Morphology of neurons in the superficial layers of the rabbit's superior colliculus projecting ipsilaterally to the pons.

Abstract

The superior colliculus (SC) is a laminar structure which can be separated into superficial and deep layers according to morphological and functional criteria. The superficial layers are interconnected with many nuclei, most of which process visual information. Also, these layers are known to project to the pontine grey. The topography of the projection is well established in many mammals but little is known about the morphology of the cells of origin of the projection. The pons of two adult albino rabbits were injected with Rhodamine latex microspheres and the retrogradely labeled cells in the SC were intracellularly filled with Lucifer yellow. The distribution of the labeled cells was organized into two bands. The upper one (71% of the cells) was located in the middle third of the stratum griseum superficiale (SGS). The lower band was less dense (29% of the cells) and located in the lower third of SGS and in the upper layers of the stratum opticum. Neurons corresponding to the upper band were preferentially vertically oriented. In particular, 47% were vertical fusiform, 23.5% globular/pyriform, 17.7% horizontal and 11.8% stellate in their morphology. Neurons corresponding to the lower band did not present so clear polarization. Thus, 57.1% were stellate, 28.6% were horizontal and 14.3% globular/pyriform. On the basis of their morphology, we consider that the cells of origin of the tectopontine projection form a particular population in the SC.