Summary
Three types of glial cells can be recognized on the basis of their form, the size and shape of the nucleus, and distinctive cytoplasmic characteristics when reconstructed from serial electron micrographs of the toad spinal cord. They correspond to astrocytic neuroglial cells, oligodendrocytes, and microgliacytes described with the light microscope.
Astrocytic neuroglial cells are most common near the central canal. Abundant cytoplasm containing fibrils surrounds the large, darkly rimmed nucleus and extends into the long, radially oriented processes. The latter give rise to many fine laminate extensions which cover the surface of nerve cells, are intercalated between components of the neuropile, and are united to one another by tight junctions. Although the processes terminate at the pia in large end-feet, similar enlargements have not been seen near the surface of capillaries.
Cells resembling type I oligodendrocytes have long, thin, processes of small diameter, dense cytoplasm, and round nuclei which commonly bear one or more invaginations. Although small round processes similar to those of oligodendrocytes form the outer cytoplasmic tongue of myelin it was not possible to trace them far enough to demonstrate continuity with oligodendrocyte somata. Inner cytoplasmic tongues reconstructed from serial sections are continuous with nodes of Ranvier and have a form resembling the tubular reticulum of type IV oligodendrocytes. The fact that the tubular reticulum is located on the inner surface of myelin and lacks cytoplasmic continuity with the outer tongues suggests that type IV oligodendrocytes are not identical to Schwann cells of peripheral nerves as they have been classically described.
Cells identified as microgliacytes have dark, elongate nuclei, scant perinuclear cytoplasm containing numerous dense bodies, clear vesicles and multivesicular bodies, and characteristic irregular processes. It is usually possible to distinguish microgliacytes from oligodendrocytes without analysis of serial sections by characteristic features of the cytoplasm and by the elongate shape of the nucleus.
Capillaries in the toad spinal cord resemble venules. They are larger than most mammalian capillaries, are surrounded by collagen, and are not invested by a continuous layer of astrocytic neuroglial cell processes. Cells tentatively identified as pericytes are frequently associated with them, but they are not separated from the surrounding neuropile by a basal lamina.
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We wish to express our gratitude to members of the Department of Ultrastructure for their cooperation. We also wish to thank Dr. Omar Trujillo-Cenóz for his advice and for his review of the manuscript.
Supported by a Postdoctoral Fellowship from the Cerebral Palsy Research and Educational Foundation.
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Stensaas, L.J., Stensaas, S.S. Astrocytic neuroglial cells, oligodendrocytes and microgliacytes in the spinal cord of the toad. Zeitschrift für Zellforschung 86, 184–213 (1968). https://doi.org/10.1007/BF00348524
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DOI: https://doi.org/10.1007/BF00348524