Hi,
One of my collegues has recently started to get perfect cuboid crystals appearing in his cultures. He is using DMEM with 20% FCS. Sometimes they will grow in only one of 6 wells treated the same way and with the same media.
Has anyone else had these or know what they are.
Many thanks,
Nadia Lovegrove

This might be the crystal of NaCl, which can form due to the higher
osmotic pressure.

Pls check the tray on the bottom to confirm that the water did not
evaporated.

I also think that they are NaCl crystals but you can never be sure.
How long since the medium was added you saw the crystals?

Those seem somewhat large to be NaCl crystals - which I would expect to see at the edges anyways. What is the ~size - ~20 uM? Are those hard crystals that you can pick with forceps, or soft and squishable?
It might be a good idea to check for water/salt content and incubator humidity. With 20% FBS, you do have a lot of protein in there, which might conceivably flocullate with other medium components.

Actually, those do look suspiciously like salt crystals - and if so, your cells must be under considerable osmotic stress!
I've attached a picture of crystallized NaCl with this post.