Glycogen metabolism in the rat visceral yolk sac. I. Glycogen content and gestational age.

The glycogen content of the rat visceral yolk sac was determined between 13.5 and 20.5 days of gestation by the best available colorimetric method. The concentration of glycogen in the tissue increased ten-fold between 13.5 and 18.5 days, to reach a value similar to that for mammalian muscle, but then decreased by 50 per cent between 18.5 and 20.5 days. Determination of the iodine-iodide spectra and fractionation of the glycogen particles by a novel sodium citrate centrifugation method indicated broad similarities between the structures of glycogen particles, isolated by a mild phenol-water method, from the yolk sac and the liver of the rat. However, the proportion of 'high'-molecular-weight glycogen in the yolk sac increases between 18.5 and 20.5 days, as a result of the preferential loss of 'low'-molecular-weight glycogen, so that at term the proportion approaches that found in liver glycogen.