I've been using THP-1 cells for the past 3 years and up until recently have had little to no trouble differentiating them. However, 6 weeks ago, I noticed that black dots are now appearing within the differentiated cells. These particles are static, are not causing apoptosis, but results generated since the appearance of these particles don't correlate with previous work - i.e. qRT-PCR data that has remained unchanged. I've changed media, FCS, broke out new cells, all to no avail.
I have attached a 'before and after' picture. I am going out of my mind. If anyone can shed light on this I would be extremely grateful.