The glygogen content of skeletal muscle, liver and heart in late chick embryos

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Abstract
The liver, heart and skeletal muscle (thigh muscles) contain appreciable quantities of glycogen at the 432nd hr. of incubation. From this time, when pulmonary respiration begins, until “ pipping “ there is a decline in the glycogen content of the liver and heart. Between “ pipping “ and hatching there is evidence of the repletion of the glycogen stores in these tissues. When the shell and shell membrane above the air space are removed at the 432nd hr. there is a progressive increase in the glycogen content of the liver, heart and skeletal muscle until the time of hatching. The depletion of the glycogen stores in normal embryos before “ pipping “ appears to be due to an hypoxic state existing before “ pipping” and relieved when “ pipping “ takes place. When the gas exchange through the air space is reduced by waxing the shell overlying the air space, the fall in the glycogen stores of all three tissues is severe and would appear to be due to accentuation of the hypoxic state prevailing before “ pipping “. With the exception of a period of about 30 min. after “ pipping”, and for brief periods in the hours before hatching the respiratory quotient remains constant at or around 0–7. This low RQ, is consistent with the synthesis of glycogen from lipid material, derived presumably from the contents of the yolk sac. The evidence is reviewed for believing that synthesis of glycogen proceeds even when depletion of the glycogen stores in the tissues occurs. The low levels of cardiac glycogen found in those embryos which had restricted access to oxygen (waxed shells) combined with the high mortality in such experiments suggests that survival through the critical period of hypoxia before “ pipping “ depends on the level of the stores of glycogen in the heart at the start of this period of hypoxia.