Chandelier and Interfascicular Neurons in the Adult Mouse Piriform Cortex

Jorge A Larriva-Sahd

doi:10.3389/fnana.2010.00148

Abstract

The structure of two neuron types native to the adult mouse piriform cortex (PC) is described. The first cell, termed an interfascicular neuron (IFN), lies between the axon fascicles of layer I. The IFN axon divides dichotomously and daughter fibrils run horizontally in the domain of layer Ia. The frequent apposition of the IFN axon to distal dendrites of the underlying pyramidal cells suggests an en passage synaptic interaction with them. A second neuron observed in layer II, or less frequently in layer III, matched in most respects the structure of the chandelier cell (CC) described elsewhere in the neo- and archi-cortex. In the PC, chandelier cells (PC-CC) display the following peculiarities. First, the PC-CC axonal field distributes in the neuropil of layers II and III and candlesticks are in close apposition to the initial axonal segment of the pyramidal cell, although somatic interactions cannot be rule out. Second, the PC-CC ascending dendrites pierce layer I, receiving short collaterals and boutons en passage from the olfactory axons therein. The possible role of IFN's and PC-CC and their interactions with the adjacent cells is discussed in the broad context of the cellular organization of the PC.

Highlights

The cytological organization of the piriform cortex (PC) has been the subject of numerous Golgi studies
Earlier works (Haberly, 1983) and recent immunohistochemical studies (Zhang et al, 2006; Gavrilovici et al, 2010; Suzuki and Bekkers, 2010a,b) have described native neurons in the superficial layer of the PC, their somatodendritic structure and axonal fields encompassing layers I or II (Price, 1973), differ from those observed in the interfascicular neuron (IFN) described here
Layer III has a limiting, cell-sparse area that is rich in pyramidal cell axon collaterals and interneurons and a deep portion composed of pleomorphic cells that form an indistinct boundary with the underlying endopiriform nucleus or layer IV

Summary

Introduction

The cytological organization of the piriform cortex (PC) has been the subject of numerous Golgi studies. Perhaps the first detailed work on the primary olfactory cortex was that performed by Calleja (1893) a young Ramón y Cajal (1904) protégé and by himself In these studies utilizing prepubertal rodents and carnivores, the PC was defined for the first time as the “cerebral cortex covered by the external root” (i.e., lateral olfactory tract) and was divided into five layers, from pia to white matter: fibrillar zone, molecular layer, layer of small and large pyramids, plexiform or polymorph layer, and white matter. O’Leary documented the existence of interneurons with ascending axons resembling those of Martinotti cells found in the neocortex (see Wang et al, 2004) In his memorable monograph Valverde (1965) highlighted six types of interneurons, utilizing primarily the prepubertal mouse brain. Earlier works (Haberly, 1983) and recent immunohistochemical studies (Zhang et al, 2006; Gavrilovici et al, 2010; Suzuki and Bekkers, 2010a,b) have described native neurons in the superficial layer of the PC, their somatodendritic structure and axonal fields encompassing layers I or II (Price, 1973), differ from those observed in the interfascicular neuron (IFN) described here

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