608 



VERTEBRATE LIFE AND ORGANIZATION 



Pitixifcajry 



Thymus "" ■ -"Y^ 



roi 



ds 



R. Kidney 



Ovaries (^Female) — 



Stomach* 



Pa.ncreas 



Intestine* 



-Xv 



XK 



Te-stes Cmale) 



Figure 30.1. The human body showing the location of the endocrine glands. The 

 starred organs, though not primarily endocrine glands, do secrete one or more hormones. 



249. The Thyroid 



All vertebrates have a pair of thyroid glands located in the neck. 

 In mammals the two glands are located on either side of the larynx 

 and are joined by a narrow isthmus of tissue which passes across the 

 ventral surface of the trachea near its junction with the larynx. The 

 thyroid has an exceptionally rich blood supply, which reflects its func- 

 tion as an endocrine gland. The thyroids develop as a ventral outgrowth 

 of the floor of the pharynx but the connection with the pharynx is 

 usually lost early in development. In a microscopic section the thyroid 

 is seen to consist of many hollow spheres, called follicles. Each follicle is 

 composed of a single layer of cuboidal epithelial cells surroiniding a 

 cavity filled with a gelatinous material called colloid, secreted by the 

 follicle cells (Fig. 30.2). 



The follicle cells have a remarkable ability to accumulate iodide 

 from the blood. This is used in the synthesis of the protein thyro- 

 globulin which is secreted into the colloid and stored. Thyroglobulin 

 is a large molecule and not readily diffusible into the blood stream, 

 but proteolytic enzymes in the colloid hydrolyze thyroglobulin to its 

 constituent amino acids, one of which is thyroxin, a derivative of the 

 amino acid tyrosine containing 65 per cent iodine. Thyroxin passes into 

 the blood stream where it is transported loosely bound to certain 

 plasma proteins. In tissues thyroxin, which contains four atoms of 



