and Jürgen Roth2
Medical University of Vienna, Vienna, Austria
University of Zurich, Zurich, Switzerland
Red Blood Cells and Cells of the Erythroid Lineage
The cells of the erythroid lineage develop from a multipotential myeloid stem cell under the influence of the major regulator erythropoietin, a glycoprotein hormone synthesized in the kidney. Erythropoiesis is stimulated in states of a disproportion of oxygen need and supply, e.g., hypoxia resulting from a decrease in oxygen level in the inspired air or a decreased number of erythrocytes in the circulating blood, due to bleeding. The erythropoietin-sensitive erythrocyte progenitor cell CFU-E (erythroid colony forming unit) differentiates to form a proerythroblast. Further differentiation stages include the basophilic, the polychromatophilic, and the orthochromatophilic erythroblast, as well as the reticulocyte, which already is able to leave the bone marrow to enter the circulating blood. Erythropoiesis takes place in a period of 4–6 days and is characterized by the synthesis and increased accumulation in the cytoplasm of hemoglobin, the protein specialized for transporting oxygen and carbon dioxide. Concomitantly, programmed cell changes take place, leading to the exclusion of the nucleus and disappearance of all cell organelles.
Panels A and B show electron micrographs of cells of the erythroid lineage taken by puncture of human bone marrow. Erythroblasts of earlier and later states of differentiation are shown in panel A. The electron density of the cytoplasm increases concomitantly with the increased accumulation of hemoglobin. The partially sectioned erythroblast on the right-hand side shows a less dense cytoplasm compared with the two erythroblasts visible in the center. The former presumably corresponds to an erythroblast of the polychromatophilic type (pE), the latter to orthochromatophilic erythroblasts (oE). Different parts of the cytoplasm are involved in hemoglobin production, including mitochondria (M) where protoporphyrin is synthesized and combined with iron to form the hem part of the hemoglobin molecule. Synthesis of the globin chains takes place on free ribosomes in the cytoplasm (arrows in the inset). By receptor-mediated endocytosis via coated vesicles (arrowhead), iron-transferrin complexes are taken up; after iron is released, transferrin recycles to the cell surface along with the receptor. In the nuclei, condensed chromatin (C) occupies extensive areas leading to a “checkerboard” pattern. Nuclei are delineated by distended perinuclear cisterns. At this stage of development, the cells are no longer able to divide. One of the orthochromatophilic erythroblasts shows a bizarre surface with irregular extensions. They reflect the surface dynamics occurring when the cells start to push out the nuclei. Orthochromatophilic erythroblasts extrude their nuclei, giving rise to reticulocytes (panel B and inset in A). In panel B, a reticulocyte can be seen that shows the characteristic fimbriated processes that appear just after extrusion of the nucleus. Mitochondria and polyribosomes (arrows in the inset) are retained, endocytosis takes place (arrowhead in the inset), and hemoglobin is still synthesized. Within a few days, the reticulocytes mature to form erythrocytes. During this final stage, the reticulocytes remodel their surfaces and get rid of all intracellular compartments. By binding of ubiquitin to proteins of cell organelles, their destruction is initiated. Membrane proteins are endocytosed and sorted into special domains of endosomal membranes, which bud into the lumen. This leads to the formation of multivesicular or multilamellar organelles (arrowheads in panel B). By fusion with the plasma membrane, their contents are released then called exosomes.
The mature erythrocyte (right segment of panel B) is a discoid biconcave “hemoglobin bag.” It is devoid of cell organelles, and the cytoplasm shows a homogenous electron density caused by the high hemoglobin concentration. A unique membrane skeleton consists of spectrin, a heterodimeric protein interacting with actin and plasma membrane proteins, band 3 protein and glycophorin via ankyrin and protein 4.1. The membrane skeleton is responsible for the biconcave erythrocyte shape that provides a large surface-volume ratio facilitating gas exchange. The elastic submembranous matrix formed by the spectrin lattice returns shear-deformed cells to the biconcave shape.
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Magnification: ×10,500 (A); ×39,200 (inset upper right corner); ×17,200 (inset lower left corner); ×15,000 (B)
Among the white blood cells (leukocytes), the neutrophilic granulocytes are the most numerous, and they also are the most common granulocytes. Neutrophilic granulocytes make up some 65 % of all blood leukocytes. Because of their multilobed nuclei, they are also called polymorphonuclear neutrophils or polymorphs. Neutrophilic granulocytes are motile cells. By complex interactions with endothelial cells combined with extensive endothelial membrane reorganizations and remodeling, and binding of extracellular matrix and chemoattractant molecules (chemotaxis), neutrophils leave the circulation and migrate to the sites where their action is needed. Neutrophils are the initial cells at sites of infection. They are active phagocytes (microphages) capable of engulfing foreign material and organisms, mainly via Fc receptors present in the plasma membrane, and interacting with the Fc region of antibodies bound to antigen, e.g., antibodies decorating the surfaces of bacteria. Neutrophilic granulocytes have a central role in microbial defense and, together with other leukocytes – such as eosinophilic and basophilic granulocytes, monocytes/macrophages, lymphocytes, and fibroblasts – hold key functions in inflammation and wound healing.
The electron micrograph shows a neutrophilic granulocyte of the human blood. Two of the nuclear lobes can be seen. The condensed chromatin is apparent mainly in the nuclear periphery, and only small euchromatic regions are in contact with the nuclear envelope. The Golgi apparatus (Golgi) is in a central position between and close to the nuclear segments. Most of the cytoplasm is occupied by the densely packed granules, the contents of which are responsible for the homing and antimicrobial functions of the neutrophils. Three main kinds of granules are accumulated in the neutrophilic granulocyte cytoplasm, the specific neutrophilic granules (arrowheads), azurophilic granules (arrows), and tertiary granules (open arrows), among which also different subtypes exist. It is difficult and perhaps impossible by ultrastructural examination to distinguish all classes of granules. The specific granules show globular or ellipsoidal shapes. Among the diverse granules, they are the smallest, and the most numerous, and contain a range of enzymes, such as phospholipases and type IV collagenase, bacteriostatic and bacteriocidal substances, such as lysozyme, as well as complement activators. The azurophilic granules are larger and less numerous than the specific granules. They appear most early during granulopoiesis (primary granules), but they are not confined to neutrophilic granulocytes. They arise in all types of granulocytes, and also are present in monocytes and lymphocytes. Azurophilic granules are lysosomes. They contain a range of typical acid hydrolases, antibacterial agents, including azurocidin, as well as myeloperoxidase, which is responsible for the generation of highly bactericidal hypochlorite and chloramines. Specific and azurophilic granules are designated as special types of secretory lysosomes (cf. Figs. 65 and 66). They fuse with phagosomes and release their contents, forming a phagolysosome. This process is called degranulation. After digestion, the degraded materials are either stored in residual bodies, or exocytosed. Subtypes of tertiary granules contain phosphatases and metalloproteinases, such as collagenases and gelatinases, which are assumed to facilitate the migration of the cells through the connective tissue.
Aside from the various granules, other cell compartments and organelles – including endoplasmic reticulum and mitochondria – are sparse, but the cytoplasm contains a considerable amount of glycogen, which can be seen in the form of numerous electron dense particles. Glycogen is broken down for yielding energy enabling neutrophilic granulocates to survive in an anaerobic environment.
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Eosinophilic granulocytes make up approximately 2–4 % of the total leukocytes in the blood. The count of eosinophils in blood samples is usually increased in patients suffering from parasitic infections and allergies. Eosinophils are involved in immunological responses and have an important role in the host defense against helminthic parasites. Like neutrophils, eosinophilic granulocytes leave the circulation and populate connective tissues at sites of potential foreign invasion. They are particularly abundant in the lymphoreticular tissue of the intestinal mucosa. Eosinophilic granulocytes are named for their large eosinophilic granules occupying the cytoplasm. The nucleus is often bilobed and, as in neutrophils, condensed chromatin is concentrated mainly in the periphery of the nuclear space.
One of the nuclear segments is visible in the eosinophilic granulocyte of the human blood shown in the micrograph. The ultrastructure of the cytoplasm is dominated by the large specific eosinophilic granules, which contain a characteristic electron-dense central crystalloid body that makes them easy to discern. The crystalloid body contains the major basic protein, which also accounts for the eosinophilia of the granules. The major basic protein, and other proteins residing in the matrix of the specific granules, such as the eosinophil cationic protein, are particularly toxic for helminthic parasites and protozoans. Eosinophil peroxidase and eosinophil-derived neurotoxin are other contents of eosinophilic granules, both attacking parasitic organisms. After binding to the surface of parasites, the granule contents are released directly onto the parasites’ membranes. Other enzymes contained in the specific granules neutralize and moderate potentially deleterious effects of vasoactive agents released during inflammation. These include cathepsins, arylsulfatase, and histaminases, which are particularly important at sites of allergic reactions. Eosinophilic granulocytes also contain azurophilic granules. Some azurophilic granules are visible aside the nucleus and between the large specific granules. They are lysosomes containing a range of acid hydrolases, which are active in the destruction of parasites and capable of decomposing antigen-antibody complexes. Both specific and azurophilic granules of eosinophils belong to special classes of secretory lysosomes.