Histology of Blood (Part 1) PDF

JABIR IBN HAYYAN MEDICAL Lecture By : Alaa AL Hussainy UNIVERSITY COLLEGE OF MEDICINE Lec :4 DEPARTMENT OF HUMAN ANATOMY Section of Histology...

JABIR IBN HAYYAN MEDICAL Lecture By : Alaa AL Hussainy UNIVERSITY COLLEGE OF MEDICINE Lec :4 DEPARTMENT OF HUMAN ANATOMY Section of Histology HISTOLOGY OF BLOOD (part 1) Blood is a specialized connective tissue in which cells are suspended in fluid extracellular material called plasma. Propelled mainly by rhythmic contractions of the heart, about 5 L of blood in an average adult moves unidirectionally within the closed circulatory system. The so-called formed elements circulating in the plasma are erythrocytes(red blood cells), leukocytes(white blood cells), and platelets. When blood leaves the circulatory system, either in a test tube or in the extracellular matrix (ECM) surrounding blood vessels, plasma proteins react with one another to produce a clot, which includes formed elements and a pale yellow liquid called serum. Serum contains growth factors and other proteins released from platelets during clot formation, which confer biological properties very different from those of plasma. 1 Collected blood in which clotting is prevented by the addition of anticoagulants (eg, heparin or citrate) can be separated by centrifugation into layers that reflect its heterogeneity. Erythrocytes make up the sedimented material and their volume, normally about 45% of the total blood volume in healthy adults, is called the hematocrit. The straw-colored, translucent, slightly viscous supernatant comprising 55% at the top half of the centrifugation tube is the plasma. A thin gray-white layer called the buffycoat between the plasma and the hematocrit, about 1% of the volume, consists of leukocytes and platelets, both less dense than erythrocytes. COMPOSITION OF PLASMA Plasma is an aqueous solution, pH 7.4, containing substances of low or high molecular weight that make up 7% of its volume. The dissolved components are mostly plasma proteins, but they also include nutrients, respiratory gases, nitrogenous waste products, hormones, and inorganic ions, collectively called electrolytes. Through the capillary walls, the low-molecular-weight components of plasma are in equilibrium with the interstitial fluid of the tissues. The composition of plasma is usually an indicator of the mean composition of the extracellular fluids in tissues. The major plasma proteins include the following: Albumin αGlobulins and β Globulins. γGlobulins. Fibrinogen. Complement proteins. 2 BLOOD CELLS Blood cells can be studied histologically in smears prepared by spreading a drop of blood in a thin layer on a microscope slide. In such films the cells are clearly visible and distinct from one another,facilitating observation of their nuclei and cytoplasmic characteristics. Blood smears are routinely stained with special mixtures of acidic (eosin) and basic(methylene blue) dyes. These mixtures may also contain dyes called azures that are more useful in staining cytoplasmic granules containing charged proteins and proteoglycans. Erythrocytes Erythrocytes (Red Blood Cells or RBCs) are terminally differentiated structures lacking nuclei and completely filled with the O2-carrying protein hemoglobin. RBCs are the only blood cells whose function does not require them to leave the vasculature. Human erythrocytes suspended in an isotonic medium are flexible biconcave discs. They are approximately 7.5 μm in diameter, 2.6 μm thick at the rim, but only 0.75 μm thick in the center. 3 The biconcave shape provides a large surface-to-volume ratio and facilitates gas exchange. The normal concentration of erythrocytes in blood is approximately 3.9 to 5.5 million per microliter (μL, or mm3) in women and 4.1-6.0 million/μLin men. Erythrocytes are normally quite flexible, which permits them to bend and adapt to the irregular turns and small diameters of capillaries. Observations in vivo show that at the angles of capillary bifurcations, erythrocytes with normal adult hemoglobin frequently assume a cuplike shape. In larger blood vessels RBCs often adhere to one another loosely in stacks called rouleaux. The plasmalemma of the erythrocyte, because of its ready availability, is the best-known membrane of any cell. It consists of about 40% lipid, 10% carbohydrate, and 50% protein. Most of the latter are integral membrane proteins, including ion channels, the anion transporter called band 3 protein, and glycophorin A. The glycosylated extracellular domains of the latter proteins include antigenic sites that form the basis for the ABO blood typing system. Several peripheral proteins are associated with the inner surface of the membrane, including spectrin, dimers of which form a lattice bound to underlying actin filaments, and ankyrin, which anchors the lattice to the glycophorins and band 3 proteins. This submembranous 4 meshwork stabilizes the membrane, maintains the cell shape, and provides the cell elasticity required for passage through capillaries. Erythrocyte cytoplasm lacks all organelles but is densely filled with hemoglobin, the tetrameric O2-carrying protein that accounts for the cells’ uniform acidophilia. When combined with O2 or CO2, hemoglobin forms oxyhemoglobin or carbaminohemoglobin, respectively. Erythrocyte differentiation includes loss of the nucleus and organelles, shortly before the cells are released by bone marrow into the circulation. Lacking mitochondria, erythrocytes rely on anaerobic glycolysis for their minimal energy needs. Lacking nuclei, they cannot replace defective proteins. Human erythrocytes normally survive in the circulation for about 120 days. By this time defects in the membrane’s cytoskeletal lattice or ion transport systems begin to produce swelling or other shape abnormalities, as well as changes in the cells’ surface oligosaccharide complexes. Senescent or worn-out RBCs displaying such changes are removed from the circulation, mainly by macrophages of the spleen, liver, and bone marrow. 5 ❯❯ MEDICAL APPLICATION Inherited alterations in hemoglobin molecules are responsible for several pathologic conditions, an example of which is sickle cell disease. This disorder is caused by a mutation of one nucleotide (a point mutation) in the gene for the hemoglobin β chain. The consequences of this single substitution are profound. When the altered hemoglobin (called HbS) is deoxygenated in capillaries, it polymerizes and forms rigid aggregates that cause a characteristic sickle shape. The sickled erythrocyte is less flexible and more fragile and has a shortened life span that can lead to anemia. It increases the blood viscosity and can damage the wall of blood vessels, promoting blood coagulation. Sickle cells can block capillaries, restricting O2 delivery to tissues and leading to varying degrees of ischemia or anoxia and organ damage. 6