512 MOLECULAR MECHANISMS OF DIFFERENTIATION 5 



have been stated earlier. In spite of these reservations Waddington's resuhs are 

 of interest because they raise the question as to whether the embryonic cells are 

 particularly sensitive to the action of nucleic acid analogs at the time when pro- 

 duction of new types of proteins might be expected. If further research leads to a 

 corroboration of these results by the use of more specific purine analogs and a 

 quantitative detailed analysis of protein formation in different organs, some prog- 

 ress in the clarification of the role of nucleic acids in protein synthesis in embryo- 

 nic tissues could be expected. Some explorations, described later on, of the relation- 

 ship of RNA content and protein synthesis, have been carried out in connection 

 with erythropoiesis and regeneration of the liver and an extended examination of 

 these tissues should prove to be a valuable approach in exploring this problem. 

 Other experimental material suggesting a relationship of RNA to the control of 

 protein formation in induction is more fully discussed in Chapter 4. 



{b) Structural aspects oj the PFS: Protein formation in microsomes a?icl mitochondria 



In electron microscope studies mitochondria appear as separate units with a 

 distinct ultra-structure whereas particulate elements of the size of microsomes 

 apparently are part of a fine network which has been designated as the endoplas- 

 mic reticulum. The microsomes are apparently released from this cytoplasmic 

 network by the conventional homogenization processes (Palade and Siekevitz, 

 1956; Novikoff, 1956) 



The investigation of protein formation in isolated cytoplasmic particles was 

 initiated by Siekevitz (1952) who measured the uptake of labelled amino acids 

 into these cell components^ The most rapid uptake was found to take place in the 

 microsome fraction while mitochondria and nuclei showed less activity. Resvdts 

 similar to those of Siekevitz were obtained by Zamecnik and his group who could 

 demonstrate that a rapid incorporation into microsomal proteins occurs in vivo 

 (Keller, Zamecnik and Loftfield, 1954). Incorporation into microsomes in vitro 

 will proceed even anaerobically if the non-particulate fraction and ATP are added 

 to the preparation as the sole source of energy (Zamecnik and Keller, 1954; 

 Zamecnik et al., 1956). Aerobically, a sufficiently high ATP level can also be main- 

 tained by the addition of a more complex source of metabolic energy such as 

 mitochondria and some utilizable substrate. 



The role of microsomes in the net synthesis of the specific proteins of differenti- 

 ating cells was elucidated in a series of interesting experiments with pancreas 

 cells where formation of the enzymatic cell proteins such as the pancreatic enzymes 

 can be influenced by various drugs and by fasting and feeding (Allfrey, Daly and 

 Mirsky, 1953). These authors found that the same stimuli which enhance the 

 quantity of secreted enzymes also increase the rate of amino acid incorporation into 

 the proteins of the microsome fraction. Time curves of the uptake into the micro- 



^ From nuclei, at least active enzyme proteins are lost during their isolation unless special 

 precautions are taken. Because of the possibility of similar losses it is not certain whether 

 the differences in the activities of the cell particles, when isolated according to more con- 

 ventional techniques, give a correct picture of the tracer amino acids in the total protein 

 moiety. 



