This article reviews current knowledge on the origin and development of microglia as well as on regulatory factors that influence microglial development within the neural tube during embryogenesis. Ameboid microglia found in the developing neural tube originate from mesodermal precursors derived from the yolk sac. These microglial cells, which exhibit characteristic features of reactive microglia, undergo mitosis in situ in the nervous parenchyma and function as the full-blown phagocytes involved in the removal of cellular debris resulting from neural tube defect or normal cellular turnover. During embryogenesis, the microglia express various cytokines, growth factors and chemokines. Some of those factors together with other local factors may influence the proliferation and activation of microglia in the developing neural tube.