Specificity of Tissue Response to Induction

The use of chick embryo liver culture in vitro as contrasted to induction in the whole animal permitted the inference that the inducing chemicals acted directly on the hepatic cells and not via other stimuli generated from other organs. The porphyrins formed could be seen by fluorescence microscopy to accumulate in the c)doplasm of the liver parenchyma cells and were identified mainly as coproporphyiin with some protoporphyrin [25]. However, Doss [70] found mainly protoporphyrin. This difference may depend on conditions of culture the older cultures had more coproporphyrin and uroporphyrin. The increase in porphyrins in tissue culture has been assumed to be a result of an increase in ALA-synthetase. With the aid of 13-cm-diameter petri dishes, it has been possible to grow sufficient numbers of cells and demonstrate with inducing chemicals an increase in vitro of ALA-synthetase activity of seven to ten times that of controls, in a 20-hour period [21]. 

The chemicals induce only in liver, not in other tissues. The steroids, as will be discussed later, induce ALA-synthetase in liver cells as well as in erythroid cells of the chick blastoderm. Although the kidneys of the chick embryo may rapidly convert ALA to porphyrins, neither steroids nor other inducing chemicals have an inducing effect on this tissue. Thus, response to inducing compounds depends on the tissue. In the liver, at least of the chick embryo, both steroids and other chemicals induce in erythropoietic blastoderm only steroids induce and in kidneys and other tissues none of these compounds induce. Steroid induction in liver and erythropoietic tissue appears to involve a transcriptional mechanism induction by chenucals appears to involve mainly the translational mechanism (see Section V). 

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