Neocytolysis: physiological down-regulator of red-cell mass

It is usually considered that red-cell mass is controlled by erythopoietin-driven bone marrow red-cell production, and no physiological mechanisms can shorten survival of circulating red cells. In adapting to acute plethora in microgravity, astronauutsʹ red-cell mass falls too rapidly to be explained by diminished by red-cell production. Frrrokinetics show no early decline in erythropoiesis, but red cells radiolabelled 12 days before launch survive normally. Selective destruction of the youngest circulating red cells-a process we call neocytolysis-is the only plausible explanation. A fall in erythropoientin below a threshold is likelhy to initiate neocytolysis, probably by influencing surface-adhesion molecules. Recognition of neocytolysis will require re-examination of the pathophysiology and treatment of several blood disorders, including the anaemia of renal disease.