RNA AND CONTROL OF CELLULAR PROCESSES 119 



a big class of them belonging to steroids, and the activation of their 

 production may be at a level quite remote from the nucleus. Still, 

 specific enzymes are needed for production of these hormones and 

 activation may mean new enzyme synthesis. The possibilit) that 

 the nucleus, through RNA "synthesis, exerts control over hormone 

 production, will be examined. 



One of the best studied examples of hormone production is that 

 of the thyroid gland. Besides the active principle, thyroxin, the 

 gland produces also a specific thvreoglobulin which functions in 

 hormone accumulation and excretion in a still unknown manner. 

 Consideration of the tlnroid gland can be particularly instructive 

 because of a mutual control system between the thyroid and the 

 hx'pophysis. Pituitary thyreotropic hormone stimulates thvroxin pro- 

 duction while thyroxin inhibits the production of thyreotropic hor- 

 mone. The relation between these two glands shows a feedback 

 system similar to that found in many biological processes, for exam- 

 ple, inducible or repressible enzymes. 



Does the thvreotropic hormone activate the thyroid gland at the 

 nuclear level? In a resting thyroid gland, the acini have a flat epi- 

 thelium surrounding large vesicles of "colloid." Upon activation, the 

 colloid shows signs of resorption: vacuoles appear near the cellular 

 peripherv and the amount of colloid slowly diminishes (Ponse, 1951; 

 Gross, 1957). At the same time, the epithelial cells grew cubic and 

 finally prismatic. The nuclei become enlarged and the nucleoli more 

 prominent (Koch, 1958; Herman, 1960). Cytoplasmic basophilia 

 increases together with ergastoplasm, which becomes vesiculated 

 ( Dempsey and Peterson, 1955 ) . The cytological picture gives every 

 evidence of active RNA production. Direct measurements of RNA 

 have confirmed this: RNA increased in thyroid glands activated with 

 thvreotropic hormone and decreased in glands suppressed by the 

 injection of thyroxin (Matovinovic and Vickery, 1959). There is no 

 doubt that activated thyroid glands produce large quantities of hor- 

 mones (including thyreoglobulin) and this increase in hormone 

 production is paralleled by RNA synthesis. 



As pilocarpin activates the cells in the pancreas, so does thyreo- 

 tropic hormone in the thyroid gland. The exact mechanism of 

 action of thvreotropic hormone is still unknown. The application 

 of this hormone is followed by so many events that it is difficult to 

 separate the primarv from the secondary effects. Consideration of 

 the exact timing of all the effects of activation, starting at very short 



