Estimating object number in biological structures

The number of cells and subcellular structures can often be readily related to quantitative evaluations of organ and tissue function. Neurons and synapses, for example, are directly involved in the integration and transfer of information in neural systems. Their numbers are consequently important parameters in the evaluations of the functional capacity of neural systems. Only information regarding the total number of objects, such as synapses and neurons, can be used to draw conclusions regarding changes or differences in the number of these structural entities. The large numbers of neurons and synapses in the vast majority of neural systems preclude absolute determinations of their total number, that is, counting each and every neuron or synapse. However, estimates or approximations based on limited sampling can be useful if the estimates are unbiased and if the individual estimates have an acceptable amount of precision. This article discusses the estimation of object number, including sampling, indirect and direct counting techniques, sources and types of bias, and the disector counting technique. An example is also given.