Glycogen synthesis, storage and transport mechanisms in the yolk-sac membrane of the chick embryo.

This paper deals with the relationship of structure and function in the yolk-sac membrane (YSM), a living "way station" between yolk and embryo of the chick. Through its parenchyma (entoderm) cells all yolk substances, either unchanged or enzymatically transformed into simpler metabolites, are transported to the blood of the growing embryo, wherein they are preeminently utilized as building blocks. Since glycogen must be derived from yolk and is readily visualized in entodermal cells (glycogenic cells of Claude Bernard) of the YSM it was chosen as a marker of functional activity.By a combination of methods of cytochemistry, radioautography and electron microscopy, glycogen localization in the entodermal cells and probable mode of glucose transport into the blood has been investigated during the middle third of embryonic life. The principal findings are: (1) Tritiated glucose injected into the yolk sac is incorporated into glycogen granules in the cytoplasm of entodermal cells. The labeled glycogen granules are arranged in circles about yolk spheres ("perilipid pattern") and in basal concentrations next to the plasma membrane ("basal pattern"). These patterns are identical to those obtained by the periodic acid Schiff test. (2) In electron micrographs glycogen commonly appears as tiny packets of rosettes typically disposed in the cytoplasmic matrix between the yolk spheres but not within them. Differences in patterns of glycogen-organelle organization between apex and base of the entodermal cell suggest a state of flux of glycogen in an apical-basal direction, to wit-from initial synthesis and deposition of glycogen in the apical cytoplasm, to an area rich in glycogen deposits associated with agranular ER, to a basal area of glycogen, ready for breakdown into glucose and transport to blood.Between the glycogen of the basal cytoplasm and the blood of the venous capillary is an ultrastructural complex of perivascular spaces and cell surface membranes having micropinocytic vesicles that are seemingly adapted for the transport of glucose or other metabolites to the blood as well as for the exchange of substances between the entodermal cells and the embryo. As to the mode of uptake of yolk substances from the yolk sac into the entodermal cells, it has been established for the first time thatwhole yolk spheres of different kinds are engulfed by the coalescence of the edge of a cup-shaped fold of the plasma membrane. Pinocytic vesicles of the cell surface membrane seem to provide devices for the uptake of soluble yolk substances.