An electron microscopic study of neurons during postnatal development of the rat cerebral cortex

Abstract

The differentiation and maturation of the neurons of rat cerebral cortex during the first three weeks of postnatal life has been studied with the light and electron microscopes. At birth, the superficial cortex is largely made up of undifferentiated cells, tightly packed together in vertical columns between which the developing process and blood vessels, and relatively extensive extracellular spaces are found. During the first two weeks, these cells differentiate into either neuroblasts and neurons, or into spongioblasts and neuroglia. In the case of neurons but not neuroglia, this maturation occurs in a gradient from the depths toward the surface. The round undifferentiated cells are characterized by an absence of processes, a thin rim of perinuclear cytoplasm containing few organelles, and a peripheral clumping of condensed nuclear chromatin. The differentiating neuroblasts display increasing numbers of cytoplasmic organelles, especially endoplasmic reticulum, evenly dispersed nuclear chromatin, and presumptive apical dendrites. The rough endoplasmic reticulum of neuroblasts become swollen during the transition into neurons during the second week. The endoplasmic reticulum forms subsurface cisterns that transiently evaginate the cell membrane, forming conspicuous diverticuli in the neuropil at the same time that the cells display an increase of cytoplasmic matrix density and increasing numbers of ribosomes. During the three weeks following birth, organelles increase in number and complexity, synapses develop, the extracellular spaces disappear, and the maturing neurons become separated from one another by the growth of neuropil and nonnervous elements. At the end of this three week period the cortical tissue and its neurons are apparently mature and the adult pattern of cortical fine structure is established.The purpose of this paper is to report the ultrastructural findings from a study of developing rat cerebral cortex between birth and three weeks of age and to the limited extent possible, to relate those findings to the light microscopic literature. However, a thorough review and consideration of the enormous literature on cortical histogenesis would not be feasible in a report such as this. Furthermore, there are inherent difficulties in relating electron microscopic studies to this literature. Classical methods of tissue preservation, paraffin embedment, metallic impregnation procedures and aniline stains fail to preserve or stain simultaneously all of the components of central nervous system tissues; and as a result, identification of cell types and interpretation of their relative development is difficult and largely subjective (Uzman, '60). By employing improvements in methods of tissue preservation that have paralleled the development of the electron mi-J. COMP. NEUR., 133: 17-44.croscope, especially in the use of osmium fixation and plastic embedment, there results a significantly different and improved image of developing cortex. It is interesting, however, that the microscopic images of the...

Differential Proteomics of the Cerebral Cortex of Juvenile, Adult and Aged Rats: An Ontogenetic Study

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Local Gene Expression in Axons and Nerve Endings: The Glia-Neuron Unit

“…As mentioned earlier, the most important single reason to consider the axon incapable of protein synthesis was the alleged absence of ribosomes, as determined by conventional EM methods. This view only regarded mature axons, as ribosomes were detected in developing axons (40,294). Hence, it was generally believed that ribosomes disappear as the axon matures (150) and, as a corollary, that the mature axon is devoid of rRNA and mRNA (31,199).…”

The functional status and columnar organization of single cells responding to cutaneous stimulation in neonatal rat somatosensory cortex S1.

“…No direct evidence for lateral inhibition was found although intracellular recordings would probably be necessary to confirm its absence. Caley & Maxwell (1968) have commented on the appearance of columns of cells in their histological investigation of immature cortex, as has for somatosensory cortex of the kitten. However, the width of these workers columns are an order of magnitude less than the apparent width of functional columns found in the present study.…”