Myelin tubes

Phospholipids are important components of biological membranes. Various classes of phospholipid occur, and within each class, a distribution of fatty acid residues is found. A variety of physical techniques have shown that a number of pure phospholipids undergo a transition from a crystalline to a liquid crystalline form at a temperature dependent upon the presence and type of unsaturation in the fatty acid residues. The implications of these results to the dispersibility of phospholipids in water, the formation of myelin tubes, the production of model membranes, and to the natural biological systems, are discussed. 

Pure phospholipids all undergo a transition from crystalline to the liquid crystalline phase many degrees below the capillary melting point. In the liquid crystalline phase the hydrocarbon chains are in a fluid condition. In the presence of water this transition temperature is lowered. At this temperature myelin tubes are formed, and the phospholipids can be more easily dispersed. The temperature for lyotropic transitions from lamellar to hexagonal phases is alsoTelated to this temperature. 

The swelling of phospholipids in water, with the formation of myelin tubes, is also related to this transition temperature. With highly unsaturated phospholipids, such as egg yolk lecithin, myelin tube formation occurs readily at room temperature, but with the fully saturated phospholipids it occurs at higher temperatures. 

Schmidt-Lantermann clefts are structures where the cytoplasmic surfaces of the myelin sheath have not compacted to form the major dense line and therefore contain Schwann or glial cell cytoplasm (Fig. 4-9). They are common in peripheral myelin but rare in the CNS. These inclusions of cytoplasm are present in each layer of myelin. The clefts can be visualized in the unrolled myelin sheet as tubes of cytoplasm similar to the tubes making up the lateral loops but in the middle regions of the sheet, rather than at the edges (Fig. 4-9). 

Cell adhesion molecules (CAMs) play critical roles in all facets of nervous system development and maintenance. Important phenomena in which CAMs are involved include initial formation of the neural tube and the neural crest, migration of all neurons and glial cells, axonal outgrowth and guidance, target selection, synaptic stabilization and plasticity, myelination and nerve regeneration after injury (see Chs 4,24,28-30 and 53). Adhesion molecules interact with each other and with nonadhesive cell-surface and/or cytoplasmic molecules, and, in the two... 

The appearance of tubular myelin-like structures in swollen lecithin was observed by light microscopy well before the systematic investigation of liposomes [351-352]. Similarly, it was also demonstrated some time ago that the addition of calcium ions converted phospholipid liposomes to cochleate cylinders [353]. Subsequent studies have, however, revealed that the system is extremely complex. For example, examination of the phase-transition behavior of synthetic sodium di-n-dodecyl phosphate [(C12H2sO)2PO2Na+ or NaDDP] and calcium di-n-dodecyl phosphate [Ca(DDP)2] showed the presence of many diverse structures [354]. In particular, hydrated NaDDP crystals were shown to form lyotropic liquid-crystalline phases which transformed, upon heating to 50 °C, to myelin-like tubes. Structures of the tubes formed were found... 

As mentioned previously, myelination is carried out by highly specialized glial cells, oligodendrocytes in the CNS and Schwann cells in the PNS. These cells differentiate from precursor cells of different origin during development the neural crest for Schwann cells, and neural tube for oligodendrocytes. Myelin characteristics differ morphologically in the CNS and in the PNS, and differ also inside the CNS. Thus... 

Decant the supernatant into a fresh tube and spin for 12 min at 14 000 g (e.g., SS34 rotor at 11 000 r.p.m). A pellet is generated in which three layers can be distinguished a dark brown bottom part (mostly mitochondria), a lighter brown middle part (synap-tosomes) and a whitish top layer(mostly myelin). 

The separation of the components of the myelin-free crude mitochondrial fraction of whole brain tissue in centrifuge tubes is compared with a separation by zonal centrifugation. On a shallow, step-wise gradient of 0.8-1.7 M sucrose in a BXIV rotor of 650 ml capacity, it was possible to obtain lysosomal, mitochondrial, synaptosomal and plasma membrane fractions after spinning for 2 h at 67,000 x g. These fractions were characterised by enzyme markers and other means. At least two synaptosomal populations could be clearly separated, one of which could actively take up a-methylnoradrenaline. Some preliminary studies on the uptake of [ A/e-14C]-choline into sub-cellular components after intracerebral injection are also described. 

Events surrounding Wallerian degeneration in the nerve are well described (Lampert, 1967 Sal-zer and Bunge, 1980 Lubinska, 1982). Schwann cells proliferate in the basement membrane tubes of the formerly myelinated nerve fibers (Asbury and Johnson, 1978). Premitotic incorporation of thymidine in Schwann cells spreads proximo-... 

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