102 The Chemistry of the Injured Cell 



CHEMISTRY OF MYELINATION 



The myelin sheath is composed largely of lipids radially 

 orientated in the more or less concentrically arranged lipoprotein 

 laminae. The lipid components include cholesterol, sphingomyelin, 

 cerebroside and probably phosphatidyl serine. Sphingomyelin is 

 composed of fatty acid— probably C 2 4 lignoceric acid or stearic or 

 neuronic acid— sphingosine and phosphoryl choline. Cerebroside is 

 a component of fatty acid, sphingosine and hexose. The sphingo- 

 sine-containing lipids are among the most characteristic constitu- 

 ents of the myelin sheath. Serine is an effective precursor of sphingo- 

 sine. After parenteral injection of [3- 14 C] serine into very young 

 rabbits radio-active cholesterol, sphingomyelin, lecithin, cerebro- 

 side and cephalins can be recovered from their brain, spinal cord 

 and sciatic nerves. There is little change in the radioactivity of these 

 compounds over the next 192 days so that it seems that once de- 

 posited in the myelin sheath at the time of its formation they under- 

 go little subsequent turnover. (Davison et al., 1959) . 



Cholesterol 



Sphingomyelin 



Serine — ^ Sphingosine 



Cerebrosides 



Phosphatidyl serine 

 Phosphatidyl ethanolamine 



So, too, radio-active-C labelled cholesterol when injected into the 

 yolk sac of newly hatched chicks disappears much slower from the 

 brain than from the liver and is still present in the central nervous 

 system after 320 days (Davison et al., 1958, 1959) . A slow turn- 

 over has also been demonstrated in the grey matter of the brain of 

 young rabbits, but not in the white matter. 



WALLERIAN DEGENERATION OF THE 

 PERIPHERAL NERVES 



When a peripheral nerve is divided or crushed it undergoes 

 Wallerian degeneration. The axon disintegrates and disappears 

 while the myelin sheath at first fragments and ultimately is de- 



