I guess it's more meaningful for you to quantify the area covered by cells (which is increasing as the wound is being healed), rather than focus on segmentation of every single cells, isn't it?
If so, you may try to increase the contrast of the cellular area vs. the empty background. For example by using Smooth, or using some series ImageMath as I made here wound_heal.cpproj (642 KB)
Here I ignore the coarse segmentation of single cells, but focus on identifying the area covered by cells.
Hope your research heal every wounds !