I don’t know if I understood your problem right (not to measure the individual cells but the cell aggregate).
However to extract the aggregate in the center of the image you can apply
the “Process->Find Edges” action which gives you the following result:
Then you can apply a Gaussian Blur filter “Process->Filters->Gaussian Blur” (or try the Median filter) to work out the center structure:
Finally convert the image to a binary for measuring “Process->Binary->Make Binary”:
Use the Wand tool (in the toolbar) to select the ring or the center to make some measurements or use the Particle Analyzer to exclude particles of a certain size and automize the whole procedure on a folder of images.
Here is the recorded macro of all steps (use “Plugins->New->Macro” and paste the following lines):
run("Gaussian Blur...", "sigma=4");
Play with some filters and their adjustments to get more appropriate results! If you need a more fine grained area you can also work with a threshold instead of the edges procedure and then work with some filters to get an satisfying result.
In my example you can at least estimate an area in which the aggregation occurs.