What we found is that the boundaries of medical images are fuzzy in nature --and this is the major problem. As traditional methods are unable to deal with this fuzziness. Some pre-processing techniques are utmost required before going or final segmentation. Few are discussed in the following paper: Article Importance of Image Enhancement Techniques in Color Image Se...
Proper pigmentation of the cell by the expert if you are working on your own dataset. It will help you lot in finding optimal threshold value so that you can extract image as per your wish.