Neurons of the basolateral amygdala: a Golgi study in the opossum (Didelphis virginiana)

The cytoarchitecture of the opossum basolateral amygdala was studied using Golgi techniques. The neuronal morphology was similar in all nuclei of the basolateral complex, an three distinct cell classes were recognized. Class I neurons, which vary in size in different nuclei, have spiny dendrites and long, projection axons. Axon hillocks and initial axonal segments often have spinous protrusions, while more distal portions of the axon give off several beaded collaterals that arborize primarily in the vicinity of the cell. Class II neurons are smaller, spine-sparse cells that are found in all nuclei of the basolateral amygdala but are greatly outnumbered by class I neurons. Axons branch and give off beaded collaterals which form a moderate to dense arborization within the dendritic field of the cell. Class II neurons exhibit considerable morphologic variability including one subtype that resembles the chandelier cell of the cerebral cortex. Varicosities (1.0 - 1.5 micrometers swellings) found along the axonal collaterals of these amygdaloid chandelier cells do not have a uniform distribution but tend to be aggregated. Segments of the collaterals displaying such clustered varicosities sometimes form nest-like entanglements. Clusters of varicosities have been observed forming multiple contacts with initial segments of class I axons. Class III neurons are neurogliaform cells which have many short, varicose dendritic branches that contact dendrites of class I neurons. Only the initial portions of their axons were impregnated. This study indicates that many of the cell types seen in the generalized, metatherian opossum are similar to those described in more specialized, placental mammals. This is the first description of amygdaloid chandelier cells and their contacts with the spiny initial segments of class I projection neurons.