Electron microscope study of development of cat superficial neocortex

Abstract

Electron microscope observations have been made on neuron cell bodies, dendrites and synapses in the superficial neocortex of near-term fetal, neonatal and 1- and 2-week-old kittens. Differences in cell bodies in fetal, newborn and older kittens are seen in the volume of cytoplasm and characteristics of the cisternae of the endoplasmic reticulum and the relationship of RNP rosettes to intracytoplasmic membranes. Cell bodies and dendrites with characteristic tubules are densely packed in the immediate neonatal period. Close apposition (150–200 A) of dendrites and cell bodies is commonly seen at this developmental stage. Dendritic processes in the neuropil are relatively large and homogeneous in fetal and newborn kittens. Dendritic terminals 300 A to 400 A in diameter are not observed until the sixth to ninth postnatal day. The development of such fine processes accounts in part for the elaboration of neuropil and the postnatal decrease in cell body density. Axodendritic synapses with characteristics similar to those observed in adult animals are seen in fetal and newborn kittens. Axosomatic synapses are rarely observed until the end of the first postnatal week. The differential development of axodendritic and axosomatic synapses is discussed in terms of the electrophysiological properties of immature cat neocortex.