The RBCs possess a unique structure responsible for their remarkable mechanical properties (de-formability, elasticity, shear resistance, etc. . . ) allowing them to flow through capillaries of the micro-circulation. Their membrane encloses a cytoplasm made of hemoglobin, allowing the transportation of oxygen through the organism. It is constituted of an elastic 2-D mesh-like spectrin cytoskeleton
CHAPTER 2. STRUCTURAL AND MECHANICAL PROPERTIES OF RED BLOOD CELLS
anchored to the internal side of a lipid bilayer [18]. Together with the cytoplasmic fluid and the absence of nucleus or other organelles, the membrane controls the overall deformability of the cell.
2.3.1 The RBC membrane
The membrane of human RBC is a multicomponent structure consisting essentially in three layers, as illustrated in Figure 2.3:
(i) An external layer with a forest of peptidoglycans exceeding the proteolipid layer bilayer by about 10 nm of thickness [19].
(ii) A phospholipid bilayer of 5 nm in thickness, composed predominantly of phosopholipids (55%), cholesterol (25%) and dissolved proteins [20]. This lipid bilayer is embedded with transmembrane proteins and junction complexes, which can allow anchoring the cytoskeleton to the lipid bilayer, transporting ions through the lipids (i.e.chlorides and bicarbonates, etc. . . ) or expressing the blood group and rhesus.
(iii) A cytoskeleton, mainly consisting in a triangular network of spectrin which maintains the mem-brane cohesion and the integrity of the cell in flow. It is attached to the lipid bilayer at a distance of about 10 nm through the junction complexes.
Figure 2.3: Schematic representation of the RBC membrane highlighting the three constituent layers with the spectrin network and the junction complexes, adapted from [20].
The cytoskeleton consists in a network of proteins that underlines the internal surface of the lipid bilayer. This 2-D network (about 10 nm in thickness [21]) is mainly composed of long spectrin filaments (200 nm of contour length2) which are interconnected by junction complexes. The spectrin is a heterodimeric fibrous protein, consisting of two stringsαandβof about 100 nm in length and 5 nm in diameter wrapped one around another to form anα-βdimer shaped like an helix [21].
Scanning electron microscopic images of the RBC cytoskeleton presented in Figure 2.4-a, highlighted the organization of the skeleton in a triangular-based structure: the ridges consisting in spectrin filament and the vertices corresponding to the junction complexes.
The RBC membrane is considered semi-permeable since it is permeable to water molecules, but impermeable to most ions. This water exchange with the external environment allows the cell to
2The contour length is defined as the sum of the length of the twoαandβstrings constituting the spectrin.
2.3. STRUCTURE OF THE RBC
Figure 2.4: (a) Negatively stained electron micrographs and (b) schematic representation of the 2-D network of spectrin with the triangular-based structures, from [22].
support suspending buffer whose salt concentration is abnormal, too high or too low. Physiological media match the osmolarity of the cytoplasm (∼300 mOsm/kg) hence ensuring isotonic conditions.
Depending on the osmolarity (i.e.salt concentration) of the surrounding environment, the RBC membrane exchanges water with the external environment in order to balance the osmotic pressure on both sides. Therefore, by respecting the energy minimization principle, the cell adapts its shape to its new volume as illustrated in Figure 2.5. As stated earlier, in an isotonic medium, the RBC has a discoidal shape, whereas in a medium with low osmolarity (i.e.hypotonic buffer), the water of the surrounding medium crosses the membrane to enter the cytoplasm and dilute its salt concentration;
in consequences the cell inflates to become a stomatocyte (Fig. 2.5 right). At 150 mOsm/kg, it is perfectly spherical and if the osmolarity is further decreased, the cell membrane ruptures under the tension and the hemoglobin is released: its the lysis phenomenon. At osmolarity greater than 300 mOsm/kg (i.e.hypertonic conditions), the water molecules contained in the RBCs exit the cell towards the external environment and the cell deflate to an echinocyte (Fig. 2.5 left).
2.3.2 The RBC cytoplasm
The cytoplasm of the RBC is mainly composed of hemoglobin which account for about 33% of the weight of cell [17]. Hemoglobin determines the internal viscosity of the cell which is∼6.5 mPa.s at 37◦C and 10 mPa.s at 25◦C [19]. Hemoglobin consists in the major part of proteins, calledglobins, itself composed of four polypeptides (long chains of amino acids), two stringsαand two stringsβ, attached to each other. To each chain of globin is fixed a molecule of heme, which serves to fix oxygen on its atom of iron. Hemoglobin has a bright red color, when it is oxygenated, and becomes brown when it has lost its oxygen. Abnormal structure of the hemoglobin may influence the shape and deformability of the RBC, as for example in sickle cell anemia where the physiological hemoglobin
"A", normally found in RBCs, is replaced by the pathological hemoglobin "S", also calledsickle cell hemoglobin, capable of polymerizing into rigid fibers hence deforming the erythrocyte into a sickle cell [24].
CHAPTER 2. STRUCTURAL AND MECHANICAL PROPERTIES OF RED BLOOD CELLS
Figure 2.5: Illustration of the effect of the osmolarity of the surrounding buffer on the shape under-gone by RBCs. In an isotonic buffer (∼300 mOsm/kg), the RBC is discosyte. It becomes equinocyte when suspended in a hypertonic buffer (> 300 mOsm/kg) and stomatocytye in hypotonic conditions (< 300 mOsm/kg), from [23].