Morphology and axon terminal pattern of glutamate decarboxylase-immunoreactive cell types in the white matter of the cat occipital cortex during early postnatal development.

During early postnatal development glutamate decarboxylase (GAD)-immunoreactive (ir) neurons are present in the white matter of the kitten occipital cortex. Most neurons are located below layer VI in the 'upper subplate' zone, others are located deeper in the white matter. In animals at postnatal (P) days 10 and 20 we classified two cell types on the basis of their axonal pattern. One type, the axonal loop cells, displays loops of 180 degrees formed either by the main axonal stem or by a major recurrent collateral. The neurons do not form terminals in the white matter. The other type, the local axon cells, have frequently branching axons giving rise to terminal varicosities contacting other white matter neurons in a basket-like manner. The local cells form terminal plexuses which occupy the white matter of younger kittens and are concentrated in a 100-150 micron wide zone subjacent to layer VI. In P48 kittens, density and width of the plexus is reduced and axonal loop cells and the local axon cells have disappeared. Some GAD-ir white matter neurons observed at this age have large fusiform somata and straight projecting axons. The origin and fate of the early postnatal white matter neurons will be discussed.