Myelinating Schwann Cell

Phylogenetically, myelinating Schwann cells appear first in gnastostomes. Agnatha do not have myelinating glial cells at all [23, 24]. Myelination of the axons is a prerequisite for rapid saltatory signal propagation in the neuronal network. During the ontogenetic development myelination of axons occurs in complex interactive networks where neurons and glial cells are dependent on reciprocal signals in a time and a space specific manner [25]. The molecular analysis of the myelination process is a hot topic and under intense investigation [26, 27]. The myelinating Schwann cell is a remarkable polarized cell (fig. 2a). During the developmental process it forms the compact myelin and wraps up the axon with loops of its cytoplasmic processes which after extrusion of the cytoplasm and the reduction of extracellular space form the tightly apposed membranes [28]. The axon defines the thickness of the myelin sheath and the internodal length. The longer and thicker the nerve fiber is, the longer is the internodal segment. The node of Ranvier is defined as a small gap where the myelin sheath is interrupted between successive Schwann cells along the axon (fig. 2c). In this location, the axonal membrane is only partially covered by few and small interdigitating cell processes of the adjoining Schwann cells. At these sites the compact myelin of the Schwann cell breaks up into paranodal loops and several glial endfeet are sealed to the paranodal axonal membrane via the septate-like junctions. The neuronal glycoprotein Contactin associated protein

Myelin Sheath Measurement

Fig. 2. Myelinating Schwann cell and non-myelinating Schwann cell. a Myelinating Schwann cell with the inner and outer mesaxon (arrrow). Note the basement lamina (bl) in close apposition with fine collagen fibrils of the endoneurium. Compact myelin (m), axon (ax). Electron micrograph, bar 1 |xm. b Non-myelinating Schwann cell with numerous axons (ax) invaginated in its cytoplasm. Electron micrograph, bar 1 ^m. c Node of Ranvier and paranodal segment with paranodal strands and inserting glial lamellae (gl). Arrows indicate paranodal strands. Small cytoplasmic processes of the adjoining Schwann cells (psc) cover the node of Ranvier. Axon (ax), myelin (m), endoneurium (en) with collagen fibrils. Electron micrograph, bar 1 ^m.

Fig. 2. Myelinating Schwann cell and non-myelinating Schwann cell. a Myelinating Schwann cell with the inner and outer mesaxon (arrrow). Note the basement lamina (bl) in close apposition with fine collagen fibrils of the endoneurium. Compact myelin (m), axon (ax). Electron micrograph, bar 1 |xm. b Non-myelinating Schwann cell with numerous axons (ax) invaginated in its cytoplasm. Electron micrograph, bar 1 ^m. c Node of Ranvier and paranodal segment with paranodal strands and inserting glial lamellae (gl). Arrows indicate paranodal strands. Small cytoplasmic processes of the adjoining Schwann cells (psc) cover the node of Ranvier. Axon (ax), myelin (m), endoneurium (en) with collagen fibrils. Electron micrograph, bar 1 ^m.

Nociceptor Types

Fig. 3. Two types of terminal Schwann cells of a mechanoreceptor and a nociceptor. a Terminal Schwann cells (ts) of the Vater Pacini corpuscle form the inner core (l) around the sensory axon terminal (ax). b Gap junctions (arrows) are numerous between the inner core lamellae (l). Electron micrograph, bar 100 nm. c Numerous nociceptive axons (arrows) close to a mast cell (mc) in the Achilles tendon. Terminal Schwann cell (ts). Electron micrograph, bar 1 ^m.

Fig. 3. Two types of terminal Schwann cells of a mechanoreceptor and a nociceptor. a Terminal Schwann cells (ts) of the Vater Pacini corpuscle form the inner core (l) around the sensory axon terminal (ax). b Gap junctions (arrows) are numerous between the inner core lamellae (l). Electron micrograph, bar 100 nm. c Numerous nociceptive axons (arrows) close to a mast cell (mc) in the Achilles tendon. Terminal Schwann cell (ts). Electron micrograph, bar 1 ^m.

(Caspr)/paranodin, Caspr2 and protein 4.1B are essential for the structure and function of these axo-glial junctions [29] in interaction with neurofascin of the Schwann cell [30]. These septate-like junctions are necessary to concentrate the voltage-gated Na+ channels of the node of Ranvier and to avoid their lateral diffusion. Multiple adherens junctions are obvious between the glial endfeet containing E-cadherin, catenin and F-actin [31]. Schmidt-Lanterman incisures are small cytoplasmic funnels of non-compact myelin. In this area, the individual lamellae are connected by gap junctions (connexin32) allowing several metabolic

short-circuits between the outer and inner cytoplasmic compartments of the Schwann cell [32].

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  • Callum
    Does endoneurium cover the nodes of ranvier?
    6 years ago

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