Quantitative Nissl study of the neuronal types, and recognition of cytoarchitectural subdivisions, within the rabbit periaqueductal gray.

A quantitative analysis of 4,621 Nissl-stained neurons within the periaqueductal gray of the rabbit found that there were four main cell types (stellate/round, ovoid, spindle, and triangular) distributed throughout this complex. Further statistical analysis on these neurons confirmed that there were morphological grounds to subdivide the periaqueductal gray into four cytologically distinct regions: ventral, lateral, dorsal, and medial. Neurons in the narrow medial zone, which completely surrounds the aqueduct, were orientated essentially parallel to the aqueduct. The majority of these neurons were small, ovoid, or spindle in shape, and highly basophilic. The cells in this region had the lowest packing density of those in any periaqueductal gray subdivision. The dorsal subdivision, a small midline region, contains the largest cells of any division and the highest packing density of glial cells. The neurons in this region show no preference for orientation, tend to be round, and are moderately basophilic. Cells in the lateral zone are also quite large and demonstrate a preferred orientation either parallel or perpendicular to the aqueduct. The average cell density within lateral PAG is considerably higher than in other regions. Most of these neurons are round or ovoid, and moderately basophilic. Neurons in the ventral zone are mainly ovoid, of medium size, highly basophilic, and lie fairly sparsely arrayed and are orientated essentially parallel to the aqueductal surface.