Ill PFS OF DIFFERENTIATED AND EMBRYONIC CELLS 513 



somal and unbound (supernatant) proteins of the pancreas cell showed that the 

 incorporation into the microsomal material took place more rapidly and reached 

 a higher maximal value than the tissue proteins. Accordingly, the microsomal 

 proteins could have been the precursors of the pancreatic enzymes but this ex- 

 pectation was not borne out by the analysis of the purified proteins trypsinogen 

 and chymotrypsinogen (Daly et al., 1955). However, in organs in which feeding or 

 stimulation by pilocarpine does not cause increased protein formation, no increased 

 incorporation into the microsomal protein fraction was observed. Therefore the 

 microsomal activity still seems essential for the formation of the secretory products. 



Rather far reaching differences in the state of the structural elements of the PFS 

 in the embryonic cell may be indicated by the observation that the amount of 

 free cytoplasmic RNA (RNA unattached by particulate material sedimented 

 below 105,000 g) is relatively large in embryos of the movise and of amphibians; 

 which diminishes with ensuing development and is practically absent in the fully 

 differentiated tissues of the adult animal (Brachet and Chantrenne, 1942). This 

 may mean that parallel with the decrease of unattached RNA there is an increase 

 of cytoplasmic, RNA containing, particles. While no direct quantitative measure- 

 ments of the microsomal fraction have been carried out in the experiments cited 

 above, in the sea urchin embryo it was found that particulates of the size and 

 the staining properties of mitochondria become morphologically apparent at 

 the late blastula stages (Gustafson and Lenicque, 1952; Lenicque, Horstadius 

 and Gustafson, 1953; Gustafson and Lenicque, 1955; Shaver, 1956). As mentioned 

 previously the mitochondria in mature cells are the primary sites of the metabolic 

 organization of the energy supply needed for the formation of proteins and there- 

 fore are only indirect indices of the overall capacity of protein formation in a cell. 

 However, the appearance of mitochondria may be indicative of the progressing 

 organization of other particulate elements. In this connection it is of interest 

 that, similar to RNA, cytochrome oxidase accumulates first in the cell matrix 

 and becomes incorporated only secondarily into mitochondria (Boell and Weber, 

 1955). The cytochrome oxidase activity was found actually to increase if cal- 

 culated per unit quantity of mitochondrial material. 



Somewhat more information has become available about the protein forming 

 capacity of the different particulate fractions of embryonic cells as indicated by 

 measurements of amino acid incorporation into their proteins (Hultin, 1953a). 

 During cleavage of the sea urchin egg the incorporation of alanine- i-''*C into 

 proteins of a cell fraction collected at 105,000 g (microsome), is considerably 

 higher than the incorporation into either the fraction which is brought down at 

 a speed as low as at 20,000 g (mitochondria), or the fraction which cannot be 

 sedimented at higher speeds (supernatant). With reference to the discussion of 

 the pathways of protein synthesis in the previous section the suggestion of an exten- 

 sive transamidation during this early phase of sea urchin development is of 

 interest (Hultin, 1953b). Beginning with the development of the mesenchyme 

 blastula, the incorporation of amino acids into the mitochondrial proteins in- 

 creases rapidly and reaches the level of the microsomal fraction. Evidence for an 

 increase of mitochondria has been mentioned above. An increase in the in- 

 corporation of NHj into RNA and into the soluble proteins (Hultin, 1952; 



Literature p. 559 



