Histology of Blood PDF

HISTOLOGY The BLOOD  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 5L of blood in an average adult moves unidirectionally within the closed circulatory syst...

HISTOLOGY The BLOOD  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 5L 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. Blood is a distributing vehicle, transporting O2, CO2, metabolites, hormones, and other substances to cells throughout the body. O2 is bound mainly to hemoglobin in erythrocytes and is much more abundant in arterial than venous blood, while CO2 is carried in solution as CO2 or HCO3 -, in addition to being hemoglobin-bound.  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. composition of the extracellular fluids in tissues. The major plasma proteins include the following: Albumin, the most abundant plasma protein, is made in the liver and serves primarily to maintain the osmotic pressure of the blood. Globulins (α- and β-globulins), made by liver and other cells, include transferrin and other transport factors; fibronectin; prothrombin and other coagulation factors; lipoproteins and other proteins entering blood from tissues.  Immunoglobulins (antibodies or γ-globulins) secreted by plasma cells in many locations. Fibrinogen, the largest plasma protein (340 kD), also made in the liver, which, during clotting, polymerizes as insoluble, cross-linked fibers of fibrin that block blood loss from small vessels. Complement proteins, which comprise a defensive system important in inflammation and destruction of microorganisms. A tube of blood after centrifugation (center) has nearly half of its volume represented by erythrocytes in the bottom half of the tube, a volume called the hematocrit. Between the sedimented erythrocytes and the supernatant light-colored plasma is a thin layer of leukocytes and platelets called the buffy coat. The concentration ranges of erythrocytes, platelets, and leukocytes in normal blood are included here, along with the differential count or percent range for each type of leukocyte represented in the buffy coat. A cubic millimeter of blood is equivalent to a microliter (µL). (Complete blood count [CBC].  › 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. Blood smears are routinely stained with 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 O 2-carrying protein hemoglobin.  Leukocytes: Leukocytes (white blood cells or WBCs) leave the blood and migrate to the tissues where they become functional and perform various activities related to immunity. According to the type of cytoplasmic granules and their nuclear morphology, leukocytes are divided into two groups: granulocytes and agranulocytes. Granulocytes possess two major types of cytoplasmic granules: lysosomes (often called azurophilic granules in blood cells) and specific granules that bind neutral, basic, or acidic stains and have specific functions.  Granulocytes have polymorphic nuclei with two or more distinct (almost separated) nuclear lobes and include the neutrophils, eosinophils, and basophils. All granulocytes are terminally differentiated cells with a life span of only a few days. Their Golgi complexes and rough ER are poorly developed. They have few mitochondria and depend largely on glycolysis for their low energy needs. Granulocytes normally die by apoptosis in the connective tissue each day. The resulting cellular debris is removed by macrophages and, like all apoptotic cell death, does not itself elicit an inflammatory response. The number of leukocytes in the blood varies according to age, sex, and physiologic conditions. Healthy adults have 4500 to 11,000 leukocytes per microliter of blood.  Neutrophils (Polymorphonuclear Leukocytes) Mature neutrophils constitute 54% to 62% of circulating leukocytes; circulating immature forms raise this value by 3% to 5%. Neutrophils are 12-15 μm in diameter in blood smears, with nuclei having two to five lobes linked by thin nuclear extensions. Neutrophils are inactive and spherical while circulating but become actively amoeboid during diapedesis and upon adhering to solid substrates such as collagen in the ECM. Neutrophils are active phagocytes of bacteria and other small particles and are usually the first leukocytes to arrive at sites of infection, where they actively pursue bacterial cells using chemotaxis.  They release polypeptide chemokines that attract other leukocytes and cytokines that direct activities of these and local cells of the tissue. Important lipid mediators of inflammation are also released from neutrophils. Neutrophils are short-lived cells with a half-life of 6 to 8 hours in blood and a life span of 1 to 4 days in connective tissues before dying by apoptosis.  Eosinophils Eosinophils are far less numerous than neutrophils, constituting only 1% to 3% of leukocytes. In blood smears, this cell is about the same size as a neutrophil or slightly larger. The main identifying characteristic is the abundance of large, acidophilic specific granules typically staining pink or red. Eosinophilic peroxidase, other enzymes and toxins, act to kill parasitic worms or helminths. Eosinophils also modulate inflammatory responses by releasing chemokines, cytokines, and lipid mediators, with an important role in the inflammatory response triggered by allergies. These leukocytes also remove antigen-antibody complexes from interstitial fluid by phagocytosis. Eosinophils are particularly abundant in connective tissue of the intestinal lining and at sites of chronic inflammation, such as lung tissues of asthma patients.  Basophils Basophils are also 12 to 15 μm in diameter but make up less than 1% of blood leukocytes and are therefore difficult to find in normal blood smears. The nucleus is divided into two irregular lobes, but the large specific granules overlying the nucleus usually obscure its shape. The specific granules (0.5 μm in diameter) typically stain purple with the basic dye of blood smear stains and are fewer, larger, and more irregularly shaped than the granules of other granulocytes. The strong basophilia of the granules is due to the presence of heparin and other sulfated GAGs. Basophilic specific granules also contain much histamine and various other mediators of inflammation, including platelet activating factor, eosinophil chemotactic factor, and the enzyme phospholipase A.  By migrating into connective tissues, basophils appear to supplement the functions of mast cells. Both basophils and mast cells have metachromatic granules containing heparin and histamine, have surface receptors for immunoglobulin E (IgE), and secrete their granular components in response to certain antigens and allergens.  Lymphocytes By far the most numerous type of agranulocyte in normal blood smears or CBCs, lymphocytes constitute a family of leukocytes with spherical nuclei. Lymphocytes are typically the smallest leukocytes and are abundant, constituting up to a third of these cells. Although they are morphologically similar, lymphocytes can be subdivided into functional groups by distinctive surface molecules (called “cluster of differentiation” or CD markers) that can be distinguished using antibodies with immunocytochemistry or flow cytometry. Major classes include B lymphocytes, helper and cytotoxic T lymphocytes (CD4+ and CD8+, respectively), and natural killer (NK) cells. Though generally small circulating lymphocytes have a wider range of sizes than most leukocytes.  Many have diameters similar to those of RBCs; medium and large lymphocytes are 9 to 18 μm in diameter, with the latter representing activated lymphocytes or NK cells. The small lymphocytes are characterized by spherical nuclei with highly condensed chromatin and only a thin surrounding rim of scant cytoplasm, making them easily distinguishable from granulocytes. Larger lymphocytes have larger, slightly indented nuclei and more cytoplasm that is slightly basophilic.  Monocytes Monocytes are a granulocytes that are precursor cells of macrophages, osteoclasts, microglia, and other cells of the mononuclear phagocyte system in connective tissue. All monocyte-derived cells are antigen-presenting cells and have important roles in immune defense of tissues. Circulating monocytes have diameters of 12 to 15 μm, but macrophages are somewhat larger. The monocyte nucleus is large and usually distinctly indented or C- shaped. The chromatin is less condensed than in lymphocytes and typically stains lighter than that of large lymphocytes. The cytoplasm of the monocyte is basophilic and contains many small lysosomes or azurophilic granules, some of which are at the limit of the light microscope’s resolution.  Platelets Blood platelets (or thrombocytes) are very small nonnucleated, membrane-bound cell fragments only 2 to 4 μm in diameter. Platelets originate by separation from the ends of cytoplasmic processes extending from giant polyploid bone marrow cells called megakaryocytes. Platelets promote blood clotting and help repair minor tears or leaks in the walls of small blood vessels, preventing loss of blood from the microvasculature. Normal platelet counts range from 150,000 to 400,000/μL (mm3) of blood. Circulating platelets have a life span of about 10 days. In stained blood smears, platelets often appear in clumps.