The basilar pontine gray of the opossum: a correlated light and electron microscopic analysis

Neurons within the basilar pontine gray (BPG) of the American opossum can be subdivided into four major nuclei which are named medial, lateral, ventral and peripeduncular in accordance with previous studies. In addition, several smaller subnuclei, such as the median and dorsolateral cell groups, are present, as well as two longitudinal columns of neurons within the ventral nucleus. Neurons in the BPG range in size from 9 to 35 mu and appear randomly distributed so that none of the subdivisions contains exclusively nerve cells of the same perikaryal dimension. Projection neurons as shown in Golgi impregnations have a variable dendritic pattern; those in peripeduncular zones exhibit dendrites closely applied to the surface of the cerebral peduncle, whereas those in other regions generally have a radial type of arrangement. Certain projection neurons can be distinguished on the basis of their dendritic surface, which bears either claw-like protrusions or stalked appendages. Smaller nerve cells measuring less than 18 mu may be intrinsic neurons, since axon-like processes arise from their dendrites and course for some distance near the parent cell before becoming thin and beaded. Ultrastructural observations show profiles of neurons comparable in size to those seen in Golgi impregnations and suggest at least four classes of presynaptic profiles. One category ranges in size from 2 to 8 mu, contains round vesicles (average diameter 450 A) and characteristically forms multiple asymmetric synaptic contacts with several small postsynaptic profiles, some of which appear to be the dendritic claws mentioned above. The other three types of axon terminals measure less than 2 mu in their greatest dimension and are distinguished by their synaptic vesicles; one group containing round vesicles with an average diameter of 380 A, a second group exhibiting larger round vesicles with an average diameter of 500 A and a third group containing flatened or eliptical vesicles. Transection of the superior cerebellar peduncle produces early filamentous and later electron dense degenerative changes in some, but not all, of the larger types of presynaptic profiles. Subsequent to large motor-sensory cortex ablations both filamentous and dark degenerating profiles are simultaneously observed at all survival times. In one case with a cortical lesion restricted to the motor-sensory cortex, mainly dark degenerating terminals are apparent in the ipsilateral pontine gray, whereas in a lesion confined to the visual cortex only filamentous degeneration was observed. It is suggested, therefore, that some of the dark degenerating profiles represent the terminals of collaterals of corticospinal axons and the filamentous boutons are terminal expansions of direct corticopontine fibers.