MC3T3-E1 (Mouse osteoblast) is cultured using Alpha-MEM, 10% FBS, 1% P/S.

I'm looking at reactions to drugs, and black dots like that only appear on certain drugs.

Seeing that it is reoccurring not just once but several times over and over again, I think it is not contamination but a cellular reaction.

Has anyone experienced this phenomenon, even if it is not the MC3T3-E1?

Why does something like that happen? Why? What ingredients or factors does it react to?

All three photos are cultured on different plates and contain the same drugs.

It is not contamination of medium or serum.

No such black dots are observed in other culture dishes.