Development of distinct cell types in the feline red nucleus: a Golgi-Cox and electron microscopic study.
The feline RN contains neurons which fall into three size categories: giant (40--80 micrometer), medium (25--35 micrometer) and small (6--20 micrometer). These three populations of rubral neurons are distinguishable on the basis of a number of ultrastructural criteria and form classes not dissimilar from the traditional three divisions of cell types. Each of the three populations of rubral neurons can be further divided into three subgroups on the basis of a large number of configurational criteria discernible by the Golgi-Cox method. Each of these nine cell types are clearly separate, distinguishable by at least three criteria, and are found in different regions of the RN. It is shown that in the 5-day prenatal kitten, rubral neurons are already organized into the aforementioned three size categories. At this age most of the subpopulations are also distinguishable by the Golgi-Cox method. However, the giant rubral neurons (about 30 micrometer) and the medium sized cells (about 20 micrometer) are much smaller than in the adult cat. The dendrites elaborate many fine processes which emerge from multiple varicosities. The neuropil differs strikingly from that of the adult in that the vast majority of axons are small and unmyelinated. A number of changes in the RN are apparent as the kitten matures. The larger rubral cells undergo configurational changes before the smaller neurons, yet the giant cells continue to grow for a longer period of time. In the perinatal period, the extent of arborization of the dendritic trees diminishes, the number of spines decreases, and the long dendritic spines shorten. Somatic spines first appear in the giant cells at about one week after birth. In prenatal kittens, large cells frequently elaborate a tuft of fine branching processes in one region of the soma. These tufts later diminish in size and disappear by one week postnatal. Recent investigations (Pompeiano, '59; Condé, '66; King et al., '73' Sadun, '75) indicate that the RN of the cat is highly organized and very heterogenous. Afferent terminals are restricted to certain regions of certain cell types which are themselves specifically located within the RN. This specificity is apparent in perinatal kittens, despite the manifest immature appearance of the RN.