Morphology of identified projection neurons in layer 5 of rat visual cortex.

We studied the morphology of neurons in layer 5 of rat visual cortex (area 17) projecting to the contralateral hemisphere and the superior colliculus. Double labelling with fluorescent tracers indicated that these projections arise from different populations of cells. To reveal the morphology of the cells we stained retrogradely labelled neurons intracellularly in living brain slices. Callosal projecting pyramidal cells have 3-6 basal dendrites and an apical dendrite which never reaches higher than layer 3. Corticotectal cells have 6-8 basal dendrites and a prominent apical dendrite which always forms a large tuft in layer 1. Thus, neurons in the same cortical layer that give rise to different projections also differ in their morphology. However, each population of neurons has a rather stereotyped dendritic branching pattern, despite a large variation in soma size.