DIFFERENTIATION AND PROTEIN SYNTHESIS 115 



Retaining cells do not possess a reticulum in association with RNA 

 particles, but numerous smaller vesicles with smooth-surfaced membranes, 

 i.e. membranes not associated with dense particles, are observed (Plate 11). 

 In secreting cells such vesicles also occur, often with an elongated flattened 

 profile, in compact clusters located adjacent to and distal to the nucleus 

 (Fig. 49). This location is that of the Golgi apparatus of the classical 

 histologists, who have long disputed its structure and function (Baker, 

 1955); very probably this group of smooth membranes is the electron 

 microscopic image of the Golgi apparatus. A certain unity of organization 

 and coherence is suggested by the fact that the clusters can be separated 

 from homogenized cells (Dalton and Felix, 1956), but the greater struct- 

 ural detail revealed by electron microscopy has not finally clarified their 

 function. No connexion with protein synthesis has been demonstrated, 

 though the opinion prevails that they are associated with the secretory 

 phase and probably it is the cell centre for membrane assembly when these 

 are required for special functions. 



Biochemistry of Protein Synthesis 



The combination of the results given by the various forms of microscopy 

 has yielded a sufficiently-detailed and usable picture of the intracellular 

 structures associated with at least one pathway of protein synthesis. 

 Unfortunately, in the present state of histochemistry, it is impossible to 

 investigate the composition and activity of the cellular organelles with the 

 same degree of resolution. For most biochemical work, a considerable 

 weight of any cell derivative is required and this has led to the need to 

 separate from large numbers of cells a sufficient weight of well-defined, 

 selected cellular constituents, such as nuclei or mitochondria for analysis. 

 The most commonly-employed procedure introduced by Claude (1938) is 

 that of homogenizing and fractioning a mass of cells, i.e. of rupturing the 

 cell membranes mechanically and fractionating the homogenate by 

 centrifugation. Most of the work on mammalian tissues has been carried 

 out on the liver, since this large organ is easily homogenized, and nothing 

 comparable has been attempted with an epidermal tissue. 



When liver is homogenized mechanically and the product suspended in 

 a sucrose solution (0-25-O88M) four fractions are conventionally recog- 

 nized in a fractional centrifugation: (a) a nuclear fraction, (b) a mito- 

 chondrial fraction, (c) a microsome fraction, and (d) a supernatant. Such 

 fractions form the basis of most biochemical studies on the activity of the 

 intracellular elements recognized microscopically. The fractions are by no 

 means pure, nor do they consist necessarily of single components. It is 

 preferable to control their composition by electron-microscopical methods 

 (e.g. Palade and Siekevitz, 1956). 



A study of the mitochondrial fraction has shown that many of the 



