THE INHERITANCE OF BIOCHEMICAL CHARACTERISTICS 307 



The neurones or nerve cells are characterized biochemically by the 

 nature and concentration of the complex lipides they contain and by an 

 exceptionally high concentration of the enzymes of the priming reactions 

 which consume glucose, and notably of hexokinase. Also the concentration 

 of the a-decarboxylase of glutamic acid (coenzyme: pyridoxal-5 -phosphate) 

 is particularly high in neurones. The system for the amidation of glutamic 

 acid is also very abundant. The neurones are of two types. All the pregang- 

 lionic fibres of the autonomic nervous system are cholinergic as also are the 

 postganglionic fibres of the parasympathetic system and certain postgang- 

 lionic fibres of the sympathetic system (those serving the sweat glands 

 for instance). The motor nerves are also cholinergic. Besides the 

 quantitative aspect of acet^'lcholine synthesis, the cholinergic fibres are 

 characterized biochemically by the mechanism of the transport of this 

 substance along the fibre and its liberation at the extremities. The adrener- 

 gic nerv^es synthesize noradrenaline, apparently by decarbox}4ation of 

 dihydroxyphen^'lserine. The cells of the adrenal medulla synthesize 

 noradrenaline and adrenaline, the latter from phenylalanine as has been 

 proved by using phenylalanine marked with ^*C in the a-carbon, the 

 isotope later appearing in the a-carbon of adrenaline. 



The argentaffin cells of the intestine are differentiated in that they can 

 synthesize 5-hydroxytryptamine, probably via 5-hydroxytryptophane. 

 Another biochemical differentiation of nervx cells is observed in the den- 

 dritic melanocytes which, in the skin and the eye, are specialized for the 



HO 



/\ 



-CHo — CHo — Nrin 



NH 



5 -hydroxy tryptamine 



biosynthesis of m.elanin. By means of their t^Tosinase these cells transform 

 tyrosine to dihydroxyphenylalanine, or DOPA, a substance similar in 

 structure to noradrenaline. The cells of the neurohypophysis, another type 

 of specialized nerve cell, perform a special synthesis, that of vasopressin 

 and oxytocin. 



The cells of the endoderm differentiate into the cells of the salivary 

 glands, the mucous cells of the digestive tract, the exocrine pancreas, the 

 hepatic parenchyma, the parathyroid, the insulin-producing tissue, the 

 thyroid and the pulmonary epithelium. 



The cells of the mucous membrane of the digestive tract and its neigh- 

 bouring glands show many types of biochemical differentiation. The cells 

 of the salivary glands are either of the mucous type or of the serous type. 



