Morphological and metabolic responses of embryonic hearts to administration of exogenous L-thyroxine

Injection of 1 microgram L-thyroxine (T4) into the yolk sacs of embryonated chicken eggs at 3 to 6 days of incubation not only induced cardiomegaly but also instigated more rapid differentiation of the heart as an organ and of the individual myocytes per se. Myocytes showed evidence of responding to this dose of exogenous T4 as early as 5 to 6 days of incubation, even though endogenous T4 was not normally forthcoming (in amounts sufficient to provoke organ changes) until 11 to 12 days of incubation. By 7 days of incubation the hearts, conditioned by a single 1 microgram dose of T4, exhibited larger areas occupied by myofibrillar material than controls. Measurements, beginning at 9 days of incubation, indicated the presence of greater amounts of RNA, total non-lipid solids and total lipids. Early increases in DNA in T4-conditioned embryos, compared with controls, indicated that hypertrophic hearts had reached this condition, at least in part, by increased cell division. By 12 days of incubation, hearts pre-treated with T4 showed conversion of many mitochondria to vesicles resembling smooth endoplasmic reticulum. No evidence of classical sarcoplasmic reticulum was seen through hatching.