Modulation of red cell mass by neocytolysis in space and on Earth

Astronauts predictably experience anemia after return from space. Upon ente ring microgravity, the blood volume in the extremities pools centrally and plasma volume decreases, causing plethora and erythropoietin suppression. T here ensues neocytolysis, selective hemolysis of the youngest circulating r ed cells, allowing rapid adaptation to the space environment but becoming m aladaptive on re-entry to a gravitational field. The existence of this phys iologic control process was confirmed in polycythemic high-altitude dweller s transported to sea level. Pathologic neocytolysis contributes to the anem ia of renal failure. Understanding the process has implications for optimiz ing erythropoietin-dosing schedules and the therapy of other human disorder s. Human and rodent models of neocytolysis are being created to help find o ut how interactions between endothelial cells, reticuloendothelial phagocyt es and young erythrocytes are altered, and to shed light on the expression of surface adhesion molecules underlying this process. Thus, unraveling a p roblem for space travelers has uncovered a physiologic process controlling the red cell mass that can be applied to human disorders on Earth.