Summary
The postnatal development of neuronal perikarya in layers II–VI of the visual cortex of perfusion-fixed albino rats, 12 h to 180 days old, has been studied by electron microscopy. Particular attention was paid to cells in photographic montages of 75μm wide strips extending through the full depth of the occipital cortex, cut from 100 μm vibratome sections of the brain.
At birth, and during the first few postnatal days, most of the neurons present in the cortex are small, tightly packed ‘indifferent’ cells with scanty cytoplasm containing mitochondria and chiefly free ribosomes; a few presumptive pyramidal cells with a developing apical dendrite and more voluminous cytoplasm can be recognized in deep cortex. Non-pyramidal cells can be identified on postnatal day 6, when although scarce and with immature cytoplasmic features, they already display a more electron opaque chromatin pattern than developing pyramidal cells and receive axo-somatic contacts of Gray's type I.
During the second postnatal week there are conspicuous increases in the maturity of the cells, which acquire a rich complement of cytoplasmic organelles: in general cells situated in the deep cortical plate are larger and better differentiated than those in the superficial plate, and non-pyramidal cells are less well differentiated than the associated pyramidal cells. By the end of the second week, differences in cytoplasmic maturity between superficial and deep, and between pyramidal and non-pyramidal cells are less evident.
Maturation proceeds during the third postnatal week; both types of cells acquire an adult complement of axo-somatic synapses and their mature nuclear and cytoplasmic features, and by day 24 are indistinguishable from their adult counterparts. In keeping with previous Golgi studies of this same cortex, the non-pyramidal cells did not acquire mature ultrastructural features significantly later than the pyramidal cells. A possible correlate of particularly active synaptogenesis and plasticity in the population of nonpyramidal, cells during the third postnatal week (immediately after eyeopening), was that at this time these cells contained very prominent accumulations of granular reticulum, ribosomes and Golgi apparatus, and appeared hypertrophic.
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Parnavelas, J.G., Lieberman, A.R. An ultrastructural study of the maturation of neuronal somata in the visual cortex of the rat. Anat Embryol 157, 311–328 (1979). https://doi.org/10.1007/BF00304996
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DOI: https://doi.org/10.1007/BF00304996