120 CONTROL MECHANISMS IN CELLULAR PROCESSES 



time intervals, may show the real first cause. Unfortmiately, most 

 endocrinological studies are concerned with effects following experi- 

 mental interference after hours or even days. Many effects of acti- 

 vation may be too small to be detected in a shorter time, but with 

 refined techniques, especially the use of tracers, it should be pos- 

 sible to study the response within minutes. 



Some responses to activation are slow enough so that longer ob- 

 servation periods have been sufficient. Cytological changes appear 

 first. The total uptake of P^- is activated immediately (Lamberg 

 et al., 1955), but it is a question whether this reflects an increase 

 in nucleic acid synthesis. Two hours after the injection of thyre- 

 otropic hormone, the colloid content decreases and a release of 

 iodine, as measured by P^\ is observed (Wolff, 1951; Reichlin and 

 Reid, 1955). An increase in the uptake of P^^ by the gland starts 

 24 to 48 hours later (Wahlberg, 1955). Activation of iodine metab- 

 olism must really mean the activation of the corresponding enzymes 

 and it would not be impossible that activation of RNA production 

 is the real cause. 



It is estimated that several enzymes are needed in the iodine 

 incorporation process, so that activation should act on several sites 

 simultaneously. However, it would suffice to activate one key en- 

 zyme of the chain which would induce the others in sequence, as 

 in sequential enzyme induction studied by Stanier ( 1950 ) . Which- 

 ever the case, thyreotropic hormone can be considered as a specific 

 activator of genie activity in the thyroid cell. It is interesting to 

 note that this hormone is a polypeptide of relatively low molecular 

 weight. 



Thyroxin acts on the hypophysis by suppressing the production 

 of thyreotropic hormone. It has been possible to find which cells 

 in the hypophysis are producing thyreotropic hormone and follow 

 their behavior under different conditions of hormone production 

 (Purves and Griesbach, 1951 ) . In the normal hypophysis these cells 

 are characterized as basophil cells, rich in RNA. After suppression 

 with thyroxin, the percentage of basophil cells diminishes and after 

 stimulation of thyreotropic hormone production, it increases ( Gries- 

 bach and Purves, 1943). Basophils of the stimulated hypophysis 

 increase in size and their nuclei become bigger and contain several 

 prominent nucleoli (Stevens et al., 1955). This cytological picture 

 again indicates direct involvement of RNA production in the activa- 

 tion of the hormone synthesis of the hypophysis. 



