The Endocrine Glands - 413 



The production or release of TSH and 

 ACTH respectively is controlled by the levels 

 of thyroxin and Cortisol present in the cir- 

 culation. If the level falls off, the production 

 of the particular trophic hormone is stimu- 

 lated. Then, as the level rises, production is 

 inhibited. In other words, a balance, or 

 homeostasis, is maintained by direct inter- 

 action between the product of the target 

 gland and the particular group of pituitary 

 cells responsible for producing and releasing 

 the particular trophic hormone of that gland. 

 Control of the other pituitary products is 

 more obscure. The hypothalamus — that part 

 of the brain to which the pituitary is con- 

 nected (Fig. 22-8) — appears to be important. 

 But whether control is mediated by nerve 

 impulses, by neurosecretions (see below), or 

 by both, remains an open question. The 

 pituitary receives a small portal vein (p. 333) 

 directly from the capillary bed of the hy- 

 pothalamus and therefore it seems likely that 

 neurosecretions are at least partially involved. 

 Moreover there is good evidence to indicate 

 that vasopressin and oxytocin are synthesized 

 in the hypothalamus and merely accumu- 

 lated and released by the posterior lobe of 

 the pituitary gland. 



OTHER ENDOCRINE ACTIVITIES 



The placenta augments the hormonal out- 

 put of the ovaries, and in addition, produces 

 a potent gonadotrophic hormone, which 

 simulates the action of the luteinizing hor- 

 mone (LH) of the pituitary. During preg- 

 nancy large amounts of this placental hor- 

 mone are present in the urine, and this fact 

 forms the basis for a widely used pregnancy 

 test. Pregnancy urine, injected into a mature 

 but virgin rabbit, stimulates ovulation. A 

 blutpunkt on the rabbit's ovary indicates the 

 ovulation, and the female urine usually shows 

 a positive reaction by the end of the sixth 

 week of pregnancy. 



A recently recognized hormone of the 

 mammalian endocrine system is relaxin. Re- 

 laxin acts upon the pubic symphysis by 



loosening this ligamentous connection and 

 allowing the pelvic bones to separate slightly 

 during childbirth (Fig. 21-17). The hormone 

 appears to be a complex polypeptide pro- 

 duced partly by the ovary and uterus and 

 partly by the placenta. Injections of relaxin 

 extracts cause a swelling of the cartilage of 

 the pubic symphysis and a softening of the 

 fibrous elements. However, these changes do 

 not occur unless the animal first has been 

 primed by a series of estrogen treatments. 



Erythropoietin. Erythropoietin, a powerful 

 stimulant to the production of erythrocytes 

 by the red bone marrow, represents another 

 hormone that is beginning to receive wide- 

 spread recognition. This substance, appar- 

 ently a glycoprotein of low molecular weight, 

 has been extracted from the plasma and 

 urine of anemic animals (including man) and 

 studied intensively by A. S. Gordon and co- 

 workers at New York University. The stimu- 

 lus for the production of erythropoietin, 

 probably by the kidney, is any form of oxygen 

 deficiency — imposed by high altitude, exces- 

 sive loss of blood, or other condition of 

 anemia. Extra red cell production at high 

 altitude or after hemorrhage has been ob- 

 served for many years, but the basic mech- 

 anism for the response remained obscure 

 prior to these recent studies. 



The Pineal Body. The pineal body (Fig. 

 22-1) arises as an outgrowth from the roof of 

 the forebrain, although later the pineal be- 

 comes flanked by overgrowth of the cere- 

 bral hemispheres. In the ancient reptiles, the 

 pineal was associated with the development 

 of a median sense organ, situated on the roof 

 of the skull, but in modern vertebrates the 

 pineal body has until recently been regarded 

 as a vestigial structure. Now, however, it is 

 known that the pineal produces at least one 

 hormone, namely, melatonin. 



The work of Lerner, referred to previously 

 (p. 412), definitely establishes melatonin, the 

 melanocyte pigment-concentrating factor, as 

 an important neurohormone, at least among 

 fish, reptiles, and amphibians. Moreover, re- 

 cent studies, utilizing the isolated product, 



