Introduction to Hematopoiesis and Blood Cells

Clinical Terminology for RBCs

Clinicians use specific terms to describe deviations in RBC volume and mass:

• Anemia: A reduced volume of RBCs or a decrease in the quantity of hemoglobin in the blood.
• Polycythemia: An increase in the circulating mass (amount) of red blood cells.

Morphological Variations

RBCs are evaluated based on their arrangement, size, color, and shape.

1. Arrangement

• Rouleaux: A linear, "stack of coins" arrangement. While normal in horses, increased rouleaux in other species can be associated with inflammatory diseases.
• Agglutination: An unorganized, 3-D clumping of cells. This is generally abnormal and highly suggestive of immune-mediated disorders where antibodies coat the RBCs.

2. Size (Anisocytosis) Variation in size is called anisocytosis.

• Macrocytic: Larger than normal RBCs (often seen in regenerative anemia).
• Microcytic: Smaller than normal RBCs (often seen in iron deficiency).

3. Color (Chromasia)

• Normochromic: Normal staining with normal hemoglobin concentration.
• Hypochromasia: Increased central pallor and decreased staining, often due to low hemoglobin (common in iron deficiency).
• Polychromasia: A bluish/purplish tint in RBCs, indicating the presence of immature cells (polychromatophils) still containing intracellular structures.

4. Shape (Poikilocytosis) Abnormally shaped cells are termed poikilocytes. Notable examples include:

• Schistocytes: Fragmented "helmet cells" caused by intravascular trauma.
• Acanthocytes: "Spur cells" with irregular, spiny projections.
• Echinocytes: "Burr cells" with uniform, small projections; often an artifact of poor slide preparation (crenation).
• Spherocytes: Small, round, densely staining cells that lack central pallor, often caused by immune-mediated destruction.

White Blood Cells (Leukocytes)

White blood cells (WBCs) are the nucleated cells central to the immune response. Unlike RBCs, they exist in various "pools" throughout the body:

• Proliferation & Storage Pools: Located in the bone marrow and spleen.
• Circulating Pool: Cells freely moving in the blood (the ones we see in a sample).
• Marginal Pool: Cells "rolling" along or stuck to vessel walls.
• Tissue Pool: Cells that have left the circulation to enter body tissues.

Granulocytes

Granulocytes are characterized by the presence of visible granules in their cytoplasm.

1. Neutrophils: The most common WBC. They feature a segmented nucleus (3–5 lobes) and function in inflammation and phagocytosis (eating bacteria/fungi).
   • Band Neutrophils: Immature, non-segmented neutrophils with kidney-bean-shaped nuclei. An increase in these is called a "left shift".
   • Toxic Changes: Morphological abnormalities (cytoplasmic vacuolation, Döhle bodies, or basophilic staining) seen in neutrophils released rapidly during severe infection.
2. Basophils: The least common WBC. They contain bluish/purple granules containing histamines and are involved in allergic and inflammatory responses.
3. Eosinophils: Contain pinkish/red granules and are primarily involved in fighting parasitic infections and allergic reactions.

Agranulocytes

Agranulocytes lack visible granules under standard light microscopy.

1. Lymphocytes: Key to the immune response (T and B-lymphocytes). They are characterized by a very large nucleus with minimal cytoplasm.
   • Reactive Lymphocytes: Slightly larger cells with a bluish cytoplasmic rim, indicating recent antigenic stimulation.
2. Monocytes: The largest of the WBCs. They have variable nuclear shapes (round or kidney-shaped) and function as phagocytes. When they move into tissues, they are called macrophages.

Thrombocytes (Platelets)

Platelets are not complete cells but rather cytoplasmic fragments derived from much larger cells called megakaryocytes in the bone marrow. Their primary function is hemostasis (the control of bleeding) and the formation of clots.

Clinical Sign: A deficiency in platelets (thrombocytopenia) can lead to petechiae, which are pinpoint, round, red spots on mucus membranes caused by submucosal hemorrhage.

Summary of Clinical Terminology

When reviewing blood counts, clinicians use specific suffixes to denote changes in cell numbers: