Histological Structure and Classification of Blood Cells

General Introduction to Blood

  • Definition: Blood is considered the fluid of life, contained within the closed circulatory system.

  • Constituents of Blood Volume:

    • Plasma: Comprises 55%55\% of the total blood volume.

    • Blood Cells: Comprise 45%45\% of the total blood volume.

  • Blood Cells Definition: These are free connective tissue (C.T.) cells produced by hematopoietic tissue and suspended in the blood plasma.

  • Classification of Blood Cells:

    • Red Blood Cells (RBCs) or Erythrocytes.

    • White Blood Cells (WBCs) or Leukocytes.

    • Platelets or Thrombocytes.

Red Blood Cells (Erythrocytes)

  • General Characteristics:

    • Life Span: Approximately 120120 days.

    • Shape: Rounded, biconcave discs.

    • Rouleaux Formation: This refers to the arrangement of RBCs when they appear in a group resembling a stack of coins.

  • Physical Dimensions:

    • Diameter: Ranges from 6−9 μm6 - 9\,\mu m, with an average of 7.5 μm7.5\,\mu m.

    • Clinical Correlates (Size):

      • If the diameter is less than 6 μm→6\,\mu m \rightarrow Microcytic anemia.

      • If the diameter is greater than 9 μm→9\,\mu m \rightarrow Macrocytic anemia.

  • Color Appearance:

    • Single RBC: Appears greenish-yellow.

    • Blood Drop: Appears red due to the presence of a large number of RBCs.

  • Staining Characteristics (Light Microscopy):

    • In stained sections, normal Hemoglobin (HB) appears acidophilic with a pale center.

    • Hypochromic: Less HB results in a pale appearance.

    • Hyperchromic: More HB results in a deeply stained periphery with a stained center.

  • Total Counts (via Hemocytometer):

    • Males: 5−5.5×106/mm35 - 5.5 \times 10^{6}/mm^{3}.

    • Females: 4.5−5×106/mm34.5 - 5 \times 10^{6}/mm^{3}.

  • Structure of RBCs:

    • They are non-nucleated cells.

    • They contain no organoids (organelles).

    • The cells are filled with Hemoglobin (HB).

    • They are surrounded by a cell membrane that is characterized as being plastic and having selective permeability.

  • Adaptability to Function:

    1. Biconcave Shape: Enhances the surface area through which gas exchange occurs.

    2. Selective Membrane: The cell membrane is highly selective.

    3. Plasticity: The membrane is highly plastic, allowing the cell to deform and pass through narrow capillaries.

    4. Absence of Internal Structures: The absence of a nucleus and all organoids provides more internal space for HB.

    5. Enzymatic Activity: Carbonic anhydrase enzyme is present, playing a vital role in the transportation of CO2CO_{2} from tissues to the lungs.

White Blood Cells (Leukocytes)

  • General Characteristics:

    • These are true nucleated cells containing organoids and inclusions.

    • They do not contain any Hemoglobin (HB).

    • Color: A single WBC is colorless, but aggregated WBCs appear white.

  • Total Leucocytic Count: The total number of leukocytes in blood ranges from 4000−11000/mm34000 - 11000/mm^{3}.

  • Classification of WBCs:

    • Classified according to the staining reaction of their granules into Granular and Non-granular types.

Granular Leukocytes
1. Neutrophils

  • Percentage: 60−75%60 - 75\%.

  • Diameter: 10−12 μm10 - 12\,\mu m.

  • Light Microscopy (LM):

    • Nucleus: Darkly stained and segmented, containing 2−52 - 5 segments (lobes).

    • Cytoplasm: Contains fine neutrophilic granules.

  • Functional Pathway and Phagocytosis:

    • Bone marrow produces Neutrophils.

    • Neutrophils are attracted to microorganisms.

    • They engulf microorganisms (phagocytosis) and are termed "microphages."

    • They secrete proteolytic enzymes.

    • They are involved in pus formation.

    • Neutrophils stimulate Monocytes to transform into Macrophages.

    • Neutrophils produce "trephone" which stimulates tissue healing.

2. Eosinophils

  • Percentage: 2−5%2 - 5\%.

  • Diameter: 12−15 μm12 - 15\,\mu m.

  • Light Microscopy (LM):

    • Nucleus: Less darkly stained than neutrophils, typically bilobed or horseshoe-shaped.

    • Cytoplasm: Large acidophilic granules.

  • Enzymatic Content: The granules contain Histaminase and Sulphatase enzymes.

  • Function: They are attracted to sites of allergy and exert an anti-allergic action by destroying histamine.

3. Basophils

  • Percentage: 0.5−1%0.5 - 1\%.

  • Diameter: 10−12 μm10 - 12\,\mu m.

  • Light Microscopy (LM):

    • Nucleus: Irregular in shape.

    • Cytoplasm: Large basophilic granules.

  • Chemical Content: The granules contain Histamine and Heparin.

  • Function: They produce histamine and heparin during allergic reactions.

Non-Granular Leukocytes
1. Lymphocytes

  • Total Percentage: 25−30%25 - 30\%.

  • Sub-classification by Size:

    • Small Lymphocytes:

      • Percentage: 15−20%15 - 20\%.

      • Diameter: 4−7 μm4 - 7\,\mu m.

      • Nucleus: Central, rounded, and dark.

      • Cytoplasm: A little rim around the nucleus.

    • Medium/Large Lymphocytes:

      • Percentage: 5−10%5 - 10\%.

      • Diameter: 10−15 μm10 - 15\,\mu m.

      • Nucleus: Large, indented, and lightly stained.

      • Cytoplasm: Abundant.

  • Functional Types of Small Lymphocytes:

    • B-Lymphocytes:

      • Origin: Bone Marrow (B.M.).

      • Percentage: 30%30\% of small lymphocytes.

      • Life Span: 33 months.

      • Role: Responsible for humoral immunity. When exposed to an antigen, they activate into medium-sized lymphocytes; some differentiate into plasma cells to produce antibodies (Primary immune response). Others remain as B memory cells for secondary responses (rapid/extensive response on re-exposure).

    • T-Lymphocytes:

      • Origin: Thymus gland.

      • Percentage: 70%70\% of small lymphocytes.

      • Life Span: Many years.

      • Role: Responsible for cell-mediated immunity.

      • T-Killer Cells: Activated by cellular antigens, they secrete cytotoxic substances to destroy the antigen.

      • T-Memory Cells: Attack the same antigen after long periods.

      • T-Helper Cells: Help B-lymphocytes become activated by antigens.

2. Monocytes

  • Percentage: 3−8%3 - 8\%.

  • Diameter: 15−20 μm15 - 20\,\mu m.

  • Light Microscopy (LM):

    • Nucleus: Large, horseshoe-shaped or kidney-shaped.

    • Cytoplasm: Non-granular and pale blue.

  • Electron Microscopy (EM): Shows the presence of lysosomes.

  • Function: They migrate via amoeboid movement through the walls of capillaries and venules into the surrounding connective tissue where they become macrophages.

Platelets (Thrombocytes)

  • Definition: These are fragments of cytoplasm covered with a membrane; they have no nucleus and are not considered true cells.

  • Dimensions and Shape:

    • Size: 2−4 μm2 - 4\,\mu m.

    • Shape: Oval or rounded discs.

  • Count: 150,000−400,000/mm3150,000 - 400,000/mm^{3}.

  • Electron Microscopy (EM) Structure:

    1. Cytoskeleton: Contains actin-like filaments and microtubules to maintain the ovoid shape.

    2. Granules:

      • α\alpha granules: Contain fibrinogen.

      • δ\delta granules: Contain serotonin.

  • Functions (Haemostasis):

    1. Vasoconstriction (V.C.): Mediated by the secretion of serotonin.

    2. Platelet Aggregation: Platelets clump together at the site of injury.

    3. Coagulation (Red Thrombus):

      • Thromboplastin changes prothrombin into thrombin.

      • Thrombin changes fibrinogen into a fibrin network.

      • The fibrin network attracts RBCs to close blood vessels and stop bleeding.