ANIMAL COLLAGENASE AND COLLAGEN METABOLISM 689 



differently to thyroid hormone. Perhaps in the loci where very in- 

 tricate remodeling takes place, as in the mouth parts, the differential 

 response to thyroid hormone in highly localized regions plays a key 

 role. By analyzing small in vitro systems we hope to be able to map 

 out regions of differential response and ultimately determine the 

 nature of this selectivity. 



It might also be asked whether the cells responsible for collagen 

 synthesis can produce collagenase or whether there is segregation 

 of function in different cell tvpes. In either case one wonders what 

 the stimuli are which tell the cells to synthesize or digest collagen. 

 Would both potentialities exist in the same cell or would there be 

 further specialization? At the present time we have no idea as to 

 which cells produce collagenase. We hope to answer the question 

 with cell suspensions and single-cell cultures. 



With regard to the beha\dor of mesenchymal tissues in morpho- 

 genesis, it is conceivable that the extracellular framework plays a 

 role in limiting growth and form of an organ, either as competitor 

 for space, or as a contact inhibitor. If this turns out to be the case, 

 then the remodeling of mesenchvme under hormonal control may 

 have significance beyond that of a model system. 



Summary 



Resorption and new formation of large collagen-containing struc- 

 tures have been explored at the chemical level in a controllable 

 remodeling svstem, the metamorphosing and resting tadpole. Ex- 

 periments on collagenolvtic activity and metabolic turnover of col- 

 lagen have been performed in thyroxin-induced metamorphosis, 

 comparing tadpole tail fin collagen (which is completely removed 

 during metamorphosis) with back skin collagen (which undergoes 

 increased production ) within the same animals. 



The most dramatic change in composition in the tissues of meta- 

 morphosing animals is a considerable loss of water, with collagen 

 and cell concentrations proportionately increased. 



Tritiated proline was injected intraperitoneally into thyroxin- 

 treated and control animals. There was greater net incorporation of 

 labeled proline in the back skin collagen of metamorphosing animals 



