Structure and shape of erythrocytes

The final stage of erythroid maturation is the circulating erythrocyte which performs the basic function of oxygen delivery. Normal human erythrocytes appear as circular, flexible, biconcave discs, lacking a nucleus and most of the cytoplasmatic organelles accompanying the erythroid precursors during their differentiation and maturation processes. These cells stain red-orange with a relatively pale centre in normally prepared smears (Giemsa staining), while they show the characteristic biconcave shape (discocyte) by scanning electron microscopy (SEM).

Resting cells have a diameter of 7.5-8.3 |xm. They are 1.7 pm thick (0.6 |xm on dried blood films) and have an average volume of 83 jjim3. When first formed and released from the bone marrow, the young erythrocyte is subjected to some surface remodeling, due to numerous events (mechanical fragmentation, immunologic reactions, chemical alterations) which may irreversibly alter the membrane structure. The progressive loss of membrane fragments, with subsequent reduction of the cell surface area, leads to the formation of a rigid spherocyte, unable to withstand continuous deformations required by circulatory stresses. The maintenance of the normal shape of the erythrocytes is dependent not only on the environment of the cell but also on its metabolic condition and age. Thus modification of the cellular shape appears as a dynamic process which allows the red cell to be stretched (up to one-twentieth of the ccll diameter) when passing through circulatory obstructions or narrowed vessels, and to undergo, under physiologic conditions, different reversible changes.

The availability of three-dimensional morphologic features obtained by SEM has provided a careful analysis of a spectrum of red cell shape changes occurring in normal and pathologic conditions. A visual demonstration of the deformation processes of the red cells and of the mechanisms underlying the fragmentation and resealing of these membranes has been obtained from several human and experimental models. Under physiologic conditions the discocyte may undergo two distinct reversible changes, i.e. the echinocytic (spiculated) and the stomatocytic (cup-shaped) forms, which are strictly dependent on the pH of the medium and intrinsic metabolic rate of the cells. According to Bessis, these two basic features can be viewed as opposite phenomena, resulting from the exposure of the red cells to conditions or substances which are antagonistically active (Figure 1). The persistence of the action of transforming

Figure 1 Echinocytic (A) and stomatocytic (C) cells represent distinct morphologic changes of the normal erythrocyte

(discocyte) (B), possibly related to the selective expansion of the outer or inner leaflets of the lipid bilayer.

Figure 1 Echinocytic (A) and stomatocytic (C) cells represent distinct morphologic changes of the normal erythrocyte

(discocyte) (B), possibly related to the selective expansion of the outer or inner leaflets of the lipid bilayer.

agents is associated with a progressive deformation of these configurations, leading through morphologically recognizable stages (spheroechinocyte and spherostomatocyte) to a fully developed spherocytic appearance which is a nonreversible condition. Spherocytes are easily trapped by reticuloendothelial cells, mainly in the spleen. The mechanism of the reversible changes is possibly due to the differential expansion of either the outer or inner leaflet of the lipid bilayer. The expansion of the outer half is associated with membrane spiculation, whereas the expansion of the inner half leads to membrane invagination and cup-shaped cells.

Beside these physiologic (reversible) modifications of the red cell shape, some cleavage processes of the membrane or metabolic events may cause a persistent (irreversible) deformation of the cell surface. The variety of the morphologic features of distorted erythrocytes has led to a classification (according to an international Greek nomenclature), which reflects the prominent characteristics of the pathologic cells. The detection of these features offers a key for the interpretation of some underlying disorders in many patients (for example, the presence of acanthocytes in abetalipoproteinemia in liver alcoholic cirrhosis, of codocytes (target cells) in thalassemia, hemoglobinopathies or in hereditary lecithin-cholesterol acyltransferase (LCAT) deficiency, of keratocytes (helmet cells) and schizocytes in microangiopathic hemolytic anemia and disease associated with intravascular deposition of fibrin, of dacryocytes (tear drop cells) in myelofibrosis, of drepanoevtes in sickle cell anemia.

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