The ultrastructural organization of lamina VI of the spinal cord of the cat. Morphological characterization of the synaptic population.

The synaptic population of lamina VI of the cervical enlargement of the cat spinal cord has been studied analysing 9 different morphological features and determining the incidence of possible combinations. The most important factors to characterize the synaptic boutons were the shape and size of pre-synaptic vesicles and the size of the synaptic boutons. The incidence of features like the density of the synaptic vesicles, the characteristics of the synaptic junction, the occurrence of post-synaptic specializations, the fibrillar or glycogen content of the pre-synaptic bag and the presence of dense-core vesicles, was unimportant. Most of the boutons (69%) contain pleomorphic vesicles (P boutons), 22% round vesicles (R boutons) and 9% flattened vesicles (F boutons). These synaptic types were further subdivided on the basis of the size of their synaptic vesicles. Most of the synapses (84%) were symmetrical, without relation to the shape of the pre-synaptic vesicles. Dense-core vesicles were found in 15% of the synaptic boutons and occurred most frequently in R boutons. On the basis of the analysis performed, we propose to classify synaptic boutons of lamina VI in 13 different types.