Hello Sage Kim

The image is not so clear. Nevertheless, it seems to me like cell debris.

Hello Malcolm Nobre

Thank you so much for your response. I have attached another image which I hope might help. I feel it is cell debris too but seeding 1 cell per well and getting a lot of debris doesnt feel right :(

Have you checked your FBS debris? Frozen and defrosted serum can form sheets of denatured protein, requiring filtration of the DMEM + FBS.

Hello Ellen A G Chernoff

Thanks for your answer. I don't think it is caused by FBS because I would filter it.