Ultrahistochemical studies on the moderately electron dense bodies in teleostean endocardial cells.

The staining properties of phosphotungstic acid (PTA) on moderately electron dense bodies (MDB), are studied in the endocardium of Gadus morrhua. In MDB fixed in aldehydes only, and stained with PTA at a low pH (0-1), Intensly electron dense material occurs on and beneath the limiting membrane. This latter area displays a declining electron density when stained with PTA solutions in which the pH is raised from 1 to 4. At pH greater than 5 the peripheral matrix appears nearly unstained. Collagen fibres fixed as above, and then stained with PTA at a low pH, appear electron dense. These results suggest that the peripheral matrix of the MDB consists mainly of basic proteins.