Comparative anatomy of the ventromedial mesencephalic tegmentum in the rat, cat, monkey and human.

The five component nuclei of the ventromedial mesencephalic tegmentum (VMT) were studied on Nissl stained serial sections of the brain stem of rat, cat, monkey (Macaca nemestrina) and human. Models of the VMT nuclei were constructed to compare their size, shape and disposition across species. For each nucleus in each species the following were calculated: the volume, the number of neurons, the size distribution of neurons, the mean soma size and the packing density of neurons. The morphology of the cells in the different nuclei is also described. The parabrachial pigmented nucleus (PBP) forms, on average, 51% of the VMT volume and cell number. The paranigral nucleus (PN) and the central linear nucleus (LC) formed 19% and 14% of the VMT volume and cell number respectively. The relatively small, but compact interfascicular nucleus (IF) was on average 9% of the VMT volume and cell number and the rostral linear nucleus (LR) formed its remaining 7%. However, in different species the relative prominence varies between species. Thus PBP is the largest of the VMT nuclei in the monkey, PN is particularly well developed in the human, IF contains a particularly large number of cells in the rat, and LR and LC are strongly developed in the cat. This study presents a cytoarchitectonic description of the five nuclei in each species. The distinctive cytoarchitectonic appearance of each nucleus suggests that their functions may differ. This possibility, which is strengthened by evidence that the projections of the VMT nuclei are differential, may need to be considered in the interpretation of the results of experimental investigations using stimulation and/or lesion experiments in the VMT region and in the interpretation of pathological findings in the human brain.