Corticopontine projection in the guinea pig. Electron microscopic study

Guinea pigs were subjected to unilateral thermocoagulation of the frontal, parietal, temporal and occipital cortex, and were allowed to survive 4 and 7 days. Routine electron microscopic technique was employed to examine orthograde degenerative changes in the ipsilateral pontine nuclei. Following four days survival the degenerating corticofugal synaptic boutons (d.s.b.) exhibited features attributed to all three basic degeneration types: dark, filamentous, and light. Most of the synaptic boutons arising in the frontal, parietal and occipital cortex display filamentous degenerative changes, and measure approximately 3 micrometers. The temporopontine axons terminate as dark d.s.b. Also, some d.s.b. following frontal ablation (collaterals of the corticospinal tract?), and a small number of occipitopontine d.s.b. (visual associative cortex?), develop dark degenerative changes. Most of the dark d.s.b. measure less than 2 micrometer. The light d.s.b., with mean diameter 2 micrometer, are rarely found following frontal and occipital lesions. Following 7 days survival almost exclusively dark d.s.b. are to be observed--a great part of them, apparently, representing the late stage of the evolution of the filamentous and light degeneration. No d.s.b. were encountered in the pontine nuclei contralateral to the cortical lesion. In good agreement with preceding studies in other animal species, the present study provides a morphological evidence for a complex, multichannel relationships between the various regions of the cerebral cortex and the pontine nuclei.