what are these cells?? - (Nov/11/2011 )

hi everybody, i hope someone can help...
we did a ficoll gradient with rat blood, to isolate endothelial progenitor cells... it was the first try, i didnt do it before. Now we have one colony of cells, that are proliferating very good, but in the background there are big round cells, and i cant figure out what that might be... also the colony doesnt look like endothelial cells i have seen so far, but the morphology can be diferrent due to the circumstances, right? maybe someone of you could take a look at this picture and help me with an idea? i didnt do any further tests so far... am i right, that it cant be fibroblasts? because they arent present in the blood, right?
i would reeeeaallly like your help! thx everybody!!
ps the picture is from day seven of culture...

The living cells in that culture look very fibroblastic to me, the rounded floating ones are probably dead (but you could assay this) and the fuzzy ones which appear to still be attached are probably apoptotic.