Myelin content

Myelin in situ has a water content of about 40%. The dry mass of both CNS and PNS myelin is characterized by a high proportion of lipid (70-85%) and, consequently, a low proportion of protein (15-30%). By comparison, most biological membranes have a higher ratio of proteins to lipids. The currently accepted view of membrane structure is that of a lipid bilayer with integral membrane proteins embedded in the bilayer and other extrinsic proteins attached to one surface or the other by weaker linkages. Proteins and lipids are asymmetrically distributed in this bilayer, with only partial asymmetry of the lipids. The proposed molecular architecture of the layered membranes of compact myelin fits such a concept (Fig. 4-11). Models of compact myelin are based on data from electron microscopy, immunostaining, X-ray diffraction, surface probes studies, structural abnormalities in mutant mice, correlations between structure and composition in various species, and predictions of protein structure from sequencing information [4]. 

Table 36-1 lists some of the mechanisms important in governing the susceptibility of the PNS to disease and injury. Peripheral nerves, although toughened by their high content of collagen, are prone to injury to myelin by compression (e.g. carpal tunnel syndrome and tardy ulnar palsy) and to axons by excessive stretch (e.g. brachial plexopathy in newborn infants following a difficult delivery). Subcutaneous nerves, because of their exposed position, are also vulnerable to cold or heat injury. 

Lamellar phases of phospholipids often exhibit myehnic figures when contacted with water. Electron micrographs [24,26] showed that each tubular myehnic figure in the egg-yolk phosphatidylcholine/water system consisted of a water core surrounded by many concentric bilayers. More recently Raman spectroscopy techniques have confirmed the concentric bilayer arrangement [1,18]. Myelinic figures are not equilibrium structures, however, and eventually break up to form vesicles or other lamellar structures. Indeed, adding water to a vessel whose inner walls are coated with a thin layer of a lamellar phase of low water content is a well-known way of forming vesicles. 

The lipid, water, and protein contents of certain tissues vary markedly as a function of age. For example, the adipose tissues of neonates contain about 55% water and 35% lipids, whereas the corresponding figures for the adult are about 25% and 70%, respectively (Friis-Hansen, 1971). The proportion of water in skin falls as a function of age, due to an increase in collagen. The water contents of liver, brain, and kidneys decrease from birth to adulthood by 5-15%. The decrease in water contents of liver and kidneys is primarily due to an increase in protein, whereas this change in the brain is due to an increase in myelin. The overall composition of muscle in terms of lipid and water does not seem to vary with age (Dickerson Widdowson, 1960). 

Membranes, which are the subject of this section, can be relatively thick (0.1 mm) if made chemically (see their use in the PEM fuel cell, (Section 13.7.3). Biological membranes are very much thinner (50-100 A), of the same (3-5 nm) range as that of passive oxides (Section 12.5). Of what do biological membranes consist Figure 14.6 shows the essential constituents. They are lipids and proteins. How much there is of one and how much of the other varies widely. Thus, in a myelin membrane the lipid content is 80% while at the other end of the range, in mitochondria, there is an inner membrane containing only about 20% lipid. There are many kinds of lipids (as well as very many kinds of proteins), but those in membranes are usually phospholipids and are represented in Fig. 14.7. The structure often contains an H atom and this allows... 

Copper The content of Cu in the human body is estimated to range between 50 and 80 pg. The RDA is 1.5-3.0 mg per day for adults. Copper is an essential component of several enzymes and is required in bone formation, cellular respiration, cardiac function, connective tissue development, and myelination of the spinal cord [11-14]. This metal is also necessary for Fe absorption and mobilization. Again, Cu content in milk differs with the biological species, stage of lactation, and diet intake. In all species colostrum is substantially richer in Cu than mature milk is. 

Myelin is modified plasma membrane. Myelin of the PNS resembles that of the CNS with respect to lipid composition. There is an enrichment in such specialized lipids as cerebroside and ethanolamine plasmalogen, and the high content of cholesterol plays an important role in control of membrane fluidity. The protein composition of PNS myelin is, however, distinct from that of CNS myelin. A single protein, P0, accounts for half of all protein of PNS myelin. Of the other proteins present, most are expressed in the CNS as well as the PNS but in quantitatively different amounts. Prominent among these proteins are myelin basic proteins and myelin-associated glycoprotein. 

MBP constitutes about 30% of the protein content of myelin. In fact, the MBPs constitute a family of proteins comprising many isoforms (reviewed in Campagnoni... 

Peripheral myelin protein 22 (PMP22) has a molecular weight of 22 kDa and represents 2-5% of the total myelin protein content. Despite its name, PMP22 is not specific to the PNS inasmuch as it is expressed, albeit at low levels, in other... 

Cholesterol and phospholipids. Most lipids found in myelin are common to other cellular membranes. Cholesterol content is high and cholesterol esters are not present in normal myelin. Phospholipids are also common to other cellular membranes, except for the great quantity of ethanolamine phosphoglycerides in the plasmalogen form. The synthesis of plasmalogens is modified in Zellweger syndrome which is a peroxisomal syndrome that also increases VLCFA. This syndrome and other peroxisomal diseases may cause demyelination (Powers, 2005). 

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