218 



E. V. COWDRY 



Secretory Phenomena. Cell Polarity and Mode of Secretion. Bens- 

 ley (6) has* advanced the following hypothesis as to the mode of secretion. 

 He claims that the original polarity of the thyroid .cell has been reversed, 

 so that it now secretes in the direction of the perifollicular blood vessels, 

 and not invariably into the lumen of the follicle, as has been generally 

 supposed. He compares it with an acinus cell of the pancreas, which 

 possesses ordinary proximodistal polarity, substances passing from the 

 blood stream (B. V.) through the basement membrane into the cell 

 and through it in a distal direction toward the duct, as is illustrated 

 by the direction of the arrow in Fig. 8A. In the thyroid cell, on the 



km. 



A 



B 



Kig. 8. Diagram illustrating the polarity of an acinus cell of the pancreas (A) as 

 compared with a cell of the thyroid epithelium (B). 



other hand (Fig. 813), the condition is reversed, the materials passing 

 from the blood stream (B. V.) into the cell, then out of the cell again 

 and back into the blood stream, as is also indicated by the arrow. As 

 structural evidence of this reversal in polarity, he cites the location of the 

 secretion antecedents next the basement membrane, just as in the pancreas 

 the zymogen granules are heaped up near the lumen of the duct ; and the 

 presence of fat droplets at the opposite end of the cell, as in the pancreas. 

 His contention is also supported by convincing experimental evidence. 



Colloid Formation and Resorption. In explanation of the occurrence 

 of intrafollicular colloid, he suggests that, when the rate of secretion is 

 in excess of the demand, it is passed in the reverse direction into the 

 follicular cavity. He writes: "According to this conception of thyroid 

 secretion, the colloid in the thyroid vesicles is per se no measure of the 

 activity of the gland at the moment of observation, though its consistence 





