For promotion of the optical fractionator,an unbiased stereological method of the optical disector combined with the fractionator that is commonly used to estimate the total number of cells in an organ(tissue),the present article exemplified its use in a total number estimation of neurons.Serial frozen sections of the dentate gyrus(DG region)of the hippocampal formation on one side were obtained from an adult mouse,1/5 of the sections were sampled in a systematic random manner,with 14 sections being sampled.The sections were stained using an immunohistochemical method and counterstained with cresyl violet to show DCX-positive neurons.The whole DG regions were observed and the somas(cell bodies)of DCX-positive neurons were counted within 15 μm(thickness)of the section tissue with the optical disector,meanwhile the actual thickness of the section tissue was measured with a microcator.As a re-sult,291 neurons(somas)were counted and the average of measured section thicknesses was 24.22 μm.Therefore,according to the op-tical fractionator,the total number of DCX-positive neurons in the hippocampal DG region was:291×(5/1)×(24.22/15)=2,439.With this example,we discussed in more details about the theoretical foundation and practice-related issues of the optical fractionator.