The time of origin for astrocytes and oligodendrocytes in rat optic nerve was studied by 3H‐thymidine autoradiographic techniques similar to those used in dating the time of origin for neurons. This study shows that astrocytes are formed throughout late embryonic and all of postnatal development, while oligodendrocytes are generated only during the postnatal period. A few astroglia undergo their final cell division as early as 15.5 days of gestation, but most astrocytes are not generated until the first week of postnatal development. Although the final cell division for more than half of the astrocytes takes place before the end of the first postnatal week, fully mature, fibrous astrocytes are not observed in electron micrographs until after 14 days of age. This time lag implies that the differentiation of these early generated cells takes place gradually over a 2‐to 3‐week interval. Oligodendroglia begin their final division a day or two before the onset of myelination (6–7 days postnatal), but the vast majority are produced during the period of myelinogenesis. After almost all of the axons have been myelinated, oligodendrocytes are still being generated in small numbers. These late forming cells are generally less differentiated in appearance than those formed earlier; this suggests that the degree of differentiation of oligodendrocytes may be dependent upon the number of axons available for myelination. As with astrocytes, oligodendrocytes show a lag of about two weeks from the time of final cell division until they transform into morphologically differentiated cells. In transverse sections of the optic nerve heavily labeled neuroglia are randomly distributed, indicating there are no temporal‐radial gradients for the individual cell types. This observation taken together with the other information obtained from the present and the previous study (Skoff et al., '76) strongly suggest that the factors controlling gliogenesis are different from those governing neuronogenesis.
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