16.7:
Nervous Tissue: Glial Cells
Glial cells or neuroglia are non-neuronal cells in the nervous system that outnumber neurons. These include astrocytes, oligodendrocytes, microglia, and ependymal cells in the CNS, and satellite and Schwann cells in the PNS.
Astrocytes are star-shaped cells with cytoplasmic processes that contact neurons, capillaries, and the pia mater— the covering of the brain and spinal cord.
Astrocytes support neurons, help to form the blood-brain barrier, regulate the concentration of ions, such as K+, and recycle released neurotransmitters.
Oligodendrocytes have flat cytoplasmic processes that wrap around axons, producing myelin, which protects and electrically insulates axons.
Microglia are small phagocytic cells that remove dead cells and pathogens from the CNS.
Ependymal cells are cuboidal or columnar cells that line the CNS ventricles and central canal. They contribute to cerebrospinal fluid production and monitor its circulation.
In the PNS, satellite cells surround and support neuronal cell bodies and regulate their microenvironment. Schwann cells or neurolemmocytes either myelinate single peripheral axons or enclose several unmyelinated axons. They also participate in axon regeneration.
16.7:
Nervous Tissue: Glial Cells
Glia, or neuroglia, are vital support cells that assist neurons in their functions. The term "glia" originates from the Greek word for "glue," reflecting their role in holding the nervous system together. These cells can be categorized into six types: four in the central nervous system (CNS) and two in the peripheral nervous system (PNS).
The CNS glial cell includes the astrocytes, the oligodendrocytes, the microglia, and the ependymal cells.
Astrocytes are star-shaped glial cells that interact with neurons, blood vessels, and the pia mater, a connective tissue covering the CNS. Their primary role is maintaining chemical balance, removing excess signaling molecules, contributing to the blood-brain barrier, and guiding neuronal growth and connections during embryonic development. Oligodendrocytes, smaller than astrocytes, form and maintain the myelin sheath around CNS axons. This myelin sheath is a multilayered lipid and protein covering that insulates axons. Microglial cells or microglia function as phagocytes, removing cellular debris and damaged nervous tissue. Ependymal cells, arranged in a single layer, line the brain's ventricles and the spinal cord's central canal. These cells produce, monitor, and help circulate cerebrospinal fluid. They also form the blood-cerebrospinal fluid barrier.
In the PNS, Schwann cells encircle axons, similar to oligodendrocytes, forming the myelin sheath around them. However, while a single oligodendrocyte can myelinate several axons, each Schwann cell myelinates only one axon. Finally, satellite cells surround the cell bodies of neurons in the PNS ganglia. They provide structural support and regulate material exchange between neuronal cell bodies and interstitial fluid in the PNS.