Microglia Morphological Development At Early Stage Of Life In Rat Brain

Background and Purpose: Microglia are widely distributed in the central nervous system(CNS) . The number of microglia accounts for 5-20% of the total number of glial cells. The origin of microglia are still controversial now. Some people believe that the origin of microglia are from the ectoderm. It was also suggested that microglia are derived from bone marrow hematopoietic stem cells. Hortega's classical view think that such cells are derived from mesoderm. At beginning of embryonic development when blood vessels were formed, microglia invaded into the brain. Microglia are inherent components of the immune cells in the CNS. Under normal circumstances or static state, they lack of phagocytic function, but with pinocytosis and migration. Microglial morphology at resting state are ramified, and activated in many pathological conditions and become "amoeboid-like" which have migration and phagocytic function.The purpose of this experiment was to observe the characteristic changes of microglial morphology at early postnatal period of the rat brain, and provide the basis of further research of their function.Material and Methods: 15 Sprague-Dawley rats of postnatal 1days, 3days, 10days, 14days and 21days were used in the experiment. The rats less than 10days were decapitated rapidly and the brain tissue immersed in 4% paraformaldehyde and fixed for about 12 hours. Rats equal to or more than 10 days were subjected the perfusion with 4% chloral hydrate (1ml/100g), then fixed with 4% paraformaldehyde. 12 hours later, the brain tissue were dehydrated by 20% and 30% sucrose. Constant frozen slices with 10μm thick were tested with nissl staining and immunohistochemical staining for OX-42 microglia. Results: 1. Brain microglia of Sprague-Dawley rats have undergone dynamic changes in morphology at early stage of life from postnatal day 1 to 21. They were round, amoeba-like and promitive ramified at white matter with the numbers increased and reached the peak at postnatal 10. While pleomorphic cells were seen at gray matter, mainly promitive ramified. The number was increased gradually. 2. At postnatal day 21, the promitive ramified and ramified microglia could be seen at white matter, But at gray matter, only mature ramified cells could be observed. 3. Microglial morph- ological changes in cerebellum at such stage were similar with that of the cerebrum. 4. Microglia migration into the brain from ventricle, meninges and blood vessels were all seen in present study and the migration style showed the "tangent-type" and "radial" like.Conclusions: 1.Cerebrum microglia of Sprague-Dawley rats have under- gone continuous and dynamic changes in morphology at early stage of life. The microglia are almost mature at postnatal 21 days. 2. In this developing stage, the amoeboid or promitive ramified microglia migrate along the white matter and reach the peak of migration and proliferation at postnatal 10 days. 3.Microglial morphological changes in cerebellum at such stage were similar with that of the cerebrum.