Relationships between dendritic morphology and cytochrome oxidase compartments in monkey striate cortex.

In primate striate cortex, staining for the mitochondrial enzyme cytochrome oxidase reveals a regular pattern of intense staining, the blobs, which are surrounded by the lighter stained interblob regions. Neurons in both compartments exhibit profound functional differences: blob cells have color selective, unoriented receptive fields, whereas interblob cells are usually not color selective and have oriented receptive fields. Neuroanatomical tracing studies have shown that blob and interblob cells receive different inputs and participate in different projections. It is not known, however, whether this compartmental organization is also reflected in the dendritic morphology of individual cells. We therefore combined intracellular staining with cytochrome oxidase histochemistry to study the relationship between cell morphology and blob pattern in layers 2 and 3 of macaque striate cortex. Single cells were injected with the fluorescent dye lucifer yellow in lightly fixed tangential sections. Adjacent sections were reacted for cytochrome oxidase to reveal the blobs. The spatial relationship between stained cells and the pattern of the blobs were subsequently determined by aligning the sections by using radially running blood vessels as landmarks. Our results show that pyramidal cells located in blob and interblob regions do not differ in their soma size, spine density, and basal dendritic field structure. This indicates that the characteristic functional properties of the neurons in both compartments do not depend on the morphology of their dendritic trees. Since the elongation of the dendritic fields of blob and interblob cells was also found to be similar, we conclude that cortical orientation selectivity is not generated through elongated dendritic fields. We found several cells with dendrites freely crossing the borders between blob and interblob regions. These cells might correspond to cells with "mixed" receptive field properties, e.g., color selective oriented cells, which in physiological studies were found at the transition from blob to interblob regions. However, there were also a number of cells that respected the borders. A quantitative analysis of the dendritic fields revealed that 67% of the cells located close to the borders have a tendency to confine their dendrites to only one compartment. Thus the pattern of basal dendrites of these cells might be shaped by the parcellation of the striate cortex in blob and interblob regions. These dendritic field asymmetries may help to maintain the segregation at the single cell level into different processing channels in monkey striate cortex.