Thalamic projections of the dorsomedial prefrontal cortex in the rhesus monkey (Macaca mulatta).

Cortical aspirations were made of the dorsomedial prefrontal sector in the rhesus monkey and the resultant anterograde and retrograde degeneration plotted. Retrograde changes were mapped using both cresyl violet and modified Fink-Heimer techniques. Following large dorsomedial lesions, small numbers of degenerating fibers were traced through the medial part of the magnocellular ventral anterior nucleus (VAmc) and the ventral part of the internal medullary lamina surrounding the anterior nuclei. Degenerating fibers were also traced medially through the inferior thalamic peduncle and rostrodorsally through the basal thalamic region into the mediodorsal nucleus. All pathways converged on areas in the dorsal part of the parvocellular mediodorsal nucleus (MDpc) containing fine-grain dust and argyrophilic neurons, reactions usually associated with retrograde degenerative changes. Sparse fiber degeneration was also noted in the ventral part of the magnocellular mediodorsal nucleus (MDmc). After a longer survival period, slight to moderate cell loss and gliosis were seen in MDpc along with an increase in the number of degenerating fibers passing through the medial part of VAmc. Rostral dorsomedial lesions resulted in small numbers of degenerating fibers in the medial part of VAmc; no degenerating fibers appeared in the basal thalamic region. Fine-grain dust was noted in the dorsal part of MDpc along with sparse preterminal degeneration in the dorsal and ventral parts of MDpc and MDmc respectively. Following caudal dorsomedial lesions, fiber degeneration was traced through the medial part of VAmc and through the inferior thalamic peduncle and basal thalamic regions to areas of degenerating preterminals in the dorsal part of MDpc. Sparse fiber degeneration was also noted in MDmc. No evidence of cell loss and gliosis or increased numbers of degenerated fibers was noted following longer survival periods. Dorsolateral and orbital lesions resulted in large areas of fine-grain dust, argyrophilic neurons, and severe cell loss in MDpc and MDmc respectively. A large combined dorsolateral and orbital lesion made 32 days prior to sacrifice of the animal resulted in coarse fiber degeneration in the medial part of VAmc and in the anterior nuclear capsule. Severe cell loss and fiber degeneration were evident in the entire MD. These results suggest that the rostral dorsomedial prefrontal sector receives projections from MDpc while the caudal sector does not. This projection courses dorsally and rostrally through the ventral part of the capsule surrounding the anterior nuclei and into the medial part of VAmc. The entire dorsomedial sector projects sparsely to both MDpc and MDmc. The projection from the rostral medial surface passes through the medial part of VAmc while that from the caudal surface reaches MD both by the dorsal approach through VAmc and through a ventral approach via the inferior thalamic peduncle and the basal thalamic region.