An electron microscopic study of the neuroglia during postnatal development of the rat cerebrum

The differentiation of spongioblasts from undifferentiated cells and into neuroglial cells has been studied in the postnatal rat cerebral cortex with the electron microscope. The nuclei of all the non‐neural elements are characterized by clumping of dense chromatin, especially near the nuclear envelope. The undifferentiated cells possess only a thin rim of perinuclear cytoplasm containing few organelles. Spongioblasts are characterized by an increased number of cytoplasmic organelles, mostly mitochondria and Golgi complex, and by the growth of processes. These cells mature and take on either the features of oligodendrocytes or astrocytes. The multiple oligodendrocyte processes are thin and branching and develop intimate contacts with the developing axons. Early stages of myelination are characterized by the envelopment of axons by oligodendrocyte processes which overlap each other to form inner and outer tongues that are cheracteristic of central nervous system myelin. Astrocytes can be distinguished by the presence of cytoplasmic glycogen particles and fibrils. Processes of these latter cells develop intimate relations with the developing blood vessels. A variety of cell was occasionally encountered which is termed the polycystic cell. This cell may be quite large, contains numerous cysts and vesicles of various types and appears to be a form of spongioblast. No cells which would qualify as “microglia” were observed.

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