Introduction to counting principles

Abstract

It is frequently of great biological interest to know the number of structural units contained in the tissue volume, and this parameter can again be determined stereologically by appropriate analysis of sections. For a description of the available counting principles two basic situations must be distinguished: 1. The section thickness T is much smaller than the diameter Di of the structural units. This situation obtains, for instance, when cell nuclei of average diameter of 5–10μ or mitochondria are counted in electron micrographs of sections 0.05μ thick; or in counting pulmonary alveoli (Di ≈ 250 μ) on 7 μ histological sections (Weibel, 1962, 1963). In these cases, profiles of the structures on planar random probes are counted. 2. The section thickness is of the same order of magnitude or even larger than the size of the structural units. This is the case in counting cell nuclei in regular histological sections or ribosomes in ultrathin sections. Here, the number of units contained in the unit slice volume is determined.