Structural inhomogeneity in the neuropil of lamina II/III in rabbit visual cortex

Due to the bundling of apical dendrites throughout laminae IV-II of the cerebral cortex, two compartments of neuropil are distinguished: (1) the neuropil of the dendrite bundles consisting of closely associated, vertically oriented apical dendrites of pyramidal cells and the fine cell processes between them and (2) the neuropil separating the dendrite bundles. The aim of the present study is to obtain more information concerning differences in composition and arrangement of fine cell processes in the two compartments and to quantify differences in the configuration and size of the extracellular space. To this end, a comparative stereological study of the two compartments has been performed in the upper half of lamina II/III in the visual cortex of 8 adult rabbits. The various constituent elements of the neuropil have been evaluated in electron micrographs with respect (a) to their volume fractions and (b) to their surface fractions, i.e. the surface area of their membranes comprised in a given volume of tissue. It was found that the volume fractions of thin axons, glial processes and spines are significantly higher between than within dendrite bundles, whereas the volume fraction of dendrites is about 50% higher in the compartment of the bundles. However, when excluding the dendrites in both compartments from calculations, the composition of the remaining neuropil within and between dendrite bundles was found to be identical. As far as surface fractions are concerned, significant differences were shown to exist for all kinds of cell processes except dendrites and myelinated axons. Surface fractions of small axons, glial processes and spines are higher between than within dendrite bundles.(ABSTRACT TRUNCATED AT 250 WORDS)