Early postnatal development of glial cells in the canine cervical spinal cord

To study qualitative and quantitative changes in the glial cell population of young postnatal dogs, the cervical spinal cords of 20 beagle pups, ranging in age from 1 to 28 days, were prepared for light and electron microscopy. Glial cells in the lateral corticospinal tract were classified and quantified directly on the electron microscope. Quantification was performed by means of a stereological method designed to correct for sampling bias, and glia were classified according to morphological criteria as immature glial cell precursors, light and dark oligodendrocytes, astrocytes, and microglia. Glial cell precursors, which include undifferentiated glioblasts, oligodendroblasts, and astroblasts, predominated in the first few days after birth, constituting 43% of the glial cell population, and then declined to less than 5% by 28 days. Light and dark oligodendrocytes differed morphologically in their electron density and the appearance of their organelles. Light oligodendrocytes increased slightly prior to myelination, and then declined, whereas dark oligodendrocytes continued to increase throughout the 4-week period and became the predominant cell type at 28 days (66%). In contrast to the oligodendroglial population, the sizes of the astroglial and microglial cell populations were relatively stable. This study shows that the population of immature glial cell precursors, abundant at birth in the lateral corticospinal tract, appear to be differentiating primarily into oligodendroglia, because this population exhibits a rapid increase in size, and relatively little change occurs in the astrocyte population. The trends in glial cell development in the dog are similar to those reported for rodents, although there may be some variation in the maturation and activity of oligodendrocytes.