Introduction
Schwann cells refer to any of the cells within the peripheral nervous system which create a myelin sheath around the neuronal axons. These cells were named after the person who discovered them, Theodor Schwann, when he discovered them in the 1800s (Encyclopedia Brittanica). The cells are the same as a type of neuroglia known as oligodendrocytes that occur within the central nervous system. (Encyclopedia Brittanica).
Simple aspects
Schwann cells are different from cells in the neural crest in embryonic development while they are stimulated to proliferate from the surface of the axons (Encyclopedia Brittanica). When motor neurons become disconnected as thus causing the degeneration of nerve terminals, Schwann cells reside within the original space in the neuron. Degeneration in this process is then followed by a regeneration while fibers regenerate so that they return to their original target sites (Encyclopedia Brittanica). The remaining Schwann cells following nerve degeneration are thought to determine to route (Encyclopedia Brittanica).
Demyelinating neuropathies are those that affect Schwann cells and then move away from nerves (Encyclopedia Brittanica). Schwann cell-axon interactions effected in this manner while the process causes the insulation of myelin in axons to be removed as conduction is blocked in the axon for nerve impulses. Schwann cells are vulnerable to toxic and immune attacks as is evident in diphtheria and Guillian-Barre syndrome. General electrical conduction can be blocked through this as injury to axons also damage Schwann cells to create what is known as secondary demyelination (Encyclopedia Brittanica).
Complex aspects
Schwann cells cover the majority of the surface of all axons in peripheral nerves (Corfas et al). Axons and glial cells are in close physical contact and also in complex and constant communication with each other while they influence and control the maintenance, functionality and development of one another (Corfas et al). Recently progress has been made in better understanding the mechanisms at a molecular level of Schwann cell-axon interactions, particularly the neuregulin1 (NRG1)-erB signaling pathway, the role perisynaptic Schwann cells have within neuromuscular junctions, the underlying mechanisms in the formation and functions of the node of Ranvier, as well as the mechanisms generating tumors in Schwann cells (Corfas et al).
Along the whole of the length of peripheral nerves of mammals, the axons of motor, autonomic, and sensory neurons are closely associated with Schwann cells (Corfas et al). The contact between peripheral axons and Schwann cells is commonly regarded as ‘intimate’ while this relationship provides an indication that the cells interact in a number of important ways (Corfas et al). In the fully developed, mature, nervous system, Schwann cells can be split into four categories: nonmyelinating cells (NMSCs,) myelinating cells (MSCs,) satellite cells of peripheral ganglia, and perisynaptic Schwann cells (PSCs.) (Corfas et al). These categories are based on their morphology, makeup with regards to biochemistry, and the types of neurons and area of axons that they are related to (Jessen et al). MSCs wrap around all axons of large diameter, including the motor neurons as well as some sensory neurons. All MSCs associate with one axon while creating the myelin sheath required for salutatory conduction in nerves (Corfas et al).
NMSCs associate with axons with small-sized axons of c-fibers radiating from all postganglionic sympathetic neurons as well as some sensory neurons (Corfas et al). Each NMSC wraps around multiple sensory axons for a Remak bundle which are split by small-diameter extensions from the body of Schwann cells (Corfas et al). Also, located more with the peripheral areas, PSCs reside within neuromuscular junctions (NMJs) where they cover the presynaptic terminals of motor axons without completely wrapping around them (Corfas et al). Satellite cells also associate with neuron-based cell bodies within the peripheral ganglia (Corfas et al).
The diverse kinds of Schwann cells which can be found within the adult are mainly stemming from a sole precursor type of cell, which is the neural crest cell. Some Schwann cells may stem from placodes and ventral neural tubes (Corfas et al). The multipotent and actively migrating neural crest cells move to the peripheral nerves in the stages of embryonic development, as mentioned, where they will grow in steps while giving rise to all Schwann cells (Corfas et al). By the twelfth day in the embryonic process, the Schwann cell precursors start to show three separate markets. From the fifteenth day to the time of actual birth, these precursors will cause the creation of immature Schwann cells. After birth, these immature cells will separate to form myelinating, nonmyelinating, and perisynaptic phenotypes, all processes which take a number of weeks to complete (Jessen and Mirky). The axons give signals which control choices made across various phenotypes of Schwann cells, however, the identity at a molecular level of such signals is not currently understood. Certainly, however, the signals from the various types of axons and areas are critically important (Corfas et al).
The three kinds of Schwann cells are not just different with regards to the kinds of axons they interact with but are also different on many levels at a biochemical level. MSCs are composed of myelin proteins, while these proteins are essential for the creation and functionality of myelin sheaths. NMSCs and PSCs are less specifically characterized, however, while the NMSCs and MSCs contain high concentrations of glial fibrillary acidic proteins (Corfas et al).
Conclusion
As we can see, Schwann cells interact closely to such an extent with axons that this relationship is considered intimate commonly across the scientific field. While some aspects have yet to be discovered, the critical roles of such cells in the nervous system are undoubtedly essential and certain.