THE PITUITARY GROWTH HORMONE 355 



tivity. Arrest of sexual development is accompanied by an involution 

 of the thyroid and adrenal glands and a concomitant reduction in their 

 secretory functions, with resulting characteristic metabolic deficiencies. 



That not the entire pituitary gland was implicated in these pro- 

 found alterations was evident to Aschner (1912), who was unable to 

 detect skeletal changes in dogs from which only the posterior lobe 

 of the pituitary was removed. This was amply confirmed in subse- 

 quent studies, with the conclusion that removal of the anterior 

 lobe or adenohypophysis was responsible for the observed eflFects of 

 hypophysectomy. 



While the pituitary gland appears to be essential for normal 

 growth and development during infancy and adolescence, the astonish- 

 ingly rapid growth of the mammalian fetus seems to be relatively unin- 

 fluenced by its secretions, since the decapitated rat and rabbit fetus 

 continue to grow at normal or nearly normal rates (Jost, 1953). Simi- 

 larly, removal of the maternal pituitary gland does not interfere in a 

 major fashion with fetal growth (Knobil and Caton, 1953; Smith, 

 1954 ) . When rats are hypophysectomized in the neonatal period, they 

 continue to increase in size, albeit at a diminished rate, until they 

 reach 30 days of age ( Asling et al., 195^) ) . This suggests that the pitui- 

 tary-independent growth processes of the fetus remain functional for 

 some time after birth, and that at some critical time in post-natal de- 

 velopment the pituitary gland assumes the functions which theretofore 

 were exercised by more primitive regulatory mechanisms. 



The pituitary growth hormone 



Although it was clearly established by the studies of pituitary 

 gland ablation that the hypophysis secretes a substance or substances 

 which sustain the morphological integrity and secretory function of 

 the gonads, the thyroid, and the adrenal and permit normal growth to 

 proceed, it was not at all clear whether the pituitary gland influenced 

 somatic development directly or through the mediation of one or sev- 

 eral of its "target" glands. The concept that the anterior lobe of tlie 

 pituitary gland secretes a specific hormone necessary for growth had 

 its origin in the remarkable observation by Evans and his collaborators 

 (1923) that the administration of saline extracts of bovine pituitary 

 glands to normal rats accelerated the growth rate of these animals, 

 with the eventual production of giant individuals. Similar findings were 

 made in dogs (Evans et al, 1933), with the now classic conclusion 

 that the animals stimulated to achieve supranormal size by the injec- 

 tion of the extract had a conformation resembling that seen in acrome- 

 galic patients. 



In these earlv studies, evidences of increased gonadal size and 



