Metamorphosis 



649 



TISSUE SPECIFICITY AND 

 SENSITIVITY 



Amphibians. The responses of the tissues to 

 the thyroid hormone are highly specific. 

 Some tissues (i.e., leg and eye) show growth, 

 others (i.e., tail tissues) show histolysis and 

 resorption, still others (i.e., digestive organs) 

 a combination of the two processes. Further- 

 more, the response of a given organ varies 

 among the species; the tail is resorbed and 

 the legs are stimvilated to growth in anurans 

 but not in urodeles. Even among anurans, 

 the degree of dependence of a given structure 

 upon hormones appears to vary from species 

 to species. Allen ('25) foimd that in the ab- 

 sence of the thyroid gland the legs of Bufo 

 proceeded relatively further in their develop- 

 ment than did those of various Ranidae. 

 Gonadal development is generally independ- 

 ent of the thyroid and metamorphosis 

 (Swingle, '18; Hoskins and Hoskins, '19). 

 But even this varies with the species; Krichel 

 ('31) found accelerated ovogenesis in Bufo 

 viridis in normal metamorphosis and as a 

 result of thyroid feeding. Continuance of the 

 development of the brain in the absence of 

 the thyroid from the larval to the advilt con- 

 dition was reported by Hoskins and Hoskins 

 ('19a), but subsequent investigation by 

 Allen ('24) showed that only the superficial 

 form changes but no internal maturation 

 occurs in the absence of th? thyroid. 



The type of response made by a given 

 group of cells is independent of its histologi- 

 cal character. In the tadpole, tail muscle and 

 skin respond by histolysis whereas back 

 muscle or skin do not. In fact, the respon- 

 siveness of muscle varies with the region of 

 the tail from, which it is taken, anterior 

 muscle degenerating more quickly than pos- 

 terior and axial more readily than peripheral 

 (Clausen, '30). 



Histological examination of metamorphos- 

 ing tissues shows striking examples of speci- 

 ficity of response that is independent of his- 

 tological differentiation. Champy ('22) re- 

 ported a sharp line of separation between the 

 epithelium of the limb and that of the body 

 with respect to their mitotic response to thy- 

 roid treatment. Transplantation of the skin 

 shows that the capacity for forming the 

 glands of the dorsal plicae is strictly local- 

 ized (Helff, '31a), as is the capacity for pro- 

 ducing pigment (Lindeman, '29). Another 

 striking case of cell specificity is that of the 

 Mauthner cells, which degenerate under the 

 influence of thyroid whereas neighboring 

 cells may be stimulated to proliferate (Weiss 



and Rossetti, '51; Kollros and Pepernik, '52). 



The highly specific reactivity of larval 

 structures described above is acquired at a 

 definite and very brief period. The embry- 

 onic tissues are not sensitive to thyroid, but 

 they acquire their individual peculiarities of 

 reactivity soon after the larval organs are 

 definitely differentiated (Champy, '22; Etkin, 

 '50). Some experimenters report a relatively 

 slight variation in the time at which differ- 

 ent tissues in the same animal become sensi- 

 tive to the thyroid hormone. The anuran tail 

 is reported to develop sensitivity before the 

 limb buds; the skin, particularly its pigment 

 pattern, is last to become responsive (Al- 

 phonse and Bauman, '34; Kuhn, '33; Moser, 

 '50). 



A number of workers report progressive 

 sensitization of the tissues to thyroid hor- 

 mone during development (Allen, '38; 

 Geigy, '41). The writer does not regard the 

 evidence on this point as satisfactory because 

 of the difficulty of controlling the quantita- 

 tive evakiation of response and the influence 

 of environmental factors. The basic property 

 of thyroid hormone sensitivity is acquired in 

 an all-or-none fashion at the time of oper- 

 cukim formation in Rana pipiens larvae 

 (Etkin, '50). Changes in sensitivity, if any, 

 are relatively slight. As compared to the role 

 of tissue specificity, this factor can be of only 

 secondary significance in determining the 

 time and pattern of metamorphic changes. 



The problem of the acquisition of sensitivity 

 of the tissues deserves a more sustained ex- 

 perimental analysis, particularly as to its 

 biochemical basis, than has as yet been given 

 to it. 



The problems of the evolution of the mech- 

 anisms of metamorphosis do not fall within 

 the scope of this chapter, but some compara- 

 tive observations relating to tissue sensitivity 

 may be made. In permanently neotenous 

 urodeles the tissues are insensitive to the thy- 

 roid hormone. Yet the thyroids and pituitary 

 glands of these animals are effective when 

 implanted into organisms capable of showing 

 the metamorphic response (Swingle, '22; 

 Charipper and Corey, '30). Whether the hor- 

 mone contained in these glands is ever re- 

 leased during the lifetime of these animals 

 is unknown. 



In the axolotl, a facultatively neotenous 

 salamander, there is no loss of tissue sensi- 

 tivity. This is shown most clearly by the fact 

 that axolotl tissues transplanted to Triton 

 hosts at embryonic stages metamorphose 

 with the host (Geigy, '38). Similarly, skin 

 fragments transferred from axolotl to Amblys- 



