segmenting region of interest (ROI) from background
Some very cool although a bit more time consuming options: 


Active contours will try to find the best "edge" for the cells while maintaining smoothness.
and is implemented in matlab2013a ( active contour)
You might try using mser which is an approach which detects regions which are similiar and in color and stable over the image. which means they stay about the same size over a wide array of thresholds.
I used it for a similar problem with detecting ultrasound tumors.
If you have a good database of images ( good = covering most cases )  I would try some machine learning algorithms. for example, you could learn on the mean and variance of the objects in the image.
If you have trouble separating the cells after segmentation ( i.e. you have two connected cells ) I would try looking at the watershed function.