The fine structure of laminae I, II and III of the macaque spinal cord
Authors: Ralston Henry J. 
Published: January 2005

The neuronal and synaptic organization of the upper three laminae of the macaque monkey spinal cord have been examined by light and electron microscopy. Using a variety of light microscopic techniques it is possible to demonstrate a laminar pattern in the upper spinal cord which is comparable to that originally defined by Rexed ('52) in the cat. Lamina I contains an array of neurons and neuronal processes oriented chiefly in a horizontal pattern, parallel to the dorsal surface of the spinal cord. Most of the neurons of the lamina are small, 10‐15 μ in diameter, but an occasional cell exceeds 20 μ in diameter. The dorso‐ventral width of this lamina varies from 40‐150 μ, medially to laterally. Lamina II corresponds to the substantia gelatinosa and is readily recognized owing to its paucity of myelinated axons and the abundance of small neurons measuring 8‐10 μ. This layer varies in dorso‐ventral width from 150‐250 μ, medially to laterally, and may be divided cytoarchitecturally into an outer zone in which the cells are densely packed and an inner zone containing somewhat more dispersed neurons. The immediately subjacent lamina III contains a meshwork of small and medium sized axons which clearly separate it from the adjacent substantia gelatinosa. The neurons are about the same size as those of lamina II. The ventral border between lamina III and lamina IV is poorly defined as the differentiation between the two laminae is rimarily determined by the appearance of larger cells scattered in lamina IV. The dorso‐ventral width of lamina III is approximately 250 μ.

The synaptic populations, analyzed by counts of more than 10,000 synapses, were quantitatively determined in the upper three laminae of the seventh lumbar segment of three normal monkeys. Synaptic profiles with round synaptic vesicles were found to be the dominant synaptic population in lamina I and outer lamina II and gradually decrease in number in inner lamina II and throughout lamina III. Conversely, flat vesicle profiles are not numerous in lamina I but increase in numbers throughout lamina II until they become the dominant synaptic population in lamina III. Synaptic profiles with an admixture of large granular vesicles and small agranular vesicles are common in lamina I and outer lamina II, then decrease markedly in number in inner II and throughout lamina III. Synaptic profiles forming the central axon in synaptic glomeruli are present in all three laminae. The vast majority of synapses are axodendritic; axosomatic synapses are generally found only on larger neurons. Axoaxonal synapses are not common in lamina I, but are frequently seen in laminae II and III. Dendritic processes containing synaptic vesicles are present, but not in large numbers.

It is concluded that there are significant morphological differences in the neuronal and synaptic populations in various subdivisions of the three laminae of the upper dorsal horn. The differences in synaptic populations reinforce the subdivision of the upper dorsal horn into horizontal laminae, and the separation of the substantia gelatinosa (lamina II) from the subjacent lamina III. It is likely that these morphological variations underly differences in neuronal functions of the upper three laminae.