Electron microscopic detection of calcium binding to the duodenal absorptive cell plasma membranes

Electron dense structures (EDS) were observed on the plasma membranes of duodenal absorptive cells of chicks of various physiological states regarding the calcium binding protein (CaBP). EDS were seen only when calcium was added to formal-glutaraldehyde fixative in cacodylate buffer and the material was post-osmicated in phosphate buffer. Microvillar, lateral and basal plasma membranes showed the EDS and these were seen always in the inner dark layer of the three-layered unit membrane. No EDS were seen in the embryonic material and some were present in material from chicks two days after hatching. There were more in that of 6-week-old normal and vitamin D replete animals. The absorptive cells from chicks, fed on a diet supplemented with cestrum powder, known to mimic the effects of 1,25-dihydroxycholecalciferol, showed the highest number of EDS. On the basis of this distinct correlation of EDS frequency with amounts of CaBP known to be present in chick duodenum, it seems that EDS, precipitated calcium phosphate, strongly suggest the sites of CaBP.