Neuroglia in the healthy brain

The nervous tissue is composed of neurons and neuroglia, which by working in a tightly coordinated manner, define the function of the nervous system. Neuroglia, defined as homeostatic and defensive cells of the nervous system, are highly heterogeneous in form and function and are endowed with a remarkable plasticity that allows life-long adaptation to environmental challenges. Neuroglia of the peripheral nervous system are represented by myelinating, nonmyelinating, perisynaptic, and cutaneous Schwann cells, satellite glia of sensory and sympathetic ganglia and enteric glia of the enteric nervous system. Neuroglia of the central nervous system (CNS) are classified into macroglia and microglia. Macroglia in turn are represented by astroglia and oligodendroglia. Astroglia represent an extended class of homeostatic glial cells, which include astrocytes (protoplasmic, fibrous, velate, and marginal), radial astrocytes (Bergmann glial cells, glia-like nervous stem cells, and tanycytes), and ependymoglia. The oligodendroglial lineage is mainly responsible for myelination and support of central axons and is represented by oligodendrocytes and oligodendrocyte precursor cells. Microglia are the cells of nonneural, myeloid origin that invade the neural tube early in embryonic development. These cells are tissue macrophages adapted to the nervous system requirements. Microglia contribute to physiology of the nervous tissue and to the innate immunity and defense of the CNS.