228 



Colchicine 



that this is a tool for the study of mitotic activity, whether stimulated 

 or depressed. Results reported in Chapter 10 support this opinion. 

 9-4-i: Pituitary hormones. Prolactins the hormone stimulating 

 secretion of the mammary gland in mammals, was used in one of 

 the first and most spectacular experiments of this type. In birds, this 

 hormone stimulates ilip (rop-sac. This organ secretes "milk" by a 



TABLE 9.5 



MiTosii IN THE Crop-Sac of the Pigeon After Prolactin Stimulation 



(Colchicine is injected 9 to 1 1 hours before the animal is killed.) 



(After Leblond and AUen^s) 



process which has no relation to that observed in mammals. The 

 bird's "milk" is made of fat-laden cells desquamating from the thick- 

 ened epithelium of the crop-sac. Table 9.5 shows the increase of the 

 mitotic index for this epithelium in pigeons injected with prolactine 

 and colchicine. •'**• -^^ In one animal, 5-^ per cent of all epithelial crop- 

 sac nuclei were found to be in a condition of arrested mitosis. The 

 average increase of the mitotic index is 37-fold, and calculation based 

 on the assumption of arrest only, leads to the result that the pro- 

 lactine-stinuilated cells must divide in about 16 minutes. It is not 

 certain that such a calculation is correct, because many factors, for 

 instance cellular differentiation, are involved. Also, from the pub- 

 lished photomicrographs-^"' it is not evident that the thickness of the 

 control and the colchicinized epithclia are comparable. Whatever the 

 significance of these quantitative estimations may be, colchicine 

 demonstrated clearly that connective tissue cells, and muscular cells 

 of the crop-sac wall also divided under the influence of prolactine. 

 This fact had never been observed.^*'- 5" 



The thyroid-stimulating hormone, thyrotropin, also increases the 

 cell divisions in the thyroid. 1 his is made much more evident by 

 spindle poisoning. In controls 6.3 mitoses were found per 100 thyroid 



