GLYCOaENIC IXFIl/rh'ATION 433 



is quite possible that both of these processes represent merely the 

 reversible action of an intracellular enzyme, but this has not been 

 estal)li8lio(l. We do know, however, that soon after death the intra- 

 cellular glycogen is rapidly converted into dextrose.^" 



Properties of Glycogen. — Glycogen is frequently called an "animal starch," 

 having the same f^eiieral composition as the starciies (('bIIk.Oo)^, and apparently, 

 like the starches, it represents a relatively insolul)le resting stage of sugar in 

 the course of metabolism. It is readily soluble in water, forming an opalescent, 

 colloidal solution, and, therefore, has no efTect on osmotic pressure, and it is not 

 difTusible."^ Because of its solubility and the rapidity with which postmortem 

 change to dextrose occurs, specimens that are to be examined microscopically for 

 glycogen must be hardened while very fresh in strong alcohol, in which glycogen 

 is insoluble.*" One of the most characteristic reactions is the port-wine color 

 given by glycogen when treated with iodin; this reaction may be applied micro- 

 scojncally, solution of the glycogen being avoided by having the iodin dissolved in 

 a solution of gum arable or in glycerol. Salivary ptyalin rapidly converts gly- 

 cogen into glucose, and this reaction may also be used microscopically to prove 

 that suspected granules are glycogen. However, failure to find glycogen micro- 

 chemically does not alwaj's mean its absence from a tissue.*' 



Physiological Occurrence 



According to Gierke, the normal glycogen of cells resembles fat in that part of 

 it disappears during starvation, while the rest cannot be removed in this way and 

 probably is something more than a reserve food-stuff. In distribution glycogen 

 somewhat resembles fat, being abundant in the liver*'^ and muscles, but Gierke 

 considers that the microscopic evidence of the quantity of glycogen present in the 

 cell agrees better with the results of actual chemical analysis than is the case with 

 fat. Ilusk,*^ however, finds only a general agreement, with marked exceptions. 

 Neither iodin nor Best's carmin stain are absolutely specific for glycogen, but 

 Gierke believes that we may safely consider a substance as glj^cogen when it is 

 homogeneous, rather easily soluble in water and more so in saliva, gives the usual 

 iodin reaction, and stains bright red with Best's carmin solution.*^ With these 

 controls, the microscopic findings were found to agree closely with the results of 

 direct chemical analysis, and glycogen was found microscopically visible in muscle, 

 liver, lung, heart, uterus, and skin (but not in the brain,*^ where it may be demon- 

 strated chemically in minute quantities). 



Glycogen is commonly said to be especially abundant in fetal tissues, but it is 

 not present in all fetal cells, *^ nor is it always most abundant in the most rapidly 

 growing tissues. Although both fat and glj'^cogen are quite abundant in fetal 

 muscle and liver tissues, the liver of early embryos does not contain either.*' 

 Invertebrates and the lower vertebrates have more than the higher forms. In 

 mammalian adults the liver and muscle contain the most glycogen, cartilage 



'* Literature concerning physiology of glvcogen bv Pflliger, Pfliiger's Arch., 

 1903 (96), 398; and Cremer, Ergeb. der Physiol., 1902 (1. Abt. 1), 803. 



'^ See Gatin-Gruzewska, Pfluger's Arch.. 1904 (103), 282. 



*° According to Helman (Cent. f. inn. Med., 1902 (23), 1017), glycogen may be 

 found in specimens preserved in alcohol as long as fifteen years. 



*i Bleibtreu and Kato, Pfluger's Arch., 1909 (127), 118. 



*^ In the livers of two executed criminals Garnier (Comp. Rend. Soc. Biol., 

 1906 (60), 125) found respectively 4 per cent, and 2.79 per cent, of glycogen. 



»3 Univ. of California Publ., Pathol., 1912 (2), 83. 



** Concerning staining methods see Ivlestadt, loc. cil.'''' 



** Mav be present in fetal nervous tissues. (Gage, Jour. Comp. Xeurol., 1917 

 (27), 451). 



«8 See Glinke, Biol. Zeit., Moskau, 1911 (2), 1. 



" Adamof! (Zeit. f. Biol., 1905 (46), 288) contests the idea that the amount 

 of glycogen is in direct relation to growth energy; see also Mendel and Leaven- 

 worth (Amer. Jour. Physiol., 1907 (20), 117), who found no particular abundance 

 in the tissues of the fetal pig. 

 28 



