M01.10.10 Blood

Blood cells are essential components of the circulatory system and play key roles in immunity, oxygen transport, and clotting. In histology, blood cells are examined to understand their morphology, function, and the role they play in homeostasis and pathology.

1. Types of Blood Cells

Blood cells are categorized into three main types:

• Erythrocytes (Red Blood Cells, RBCs)
• Leukocytes (White Blood Cells, WBCs)
• Thrombocytes (Platelets)

Each type originates from hematopoietic stem cells in the bone marrow and can be studied under a microscope after staining, usually with Giemsa or Wright stain, which highlights different cellular structures.

2. Erythrocytes (Red Blood Cells, RBCs)

• Structure: Biconcave, anucleate cells with a diameter of about 7-8 µm, specialized for gas exchange.
• Function: Transport oxygen (via hemoglobin) and carbon dioxide.
• Histological Appearance: Lack of nucleus and organelles; appear as red or pink due to hemoglobin; uniform in shape.
• Lifespan: Approximately 120 days.

Key Points:

• The biconcave shape increases the surface area for gas exchange.
• The lack of nuclei and organelles makes them efficient at transporting gases but limits their lifespan.

3. Leukocytes (White Blood Cells, WBCs)

Leukocytes are critical to the immune system and are classified into two groups: granulocytes and agranulocytes.

A. Granulocytes

• Neutrophils: Most abundant (50-70% of WBCs), multi-lobed nucleus, pale granules.
  • Function: First responders to infection; phagocytize bacteria.
• Eosinophils: Bi-lobed nucleus, reddish-orange granules.
  • Function: Fight parasitic infections and mediate allergic responses.
• Basophils: Least common, bi-lobed nucleus, dark purple granules.
  • Function: Release histamine and other mediators in allergic reactions.

B. Agranulocytes

• Lymphocytes: Round, large nucleus with minimal cytoplasm.
  • Function: Adaptive immunity (B cells and T cells).
• Monocytes: Kidney-shaped nucleus, large cell.
  • Function: Differentiate into macrophages in tissues, phagocytize pathogens and debris.

Key Points:

• Granulocytes contain cytoplasmic granules and are primarily involved in innate immunity.
• Agranulocytes lack visible granules and play roles in both innate and adaptive immunity.

Normal peripheral white blood cell types,

High power microscopic

4. Thrombocytes (Platelets)

• Structure: Small, anucleate cell fragments derived from megakaryocytes.
• Function: Blood clotting and repair of blood vessels.
• Histological Appearance: Tiny purple-staining fragments, often clustered.
• Lifespan: Around 7-10 days.

Key Points:

• Platelets adhere to damaged blood vessel walls, forming a plug.
• They release chemicals essential for clotting and tissue repair.

5. Summary Table

Points to Remember

• Blood cells are derived from a common hematopoietic stem cell in the bone marrow.
• Each type of cell has a specialized function contributing to the overall immune response, gas transport, and hemostasis.
• Understanding the histology of blood cells provides insight into various blood disorders, infections, and immune responses.

Bibliography

• Ross, M. H., & Pawlina, W. (2018). Histology: A Text and Atlas. Wolters Kluwer.
• Gartner, L. P., & Hiatt, J. L. (2014). Color Atlas and Text of Histology. Lippincott Williams & Wilkins.

Click to identify the segmented neutrophil, band neutrophil, lymphocyte, monocyte, eosinophil, basophil, and platelet in the image above.

Normal peripheral blood, segmented neutrophil and band neutrophil,

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Normal peripheral blood, segmented neutrophil and lymphocyte,

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Normal peripheral blood, monocyte,

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Blood Normal peripheral blood, eosinophil and lymphocyte,

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Normal peripheral blood, basophil, band neutrophil, and lymphocyte,

High power microscopic