314 CM. JACKSON 



pret it as an atrophic type, due perhaps largely to the pressure 

 on the gland from adjacent organs. Although these follicles are 

 filled with dense, deeply-staining colloid, it is unlikely that the 

 flattening is due entirely to consequent endofollicular pressure, 

 as follicles are sometimes seen in which the epithelium on the ex- 

 ternal surface is much more flattened than that on the inner 

 aspect of the follicle. The socalled 'colloid' cells of Langendorff 

 (frequently described by various authors) probably belong to 

 this atrophic type, and have no specific functional significance. 



In addition to these peripheral flattened atrophic cells, men- 

 tion must be made of more advanced types of degeneration, al- 

 though the latter appear much less frequent in the young rat at 

 10 weeks than in older animals. These degenerative cells may 

 occur in any part of the gland, either singly or involving an en- 

 tire follicle (occasionally a regional group of follicles). The de- 

 generating cells may remain in the follicular wall or may be 

 desquamated into the follicular cavity. In rare cases the desqua- 

 mated epithelium may replace the colloid with an irregular mass 

 of cells in various stages of degeneration. 



In the degenerating cells, the cytoplasm loses its typical light 

 granular structure and becomes vacuolated and reticular in ap- 

 pearance, later disintegrating into irregular, usually deeply-stain- 

 ing (eosinophile) masses. The nucleus may be hypochromatic 

 (karyolytic) in type, but more frequently presents various grades 

 of pycnosis (rarely karyorrhexis) , especially in the desquamated 

 cells. 



As to the frequency with which these degenerative types of 

 cell occur in the thyroid of (apparently) normal rats at 10 weeks, 

 it may be stated that of 9 glands carefully examined in serial 

 sections, one showed rather extensive degeneration, one a well 

 marked area (much less extensive), and four showed traces or 

 small areas in early stages of degeneration. Thus in a majority 

 of the glands, at least slight traces of degeneration could be 

 fovmd, even in normal, apparently perfectly healthy animals. 



In older rats (from 3 to 15 months of age), the normal struc- 

 ture of the thyroid gland is essentially similar to that described 

 for the younger rats. The follicles average slightly larger, the 



