Emergence of callosally projecting neurons with stellate morphology in the visual cortex of the kitten.

Callosally projecting neurons in areas 17 and 18 of the adult cat can be classified into two types on the basis of their dendritic morphology: pyramidal and stellate cells. The latter are nearly exclusively of the spinous type and are predominantly located in upper layer IV. Retrograde transport of the carbocyanine dye DiI, applied to the corpus callosum, showed that, up to P6, all callosally projecting neurons resemble pyramids in the possession of an apical dendrite reaching layer I. At P10, however, callosally projecting neurons with stellate morphology were found. A study was designed to distinguish whether these neurons are late in extending their axons to the corpus callosum or, alternatively, have transient apical dendrites. To this end, callosally projecting neurons were retrogradely labeled by fluorescent beads injected in areas 17 and 18 at P1-P3 and then either relabeled with DiI applied to the corpus callosum at P10 or intracellularly injected with Lucifer Yellow at P57. Double-labeled stellate and pyramidal cells were found in similar proportions to those found for the total, single-labeled population of callosally projecting neurons. It is therefore concluded that callosally projecting spiny stellate cells initially possess an apical dendrite and a pyramidal morphology. At P6, i.e. close to the time when stellate cells appear, layer IV neurons with an atrophic apical dendrite were found, suggestive of an apical dendrite in the process of being eliminated.