ENHANCED DESOXYRIBONUCLEIC ACID ACCUMULATION AND RETARDED PROTEIN ACCUMULATION IN SKELETAL MUSCLE OF “HYPOPHYSECTOMIZED” CHICK EMBRYOS1

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Abstract
Hormonal regulation of the chemical composition of developing skeletal muscle was examined by the use of chick embryos in which the prosencephalon was surgically removed at Hamburger-Hamilton stages 10 to 12. Control and operated animals were sacrificed at two-day intervals from the 10th through the 20th day of development and homogenates of leg muscle prepared and assayed for DNA, RNA and protein nitrogen content. Muscle fresh weight in the experimental group is normal on the sixteenth day but thereafter increases at a significantly slower rate than in the control group. On the 16th day of development both the RNA and protein nitrogen content of the leg muscle in the experimental embryos are significantly lower than the control levels. However, although the differences in protein nitrogen content in the two groups becomes progressively larger the initially observed differences in RNA levels is maintained throughout the remainder of the experimental period. In contrast to the other growth parameters studied DNA accumulation is enhanced in the absence of the derivatives of the prosencephalon. DNA accumulation normally decreases sharply after the 16th day of development. In the experimental group rapid accumulation continues for at least another 48 hours. Consequently, in the operated embryos, the DNA content of the leg musculature is 16% and 22% greater at 18 and 20 days respectively than in control embryos.