Quantitative Analysis of the Superior Colliculus and the Parabigeminal

Quantitative morphological changes in the superior colliculus (SC) and the parabigeminal nucleus (PB) were studied in hereditary unilaterally microphthalmic rats. The mutant animals have one vestigial and one almost normally developed eyeball. The former eye completely lacks the optic nerve. The proportion of uncrossed to crossed retinal fibers in the mutant rats was estimated at about 6%. Conspicuous changes in SC were found only on the contralateral side of the abnormal eye. Relative volume of the superficial layers of SC (SCS) to the central gray matter (SGC) was decreased to 50% of the normal. However, the cell density in SCS increased up to 130-190% of the normal. The str. griseum superficiale consisted mainly of small roundish neurons in dense and irregular arrangements. Small amounts of fibers were observed in the medial 1/3 of the str. opticum, but very few in the lateral 2/3. No significant changes were found in the deeper collicular layers of the mutant rats. Unilaterally microphthalmic PB was subdivided normally into three parts: the dorsal (PBd), middle (PBm) and ventral subdivisions (PBv). The relative volumes of each PB subdivision on both sides had decreased to 57 to 65% of the normal, except for that of PBv on the contralateral side to the anomalous eye (26% of the normal). Cell densities of PB were slightly lowered (76-84% of the normal) in PBm and PBv, but not much in PBd (88-90% of the normal) on both sides.