Quantitative observations on the kinetics and mechanisms of binding of electron stains to thin sections through hen erythrocytes.

The kinetic studies described in this paper have enabled us to define the staining conditions which lead to occupation of all available binding sites in sections through biological material. In a hypothetical section containing stainable regions the concentration, c, of bound stain in any thin layer is a function of its distance, x, from the surface in contact with the staining solution and the staining time t. Theoretical considerations indicate that there are 2 extreme types of kinetics depending on the relative values of 2 diffusion or migration rates :rlt that of the staining solution into the depth of the section, and r,, that of the solution into the stainable regions. When rx > > r,, called type A kinetics, penetration of stain throughout the depth of the section is 'instantaneous' and binding to stainable regions is slow. Two families of curves can be constructed, each member of which has a particular value of /: first c-vs-x curves and second, derived from them by integration, E-vs-d curves, where E is the electron-scattering density of a stained region relative to clear resin and d is section thickness. When rt< <rt, called type B kinetics, the staining solution diffuses relatively slowly into the section, and all the binding sites in each stainable region are occupied 'instantaneously'. Similarly there are 2 families of curves, c-vs-x and E-vs-d, but of different shapes. When rr~rt there is an intermediate type AB kinetics. At any point on an E-vs-d curve the slope is proportional to the concentration of bound stain. The penetration time, ip, is defined as the time taken for the concentration of the staining solution to reach the same value at the bottom of a ioo-nm section as at the top surface in contact with the solution.