Early postnatal development of cholecystokinin-immunoreactive structures in the visual cortex of the cat.

The early postnatal development of cholecystokinin-immunoreactive (CCK-ir) neurons was analyzed in visual areas 17 and 18 of cats aged from postnatal day 0 to adulthood. Neurons were classified mainly by axonal criteria. According to their chronology of appearance neurons are grouped into three neuronal populations. The first population consists of five cell types which appear perinatally in areas 17 and 18. Four of them have axons terminating in layer VI. Neurons with columnar dendritic fields of layers IV and V display a conspicuous dendritic arborization with the long dendrites always arranged parallel to each other. This way they form a vertically oriented dendritic column. The neurons differentiate at around P 2 and are present until the end of the second postnatal week. They disappear possibly by degeneration and cell death. Multipolar neurons of layer VI have long dendrites and axonal domains of up to 800 micron in diameter. Three percent of these neurons send out two axons instead of only one. Neurons differentiate at P 0 and the cell type persists into adulthood. Bitufted to multipolar neurons of layer V constitute a frequent type; 10% of these cells issue two axons. They differentiate at P 2 and the type survives into adulthood. Bitufted to multipolar neurons of layers II/III appear at P 2 and send their axons into layer VI. So, early postnatally an axonal connection from superficial cortical layers to layer VI is established. The cell type persists into adulthood. The fifth cell type of the first population is constituted by the neurons of layer I with intralaminar axons which differentiate at P 2. Although they derive from the early marginal zone, the cell type survives into adulthood. The second population consists of two cell types which appear around the end of the second and during the third postnatal week in areas 17 and 18. Multipolar neurons of layer II have horizontally or obliquely arranged basket axons which, during the second postnatal month, form patches of high fiber and terminal density along the layer I/II border. Neurons with descending main axons issuing horizontal and oblique collaterals of layers II-IV form broad axonal fields. The third population in area 17 is constituted by three cell types: Bitufted neurons with axons descending in form of loose bundles of layers II/III differentiate during the fifth postnatal week. Small basket cells of layers II/III with locally restricted axonal plexuses and somewhat larger basket cells of layer IV appear during the sixth and seventh week.(ABSTRACT TRUNCATED AT 400 WORDS)