MAKING PERIPHERAL NERVES AND CENTRAL TRACTS
Another essential aspect of establishing cellular diversity in the nervous system is the differentiation of distinct classes of glial cells that associate themselves with developing axons. These glial cells are found in both the peripheral nervous system (PNS), where they are derived from the neural crest, or in the central nervous system, where they arise from local neural stem cells (thus true multipotent neural stem cells are capable of giving rise to both neurons and glia). These glial cells then interact with peripheral axons either as they form peripheral nerves or with central axons as they form central tracts. Schwann cells establish a clear relationship with unmyelinated axons in the peripheral nervous system, surrounded, or ensheathed, by Schwann cell processes that constitute the neurilemma. Each Schwann cell usually ensheathes more than one axon of this type. Most axons of postganglionic autonomic (sympathetic and parasympathetic) neurons are unmyelinated. Numerous layers of the cell membrane of Schwann cells wrap myelinated axons of the peripheral nervous system. A single neurilemmal cell typically forms a segment of myelin sheath for only one peripheral axon.
Oligodendrocytes in the CNS and Schwann cells in the PNS form myelin sheaths by similar processes. In an action similar to the continuous wrapping of a bolt of cloth, the oligodendroglial cell membrane becomes wrapped around the axon many times. As the wrapping occurs, the oligodendroglial cytoplasm retracts or is extruded so that the two layers of the cell’s plasma membrane, which originally were separated by cytoplasm, come together and fuse. Except for small islands of cytoplasm, which may be trapped between the fused membranes, the fusion is complete. The cell membrane of the myelinating oligodendrocyte, like cell membranes elsewhere, is composed of alternate layers of lipid and protein molecules. Thus myelin is made up of numerous fused layers of lipoprotein membrane.
Myelination is closely associated with the development of the functional capacity of neurons. Unmyelinated neurons have a low conduction velocity and show fatigue earlier, whereas myelinated neurons fire rapidly and have a long period of activity before fatigue occurs. Neurons that ultimately are capable of rapid transmission of impulses become fully functional at about the time their axons become completely insulated with a myelin sheath. In general, the motor neurons of cranial nerves become myelinated before their sensory counterparts. The sensory neurons of the trigeminal nerve and the cochlear division of the vestibulocochlear nerve begin to acquire myelin only in the fifth and sixth months of development. The optic nerve neurons begin to be sheathed at birth, and myelination is completed by the end of the second week after birth.
As development continues, the nerve fibers (axons) of both the CNS and the PNS eventually become sheathed or encapsulated. In the PNS, neurons become completely encapsulated by parts of other cells, except at their terminal endings and at the nodes of Ranvier. The Schwann cell ensheathes both the myelinated and unmyelinated axons of somatic motor neurons and pre- ganglionic autonomic motor neurons as they pass out of the CNS. These cells, derived from both the neural crest and the wall of the neural tube, also ensheath both the central and peripheral processes of the somatic and visceral sensory neurons, as well as the axons of post-ganglionic autonomic (sympathetic and parasympathetic) motor neurons.
|DEVELOPMENT OF MYELINATION AND AXON ENSHEATHMENT|
Another type of cell, which is derived from both the neural crest and the wall of the neural tube and which participates in covering the neurons of the PNS, is the satellite cell. Satellite cells completely encapsulate the cell bodies of sensory neurons in the sensory ganglia of both the cranial and spinal nerves, and also the post-ganglionic neurons of the sympathetic and parasympathetic ganglia. Finally, there is a specialized glial cell that shares properties of Schwann and satellite cells but is only found apposed to the unmyelinated axons of the olfactory nerve; these axons originate from receptor neurons in the nose that are continually replaced throughout life and thus must regrow into the olfactory bulb in the CNS and make new synapses. These olfactory ensheathing cells are apparently specialized to support the constant regrowth of the axons and the establishment of new connections in the CNS. Accordingly, there is great interest in these cells as a substrate for improve axon growth in other regions of the CNS after injury.