Oligodendrocyte Oligodendrocyte

Adult NG2+ Cell

Figure 15 Possible differentiation route for oligodendrocyte lineage cells. The neonatal and adult CNS contains numerous NG2+ cells with PDGF a receptors, but those present in the adult do not proliferate as rapidly as the perinatal counterpart and do not readily differentiate into oligodendrocytes. They appear to be arrested at the NG2+/PDGF aR stage and persist in the adult CNS. (With permission from Nishiyama et al., 1999.)

Figure 16 Transverse (A, B, C) and longitudinal (D, E, F) sections through nodes of Ranvier in glutaraldehyde-fixed and paraffin-embedded spinal cord white matter of the adult cat. The tissue was processed according to the procedure described by Langley and Landon (1969) for demonstration of the extracellular node gap substance in PNS nodes. Note distinct presence of reaction product in relation to nodes of Ranvier (arrowheads). (x1,500). (With permission from Hildebrand and Skoglund, 1971.)

Figure 16 Transverse (A, B, C) and longitudinal (D, E, F) sections through nodes of Ranvier in glutaraldehyde-fixed and paraffin-embedded spinal cord white matter of the adult cat. The tissue was processed according to the procedure described by Langley and Landon (1969) for demonstration of the extracellular node gap substance in PNS nodes. Note distinct presence of reaction product in relation to nodes of Ranvier (arrowheads). (x1,500). (With permission from Hildebrand and Skoglund, 1971.)

zone and the juxtaparanode (Thomas et al., 1993). As seen in longitudinal sections, each myelin lamella terminates as a lateral loop (Figs. 7B and 13B). Here, the major dense line splits and forms a membrane-bounded drop-shaped cytoplasmic compartment containing a few microtubules, an occasional vesicle and varying amounts of electron-dense particles. In cat CNS fibers of all sizes, the lateral loops appose the paranodal part of the axon, on each side of the node, over a length of about 4 |im (Hildebrand 1971). Since each loop occupies a specific length, the paranodal axon allows direct apposition of a limited number of loops. When the number of myelin lamellae exceeds that number, some loops pile up on top of each other forming the "spiny bracelets of Nageotte" (Hess and Young, 1952; Hildebrand, 1971). Adjacent loops are linked to one another through tight junctions, which separate the extracellular compartment inside myelin from the general extracellular space (Kosaras and Kirschner, 1990).

Relative to the unspecialized internodal part of the axon, the paranodal part is slightly constricted in thin fibers, and markedly so in thick fibers. In the thickest fibers the paranodal axon diameter is one-third the internodal diameter (Fig. 1) (Hildebrand, 1971). The lateral loops indent the paranodal axolemma forming scallops. Here the gap between the axolemma and the lateral loops is only 2.5 to 3 nm (Dermietzel, 1974; Schnapp and Mugnaini, 1978; Rosenbluth, 1983, 1984). The loops in direct contact with the axolemma form a junction, the features of which vary with the method of preparation. Hence, the exact structure of this region is disputed (see Schnapp and Mugnaini, 1978; Peters et al., 1991). In EM images from longitudinal sections of aldehyde- and osmium-fixed material, the paranodal axoglial gap is interrupted by thickenings protruding from the axolemma toward the plasma membrane of the terminal loops, the transverse bands (Fig. 7C) (Dermietzel, 1974).

The transverse bands form segments of a circumaxonal helix, which is interrupted between the lateral loops. As seen by freeze-fracture EM, the bands comprise rows of regularly spaced particles in glial and axonal membranes, which are associated with cytoskeletal filaments and which appear to tighten the paranodal axoglial junction (Fig. 17) (Ichimura and Ellisman, 1991). This junction is reminiscent of a septate junction, an invertebrate equivalent to the tight junction (Schnapp and Mugnaini, 1978). The complex membrane anatomy of this region as seen by freeze-fracture EM has been described in detail elsewhere and will not be further commented on here (see Wiley and Ellisman, 1980; Rosenbluth, 1983, 1984, 1995; Waxman 1997). The paranodal junction is not tight in the conventional sense since small tracer molecules, but not larger ones, can reach the periaxonal space from the node gap (Hirano and Dembitzer, 1969; see Peters et al., 1991). The node gap and the periaxonal space seem to be connected

Figure I 7 Turtle optic nerve. Freeze-fracture electron micrographs illustrating the four fracture faces associated with the paranodal axo-glial junction. (A) The glial E-face (gE) and the axonal P-face (aP) are seen. Note that the junctional pattern is more obvious in the glial than in the axonal membrane. The glial membrane is undulated and exhibits parallel chains of particles in register with shallow grooves (arrows). Arrowheads indicate strips of axolemma that face the intervals between the paranodal loops. This nonjunctional band of the axolemma contains randomly distributed particles. Compare with Fig. 13B. (x63,750.) (B) The axonal E-face (aE) appears above the glial P-face (gP). The undulations in the axolemma are more obvious in this figure. Their coincidence with the undulations in the glial membrane are clearly discernible. The chains of particles in the glial P-face are located on narrow ridges. In the middle left part of the picture the long axis of the paranodal lateral loop (lb) and of the junctional specialization (js) are indicated. The angle between them is about 30 degrees. (x57,700.) (With permission from Schnapp and Mugnaini, 1978.)

by a narrow helical channel. The paranodal axoglial junction has been attributed a variety of functions: to anchor lateral loops to the axon, to form a partial diffusion barrier from the node gap into the periaxonal space, to demarcate axonal domains by limiting lateral diffusion of membrane components, to be a site for signaling between axons and myelinating glial cells, and/or to play an active role in the ion exchange underlying saltatory conduction (Schnapp and Mugnaini, 1978; Wiley and Ellisman, 1980; Peles and Salzer, 2000).

3. Juxtaparanodal Features a. The Juxtaparanodal Segment of the Myelinated Axon The juxtaparanode is markedly different in PNS and CNS axons. In a thick myelinated PNS axon, this region extends some 35 |im in an abnodal direction from the paranode. In the juxtaparanodes of a large myelinated PNS axon, the axon and the Schwann cell form a complex structural relation, the axon-Schwann cell network, which is composed of axonal and Schwann cell processes containing lysosomes and residual bodies. In each juxtapara-node-paranode-node-paranode-juxtaparanode region, the network tends to be most prominent in the distal juxta-paranode. It has been suggested that the network takes care of (potentially injurious) material taken up by peripheral terminals and transported in a retrograde fashion toward the CNS. The transported material interacts with primary lysosomes so that secondary lysosomes form. Large myelinated CNS axons exhibit few axoglial networks and secondary lysosomes. Although such formations are prominent in the ventral root part of motor axons, they are lacking or weakly developed in the CNS segments of the same axons. Similarly, dorsal root ganglion axons show more robust network formations in their dorsal root part than in their dorsal column part (Gatzinsky and Berthold 1990; Gatzinsky et al., 1997).

There is another major juxtaparanodal PNS/CNS difference. In large myelinated PNS axons the juxtaparanodal myelin sheath exhibits three to five longitudinal crests, with intervening furrows, which extend to a distance of about 40 |im from the node. Hence, the juxtaparanodal segment of the axon has a markedly noncircular shape. The axon-Schwann cell network resides on the axonal side of the crests. The furrows on the outside of the juxtaparanodal myelin sheath are filled with mitochondrion- and glycogen-loaded Schwann cell cytoplasm. These "mitochondrion bags" are continuous with the nodal Schwann cell processes (Berthold and Rydmark, 1995). Large myelinated CNS axons are devoid of any direct counterpart to the juxtaparanodal myelin crests and mitochondrion bags (Hildebrand, 1971).

As revealed by freeze-fracture EM, the juxtaparanodal region of CNS axons shows accumulations of large E-face particles, possibly representing K+ channels (Fig. 13B) (Arroyo and Scherer, 2000; Peles and Salzer, 2000). These accumulations show a sharp boundary toward the paranode and a gradually decreasing concentration in the direction of the unspecialized internodal segment. Such juxtaparanodal particle accumulations do not occur in PNS axons (Rosenbluth, 1983; Tao-Cheng and Rosenbluth, 1984). A detailed consideration of the molecular anatomy of the juxtaparanodal region is presented elsewhere (see Arroyo and Scherer, 2000; Peles and Salzer, 2000, and elsewhere in this volume).

b. The Myelinoid Bodies Marchi's histochemical method stains degenerating PNS and CNS myelin fragments and associated lipid droplets brown or black, while normal myelin remains unstained (Adams, 1965). Marchi-positive bodies are also ubiquitous in normal white matter, where they occur in rows or clusters along juxtaparanodes of large myelinated fibers (Fig. 18 A-D) (Hildebrand and Skoglund 1971; Hildebrand, 1977). Since the Marchi-posi-tive bodies were found to correspond to myelin-like lamel-lated bodies in the EM (Fig. 18E-G) they were named myelinoid bodies (Hildebrand and Skoglund, 1971). The lamellar pattern either shows alternating minor and major dense lines, like myelin, or uniform dense lines, unlike myelin (Figs. 18F,G). The two patterns probably reflect different stages in the life history of these bodies. Myelinoid bodies have a size range of <1 |im to >25 |im, and a peak frequency at 3 | m. They occur in spinal cord white matter of various vertebrate species, including humans, being particularly prominent in areas with large myelinated fibers (Hildebrand, 1977; Corneliuson et al., 1989). In the lateral funiculus of the cat, the incidence and size of Marchi-posi-tive myelinoid bodies increase with development (Remahl et al., 1977). While some myelinoid bodies are linked to oligodendroglial/myelin units, others reside within astro-cytes or in microglial cells (Fig. 18E, H) (Hildebrand and Aldskogius, 1976; Hildebrand, 1977). Juxtaparanodal Marchi-positive myelinoid bodies occur also in the PNS (Berthold, 1973, 1974), but they are much less frequent than in white matter. This difference may be due to the greater speed with which degenerating myelin fragments are removed in the PNS compared to the CNS (Franson and Ronnevi, 1984).

Since myelinoid bodies resemble degenerating myelin fragments formed during wallerian degeneration (Fig. 18E) (Hildebrand and Aldskogius, 1976; Hildebrand, 1977), and since myelinoid bodies inside astrocytes and within microglia are surrounded by acid phosphatase activity (Fig. 18H) (Hildebrand and Skoglund, 1971; Hildebrand, 1982), these bodies seem to reflect the catabolic side of myelin turnover. This view has gained support from biochemical studies. Ultracentrifugation of a rabbit CNS homogenate in a 0.32 /0.85 M sucrose gradient gives a myelin fraction at the interface, and a small floating fraction (FF) on top of the light sucrose. The FF is highly enriched in Marchi-negative and Marchi-positive myelinoid bodies. Similar fractions from pathological CNS tissue have been interpreted as partially degraded myelin (Persson and Corneliuson, 1989; Persson, 1991). The protein composition of the FF is myelin-like, except for partly degraded myelin proteins and some nonmyelin proteins (Persson and Corneliuson, 1989; Persson, 1991; Persson et al., 1992). In addition, calpains, which participate in the degradation of myelin proteins during degeneration, are present in the FF (Persson and Karlsson, 1991). These biochemical analyses

Figure 18 Marchi-positive myelinoid bodies in longitudinal slices from cat spinal cord white matter. (A) Row of Marchi-positive myelinoid bodies (arrows); (B) Survey illustrating the high general incidence of such bodies; (C, D) Paranode-juxtaparanode region with Marchi-positive myelinoid bodies at different magnifications. Arrowheads indicate node gap. (A) (x1,000) (B) (x200) (C) (x1,500) (D) (x640). (With permission from Hildebrand and Skoglund, 1971.) (E) Microglial cell (N = nucleus) in cat white matter. Note that this cell contains a myelinoid body (arrow) and lipid droplets (L). (F, G) Examples of the lamellar pattern in the outer compact part (F) and in the inner less compact part (G) of a myelinoid body (see also Fig. 6C). Inset in (F) shows lamellar pattern in normal myelin. X in (G) indicates a dense flocculent nonlamellar material possibly cytoplasmic remnants. (E) (x11,300) (F) (x113,000) (G) (x63,400) (With permission from Hildebrand, 1971.) (H) Electron micrograph from slice of adult guinea pig white matter incubated for the demonstration of acid phosphatase activity. The picture shows two microglial cells (N = nucleus). The left cell contains a hollow myelinoid body surrounded by a dense precipitate indicating acid phosphatase activity. The right cell exhibits two massive lamellated bodies containing reaction product. In both cells lipid droplets can be seen (arrows). (x6,500.) (With permission from Hildebrand, 1982.)

Figure 18 Marchi-positive myelinoid bodies in longitudinal slices from cat spinal cord white matter. (A) Row of Marchi-positive myelinoid bodies (arrows); (B) Survey illustrating the high general incidence of such bodies; (C, D) Paranode-juxtaparanode region with Marchi-positive myelinoid bodies at different magnifications. Arrowheads indicate node gap. (A) (x1,000) (B) (x200) (C) (x1,500) (D) (x640). (With permission from Hildebrand and Skoglund, 1971.) (E) Microglial cell (N = nucleus) in cat white matter. Note that this cell contains a myelinoid body (arrow) and lipid droplets (L). (F, G) Examples of the lamellar pattern in the outer compact part (F) and in the inner less compact part (G) of a myelinoid body (see also Fig. 6C). Inset in (F) shows lamellar pattern in normal myelin. X in (G) indicates a dense flocculent nonlamellar material possibly cytoplasmic remnants. (E) (x11,300) (F) (x113,000) (G) (x63,400) (With permission from Hildebrand, 1971.) (H) Electron micrograph from slice of adult guinea pig white matter incubated for the demonstration of acid phosphatase activity. The picture shows two microglial cells (N = nucleus). The left cell contains a hollow myelinoid body surrounded by a dense precipitate indicating acid phosphatase activity. The right cell exhibits two massive lamellated bodies containing reaction product. In both cells lipid droplets can be seen (arrows). (x6,500.) (With permission from Hildebrand, 1982.)

of purified subcellular fractions agree with the view that myelin turnover in large myelinated CNS fibers includes a degradation step via formation of myelinoid bodies in the juxtaparanodal region (Fig. 19).

4. The Unspecialized Internodal Segment

The segment of a myelinated axon extending between the juxtaparanodes (Fig. 4B) has been called the internode by some workers. However, it is more appropriate to use this term for the entire myelin sheath, as we do when we talk about internodal length. Here the segment extending between the juxtaparanodes is called the unspecialized internodal segment. In this segment the CNS myelin sheath is separated from the axolemma by a space of at least 12 nm. As in myelinated PNS axons, the inner aspect includes a thin cytoplasmic lamella terminating in a small inner loop that is part of the inner mesaxon (Fig. 4B). With respect to thin myelinated axons the oligodendroglial cytoplasm associated with the outer aspect of the internodal segment of a CNS myelin sheath is limited to a small outer tongue and the outer mesaxon (Fig. 4B; see Hirano and Llena, 1995). However, in large myelinated axons associated with Type IV oligoden-drocytes, the glial perikaryon may be directly apposed to the internodal sheath in a Schwann cell-like manner (Fig. 11B; Hildebrand, 1971; Remahl and Hildebrand, 1990; Anderson et al., 2000). Otherwise the internodal anatomy of CNS fibers is devoid of specific features. The molecular anatomy of this region is considered elsewhere (Arroyo and Scherer, 2000; Peles and Salzer, 2000; Arroyo et al., 2001).

0 0

Post a comment