The cytoarchitecture, cytology, and synaptic organization of the basilar pontine nuclei in the rat. II. Electron microscopic studies.

The basilar pontine gray in adult rats was subjected to electron microscopic examination in order to characterize: 1) certain general cytologic features, 2) the principle categories of presynaptic profiles, and 3) the postsynaptic targets of the various categories of presynaptic boutons. General cytologic features included the presence of neuronal somata exhibiting either a smooth contoured nucleus and central nucleolus or an irregularly contoured nucleus with numerous invaginations and an eccentric nucleolus. In addition, dendritic protrusions with variable morphology were apparent as well as a class of dendrites and somata with a curious dense, granular matrix. Four general types of presynaptic profiles were distinguished and were classified as: round vesicle boutons, elongate vesicle, pleomorphic vesicle, and dense core vesicle boutons. The category of round vesicle boutons formed asymmetric synaptic contacts and included three subtypes. One group contained vesicles with diameters of 25-35 nm, a second group contained larger (40-60 nm) round vesicles, and a third group of much larger boutons (2.5-6.8 microns) exhibited the smaller round vesicles. Another general category of presynaptic profile contained distinctly elongated synaptic vesicles, formed symmetrical active sites, and ranged in size from 0.4-1.7 microns. A third category of synaptic end-structures was distinguished by the presence of pleomorphic synaptic vesicles and microtubules and exhibited a distinctive pale or lucent matrix, while the fourth type was characterized by a mixed population of clear and dense core vesicles. Analysis of the distribution of such bouton types over the surface of pontine neurons revealed that: somata and proximal dendrites were contacted by round, elongate, and dense core vesicle boutons; intermediate and distal dendrites received synapses from all categories of boutons including the large endings with smaller round vesicles, while dendritic spines were contacted by round vesicle boutons (large and small), the large boutons, and elongate vesicle boutons. Synapses were also observed to occur between two vesicle-containing profiles. In such situations, pleomorphic vesicle boutons were found to be postsynaptic to either elongate vesicle boutons or small boutons with round vesicles and at the same time were presynaptic to a dendritic profile.