In these images,how can I remove the backgrnd noise in matlab?

[Urmi Roy]

I have to isolate the cell(seen as a bright blob) in the attached images from the background the zig-zag.

The problem is essentially that the intensity/brightness of the cell is almost equal to its backgraound, so I can't just subtract frames!

 

[Urmi Roy]

After filtering the images with medfilt2 twice, I get the following image...
http://imgur.com/D60kG

However, the cell is no longer the brightest spot here, so do I go for some other filter or does the criterion on the basis of which I locate the cell change?

Please suggest.