Stereology

In most studies of biological samples and other materials, it is intended to obtain information or measurements of volume, surface, length or number of objects in three-dimensional structures. However, a study with sufficient resolution, for example at the cellular level, of the complete structure is not possible. Stereology evaluates these three-dimensional measurements from information obtained from 2D sections.
For this, the stereological procedure consists of:
Firstly, obtaining a sample of sections that is representative of the entire structure to be measured and whose measurements do not depend on the orientation of the sections. For this, an approach is the obtaining of sections of different randomly oriented replicas or samples while, in other occasions, samples of known orientation are used.
Second, the superposition of geometric "probes" (lines, dots, etc.) on the images of the sections and the counting of the interactions of the same with the structure to be measured. Ex: points in the structure against points in the reference volume; number of intersections of a structure with a line, etc. Finally, a series of mathematical calculations relate the data obtained in the different sections with a three-dimensional measurement.
For a more detailed overview see: M.J. West, 2012, Cold Spring Harb Protoc; doi:10.1101/pdb.top070623