Red cell membrane
The complex mixture of lipids and proteins of the red blood cell membrane is well maintained during the life of the cell. The structural backbone of the RBC plasma membrane, which separates outside from inside, comprises more than 250 molecular species of glycerophospholipid and sphingomyelin, and together with cholesterol make up most of plasma membrane lipids. Cellular organelles, including the nucleus, mitochondria, lysosomes, peroxisomes, Golgi apparatus, and endoplasmatic reticulum are lost in the final stage of red cell maturation. The adult red cell has only a plasma membrane that surrounds a cytosol highly enriched in hemoglobin. A membrane skeleton under the lipid bilayer interacts through protein interactions with integral membrane proteins, and provides both a strong and flexible structure that defines the shape of the red cell.
Red Cell membrane: The fluid mosaic model of biological membranes. Globular integral membrane proteins are mobile within a sea of phospholipids and cholesterol. The spectrin network of the membrane skeleton interacts with other membrane proteins to provide the red cell with a strong and flexible structure.
Changes in the interactions within the membrane skeleton can lead to alterations in the ability of the red cell to deform. Mutations in the horizontal plane lead to eliptocytosis, while changes in the interaction of the spectrin network with membrane components in the vertical plane lead to spherocytosis, both readily diagnosed by ektacytometry. Hemoglobin mutations affect the red cell membrane in sickle cell disease and thalassemia. The changes in the membrane lead to in-effective hematopoiesis, premature removal from the circulation, altered interactions with platelets, white cells and endothelial cells, an imbalance in hemostasis, and hemolysis. Together these changes play an important role in the pathology of hemoglobinopathies. In addition to changes in blood parameters as measured by blood analysis using the Advia 120, the hemoglobin induced changes in the cytosol and membrane lead to a dramatic shift in the ektacytometric osmotic deformability pattern, exposure of phosphatidyl serine, and a reduced red cell survival. Shifts of these and other parameters of the red cell towards normal can be used to evaluate the efficacy of treatment protocols.