Length measurement: new developments in neurostereology and 3D imagery.

Quantification of linear biological structures has important applications in neuroscience; for example, the length of neurotransmitter-specific axonal innervation or length of dendritic processes within particular brain structures. Until recently, however, there have been practical limitations in the application of stereological tools for the unbiased estimation of object length on tissue sections. The recent development of efficient new approaches allows for the wider application of theoretically unbiased sampling and estimation techniques that are devoid of the assumptions and models of earlier methods. In this review, we outline the historical background and recent advances in the estimation of total length for biological objects on tissue sections, including a practical method to estimate the length of cholinergic fibers using newly developed methods. These newer methods also take advantage of three-dimensional image datasets and virtual probes, techniques that may have wider application in quantitative morphometry.