The Effects of Thyroxine and Antithyroid Compounds on the Synthesis of Pigment Granules in Chick Melanoblasts Cultured in vitro

Abstract

Pieces of back skin from 7-day Barred Rock embryos were cultured in vitro in standard hanging-drop prepns. An analysis was made of the effects of each tissue component of the skin and of each separable constituent of the culture medium on the synthesis of pigment granules by the melanoblasts contained in the skin. Unidentified metabolites in blood plasma and embryonic extract were necessary for pigment synthesis. The presence of other types of cells and tissues was unnces-sary for pigment synthesis, and in fact, the dermis delays melanogenesis. The general antagonism between growth and differentiation of melanoblasts was demonstrated by changing the growth-promoting properties of the medium. Addition of thyroxine to the culture medium did not influence the synthesis of pigment. However, several antithyroid compounds[long dash]phenyl-thiourea, alpha-naphthylthiourea, symmetrical-diphenylthiourea, prophylthiouracil, and thiouracil[long dash]directly inhibited pigment formation. The duration of pigment inhibition by antithyroid compounds is proportional to the initial concn. of the inhibitor. The effectiveness of the pigment inhibitors is destroyed by the addition of free iodine, and reversed by the addition of appropriate concentrations of 2,4-dihydroxyphenylalanine (dopa) to the culture medium. Tyrosine, cysteine, cystine, and Ca pantothenate had no effect on inhibited cultures. The mechanism of pigment inhibition by the antithyroid compounds may be explained either as a competitive enzyme inhibition or as a direct antioxidative effect on the pigmentation process.