipofuscin pigment patterns differed distinctly from that of the pyramidal and modified pyramidal neurons. Based on their location, soma size, and dendritic arborization, they were grouped as large, medium-sized, and small neurons either of the multipolar or bipolar (vertical or horizontal) type. One type could be identified as an axo-axonic neuron, more specifically as a chandelier neuron generating axon cartridges. The dense fiber net within layer pre-gamma suggested the existence of another neuronal type, probably a neuron with an extended axonal ramification. The identified neurons were compared to neuronal types described in the literature from Golgi studies.
