Evidence for large-scale astrocyte death in the developing cerebellum.

There is increasing evidence that some glial cells die during normal vertebrate development, but the extent of the death and the types of glial cells that die remain uncertain. We have analyzed pyknotic cells in the developing postnatal rat cerebellum. During the first postnatal week, the majority of pyknotic cells are in the developing white matter where their number peaks at about postnatal day 7 (P7) and then declines sharply. Pyknotic cells in the internal granule cell layer peak at P10, while those in the molecular and external granule cell layers peak later. Both electron microscopy and in situ end labeling of DNA catalyzed by terminal deoxynucleotidyl transferase confirm that the pyknotic cells are undergoing apoptosis. Immunohistochemical staining suggests that 50-70% of the pyknotic cells in the white matter and internal granule cell layer are astrocytes. We estimate that at P7, as many as 50% of the white matter cells die and, of these, more than half appear to be astrocytes.