Credit: Nymus 3D (https://nymus3d.nl/)
The top two panels show a stressed stem cell, as indicated by the calreticulin expressed on its surface (yellow spikes), being engulfed and eaten by a macrophage. The bottom two panels show a healthy stem cell, expressing very little calreticulin, being contacted by a macrophage but not eaten. Instead, the macrophage appears to draw out some material from the cell. The calreticulin acts as an “eat me” signal.