IV REGULATING FACTORS OF THE PFS 529 



In later stages of development the endocrine glands exert characteristic effects 

 on the processes of the development of certain tissues. Some of these processes, 

 such as metamorphosis in insects and amphibians, are among the most widely 

 known phenomena of embryonic dev^elopment. Many of the endocrine glands 

 affect growth of organisms in general or stimulate growth in specific target organs 

 by an increase in protein anabolism (White, 1956). In certain organs it could 

 actually be demonstrated that hormone stimulated growth is equivalent to an 

 activation of general protein synthesis (Kochakian and Tillotson, 1956) or an in- 

 creased formation of specific proteins in one cell type (Csapo, 1950; Scow, 1951, 

 1953; Mueller, 1953; Hurley and Herrmann, 1955; Harkness and Harkness, 1956). 

 However, the mechanism of these effects has remained surprisingly obscure. There 

 is, in most instances, little information relative to the step or steps in protein synthesis 

 acted upon by those hormones which stimulate growth. At the same time it must 

 be realized that elucidation of the mechanism of hormonal action would yield 

 information on the control of the PFS which could be applied to earlier phases of 

 development. The possibility cannot even be entirely excluded that hormone-like 

 substances, e.g. steroids, may occur rather ubiquitously, possibly located in the 

 cell surfaces, in many cells of the early embryo, and exert at this time an effect by 

 transmission from cell to cell. The relationship between hormonal effects and 

 embryonic induction and the pertinence of studies of hormonal mechanisms for the 

 understanding of embryonic development is particularly evident in studies on sex 

 differentiation (Chang and Witschi, 1 956) and has been well stated by Moog. In one 

 of her papers (Moog, 1953) dealing with the control of phosphatase production in the 

 intestinal epithelium of the chick and the mouse embryo (Moog, 1950, 1951 ; Moog 

 and Richardson, 1955) she expresses her thoughts as follows: "Classical studies in 

 experimental embryology have long since established the fact that early embryonic 

 development involves the interactions of series of inductive stimuli and correlated 

 competences. Now it appears that in later stages similar situations obtain, with 

 hormones constituting the inductors and acting on target tissues that pass through 

 transient phases of sensitivity, or competence." 



In attempting to come to a better understanding of the relationship between 

 hormonal stimulation and the formation of the protein moiety of a highly special- 

 ized differentiating tissue, investigations were carried out on the hormonal 

 control of the formation of the muscle proteins in the leg musculature of the chick 

 embryo. In view of the fact that a marked increase in dry weight of the chick leg 

 musculature as well as an increase in activity of several endocrine organs of the 

 chick takes place at about the 12th day of development, the possibility was 

 considered that development of muscle after this time is, at least in part, regulated 

 hormonally. As the first step in this investigation the effect of thyroid activity on 

 muscle development was examined (Konigsberg, 1958). Thyroid activity was 

 depressed by a carefully established single dose of thiourea administered intra- 

 vascularly on the tenth day of development. Histological examination of the 

 thyroid glands of the experimental animals showed very marked suppression of 

 thyroid activity. For the present discussion the most significant finding was that 

 the increase of protein and of RNA per cell (DNA) was delayed to the i6th day 

 of development while in controls an increase in both parameters occurs after the 



Literature p. ssg 



