Model of interactions in erythroblastic islands. Erythroblasts bind macrophages via integrin α₄β₁ and VCAM interactions until they reach the stage of enucleation, at which time the erythroblast separates into a released enucleate reticulocyte and nucleated pyrenocyte. The pyrenocyte then exposes phosphatidylserine on its surface, which binds to Protein S and stimulates the MerTK receptor on the macrophage, causing its phagocytosis. See Figure 5 in the article by Toda et al that begins on page 3963.