Platelets, or thrombocytes, the smallest formed elements in the blood histology , are disklike cell fragments that vary in diameter from 2 to 5 um. In humans, they lack nuclei and originate by budding from large cells in the bone marrow called megakaryocytes. They range in number from 200,000 to 400,000 mm3 of blood and have a lifespan of about 8 days. In blood smears they appear in clumps. Each platelet has a peripheral hyalomere region that stains a faint blue and a dense central granulomere that contains a few mitochondria and glycogen granules and a variety of purple granules. Platelets have an important physical role in plugging wounds, and they contribute to the cascade of molecular interactions among the various clotting factors dissolved in the plasma blood histology
Blood Platelets
Basophils
Basophils are the least numerous of the circulating leukocytcs, constituting 1% of the white blood cells of healthy adults. Like other white blood cells, basophils may leave the circulation, but they are capable of only very limited ameboid movement and phagocytosis in the tissues. Extravascular basophils are found most often at sites of inflammation and may be the major cell type at sites of cutaneous basophil hypersensitivity. Despite structural and functional similarities between basophils and mast cells, these cells are not the same and are distinguishable on the basis of ultrastructure. blood histology
Eosinophils
Eosinophils constitute only 1-4% of the circulating leukocytes in healthy adults. They may leave the bloodstream by diapedesis, spread out, and move about in the connective tissues. They are capable of only limited phagocytosis, showing a preference for antigen antibody complexes. The number of circulating eosinophils typically increases during allergic reactions and in response to parasitic infections and rapidly decreases in response to treatment with exogenous corticosteroids in blood.
2. Granulocytes have segmented nuclei and are described as polymorphonuclear leukocytes (PMNLs). Depending on the cell type, the mature nucleus may have 2-7 lobes connected by thin strands of nucleoplasm. Granulocyte types are most easily distinguished by their size and staining properties and by the appearance (as seen by EM) of the abundant specific granules in their cytoplasm. These granules are all membrane-limited and bud off the Golgi complex.
In addition to a small Golgi complex, each granulocyte contains a few mitochondria and free ribosomes and sparse RER. a. Neutrophils are the most abundant leukocytes in the blood. They usually constitute 60-70% of the white blood cells, with a limited range of normal variation (50-75%). They are also found outside the bloodstream, especially in loose connective tissue. Neutrophils are the first line of cellular defense against the invasion of bacteria. Once they leave the bloodstream, they spread out, develop ameboid motility, and become active phagocytes. Unlike lymphocytes, neutrophils are all terminally differentiated cells and so are incapable of mitosis.
Agranulocytes - Monocytes
1. Agranulocytes have unsegmented nuclei and are described as mononuclear leukocytes. They lack specific granules, but may contain azurophilic granules (0.05-0.25 um in diameter). a. Lymphocytes constitute a diverse class of cells; they have similar morphologic characteristics but a variety of highly specific functions. They normally account for 20-25% of the white blood cells in adult blood, with a considerable range of normal variation (20-45%). They respond to invasion of the body by foreign substances and organisms and assist in their inactivation. Unlike other leukocytes, lymphocytes never become phagocytic. The 2 major functional classes of lymphocytes are T cells and B cells. Lymphocytes in the blood are predominantly (about 80%) T cells.
b. Monocytes are often confused with large lymphocytes. They are large and constitute only 3-8% of the white blood cells in healthy adults. Monocytes are found only in the blood, but they remain in circulation for less than a week before migrating through capillary walls to enter other tissues or to become incorporated in the lining of sinuses. Once outside the bloodstream, they become phagocytic and apparently do not recirculate. The mononuclear phagocyte system consists of monocyte-derived phagocytic cells distributed throughout the body. Examples include the Kupffer cells of the liver and some of the macrophages of connective tissues.
Leukocytes
Leukocytes, or white blood cells, are nucleated and are larger and less numerous (6000-10,000/Cl) than erythrocytes.
Leukocytes can be divided into 2 main groups, granulocytes and agranulocytes, according to their content of cytoplasmic granules. Each group can then be further divided on the basis of size, nuclear morphology, ratio of nuclear to cytoplasmic volume, and staining properties. Two classes of cytoplasmic granules occur in leukocytes: specific and azurophilic granules. Specific granules are found only in granulocytes; their staining properties (neutrophilic, eosinophilic, or basophilic) distinguish the 3 granulocyte types. Azurophilic granules occur in both agranulocytes and granulocytes.
Their content of lytic enzymes suggests that they function as lysosomes. Unlike the RBCs, all leukocytes can leave the capillaries by squeezing between endothelial cells, a process termed diapedesis, and enter the surrounding connective tissue in response to infection or inflamma tion. The types and levels of activity expressed by extravascular leukocytes depend upon the specific cell type
Formed elements of blood : hemoglobin
3. Hemoglobin. Each hemoglobin molecule consists of 4 polypeptide subunits, each of which includes an iron-containing heme group. Hemoglobin can bind reversibly to oxygen, forming oxyhemoglobin, and to carbon dioxide, forming carbaminobemoglobin. However, hemoglobin binds irreversibly to carbon monoxide, forming carboxyhemoglobin, which reduces the oxygen-carrying capacity of the blood. Hemoglobin (Hb) exists in a variety of forms, distinguishable on the basis of the amino acid sequence of their subunits. In humans, only 3 forms are considered normal in postnatal life: HbAI constitutes 97%, HbA2 2%, and HbF 1% of the hemoglobin of healthy adults. HbF makes up around 80% of the hemoglobin of newborns, however; this proportion gradually decreases until normal adult levels are reached at about 8 months of age. HbS is an abnormal form of HbA that is found in patients with sickle cell anemia; it differs by a single amino acid substitution in the beta chain (valine in HbS, glutamine in HbA). Unlike HbA, HbS becomes insoluble at low oxygen tensions and crystallizes into inflexible rods that deform the RBCs, giving them the characteristic sickle shape. When the rigid sickled cells pass through narrow capillaries, they cannot bend as normal RBCs do. They may become trapped, obstructing blood flow through the capillary, or rupture, decreasing the number of RBCs available for oxygen transport (anemia).
4. Plasmalemma and stroma. When placed in a hypotonic solution, RBCs swell and release their hemoglobin into the surrounding solution, a process termed hemolysis; they leave behind an empty shell, or red cell ghost, composed of the plasmalemma and the stroma.