Automated cell segmentation from DIC images is notoriously tricky, not just for us but also for those in the biological computer vision business in general. You’re right in that the IdentifyPrimAuto doesn’t work since there is not a clear foreground and background as there is in fluorescence images.
We have someone in our group working on the problem now, in fact. If his results are promising, we will certainly release it as a new module. Undoubtedly, there is a demand out there for such a thing.
In the meantime, you may have some success with the FindEdges module, which will make non-smooth areas of your image stand out above the background, such the cell perimeter.
Hope this helps,