(More technically, this is known as HISTOMETRY.) This involves counting the numbers of cells in a given piece of tissue—and of course, in biological psychology, this is typically a count of the numbers of neurons in particular regions of the central nervous system. Cell counting could be undertaken simply in order to calculate the numbers of neurons in a particular structure, but it is more commonly undertaken for any of several reasons: (1) To determine the numbers of neurons of a particular type, identified by a selective visualization procedures (such as IMMUNOHISTOCHEMISTRY, IN SITU HYBRIDIZATION or AUTORADIOGRAPHY, for example). This might involve counting neurons normally present, or the numbers that have survived and developed following TRANSPLANTATION. (2) One might wish to determine the nature of an experimental LESION placed in the central nervous system. In this case, what one is doing is counting the numbers of neurons in control tissue and then making a comparison with the numbers of neurons in the same area in tissue in which a lesion has been made—one is trying to estimate how many neurons are missing from the lesioned brains. (3) One might wish to count the numbers of neurons identified by one form or another of the TRACT TRACERS.
Cell counting used to involve exactly that: either examining tissue sections and counting the numbers of all cells present or using a grid (either in the optical system of the microscope being used or on a monitor screen if the image was being projected) and counting in a given proportion of grid squares and estimating total numbers from the sample counted.
In some cases this might be satisfactory, but there are a number of hidden problems. For example, if serial sections are taken (that is, the brain is sectioned and every SECTION is analysed, rather than say, every fifth section) then very probably, cells will be present on more than one section. The degree to which this occurs of course depends on the thickness of the tissue section and the size of the cells: with large cells and thin sections it is certain that most will be represented on a number of sections. Cell counting in such circumstances requires use of the ABERCROMBIE CORRECTION factor. Recent advances in MICROSCOPY and IMAGE ANALYSIS have led to the development of better techniques for cell counting. The method of choice now used is STEREOLOGY.
Reference
West M.J. (1999) Stereological methods for estimating the total number of neurons and synapses: issues of precision and bias. Trends in Neurosciences 22:51–61.
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