Quantitation of gold labeling and estimation of labeling efficiency with a stereological counting method.

The amount of immunolabeling over a cell compartment of an average cell was estimated by use of an adaptation of the double disector method introduced by Gundersen. The first and last sections of a stack of ultra-thin sections formed a disector in which cell number could be estimated and related to a defined reference volume to give the cell density. Another stack section, selected at random, was immunolabeled and the number of gold particles associated with unit volume reference space (gold "density") estimated in quadrats placed systematically across the section. The ratio of gold density to cell density was used to estimate the number of gold particles lying over a chosen compartment of an average cell, N(gold)/N(cell). Such estimates required neither cell volume nor section thickness measurement and were reproducible. By combining the number of gold particles per cell with estimates of the number of protein antigens per cell, the number of gold particles associated with each antigen could be found (labeling efficiency).