The fine structure of laminae IV, V, and VI of the macaque spinal cord

The normal fine structure of the deep dorsal horn (laminae IV, V, and VI) of the lumbosacral spinal cords of adult Macaque monkeys was examined. Axonal profiles with rounded synaptic vesicles (R) constitute about one-fourth of the total synaptic population in lamina IV and gradually increase in number to comprise more than a third of the synaptic population in lamina VI. Conversely, profiles with flattened vesicles (F) make up two-thirds of the synaptic population in lamina IV and slightly more than half in lamina VI. Axons which form the central profile (C) of synaptic glomeruli are more common in laminae IV and V than in lamina VI, and constitute about 5% of the total synaptic population. Profiles with large granular vesicles (LGV) are relatively uncommon in all the deep laminae. The vast majority of synaptic contacts are axodendritic. Axoaxonal synapses are formed between F, and R or C profiles, the F profile being the presynaptic component. It is concluded that there are both light and electron microscopic morphological differences among the deep laminae, as well as between the deep laminae and the superficial laminae of the spinal cord. Therefore, it appears that Rexed's original schema for laminar organization of the dorsal horn in the cat is also applicable to that of the monkey. In view of the relative paucity of LGV profiles in the deep dorsal horn, it is suggested that synapses in these laminae which contain monoamines or peptides are more likely to be associated with clear synaptic vesicles than granular synaptic vesicles.