Section X 

 ONTOGENY OF ENDOCRINE CORRELATION 



B. H. WILLIER 



One of the ways whereby cells, tissues, and 

 organs of the vertebrate organism are cor- 

 related functionally is through hormones 

 sensu strictu, i.e., by specific organic chem- 

 ical substances produced by specialized tis- 

 sues or glands, which are carried in minute 

 quantities in the blood stream and have as 

 their primary function the exertion of spe- 

 cific physiological effects on other tissues or 

 organs. During the past three decades ex- 

 tensive investigations on the fvmctions of the 

 endocrine glands of embryonic, young, and 

 adult vertebrates have resulted in the estab- 

 lishment of the concept of the integration of 

 the endocrine system, i.e., the interaction and 

 regulation of secretions of two or more en- 

 docrine glands. The relations are such that 

 the secretions of one gland act upon another 

 gland, governing its production and libera- 

 tion of a hormone entirely different in chem- 

 ical nature. The endocrine gland thus excited 

 may have in turn, through a rise in blood 

 level of the hormone released, a reciprocal 

 effect on the initiating (trophic) gland, 

 depressing its secretion. In such an interrela- 

 tionship the mechanism is one in which 

 secretion is regulated by a delicate balance 

 between the levels of trophic and "target" 

 gland hormones in the blood. Thus there 

 exists a vast interlocking of functional ac- 

 tivity of the endocrine glands. The order and 

 time at which such correlating mechanisms 

 are established in the ontogeny of the in- 

 dividual is the main problem for analysis in 

 this chapter. In treating of the subject at- 

 tention will be centered on such topics as 

 (1) the course of differentiation of functional 

 activity of endocrine glands, (2) the extent 

 to which their initial activity arises inde- 

 pendently of other hormones, and (3) the 

 time of onset of functional interaction be- 

 tween them. 



ANTERIOR PITUITARY-THYROID 

 RELATIONS 



The functional relationship between the 

 anterior pituitary and the thyroid is one 



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among others to be set up between endocrine 

 glands of the developing embryo. The basic 

 evidence for this relationship comes from a 

 study of the effects of (1) extirpation of 

 primordia of either the anterior pituitary or 

 the thyroid or both at the time of their first 

 visible appearance, in combination with 

 those of (2) grafting of the thyroid or an- 

 terior pituitary or administration of extracts 

 or the active principles of these glands. 



THYROTROPHIC EFFECTS ON THYROID 

 ACTIVITY 



Without doubt the most complete evidence 

 has come from an analysis of the anuran 

 embryo, which is particularly suitable for 

 the type of experimentation required. At 

 early stages (3.5 to 6 mm. in length) removal 

 of the anterior pituitary, an ingrowing pri- 

 mordium from the surface oral ectoderm, 

 results in changes in the thyroid and in the 

 failure of metamorphosis to take place. The 

 thyroid glands are markedly retarded in 

 growth (reduced to one-sixth the normal 

 weight in tadpoles 36 mm. or more in length). 

 The follicles which form are few in number 

 and atrophic, containing little or no colloid, 

 contrasting sharply to a larger number of 

 follicles distended with colloid in the thy- 

 roids of normal tadpoles at the time of be- 

 ginning metamorphosis (Allen, '27; Smith, 

 '20). 



The effect of the anterior pituitary in in- 

 ducing tadpole metamorphosis is indirect 

 rather than direct. Adler ('14) was the first 

 to express the view that the thyroid is de- 

 pendent upon the anterior pituitary in the 

 anuran larva. He was led to this interpreta- 

 tion by noting in three cases that hypo- 

 physectomy (destruction by electric cautery) 

 of 20 mm. Rana tadpoles not only prevents 

 metamorphosis but results in profound 

 changes in the thyroid (marked growth re- 

 tardation; small amount of abnormal colloid). 

 That the effect of the anterior pituitary is 

 through a trophic action on the thyroid gland 

 is now well established and is based on two 



