Microglia and cell death in the developing mouse cerebellum

The appearance and distribution of microglia in the developing cerebellum has been examined with the aid of a peroxidase-conjugated lectin derived from Griffonia simplicifolia. This distribution has in turn been correlated with that of pyknotic figures in the same Nissl-counterstained sections, in order to gain an understanding of the role of microglial in the developing cerebellum. Round and ameboid microglia may be recognised in the fetal cerebellum as early as E11. Numbers of microglia increase steadily from that time, with initial concentrations in the region of the dorsal and ventricular surfaces. By P1, concentrations of both pyknotic figures and ameboid microglia begin to appear in the region of the future cerebellar medulla. Ameboid microglia are recognisable in the cerebellar medulla until P10, with particular concentrations where folia branch and in the rostral cerebellar peduncles. After this time only resting microglia are found in the cerebellum. Concentrations of microglia largely match the positions of pyknotic figures throughout development, except at P10 and P14, when cell death is found in the external granular layer without an accompanying concentration of microglia. Electron microscopic examination of the phagosomes of ameboid microglia at P5 and P6 indicates that these cells are mainly concerned with the phagocytosis of entire cells rather than axons. Cell death in the cerebellar medulla may serve to clear pathways for developing cortical afferents and efferents, or to increase the mechanical plasticity of the medulla during cortical folding.