Axon Regeneration

Axons of both central and peripheral neurons in adult mammals have the intrinsic potential to regenerate after crush or transection, as indicated by the fact that they initiate sprouting after such injuries. In most vertebrates, spinal nerves and olfactory nerves are able to regenerate across a lesion and reinnervate their target tissues. The optic nerve and the ascending and descending tracts of the spinal cord in fish and larval and adult urodele amphibians are also able to regenerate. By contrast, axons of the reptilian, avian and mammalian central nervous system fail to regenerate further after initial sprouting.

A large body of evidence suggests that whether or not axons regenerate depends in large part on their associated glial cells (Yannas, 2001). Differences in the ability of peripheral and central glial cell populations to support regeneration have been well documented by experiments in which the regeneration of central axons was promoted by peripheral nerve sheaths grafted into the central nervous system, whereas central nerve sheaths inhibit the regeneration of peripheral axons (Aguayo, 1985). These differences appear to reside largely in the adhesion molecules and soluble signals synthesized by glial cells. Glial cells that support regeneration provide most or all of the molecules that are

Dendrites

Myelin sheath Synapse Oligodendrocyte

Dendrites

Astrocyte in Astrocyte in Gray mater White mater

Myelin sheath Synapse Oligodendrocyte

Astrocyte in Astrocyte in Gray mater White mater

FIGURE 5.1 Neurons and glial cells. Neurons have a large metaboli-cally active cell body with short dendrites on one side and a long axon on the other. Axons synapse with the cell bodies and/or den-drites of adjacent or more distant neurons. Axons are sheathed in myelin made by oligodendrocytes in the CNS and Schwann cells in the PNS. Astrocytes in both the gray and white matter interact with neurons in a variety of dynamic functions.

FIGURE 5.1 Neurons and glial cells. Neurons have a large metaboli-cally active cell body with short dendrites on one side and a long axon on the other. Axons synapse with the cell bodies and/or den-drites of adjacent or more distant neurons. Axons are sheathed in myelin made by oligodendrocytes in the CNS and Schwann cells in the PNS. Astrocytes in both the gray and white matter interact with neurons in a variety of dynamic functions.

important for neuron survival and axon outgrowth, whereas regeneration fails where glial cells do not make these molecules and/or synthesize molecules inhibitory to regeneration.

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