Effects of thyroxine on mitochondrial morphology and uptake of 35S in embryonic chicken livers

Ultrastructural analyses of reactions of mitochondria in hepatocytes of chicken embryos to low levels of exogenous thyroxine (T4) reveal that such reactions (overall swelling accompanied by disruption of crest geometry) first take place at about 10 days of incubation, T4 having been administered on the 6th day. Physically altered mitochondria may be seen after 11-12 days of incubation but are no longer evident by 13 days. Correlated with the initial evidence of T4 effects on mitochondria at 10 days of incubation is a spurt in hepatocyte proliferation. The time correlation observed between T4 induced mitochondrial changes in morphology and abrupt increases in rates of cell proliferation, suggests that liver nuclear receptors for thyroxine are unavailable prior to 9-10 days of incubation. Golgi complexes within the hepatocytes appear to be especially active in the production of electron-opaque vesicles from at least the 8th day of incubation to 11-12 days. Uptake of 35S (probably into chondroitin sulphates) was found to be fifteen times greater on the 8th day of incubation than at 15 days. This correlates with the period of heightened activity of the Golgi complex. In livers exposed to T4 on the 6th day, uptake of 35S was higher on the 9th and 10th days of incubation as compared to controls.