Quantitative morphological changes in the superior colliculus and the parabigeminal nucleus in the bilaterally microphthalmic rat.

Quantitative morphological changes in the superior colliculus (SC) and the parabigeminal nucleus (PB) were studied in hereditary bilaterally microphthalmic rat, which lacks the optic nerve completely. Volumes of the retinorecipient superficial collicular layers of SC (SCS) to the central gray matter were decreased by 35% with respect to the normal. However, the cell density in SCS was increased as much as 150% of the normal. The stratum griseum superficiale was packed densely and irregularly with small-sized round nerve cells. The stratum opticum of the mutant rat appeared as a narrow band with few fiber components, but it contained some medium-sized polygonal neurons. No significant changes were found in the deeper layers of the microphthalmic SC. Bilaterally microphthalmic PB reduced both its volume and cellular density per unit area (about 30 and 75% of the normal, respectively). Furthermore, in contrast to the normal rat, the mutant PB could not be subdivided into the dorsal, middle and ventral subgroups.