Fig. 1.

Axon from a single pyramidal cell in layer II of rat piriform cortex. Note that axon branches extend over nearly the entire length of the cerebral hemisphere and are widely distributed within piriform cortex and other olfactory and nonolfactory areas. The SP cell in posterior piriform cortex was stained by intracellular injection of biotinylated dextran amine in vivo, and the axon was reconstructed through serial sections with a computer microscope system. A, Spatial distribution of axon branches in surface view. The inset at top right shows the illustrated portion of the rat brain (dashed rectangle) and orientation (45° upward rotation); the hatched area is piriform cortex, and theshaded area is lateral olfactory tract.APC, Anterior piriform cortex; PPC,posterior piriform cortex. B, Depth distribution of same axon within PPC. View is parallel to layers after 90° rotation; rostral is at left as in A; branches outside PPC have been removed; size scale is expanded relative to that in A. Roman numerals indicate layers: Ib, association fiber zone in layer I (molecular layer); IIIs, IIId,superficial and deep portions of layer III; dotted lines, superficial and deep borders of layer II (compact cell body layer). Open arrowheads mark branch points for axon collaterals that ascend to layer I. Open arrows inA and B indicate cell body; dendritic tree is not illustrated (Fig. 3). Ant,Anterior; ctx, cortex; nuc, nucleus;olfac, olfactory.