Myelin structures

Information concerning myelin structure is also available from electron microscope studies, which visualize myelin as a series of alternating dark and less dark lines (protein layers) separated by unstained zones (the lipid hydrocarbon chains) (Figs 4-4 to 4-7). There is asymmetry in the staining of the protein layers. The less dark, or intraperiod, line represents the closely apposed outer protein... 

Myelin basic protein. In PNS myelin, MBP varies from approximately 5% to 18% of total protein, in contrast to the CNS, where it is close to 30% [ 1 ]. In rodents, the same four 21,18.5,17 and 14kDa MBPs found in the CNS are present in the PNS. In adult rodents, the 14kDa MBP is the most prominent component and is termed Pr in the PNS nomenclature. The 18.5 kDa component is present and is often referred to as the P, protein in the nomenclature of peripheral myelin proteins. Another species-specific variation in human PNS is that the major basic protein is not the 18.5 kDa isoform that is most prominent in the CNS but rather a form of about 17 kDa. It appears that MBP does not play as critical a role in myelin structure in the PNS as it does in the CNS. For example, the shiverer mutant mouse, which expresses no MBP (Table 4-2), has a greatly reduced amount of CNS myelin, with no compaction of the major dense line. By contrast, shiverer PNS has essentially normal myelin,both in amount and structure, despite the absence of MBP. This CNS/PNS difference in the role of MBP is probably because the cytoplasmic domain of P0 has an important role in stabilizing the major dense line of PNS myelin. Animals doubly deficient for P0 and MBP have a more severe defect in compaction of the PNS major dense line than P0-null mice, which indicates that both proteins contribute to compaction of the cytoplasmic surfaces in PNS myelin [23],... 

Increased production of metabolites of phenylalanine that inhibit synthesis of a variety of substances required for normal brain growth Inhibition of TV-methyl-D-aspartate receptors, which are involved in memory and learning Competitive inhibition of transport of other amino acids required for protein synthesis Impaired polyribosome formation or stabilization Reduced synthesis/increased degradation of myelin Decreased formation of norepinephrine and serotonin Altered myelin structure and function... 

Myelin Structure in the Central and Peripheral Nervous System. 540... 

Enhanced lysosomal storage of phospholipids due to the inhibition of phospholipases is another special form of hepatic steatosis. The hepatocytes are enlarged and exhibit a distinctive foamy lucency of the cytoplasm. Crystalline inclusions and an agglomeration of myelin structures are visible in the lysosomes by electron microscopy. Mallory bodies may also be found. This idiosyncratic metabolic liver damage may even develop into cirrhosis, (s. p. 545) (s. tab. 31.4)... 

All three of these proteins are present in the lung. The lamellar bodies are secreted into alveolar lumen where they are transformed into an extracellular form of surfactant that has a quadratic lattice structure called tubular myelin. The three-dimensional tubulin-myelin structures spread in a monolayer at the air-liquid interface. This spreading decreases the surface tension, prevents alveolar collapse at the end of expiration, and confers mechanical stability to the alveoli. The surfactant system is in a continuous state of flux, and surfactant is recycled by uptake... 

The typical symptoms of untreated individuals with PKU are the manifestation of the neurotoxic effect of phenylalanine on the central nervous system. A morphological change in the brain in patients with PKU affects both white and gray matter. Microcephaly, where the brain mass can be 80 % of that of a healthy individual, is characteristic feature for many untreated PKU patients [37]. This symptom is caused by myelin structure anomalies that result in a loss of myelin volume, disturbances in cortical neuronal development, diffuse cortical atrophy, and general abnormalities in protein synthesis [37, 39, 40]. 

Cholesterol is a zoosterol that is present in all animal cells. It has a low solubility in water, about 0.2 mg/100 ml. It is the major sterol in human beings and is important as a constituent of various biological membranes, it is particularly important in the myelinated structures of the brain and central nervous system and may constitute up to 170 g/kg. it is the precursor of the steroid hormones, it is also the precursor of the bile acids. 

Fig. 9.1. a An 11-year-old girl with an osteoid osteoma of the left pedicle of the sixth vertebra of the thoracic spine. Note the partially sclerosed nidus together with the dense surrounding hyperostosis, b The dense cortical lamella was perforated by a biopsy cannula with a crown cut. c Subsequently, a radiofrequency single electrode with an active tip of 1 cm was placed into the nidus. To protect adjacent myelin structures a 21-gauge needle was placed into the epidural space for saline flushing... 

In the areas of damage, cerebrosides and DNA are decreased [28]. In vitro experiments reveal decreased uptake of acetate into lipids and of leucine into proteins [29] of both myelin and non-myelin structures of brain but not of spinal cord, suggesting a difference in susceptibility. The concentration of cyanide in in vitro experiments, however, is probably much higher than occurs in vivo. 

---