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The present study was designed to evaluate the possibility of morphological subdivisions within the rat periaqueductal gray by using a statistical cluster analysis system. In addition both qualitative and quantitative data concerning neuronal size, shape, and density were obtained. On the basis of measurements of over 12,000 neurons in two planes of section, the mean neuronal length of cell bodies in this region was 14.82 \u03bcm and the mean neuronal area was 95.59 \u03bcm<jats:sup>2<\/jats:sup>. The mean neuronal density was found to be 16,284 cells per mm<jats:sup>3<\/jats:sup>. Neuronal density decreased from rostral to caudal in the periaqueductal gray. The data obtained from cluster maps suggest the presence of four subdivisions within this midbrain region. The medial subdivision contains the smallest neurons and exhibits the lowest cell density. The dorsolateral and ventrolateral divisions contain the largest neurons while the dorsal division displays the highest packing density. These results are discussed in light of recent receptor binding and immunohistochemical studies of this region.<\/jats:p>","DOI":"10.1002\/cne.902370403","type":"journal-article","created":{"date-parts":[[2005,1,1]],"date-time":"2005-01-01T19:34:37Z","timestamp":1104608077000},"page":"445-459","source":"Crossref","is-referenced-by-count":123,"title":["The midbrain periaqueductal gray in the rat. I. 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