THE ENDOCRINE SYSTEM 



625 



Chemical analysis of the adrenocorticotropic hormone, ACTH, has 



shown that the active fraction is a peptide containing 39 amino acids. 

 The sequence of these amino acids is now known. In recent years ACTH 

 has become famous because of the remarkable results it sometimes gives 

 in the treatment of allergies and arthritis. However, the prime, and 

 perhaps the only physiologic function of ACTH is to stimulate the 

 adrenal cortex to grow and to release cortical steroids. The injection of 

 ACTH reduces the amount of cholesterol and ascorbic acid in the 

 adrenal cortex, presumably because they are used in the synthesis of 

 steroids. The injection of ACTH stimulates, within a few minutes, a 

 marked increase in the amount of hydrocortisone in the blood. The 

 adrenal cortex undergoes a prompt atrophy after the removal of the 

 pituitary and can be returned to normal by the injection of ACTH. 



The extirpation of the pituitary also causes atrophy of the thyroid. 

 The gland decreases in size and the follicle cells become flattened. The 

 thyroid is returned to normal by the implantation of a pituitary gland or 

 by the administration of an extract containing thyrotropin. The injection 

 of thyrotropin in a normal animal causes growth of the thyroid and 

 thickening of the follicle cells so that they become columnar rather than 

 cuboidal epithelium (Fig. 30.2). 



The ovaries or testes of a hypophysectomized young animal never 

 become mature; they neither produce gametes nor secrete enough sex 



Figure 30.10. The effect of growth hormone on the dachshund. Top, normal dog. 

 Bottom, dog injected with growth extract for a period of six months. (From Evans, 

 Simpson, Meyer and Reichert.) 



