MORPHOLOGY OF ASTROCYTES AND OLIGODENDROCYTES DURING DEVELOPMENT IN THE INTACT RAT OPTIC-NERVE

The detailed three-dimensional morphology of macroglial cells was dete rmined throughout postnatal development in the intact rat optic nerve, a central nervous system white matter tract. Over 750 cells were anal yzed by intracellular injection of horseradish peroxidase or Lucifer Y ellow to provide a new perspective of glial differentiation in situ. R etrograde analysis of changes in glial morphology allowed us to identi fy developmental timetables for three morphological subclasses of astr ocytes and oligodendrocytes, and to estimate their time of emergence f rom undifferentiated glial progenitors. Glial progenitors were recogni sed throughout postnatal development and persisted in 35-day-old nerve s, where we suggest they represent adult progenitor cells. Astrocytes were present at birth, but the majority of these cells developed over the first week as three morphological classes emerged having either tr ansverse, random, or longitudinal process orientation. Several lines o f evidence led us to believe that the majority of astrocytes in the ra t optic nerve were morphological variations of a single cell type. You ng oligodendrocytes were first observed 2 days after birth, indicating that they diverged from progenitors at or near this time. During earl y development these cells extended a large number of fine processes, w hich then bifurcated and extended along axons. Later, as myelination p roceeded, oligodendrocytes exhibited fewer processes which grew symmet rically and uniformly along the axons, resulting in a highly stereotyp ic mature oligodendrocyte form. Our analysis of oligodendrocyte growth suggests that these cells did not myelinate axons in a random manner and that axons may influence the myelinating processes of nearby oligo dendrocytes. (C) 1993 Wiley-Liss, Inc.