Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Extracellular matrix wikipedia , lookup
List of types of proteins wikipedia , lookup
Cell culture wikipedia , lookup
Cellular differentiation wikipedia , lookup
Organ-on-a-chip wikipedia , lookup
Cell encapsulation wikipedia , lookup
Node of Ranvier wikipedia , lookup
Glial Cells Most neurons are surrounded by glial cells (neuroglia), the other cell type found in the nervous tissue. Glial cells are the supportive cells of the nervous system and are 10 times more numerous than neurons. The most well defined role for neuroglia is to provide structure to the delicate nervous tissue. They fill the space between neurons, serving as mortar or “glue” and thus hold nervous tissue together. Unlike neurons, glial cells retain the ability to divide throughout one’s lifetime. When neurons are injured, neuroglia is stimulated to divide and form glial scars. Glial cells have different shapes and sizes and their processes are indistinguishable in contrast to the distinct axon and dendrites found in neurons. There are 6 types of glial cells, 4 types are found in the CNS and 2 types in the PNS. The CNS neuroglia is: astrocytes; oligodendrocytes; microglia, and ependymal cells. The 2 types of glia found only in the peripheral nervous system (PNS) are satellite cells and Schwann cells. CNS Glial Cells Astrocytes are star-shaped neuroglia and are the most numerous cells in the central nervous system. They make up half of all cells in the brain. Astrocytes provide a structurally supportive framework for neurons with their processes wrapping most non-synaptic regions of neurons in gray matter and covering the entire outer surface of the brain to form the glial – pia (connective tissue meninx) interface. Astrocytes help form the protective blood-brain barrier by encircling CNS capillary endothelial cells and stimulating the cells to form tight-junctions. They help to maintain the concentration of chemicals in the extracellular space and remove excess signaling molecules. Astrocytes also react to neural tissue damage by forming scar tissue in the damaged space. Oligodendrocytes are glial cells of the CNS that wrap and insulate axons and give the CNS white matter its characteristic glossy, white appearance. Oligodendrocytes have a large soma with up to 15 processes. The processes reach out to axons of nearby neurons and wrap around them (like wrapping tape around a pencil) forming a high resistance sheath called myelin. Myelin insulates a small region of the axon (prevents ions from leaking out into the extracellular fluid), which facilitates signal propagation down the axon towards the synaptic terminal. A single oligodendrocyte’s processes will wrap axons of numerous different neurons. Processes from many different oligodendrocytes contribute to the myelin sheath of a single neuron’s axon. Microglia are small highly mobile, phagocytic neuroglia that protect nervous tissue from pathogen infection, remove debris and waste, and may play a role in remodeling of the synapse that occurs during development and with learning. About 10-15% of CNS glial cells are microglia. Microglia are derived from monocytes and thus are more closely related to white blood cells than to the other glial cells. Since cells of the immune system cannot penetrate the blood brain barrier, microglia serve as brain macrophages, destroying foreign invaders, promoting inflammation and destroying cancer cells and cells infected with virus. Clusters of microglia in nervous tissue provide pathologists with evidence of recent injury. Ependymal cells are cuboidal-shaped glial cells that are joined together to form a continuous sheet lining the fluid-filled ventricles and central canal of the brain and spinal cord. Ependymal cells produce and secrete cerebrospinal fluid (CSF), the fluid that bathes the tissues of the CNS. The basal side of the cell has rootlets that anchor the cells to the underlying tissue. The apical surface is marked by cilia, which helps circulate the CSF. PNS Glial Cells The remaining two glial cells, Schwann cells and satellite cells are found solely in the peripheral nervous system. Schwann cells are analogous in function to oligodendrocytes (found in the CNS). They insulate the axons of peripheral nerves in one of two ways. A Schwann cell can wind its way round and round the axon (up to 100 times), while squeezing its cytoplasm out of the way (much like a toothpaste tube could be wrapped around a pencil), forming a myelin sheath. Like myelinating a single fiber in the CNS, which requires many oligodendrocytes, a complete myelin sheath in the PNS requires many Schwann cells. Schwann cells can also envelop PNS axons without forming a myelin sheath. Instead of wrapping a single axon many times, the Schwann cell forms an envelope around a bundle of unmyelinated axons. Additionally, Schwann cells can also assist in the regeneration of a damaged peripheral nerve. If a peripheral nerve is damaged, it may regenerate if its soma is undamaged and the neurilemma (the plasma membrane of the Schwann cell) enveloping it is intact. Satellite cells are found surrounding neural somas in peripheral ganglia (collections of cell bodies located outside the CNS). Satellite cells resemble CNS astrocytes and are thought to have similar functions, providing structural support and regulating the chemical environment.