Morphological organization of the neuropil in laminae II-V of rabbit visual cortex.

A combined light- and electronmicroscopic study of tangential serial sections through the visual cortex of the rabbit has been performed in order to find out whether or not the vertical bundles of apical dendrites in laminae IV/V and II/III on one hand and the areas between these bundles on the other differ with respect to composition and/or spatial organization of the neuropil. Lightmicroscopically only thick profiles such as apical dendrites of pyramidal cells and myelinated axons contribute to the structural characteristics of the neuropil. The appearance of the areas between the dendrite bundles is determined by the presence or absence of radiate bundles of myelinated axons. Lamina-dependent variations were seen in the neuropil of the dendrite bundles as well as in that of the areas between them. Ultrastructurally, the dendrite bundles and the areas between them were observed to be different also with respect to the distribution of the various types of small dendritic profiles and thin axons. The neuropil within the dendrite bundles except for the shafts and thick branches of apical dendrites contains thin unmyelinated axons and numerous spines of apical dendrites contracted by axon terminals. Small irregularly shaped dendrites are few in number. The neuropil between the dendrite bundles contains a larger number of thin unmyelinated axons than that within the bundles, and, instead of spines of apical dendrites, small irregularly shaped smooth and spiny dendrites represent the prevailing postsynaptic structures. Hence, areas within dendrite bundles differ from areas between them by thickness and orientation of their profiles and by the quantitative relation of the various kinds of processes accumulating in each compartment.