Morphology and quantitative changes of transient NPY‐ir neuronal populations during early postnatal development of the cat visual cortex

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Abstract
The early postnatal development of neuropeptide Y-containing neurons in the visual cortex of the cat was analyzed. Immunohistochemistry reveals several stages of morphological differentiation and degeneration. Completely undifferentiated neurons have very small somata with nuclei surrounded by a thin rim of cytoplasm and processes unclearly differentiated into dendrites and axons. Processes bear growth cones. Differentiating neurons show an increase in soma size and complexity of processes. Axons are recognizable. Fully differentiated neurons have well-defined axonal and dendritic patterns. Degenerating neurons are identified by thick, heavily beaded processes covered by hairy appendages and vacuolar inclusions in the somata. Cell death is expressed by shrunken somata and lysed, fragmented processes. According to their postnatal time course of differentiation and/or degeneration, NPY-immunoreactive neurons, which form several morphological distinct cell types, are grouped into 3 neuronal populations, (1) Pseudopyramidal cells, bitufted “rectangular” cells with wide dendritic fields, unitufted cells, and small multipolar cells are located in the gray matter and have a rather primitive morphology resembling cell types found in lower vertebrate cortex and tectum. They constitute a first transient neuronal population, because all neurons are fully differentiated at birth and become largely eliminated by postnatal day (P) 12. (2) Axonal loop cells are mainly located in the white matter. Their most prominent feature is an often long hairpin loop formed by either the main axon itself or by a major collateral. The axonal branches pass through the cortex to connect the white matter and layer I. Axons do not form local plexuses and terminal elements in the gray matter. Neurons differentiate perinatally, form a first peak from P6 to P10, followed by a decrease in cell number and innervation density at P12, followed by second peak from P15 to P20. After P20 the number of axonal loop cells steadily decreases, and they become eliminated by P48. (3) A third population consists of neurons with a higher degree of axonal ramification and a variety of axonal patterns. Early members are located mainly at the layer VI/white matter border, differentiate during the first postnatal week, and give rise to a diffuse innervations of the gray matter without forming specific terminal elements. Some of the early axonal patterns persist into adulthood, whereas others are not found in the adult brain. Late members of the third population are located mainly in gray matter layers VI and V, and differentiate from P18 onward into short axon basket cells possessing the typical perisomatic terminal bendings. They form the adult fiber and terminal strata until P48. So, the third population comprises transient as well as persisting neurons, and finally it represents the adult neuronal stock. The origin of transient cell types from the cortical subplate, the origin of persisting neurons from the cortical plate, and some functional implications following from these results are discussed.