Morphology of normal and deafferented neurons in the chick ectomamillary nucleus

Neurons in the ectomamillary nucleus (EMN) undergo both atrophy and cell death following eye removal at hatching. It is not known whether all EMN neurons are affected uniformly by transneuronal atrophy or whether cell loss is an artifact due to misidentification of atrophied neurons as glia. In a preliminary morphological study, four types of neurons were found in the EMN by using the rapid Golgi method: A large multipolar neuron (type I); two medium-sized spindle-shaped neurons, one possessing many dendritic branches (type II) and the other possessing few dendritic branches (type III); and a small round neuron (type IV). Horseradish peroxidase (HRP) was then injected into two of the EMN projection fields in enucleated chicks in order to label retrogradely as many EMN neurons as possible. Types, I, II, and III neurons were identified both in the control and experimental EMN. The three types of backfilled neurons showed different degrees of transneuronal atrophy ranging from 12 to 47%. The type IV neuron, which could not be backfilled, was inferred to atrophy by 33%. Substantial differences in transneuronal atrophy, therefore, exist among the different types of neurons within the same nucleus. Since no glialike neurons could be retrogradely labeled it was concluded that there is a true neuron loss in the EMN following eye removal rather than mistaken identification of neurons as glia.