Microglial cells are the scions of foetal macrophages which invade the neural tube early during embryogenesis. The nervous tissue environment instigates the phenotypic metamorphosis of foetal macrophages into idiosyncratic surveilling microglia, which are generally characterised by a small cell body and highly ramified motile processes that constantly scan the nervous tissue for signs of changes in homeostasis and allow microglia to perform crucial homeostatic functions. The surveilling microglial phenotype is evolutionarily conserved from early invertebrates to humans. Despite this evolutionary conservation, microglia show substantial heterogeneity in their gene and protein expression, as well as morphological appearance. These differences are age, region and context specific and reflect a high degree of plasticity underlying the life‐long adaptation of microglia, supporting the exceptional adaptive capacity of the central nervous system. Microgliocytes are essential elements of cellular network formation and refinement in the developing nervous tissue. Several distinct patrolling modes of microglial processes contribute to the formation, modification, and pruning of synapses; to the support and protection of neurones through microglial–somatic junctions; and to the control of neuronal and axonal excitability by specific microglia–axonal contacts. In pathology, microglia undergo proliferation and reactive remodelling known as microgliosis, which is context dependent, yet represents an evolutionarily conserved defence response. Microgliosis results in the emergence of multiple disease and context‐specific reactive states; in addition, neuropathology is associated with the appearance of specific protective or recovery microglial forms. In summary, the plasticity of microglia supports the development and functional activity of healthy nervous tissue and provides highly sophisticated defences against disease.

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Although they are usually viewed as the first line of defense in response to injury, inflammation, or disease (Nayak et al., 2014), microglia, the brain immunocompetent cells, have emerged as major contributors of brain development as well as homeostasis throughout life (Cuadros et al., 1994; Lenz & McCarthy, 2014). Microglia are myeloid cells originating from the yolk sac that start colonizing the developing brain around embryonic day 9.5-10.5 in rodents (Ginhoux et al., 2010) and as early as 4.5 gestational weeks in humans (Verney et al., 2010). They enter the brain via the ventricles, meninges, and developing blood vessels under an immature ameboid form (Thion & Garel, 2017). They migrate and proliferate through the nervous system parenchyma (Nikodemova et al., 2015; Tay et al., 2016), then progressively transition to ramified microglia as they reach their final location (Lannes et al., 2017; Thion et al., 2018). After the formation of the blood-brain barrier, microglia proliferate by self-renewal throughout life (Ajami et al.,

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