Quantitative light and electron microscopic analysis of cytochrome oxidase‐rich zones in V II prestriate cortex of the squirrel monkey

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Abstract
Area 18 of V II of the prestriate cortex of the squirrel monkey was examined at both the light and electron microscopic (EM) levels for cytochrome oxidase (C.O.) activity. At the 17/18 border, the intense C.O. staining of lamina 4 abruptly ended and a new pattern continued for approximately 6 mm into the adjacent prestriate cortex. Here, periodic puffs of high C.O. activity appeared in laminae (lam.) 2 and 3, with the highest activity in lower 3 (3B) extending slightly into upper 4. There was a hint of a columnar pattern in that lam. 4 and especially 5 below the puffs were slightly more reactive than adjacent areas. A thin band of activity could also be seen in upper 5 (5A) and another one between 5 and 6. Tangential sections revealed that the puffs were arranged in alternating wide and narrow rows that radiated orthogonally from the 17/18 border. The puffs in the wider rows tended to be larger (700–1,100 μm in diameter) than those in the narrow rows (400–890 μm in diameter). The center‐to‐center spacing between the puffs was approximately 1,100 μm. Both C.O.‐reactive and nonreactive stellate and pyramidal cells were found between lam. 2 and 6. Quantitative analysis of the supragranular layers indicated that the mean area of reactive neurons was significantly larger than that of nonreactive neurons in both the puffs and interpuff (nonpuff) regions. The relative density of reactive neurons was also significantly greater than that of nonreactive neurons, and was highest within the puffs. At the EM level, reactive neurons were medium to large pyramidal cells as well as medium‐sized stellates with mild to severely indented nuclei and darker cytoplasm filled with reactive mitochondria. The majority of small stellates with scanty cytoplasm and few mitochondria were nonreactive Extensive quantitative analysis of mitochondrial number and level of reactivity in different neuronal profiles indicated that the number and area of darkly reactive mitochondria was significantly higher in the puffs than in the nonpuffs, and that the majority of them resided in dendritic profiles. Between a third to half of the mitochondria in axonal profiles were darkly reactive, the frequency being slightly higher in profiles with flattened vesicles making symmetrical synapses than those with round vesicles making asymmetrical synapses. Mitochondria in axonal trunks and myelinated axons contributed to only a small percentage of the total population. Glial cells, in general, were not very reactive. Quantitative analysis of synapses indicated that there were twice as many asymmetrical synapses as symmetrical synapses in the puffs, while the difference was much less in the nonpuffs. The majority of the synapses were axodendritic, about 1–3% were axosomatic, and less frequent ones were somatodendritic and serial synapses. The frequency of asymmetrical axosomatic synapses was much higher in the puffs than the nonpuffs. The results indicate that an organized array of metabolically active zones exists in the supragranular layers of the prestriate cortex, and that it is made up of a higher percentage of reactive neurons, reactive dendritic arborizations of local and/or deeper neurons, and axonal terminals of intrinsic and/or extrinsic sources. There is also a higher concentration of asymmetrical synapses suggestive of a greater degree of excitatory synaptic influence on both the pyramidal and nonpyramidal interneurons there. These findings may form the structural basis for some of the physiological properties recently discovered in the primate visual cortex.