Electron microscopy of the early postnatal development of fibrous astrocytes

Abstract The early postnatal development of fibrous astrocytes in rat optic nerves, has been studied with the electron and light microscopes. At birth, immature astrocytes form approximately 85% of the total cell population, and they have many features in common with the fibrous astrocytes of adult optic nerve. They are stellate cells whose processes form both the glia limitans and the glial sheets which group the axons into fascicles. However, in respect to their cytoplasmic characteristics, immature and mature astrocytes differ considerably. Early postnatal astrocytes have a more electron‐dense cytoplasmic matrix containing an extensive array of organelles. Many microtubules occur in the processes of immature astrocytes, but the characteristic filaments of adult fibrous astrocytes are very sparse. During development, the number of microtubules decreases while the filaments increase until all cytoplasmic areas not occupied by organelles are filled by filaments. This suggests that filaments may be derived from the breakdown of microtubules. This and other possible functions of microtubules in developing astrocytes are discussed.