Myelin cylinders

Noebe/s Again, there is no myelin sheath present. Myelin only comes as a condensation of membranes as the ohgodendrocyte wraps onto its compact structure. There is abundant ghal membrane, but it doesn t form a myelin cylinder, and therefore it isn t seen anatomically as white matter. 

Myelin Cylinders. Long-chain polar compounds, above their solubility limit, may interact with surfactants to form mixed micelles that separate (as a coacervate) in the form of cylinders. These are termed myelin cylinders or myelinic figures. They are usually quite viscous and may be birefringent. 

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... 

Fig. 5.4.1. (a) Smectic layers in concentric cylinders to form a myelin sheath with a singular line L along the axis (b) the cylinders are closed to form tori there are two singular lines, a circle and a straight line (c) the general case when the smectic layers form Dupin cyclides the circle becomes an ellipse and the straight... 

Histologically, a diffuse, symmetrical, extensive demyelination is seen. There may be almost complete lack of myelin sheaths in the central nervous system (Jervis 1960) with the exception, in most cases, of some U-fibers and of myelin sheaths within the central grey nuclei and the optic radiation. Demyelination is most marked in the internal capsule and pyramidal tracts spinal roots and peripheral nerves are involved to a lesser degree (Jacobi 1947 Bertrand et al. 1954). There may be partial loss, or in the center of involved regions, complete loss of axis cylinders. In the periphery of demyelinized areas they may be swollen and terminally distended (Hollander and Pilz 1964). The axis cylinders of U-fibers are, in general, well preserved. Small inflammatory lymphocytic infiltrates may be seen perivascularly. 

In addition to Mellanby s results with dogs, Aberle and Setterfield and Sutton have described degeneration of medullary sheaths of sensory tracts at the periphery of the cord. In some cases this degeneration was found in the posterior columns, and in others the posterior roots were involved. These changes were found to be associated with marked weakness or paralysis of the extremities. These authors were able to obtain a clinical cure of this condition with cod-liver oil while it was still possible to demonstrate lesions in the nervous system. Aberle suggests that this result may be due to the fact that the regenerated axis cylinder functions before remyelination of the fiber is complete. He points out that stimulation of the motor cortex in embryonic animals may elicit motor responses before the corticospinal tracts are myelinated. Modern research on nerve regeneration supports Aberle s explanation. 

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