Histology of Blood

BLOOD Blood consists of fluid element (plasma) and formed elements cells: erythrocytes, platelets and leukocytes. Adult has about 5.5 Liters Blood is usually studied in stained smears using Leishmann or Giemsa stains. Blood cells are produced in bone marrow from stem cells.

Composition of Blood

Erythrocytes (RBCS) Shape: Biconcave disks which has no nucleus (large surface area for gas exchange). Dimention: They are 7.2 - 7.4 µm in diameter and 2 µm in thickness in stained smears. Rouleaux appearance in slow circulation.

Flexible; squeeze when pass through capillaries. Survive about 120 days in circulation. Old RBC removed by macrophages in spleen and bone marrow. Contains hemoglobin which is an endogenous pigment (greenish yellow) to carry oxygen.

Adaptations of RBCS to function Biconcavity: increase surface area/decrease distance for gas exchange. Plasticity: passage through narrow capillaries Rounded edges: no injury. No organelles: space for hemoglobin. Carbonic anhydrase: carry CO2.

Numbers of RBCs Normal: 4-6 million per mm³. Polycythemia (+) Severe dehydration. Hypoxia (+BM) physilogical (high altitude) or pathological (heart and lung diseases). Anemia (-) Deficiency (iron, pr., vit., hormones….) Aplastic (b.marrow affection as in leukaemia/drugs/X-ray/.. Haemolytic anemia. Haemorrhagic anemia

Leucocytes Number: 6-10,000 per mm³ Granulocytes (polymorphonuclear leukocytes) have specific granule neutrophils 60-70% acidophils 2-4% basophils 0.5-1% Agranulocytes lack specific granule lymphocytes 20-30% monocytes 3-8% Platelets 200,000-400,000 per mm³

Granular Leukocytes Eosinophil 2-4% Neutrophil 60-70% Basophil 0.5-1%

Agranular Leukocytes Lymphocyte 20-25% Monocyte 3-8%

Neutrophils 12-15 µm in diameter. Multi-lobed nucleus. Human females may have inactivated second X chromosome (drumstick). Specific granules: Alkaline PO4ase, collagenase, lactoferrin, lysozyme and phagocytins. Azurophilic granules: deep red or purple, primary lysosomes containing acid hydrolase myeloperoxidase, lysozyme.

Neutrophils Lifespan: 1-4 days in tissue. Active phagocytes which are important in defense against infection. Form H2O2 and powerful cytotoxins. Lysozyme breaks down bacterial cell wall.

Eosinophils About 12-15 µm in diameter Bi-lobed nucleus Many granules (primary lysosomes) Kill parasites, especially worms Phagocytose Ag-Ab complexes to terminate allergic reactions.

Basophils Scarce, hard to find in smears S-shaped nucleus Ig E receptors on their plasmalemma Many blue (basophilic) specific granules with heparin and histamine Content of specific granules cause vasodilatation Nonspecific granules are lysosomes Function as initiator of the inflammatory process May act as supplement of mast cell function

Lymphocytes 20-25 % of circulating leukocytes 6-8 µm dia most common; large ones up to 18 µm dia also found in blood Dark, heterochromatic nucleus Thin layer of blue cytoplasm, many ribosomes Three types: T-cells(80%), B-cells (15%) and Null cells(remainder) Differentiation occurs in bone marrow (B cells) and thymus (T cells) T-cells may live many years, B-cells live a few months

Lymphocytes B cells form plasma cells, function in humoral immunity via immunoglobulins T cells function in cell-mediated immunity Some T cells with “memory” of antigen exposure survive long periods; immunization Null Cells are composed of: Stem cells and Natural killer cells NK cells kill some foreign and virally alerted cells

Monocytes 12-20 µm in diameter Oval, eccentric, horseshoe or kidney shaped nucleus Lighter stained nucleus than large lymphocytes Cytoplasm light blue due to azurophilic granules (lysosomes) and ribosomes In tissues, differentiate into macrophages Lifespan 12-100 hours Do not re-enter into circulation

Platelets Non-nucleated disk-shaped cell fragments 2-5 µm in diameter, derived from megakaryocyte cytoplasm Life span less than 14 days in blood Hyalomere is the peripheral clear region Granulomere is the central darker region

Platelet Function Blood clotting; aggregation of platelets Clot retraction due to actin/myosin Clot removal due to plasmin which is a proteolytic enzyme

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