Human Blood Cells: Structure and Functions

Course Information

• Faculty: Medicine
• Academic Year: 2024-2025
• Year: 1
• Semester: 1
• Module: Blood and Body Fluids (BLF) 103
• Module Topic: Histology of Blood Cells I

Objectives

• Recognize different blood elements
• Describe the histological characteristics of RBCs, leucocytes (granular, non-granular) & platelets at light microscopy (L.M.) and electron microscopy (E.M.) levels
• Correlate normal RBCs structural adaptation to proper function
• Relate RBCs structural abnormalities to clinical conditions (e.g., hemolytic anemia)
• Differentiate between RBCs and WBCs
• Recall normal RBCs, leucocytic and platelet counts
• Correlate the ultrastructure components of platelets to their function
• Discriminate different types of WBCs from each other in various blood films
• Point out abnormal variations in the count and shape of RBCs
• Identify deviations in WBCs and platelet counts and their causes
• Interpret alterations in RBCs, WBCs and platelet count and relate them to possible clinical conditions
• Define total and differential leucocytic counts
• Describe the histological organization and sites of the bone marrow
• Identify types of bone marrow
• Recognize the histological structure of the cells involved in various stages of hemopoiesis

Blood Composition

• Blood is a specialized connective tissue.
• Blood elements comprise 45% of total blood volume
• Plasma comprises 55% of total blood volume

Blood Examination Methods

• Blood film preparation
• Leishman's stain (neutral stain)
• Acidic stain (eosin)
• Basic stain (methylene blue)
• Dissolved in methyl alcohol (fixative)

Blood Smear Preparation

• Withdraw blood using micropipettes
• Place a drop of blood on a slide
• Using a second slide, spread the blood thinly on the first slide
• Allow the blood to dry
• Stain the blood film
• Add a coverslip on top
• Examine using light microscopy.

Red Blood Corpuscles (RBCs) - Erythrocytes

• L.M. characteristics:

  • Shape: biconcave discs (side view), rounded (top view); high surface area for gas exchange
  • Size: 6-9 µm diameter, 7.5 µm average; 2.2 μm thickness at edge, 0.8 μm at center
  • Adaptations: Flexible plasma membrane for squeezing through narrow capillaries, biconcave shape for maximizing surface area
  • Color: Acidophilic (hemoglobin); pale center (1/3 diameter) is normochromic
  • Life span: 120 days
  • Number: 5-5.5 million/mm³ (males), 4.5-5 million/mm³ (females).

• E.M. characteristics:

  • No nucleus or organelles
  • Hemoglobin (Hb) is electron-dense & homogeneous
  • Flexible cell membrane maintained by cytoskeleton components (actin & spectrin)
  • Glycocalyx: includes antigenic sites for blood groups and Rh factor
  • Abnormal shapes: Spherocytes (spherical), ovalocytes (oval), sickle cells (crescent-shaped), poikilocytes (pear-shaped); more fragile, prone to hemolysis.

RBC Life Cycle & Fate

• Old RBCs are phagocytosed by macrophages in the liver, bone marrow, and spleen.
• Excreted as bile pigments
• Iron is recycled for new RBC production.

Histologic Features of Osmotic Fragility of RBC

• Plasma (isotonic solution): normal shape
• Hypertonic solution: crenation (RBCs shrink, show notches)
• Hypotonic solution: hemolysis (RBCs swell, burst, leak hemoglobin)

RBC Number

• Determined by hemocytometer or electronic instruments
• Average number: 5 million/mm³
• Normal adult males: 5-5.5 million/mm³
• Normal adult females: 4.5-5 million/mm³
• Abnormal numbers:
  • Anemia: <4 million/mm³ (oligocythemia) or low hemoglobin
  • Polycythemia: > 6 million/mm³ (due to hypoxia stimulating bone marrow; physiological or pathological.)

Adaptation to Function (RBCs)

• Flexible plasma membrane: squeeze through capillaries.
• Lipoprotein in membrane: highly selective for gas exchange
• Shape: biconcave disc maximizes surface area for gas exchange; rounded edges for easy passage in vessels
• Content: no nucleus for more space for hemoglobin; enzymes (Hb reductase and carbonic anhydrase).

White Blood Cells (WBCs) - Leukocytes

• Types:
  • Granular leucocytes (neutrophils, eosinophils, basophils)
  • Non-granular leucocytes (lymphocytes, monocytes)
  • Count: 4000-11000/mm³
  • Life span: Days to years (varies by cell type)
  • Shape & size: Varies (spherical, 6-10 μm for neutrophils).

Granular Leucocytes (Overview)

• Short life span: (a few days)
• Single, condensed nucleus, segmented
• Nucleus with peripheral heterochromatin and central euchromatin
• Few organelles (Golgi apparatus, mitochondria, ER)
• Contain non-specific (azurophilic) lysosomes and specific granules; azurophilic have affinity to azure dye
• Specific granules stain with neutral, basic, or acidic stains; contain specific functions

Specific Granules & Functions (Granular Leucocytes)

• Specific granules have particular functions
• Functions of various types of white blood cells are important for determining abnormal cell count or function

Neutrophils (Granular Leucocytes)

• 60-70% of WBCs, 10-12 µm diameter
• Life span: a few days (short)
• L.M.:
  • Cytoplasm: fine, pale granules, azurophilic granules
  • Nucleus: single segmented, 2-5 lobes, connected by thin chromatin threads; may have a Barr body (inactive X chromosome, drumstick-like appendage, visible in ∼3-6% of cells)
• E.M.: Peripheral heterochromatin and small central euchromatin in the nucleus
• Cytoplasm: pseudopodia, more glycogen (energy); primary (azurophilic) & secondary (specific) granules
• Azurophilic granules: few, large, stain purple with azure dye.
• Functions (1st line): Non-specific: toxins attract neutrophils to migrate between endothelial cells; become motile with pseudopodia (microphages). Functions in the Tissue: Phagocytosis of bacteria; lactoferrin binds to iron (bacteriostatic); phagocytin kills bacteria (bactericidal); lysozyme destroys bacterial proteins; collagenase destroys collagen facilitating movement; myeloperoxidase kills bacteria, other enzymes cause lysis. Other functions: dead neutrophils form pus; pus increases body temperature; attract monocytes to the site to clean up tissue; stimulates bone marrow to form more neutrophils; secretes trephones that aid healing.
• Clinical significance: Neutrophilia (>75%), due to acute pyogenic infections (e.g., tonsillitis, appendicitis); neutropenia (<60%), due to typhoid fever, TB, viral infections (e.g., flu)

Eosinophils (Granular Leucocytes)

• 1-4% of WBCs, 10-14 µm diameter
• Life span: a few days (short)
• L.M.: Nucleus: bilobed horse-shoe-shaped, thick chromatin threads; cytoplasm: large acidophilic specific granules.
• E.M.: Peripheral heterochromatin with central euchromatin; cytoplasm: few organelles and two types of granules: azurophilic (small=lysosomes) and specific (oval).
• Functions: Terminate allergies by hydrolyzing histamine and heparin; phagocytoses antigen–antibody complexes; defends against parasites (cytotoxins, effect of internum pores and nervous dysfunction).
• Clinical significance: Eosinophilia (>5%), due to allergic diseases (e.g., urticaria, eczema, asthma); parasites (e.g., schistosomiasis).

Basophils (Granular Leucocytes)

• 0-1% of WBCs, 10-12 µm diameter. Life span: few days (short).
• L.M.: Nucleus: irregular segmented S-shaped; Cytoplasm: coarse basophilic granules obscure nucleus; metachromatic staining with toluidine blue.
• E.M.: Peripheral heterochromatin and central euchromatin Nucleus; cytoplasm: few organelles and two types of granules; specifically; azurophilic (small = lysosomes) and specific (large, rounded, electron-dense with histamine, heparin, eosinophil chemotactic factor, leukotrienes).
• Function: secrets heparin preventing clotting and promotes allergies; secretes histamine that promotes sudden decrease in BP during anaphylaxis; attracts eosinophils to allergy sites by ECF; produces leukotrienes that cause bronchospasm (bronchial asthma).
• Clinical significance: Basophilia (>2%), due to allergic diseases (e.g., Asthma); viral infection (e.g., chicken pox) and certain liver diseases (e.g., cirrhosis)

WBC Counts (Leukocytes)

• Total count: 4,000-11,000/mm³
• Leukocytosis (>11,000/mm³): Physiological and pathological (due to acute or chronic infections).
• Leukopenia (< 4,000/mm³): due to X-ray, irradiation, typhoid fever, influenza virus, or excessive antibiotic use.
• Differential count: percentage of each WBC type to the total number. Counted in a stained blood film..

Dive into the fascinating world of human blood cells with this quiz! Test your knowledge on the characteristics and functions of red blood cells and leukocytes, including their adaptations, life spans, and conditions like leukemia. Perfect for biology enthusiasts and students alike!