Morphometric parameters of the superior colliculus of albino and pigmented rats.

The superior colliculus (SC) or optic tectum of mammals consists of seven layers, numbered I-VII from superficial to deep, each of which has distinct connectivity patterns and electrophysiological response properties. The present study is devoted to a morphometrical analysis of neuronal diameters, densities, and numbers in different layers and regions of the SC of albino as well as pigmented rats in order to present a quantitative characterization of the collicular neuronal population involved in the different connectivities and functions of these compartments. The morphometric parameters were calculated from tracings of nuclei and cell bodies by means of Kontron-Videoplan equipment and a Micro PDP 11/23 computer. The mean soma diameter per superior colliculus appears to be 12.0 microns, the average neuronal density 70 cells per 0.001 mm3, and the total number of neurons about 600,000. The mean soma diameter gradually increases from superficial to deep layers (i.e., from 10.0 to 14.0 microns). Cellular density is highest in layer III, the retinal afferent layer (90 cells per 0.001 mm3), and decreases both in more superficial layers (to about 80 in layer I) and deeper layers (to about 44 in layer VII). About 25% of all collicular neurons are situated in layer II whereas layer I contains the lowest percentage of cells (4%). Rostrally within each collicular layer, cellular volumes are about 25% larger than caudally. On the other hand, neuronal densities are rostrally about 38% lower than caudally in all layers except for layers VI and VII. We conclude that collicular neurons, in contrast to collicular axons, are not arranged in distinct layers or clusters but basically establish a random network with only gradual transitions. In this respect, no statistically significant differences were observed between albino and pigmented rats.