Your problem does not look so simple and I don't think there is a matlab code that would do exactly what you want (unless another researcher worked on this exact same problem before). You could try using circular hough transform and see it it can detect cells that are roughly circular
The very simple Matlab code suggested by Dang will not work in this case, because you have overlapping cells and secondly, they don't stand out very well from the background. I don't think the Hough transform to detect circles will help you either: it will detect several circles belonging to the same cell (!!) because the cells are not enough like circles but instead have circle-like patches.