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Neuroregeneration
Neuroregeneration refers to the regrowth or repair of nervous tissues, cells or cell products. Such mechanisms may include generation of new neurons, glia, axons, myelin, or synapses. Neuroregeneration differs between the peripheral nervous system (PNS) and the central nervous system (CNS) by the functional mechanisms and especially the extent and speed. When an axon is damaged, the distal segment undergoes Wallerian degeneration, losing its myelin sheath. The proximal segment can either die by apoptosis or undergo the chromatolytic reaction, which is an attempt at repair. In the CNS, synaptic stripping occurs as glial foot processes invade the dead synapse.〔Principles of Neural Science; Kandel, Schwartz; McGraw-Hill Medical; 4 edition (January 5, 2000); Chapter 55〕
Nervous system injuries affect over 90,000 people every year.〔Stabenfeldt, S.E., A.J. Garcia, and M.C. LaPlaca, Thermoreversible laminin-functionalized hydrogel for neural tissue engineering. Journal of Biomedical Materials Research. Part A, 2006. 77: p. 718-725〕 It is estimated that spinal cord injuries alone affect 10,000 each year.〔Prang, P., et al., The promotion of oriented axonal regrowth in the injured spinal cord by alginate-based anisotropic capillary hydrogels. Biomaterials, 2006. 27: p. 3560-3569.〕 As a result of this high incidence of neurological injuries, nerve regeneration and repair, a subfield of neural tissue engineering, is becoming a rapidly growing field dedicated to the discovery of new ways to recover nerve functionality after injury. The nervous system is divided into two parts: the central nervous system, which consists of the brain and spinal cord, and the peripheral nervous system, which consists of cranial and spinal nerves along with their associated ganglia. While the peripheral nervous system has an intrinsic ability for repair and regeneration, the central nervous system is, for the most part, incapable of self-repair and regeneration. There is currently no treatment for recovering human nerve function after injury to the central nervous system.〔Recknor, J.B. and S.K. Mallapragada, Nerve Regeneration: Tissue Engineering Strategies, in The Biomedical Engineering Handbook: Tissue Engineering and Artificial Organs, J.D. Bronzino, Editor. 2006, Taylor & Francis: New York〕 In addition, multiple attempts at nerve re-growth across the PNS-CNS transition have not been successful.〔 There is simply not enough knowledge about regeneration in the central nervous system. In addition, although the peripheral nervous system has the capability for regeneration, much research still needs to be done to optimize the environment for maximum regrowth potential. Neuroregeneration is important clinically, as it is part of the pathogenesis of many diseases, including multiple sclerosis.
== Peripheral nervous system regeneration ==
(詳細はchemotactic factors secreted from Schwann cells (neurolemmocytes). Injury to the peripheral nervous system immediately elicits the migration of phagocytes, Schwann cells, and macrophages to the lesion site in order to clear away debris such as damaged tissue. When a nerve axon is severed, the end still attached to the cell body is labeled the proximal segment, while the other end is called the distal segment. After injury, the proximal end swells and experiences some retrograde degeneration, but once the debris is cleared, it begins to sprout axons and the presence of growth cones can be detected. The proximal axons are able to regrow as long as the cell body is intact, and they have made contact with the Schwann cells in the endoneurial channel. Human axon growth rates can reach 1 mm/day in small nerves and 5 mm/day in large nerves.〔 The distal segment, however, experiences Wallerian degeneration within hours of the injury; the axons and myelin degenerate, but the endoneurium remains. In the later stages of regeneration the remaining endoneurial tube directs axon growth back to the correct targets. During Wallerian degeneration, Schwann cells grow in ordered columns along the endoneurial tube, creating a band of Büngner (boB) that protects and preserves the endoneurial channel. Also, macrophages and Schwann cells release neurotrophic factors that enhance re-growth.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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