Divalent Cation-Dependent Deposits in Paramecium

Abstract

In 1972 Oschman and Wall observed the presence of electron-dense deposits (EDDs) along the plasma membranes of intestinal cells (1). They systematically tested the components of their fixative solutions and concluded that the combination of glutaraldehyde and millimolar concentrations of several divalent cations formed these deposits. Using what has become known as the Oschman and Wall fixation technique, most commonly employing glutaraldehyde and CaCl2, several workers have since reported the presence of similar EDDs which often occur at specific sites where Ca2+ action has been implicated. Examples of these reports are given in Table 1. The deposits characteristically form on the cytoplasmic side of surface membranes. X-ray microprobe analyses show that deposits contain Ca2+ (6,7,9,12,14,15,18). Sr2+, Ba2+, Co2+, Mn2+, Fe2+, Zn2+, Ni2+, and Mg2+ can also produce deposits but all monovalent and trivalent cations tested do not (1–3,11,13,14). Other Ca2+ salts such as Ca(NO3) produce EDDs, hence it is the cation and not the anion which is important for deposit formation (14). Deposits can be classified under at least two different groups—those associated with membranes and those contained within organelles with no apparent associations with membranes. In some cases membrane deposit sites are associated with sites known to contain intramembranous particles (IMPs) within the membrane bilayer and fibrillar material attached to the bilayer (8,11,14).