Histology of blood

• Learning Outcomes

  • Know the main components of peripheral blood
  • Be able to recognize all of the cellular components found in peripheral blood by light and electron microscopy
  • Know the approximate abundance and life span of various types of blood cells
  • Understand the functions of major plasma proteins and of the different kinds of cells
  • Be familiar with the general process of haematopoiesis and know the changes that occur during myeloid and erythrocyte cell differentiation
  • Be able to recognize megakaryocytes in the bone marrow and understand their function in platelet production
• Blood

  Suspension of cells in fluid circulated around the body by the heart, transport vehicle for gases, nutrients, waste products, cells and hormones
• Plasma

  Fluid component of blood: water, proteins, inorganic salts, lipids, glucose & other minor components
• Functions of blood

  • Transport of gases, nutrients, waste products, cells and hormones
• Romanowsky stain


  Main components are methylene blue and eosin, best for morphology of blood and bone marrow (Giemsa, Wright)
• Types of staining


  • Basophilia - affinity for methylene blue
  • Azurophilia - affinity for azure dyes
  • Acidophilia or eosinophilia - affinity for eosin
  • Neutrophilia - affinity for complex dyes
• Blood cells are classified by the type of stain that binds to them or their components
• Preparing a blood smear


  1. Lymphocyte

  2. RBC

  3. Platelets

  4. Neutrophil
• Erythrocytes


  • Life span in blood: About 120 days
  • Size and shape: biconcave disk, 8 µm diameter, 2 µm at thickest point, 1 µm at thinnest, shape maintained by cytoskeletal complex, flexible to pass through small capillaries
  • LM appearance in smear: Pink circle with light center, no nucleus
  • TEM appearance: Solid dark gray cytoplasm due to highly concentrated hemoglobin
  • Function: Transport of oxygen and carbon dioxide bound to hemoglobin, majority of CO2 transported as HCO3-, pH homeostasis via carbonic anhydrase
• Erythrocytes are flexible, capable of bending to pass through small capillaries
• RBC Cytoskeleton and Membrane-Associated Proteins


  • Plasmalemma contains proteins like band 3 and glycophorin A which form the basis of blood typing
  • Spectrin and ankyrin form a network that permits the flexibility of RBCs through capillaries
  • Cytoplasm is densely filled with hemoglobin - oxyhemoglobin and carbaminohemoglobin
• Platelets (thrombocytes)


  • Life Span: about 10 days
  • Shape, size, and origin: Small, biconvex disks, 2-3 µm in diameter, non-nucleated cell fragments derived from cytoplasm of megakaryocytes in bone marrow
  • LM appearance in smears: Small basophilic fragments, often appearing in clusters
  • TEM appearance: Bound by plasma membrane, bundle of microtubules around the margin, three types of granules containing fibrinogen, plasminogen, thromboplastin and other clotting factors
  • Function: Platelets initiate blood clots
• In a blood smear, platelets are often found as aggregates
• Ultrastructurally a platelet typically shows a system of microtubules and actin filaments near the periphery, an open canalicular system of vesicles continuous with the plasmalemma, and a central granulomere region containing glycogen and secretory granules
• TEM section shows platelets adhering to collagen, upon adhesion they exocytose their granules into the canalicular system, allowing rapid secretion of clotting factors
• Neutrophils (polymorphonuclear leukocyte)


  • Life Span: < 7 days
  • Granulocyte with specific and non-specific granules
  • LM appearance in smear: About 9-12 µm in diameter, nucleus long and multi-lobed, cytoplasm has small, neutrally stained specific granules and azurophilic non-specific granules
  • TEM appearance: Multi-lobed nucleus and numerous specific granules and lysosomes
  • Function: Primarily antibacterial, leave blood and follow chemotactic signals to sites of wounding or inflammation, phagocytose foreign agents, pus is composed largely of dead neutrophils
• In blood smears neutrophils can be identified by their multilobulated nuclei, with lobules held together by thin strands
• Neutrophil Granules


  • Specific granules: Type IV collagenase, lactoferrin, phospholipase A2, lysozyme
  • Non-specific granules (lysosomes): Lysozyme, acid hydrolase, myeloperoxidase, elastase
• Eosinophils


  • Life Span: < 14 days
  • Granulocyte with specific and non-specific granules
  • LM appearance in smear: About 10-14 µm in diameter, bilobed nucleus, cytoplasm has prominent pink/red specific granules
  • TEM appearance: Specific granules are ovoid in shape, contain a dark crystalloid body composed of major basic protein, cytoplasm also contains lysosomes
  • Function: Anti-parasitic activity, mediators of inflammatory/allergic responses, inactivate leukotrienes and histamine, engulf and sequester antigen-antibody complexes
• Eosinophils are about the same size as neutrophils but have bilobed nuclei and abundant coarse cytoplasmic granules
• TEM of a sectioned eosinophil clearly shows the unique specific granules as oval structures with disk-shaped electron-dense crystalline cores
• Eosinophil Granules

  • Specific granules: Major basic protein, eosinophilic cationic protein, neurotoxin, histaminase
  • Non-specific granules (lysosomes): Lysozyme, acid hydrolase, myeloperoxidase, elastase
• Basophils

  • Life Span: a few hours to a few days
  • Granulocyte with specific and non-specific granules
  • LM appearance in smear: About 8-10 µm in diameter, cytoplasm contains large, purple/black specific granules, nucleus usually bilobed
  • TEM appearance: Specific granules vary in size and shape, some with myelin figures, cytoplasm also has lysosomes
  • Function: Allergies and anaphylaxis, binding of antigens to membrane-bound IgE antibodies induces degranulation leading to allergic reaction
• Basophils are approximately the same size as neutrophils and eosinophils, but have large, strongly basophilic specific granules which usually obstruct the appearance of the nucleus
• TEM of a sectioned basophil reveals the lobulated nucleus, large specific basophilic granules, mitochondria, and Golgi complex
• Basophil Granules

  • Specific granules: Histamine, heparin, eosinophil chemotactic factor, phospholipids for synthesis of leukotrienes
  • Non-specific granules (lysosomes): Lysozyme, acid hydrolase, myeloperoxidase, elastase
• Lymphocytes


  • Life Span: variable (few days to several years)
  • LM appearance in smear: Small lymphocyte ~8 µm in diameter, round dense nucleus, narrow rim of cytoplasm, large lymphocytes up to 15 µm
• Basophils


  Have large, strongly basophilic specific granules which usually obstruct the appearance of the nucleus having two or three irregular lobes
• Basophils

  • X1500, Wright
  • X150
• Basophil


  Lobulated nucleus appearing as three separated portions, large specific basophilic granules, mitochondria, and Golgi complex
• Basophils


  Exert many activities modulating the immune response and inflammation and share many functions with mast cells
• Lymphocytes


  Variable life span (few days to several years)
• Small lymphocytes

  • About 90% of lymphocytes, ~8 μm in diameter, round dense nucleus, narrow rim of pale blue cytoplasm
• Large lymphocytes

  • Up to 15 μm in diameter, may represent activated cells that have returned to the circulation
• Lymphocytes

  Cytoplasm contains mitochondria, free polysomes, and a few lysosomes
• Monocytes

  Large agranulocytes with diameters from 12 to 20 μm that circulate as precursors to macrophages and other cells of the mononuclear phagocyte system
• Monocytes

  • X1500, Giemsa
  • X1500, Wright
• Monocyte cytoplasm


  Contains Golgi apparatus, mitochondria, and lysosomes or azurophilic granules
• Blood cell types and percentages

  • Lymphocyte: 20-25%
  • Monocyte: 3-8%
  • Eosinophil: 2-4%
  • Neutrophil: 60-70%
  • Basophil: 0.5-1%
• Blood cell development (hematopoiesis)


  1. Mesoblastic phase (yolk sac, 2 weeks)

  2. Hepatic phase (6 weeks)

  3. Splenic phase (12 weeks)

  4. Myeloid phase (marrow, 24 weeks)

  5. Mitotic stem and progenitor cells undergo increasing lineage restriction to produce committed precursors

  6. Precursors undergo cell division and differentiation into mature cells

  7. Maturation involves decrease in cell size, shutting down transcription, adoption of morphological characteristics specific to that lineage

  8. Future granulocytes produce specific and non-specific granules, and then shape their nucleus

  9. Future monocytes produce non-specific granules and shape their nucleus

  10. Future small lymphocytes decrease their size and enter the blood, but then undergo extensive further maturation at another site

  11. Future erythrocytes fill cytoplasm with hemoglobin, synthesized on free polysomes, and eventually extrude their nucleus