Hematology

Leukocyte Histology

Anisha ValliLeave a comment

EARLY ORIGINS

Hemopoietic stem cell

  • Gives rise to two types of progenitor cells:
    • The common myeloid progenitor
    • The common lymphoid progenitor

Common myeloid progenitor (CMP, aka, CFU-GEMM) gives rise to the following leukocytes:

  • Neutrophil
  • Eosinophil
  • Basophil
  • Monocyte

As we learn elsewhere, the common myeloid progenitor also gives rise to red blood cells and platelets.

Common lymphoid progenitor (CLP)

  • Gives rise to lymphocytes.

HISTOLOGICAL DETAILS

CMP Lineage:

Neutrophils

  • Comprise 60-70% of circulating white blood cells; they are the most common leukocyte.
  • Have multi-lobed nuclei.
    • As they age, the number of lobes increases: immature band neutrophils have no formed lobes, older neutrophils may have 5 lobes.
  • Various types of cytoplasmic granules lend the cytoplasm a pale pink hue.
  • Neutrophils phagocytose bacteria by engulfing them with pseudopodia; the bacteria are trapped within a phagosome, where granular digestive enzymes destroy them.

Eosinophils

  • Comprise 2-4% of circulating white blood cells.
  • Nuclei are multi-lobed.
  • Granules stain a bright orange-red color, which helps to differentiate them from neutrophils under the microscope.
  • Eosinophilic granules contain parasite-destroying enzymes and polypeptides, including major basic protein.
  • Eosinophils also participate in the immune response, and play a role in triggering bronchial asthma.

Basophils

  • Comprise 1% of circulating white blood cells.
  • Bi-lobed or bean-shaped nucleus that is obscured by an abundance of large metachromatic granules (metachromatic means that the granules stain a different color from that of the dye).
  • Basophils participate in hypersensitivity, such as bronchial asthma and dermatological reactions.
  • Their granules contain heparin, which prevents blood coagulation, and histamine, which causes inflammation.

Monocytes

  • Comprise 5% of circulating white blood cells.
  • Their bean-shaped nucleus is visible against light blue/gray staining cytoplasm; lack visible granules.
  • In the connective tissues, monocytes become macrophages, and can phagocytose bacteria and other particles (including red blood cells), clean up cellular debris, and present antigens.
  • In the bones, they become osteoclasts, which resorb bone tissue as part of growth and remodeling.

CLP Lineage:

Lymphocytes

  • Comprise 20-40% of circulating white blood cells.
  • Large, round nucleus leaves just a thin rim of cytoplasm.
  • Lymphocytes diverge to form B cells, which mature within the bone and participate in humoral immunity, and T cells, which mature in the thymus and participate in cell-mediated immunity.

Granulocytes vs Agranulocytes

Granulocytes:

Neutrophils, eosinophils, and basophils are histologically classified as granulocytes, because of the presence of primary, secondary, and, sometimes, tertiary granules in their cytoplasm.

Agranulocytes:

Monocytes and lymphocytes are called “agranulocytes,” because they have only primary granulocytes in their cytoplasm; these can be difficult to see.

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