Ultrastructure of the degradation of erythrocytes by thyroid epithelial cells in vivo.

The ultrastructure of the degradation of red blood cells (RBCs) in vivo by rat thyroid epithelial cells was studied. Two morphologically distinct degradative pathways appeared evident. The granular pathway was characterized by a finely mottled appearance of RBC phagosomes, first observed about the RBC periphery and later extending to the entire RBC matrix. Such phagosomes became gradually smaller, less electron dense, and indistinguishable from cellular lysosomes. The hemolytic degrative pathway was characterized by a progessive, usually homogenous decrease in the density of the RBC matrix except at the periphery of the RBC, where a thin, dense layer persisted for some time. Such phagosomes often appeared swollen relative to freshly ingested RBCs and resembled RBC ghosts. In later stages, they became irregular, smaller, and gradually indistinguishable from cellular lysosomes. Both degradative pathways were associated with ferritin-like particles. The earliest visualization of ferritin was in the cytoplasm, but in later stages it was also found to be concentrated within lysosomes and phagolysosomes.