Ultrastructural localization of calcium in the chick yolk sac membrane endodermal cells as revealed by cytochemistry and X-ray microanalysis.

The yolk sac membrane (YSM) of the chick embryo transports calcium from the yolk into the embryonic circulation during the first half of development, but the intracellular pathway of calcium transport is poorly understood. In the present study, the ultrastructural localization of calcium was investigated in cells of the YSM of 9-day chick embryos. X-ray microanalysis as well as cytochemical techniques performed on yolk sac membrane cells treated with potassium oxalate, potassium ferricyanide and potassium antimonate demonstrated accumulation of calcium in yolk granules, digested yolk products, electron-dense bodies (EDBs; 100-400 nm diameter) and electron-dense granules (EDGs; 30-50 nm diameter). When strontium ions were injected into the yolk, they were incorporated into the endodermal cells and sequestered specifically in EDGs. From these results, we propose that calcium enters the endodermal cells by endocytosis of calcium-containing yolk granules, as well as through calcium channels in the apical cell membrane. In the cytoplasm, digested yolk products, EDBs, and EDGs act as sites of sequestration and accumulation of calcium. Extrusion of intracellular calcium into the extracellular space and embryonic circulation is accomplished by exocytosis of calcium-containing material and via an ion pump in the basal cell membrane.