Cytoarchitecture and saccular innervation of nucleus y in the mouse.

The cytoarchitecture and saccular innervation of the mouse nucleus y were investigated by using Golgi, Nissl, and myelin stains and anterograde axonal transport of horseradish peroxidase. Nucleus y was found to be a compact group of cells in a small fiber-free region dorsal to the restiform body. Qualitative and morphometric analyses showed that most (75%) of the nucleus y neurons could not be reliably subdivided into morphologic subgroups, but varied continuously in soma size (15-25 microns), shape (fusiform to stellate), and number of dendrites (two to four), and had sparsely branched dendrites with an average of 3 to 4 spines per 10 microns of length. Three groups of cells that were identified morphometrically accounted for 10% (type I: large stellate cells), 9% (type II: long-dendrite cells), and 6% (type III: elongated soma cells) of the y neurons. Vestibular nerve axons transporting horseradish peroxidase after injury at their origin in the saccular neuroepithelium were found to form a dense terminal meshwork that was virtually co-extensive with the cytoarchitectonic boundaries of nucleus y. Nucleus y was distinguished from the overlying infracerebellar nucleus on the basis of anatomical, cytoarchitectural, and hodological features.