Colloid of the Thyroid 



495 



cells. With increasing age the thyroid again enters upon a stage 

 of colloid accumulation. In animals 4.5 years of age, 56 per 

 cent, of the thyroid mass is colloid. This relative increase 

 is not due entirely to an absolute colloid increase, but also to an 

 absolute decrease of the epithelium. 



The colloid was studied on sections stained, according to 

 Kraus' method, with polychrome methylene blue and acid 

 fuchsin. 



Colloid is elaborated long before follicles are formed, at a 

 time when the cells still contain yolk and are devoid of gran- 

 ules, a structure frequently considered to be prerequisite to col- 

 loid elaboration. In thyroids of larvae fed exclusively thymus 

 gland, the cells remain permanently primitive, consisting almost 

 entirely of nucleus, possess very small amounts of plasma and 

 are devoid of granules. Yet these cells elaborate large quantities 

 of colloid (55 per cent, of the total thyroid mass). 



At first colloid is elaborated only by the formation of intra- 

 cellular colloid globules of small size which stain red, the color 

 of newly formed colloid. Owing to the increase in the number 

 of cells containing intracellular globules, two or more globules 

 may touch each other, become free, and form, by fusion, a pri- 

 mary follicle. Or the globule may grow within the cell, become 

 free by retraction of the surrounding cell plasma and form a pri- 

 mary follicle. Primary follicles form, by fusion, the large sec- 

 ondary follicles. Colloid elaboration by intracellular globules, 

 now in the individual cells of the follicle walls and in the cells 

 of the reserve cell masses, continues, even after the formation 

 of secondary follicles. These globules may grow within the cell 

 to a large size, become an extracellular colloid mass in the folli- 

 cular wall, by retraction of the cell plasma, and fuse with the 

 main colloid mass ; or they may be extruded by the inner cell end 

 directly into the follicle recognizable for some time by the red 

 coloration. The latter is a permanent mode of colloid elabora- 

 tion. 



Another mode of colloid elaboration is the elaboration of col- 

 loid from granules, the predominant mode after secondary folli- 

 cles have formed. At first, fine granules of reddish color are 

 scattered throughout the plasma. Gradually they increase in 

 number and in size and tend to crowd near the inner surface of 

 the cell, where a reddish substance accumulates between the 

 granules, apparently the result of the fusion of the granules. 

 It seems that this substance can diffuse directly through the cell 



