118 CONTROL MECHANISMS IN CELLULAR PROCESSES 



stance originates on chromatic filaments to be concentrated into 

 droplets, finally confluent into a new nucleolus. In the meantime, 

 the cellular basophilia becomes stronger and the number of ribo- 

 some particles in the cell increases (Siekevitz and Palade, 1958). 

 These cytological observations clearly indicate a renewal of activity 

 in RNA production. An actual increase of RNA content and of P-''- 

 incorporation into RNA was detected in the activated pancreas by 

 Russian authors ( Guberniev and Iljina, 1950 ) while no appreciable 

 increase of RNA has been measured by others (Daly and Mirsky, 

 1952; Morris and Dickman, 1960). 



Despite a lack of final proof, it can be considered that the release 

 of nucleolar RNA and further RNA synthesis precede enzyme syn- 

 thesis in the activated pancreas. Pilocarpin or a nervous stimulus 

 is the first cause of activation, but the action of these agents is pri- 

 marily toward the liberation of enzvmes from the cell and the new 

 synthesis is a secondary eftect. If this is so, an intracellular control 

 system should exist, giving signal to the nucleus when enzyme re- 

 serves are depleted. Since the substrates for most of the pancreatic 

 enzymes are found only in the digestive tract, a control similar to 

 that of inducible enzymes cannot be considered. Also, the activation 

 should start the production of several quite different enzymes and 

 could not be induced by one simple substrate. Until this system of 

 activation of enzyme synthesis is better understood, we can only say 

 that the activation process should act simultaneously at several gene 

 loci and that the depletion of cellular enzymes ( or RNA ) gives this 

 signal. 



In the salivary glands, as in the pancreas, pilocarpin stimulates 

 saliva secretion, but it also induces a nearly immediate new synthesis 

 of salivary amylase (Schneyer and Schneyer, 1957). The RNA con- 

 tent of activated salivary glands increases (Laird and Rarton, 1958). 

 In the stomach, RNA production is increased during active HCl 

 secretion (Yakhnina, 1956). All these and other similar cases will 

 have to be studied in more detail before the exact relationship be- 

 tween activation of secretion and RNA production can be deter- 

 mined. 



Hormones. Endocrine glands can suddenly increase the produc- 

 tion of hormones upon activation. Some of the hormones are of a 

 proteinaceous nature, where activation would mean the induction 

 of new protein formation. Most hormones are smaller molecules, 



