Identification of pericellular baskets in the cat striate cortex: Light and electron microscopic observations after uptake of horseradish peroxidase

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Abstract
This paper presents one of the few descriptions and the first light and electron microscopic reconstruction of a pericellular basket in the cat striate cortex. A pericellular basket and its related pyramidal cell in cortical layer V were labelled by nearby injection of horseradish peroxidase. The basket and pyramidal cell, contained in a cortical transverse section embedded in plastic, were analysed by light microscopy. Subsequently, similarly transverse serial sections of 4 μm thickness were cut through a basket and pyramidal cell, and these sections were also documented by light microscopy. Finally, serial ultrathin sections were made from each 4 μm section and analysed electron microscopically. Graphic reconstructions of the basket and its pyramidal cell were then made from the 4 μm and ultrathin sections. The basket had the characteristic appearance of axon stalks and terminal boutons intertwined around the soma and basal dendritic processes of the pyramid. Basket boutons contained mitochondria and a large number of tightly packed dark vesicles, round or oval in shape. Subsynaptic densities were discrete and extended for only a fraction of the basket bouton/pyramidal cell contact. With some reservations, these synapses can be classed as symmetrical. The basket was connected to several axons, and these were myelinated right to the level of the stalks and boutons. This was an unexpected finding since pericellular baskets have been shown to arise from local interneurons.