Keep in mind that ImageJ processing is not perfect. However, the error of missing cells may be "corrected" by the error of including some noise.
Steps in Video:
Process - Subtract Background
Image - Adjust - Threshold
Process - Binary - Fill Holes
Process - Binary - Convert to Mask
Process - Binary - Watershed
Analyze - Analyze Particles
Basic description of tools:
Subtract Background - cancel out noise of the background
Threshold - change the image to binary image of red, black and white, or
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