140 SUBCELLULAR PARTICLES 



approximation. It would indeed be very astonishing if all the mitochondria from 

 all the liver cells had exactly the same ratio of, let us say, glutamic dehydrogenase 

 to cytochrome oxidase. In fact, such a possibility seems almost incompatible, at 

 least for some enzymes, with the histochemical demonstration of zonal dif- 

 ferences within hepatic lobules, as with the existence of several types of cells 

 within the liver. The homogeneity is therefore a statistical one, and the crux of 

 the matter is to appreciate at what stage our techniques become sensitive enough 

 to show up the heterogeneity which undoubtedly exists. Another point is that 

 true heterogeneity can never be entirely disproved. In the case of mitochondria 

 and lysosomes, for example, the evidence is compatible not only with the possi- 

 bility that the two populations are entirely independent of one another, but 

 also with the eventuality that they may be related genetically and linked by transi- 

 tion forms containing enzymes of both groups. This problem concerns the life 

 cycle of the particles and must be settled on other grounds. Even if a relation 

 between the two types of particles should one day be shown to exist, this would 

 not invalidate the distinction which has been made between mitochondria and 

 lysosomes, since the transition would obviously be a drastic one and involve a 

 complete change in biochemical and even structural properties. 



Morphological Identification of Lysosomes. Fractions which had been enriched 

 10- to 14-fold in acid phosphatase have been examined in the electron micro- 

 scope by Novikoflf, Beaufay and deDuve(66). They were found to contain, 

 besides a fair amount of mitochondria and a few microsomal fragments, to 

 be expected from their cytochrome oxidase and glucose-6-phosphatase activities, 

 a large number of distinctive particles which were practically absent in fractions 

 with a low content in lysosomes. These particles, representatives of which can be 

 seen in figures 9 and 10, could be identified with the dense peribiliary bodies de- 

 scribed by Rouiller (82) and by Palade and Siekevitz (73) or with the micro- 

 bodies of Rouiller and Bernhard (83). Their relationship with the newly de- 

 scribed siderosomes of Richter(8o) is not clear. 



Since no other type of particle was recognized in these lysosome-rich fractions, 

 it is tempting to conclude that the biochemical concept of lysosome and the 

 morphological one of dense peribiliary body refer to the same entity. But this can 

 by no means be regarded as proved, especially since both concepts are still some- 

 what imprecise. As mentioned above, the relationship between uricase and the 

 lysosomes is still uncertain and the latter do themselves form a complex group. 

 On the other hand, the dense peribiliary bodies are characteristically polymorphic 

 and the term could therefore cover several distinct types. A direct link between 

 the two entities is apparently provided by the histochemical observations of Holt 

 (48), who has found acid phosphatase to be concentrated around the bile canalic- 

 uli in rat liver. However, this author has observed an almost identical localization 

 for esterase, which sediments quantitatively with the microsomes (100). 



