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Hematology

Morphologies

Blood and Bone Marrow

  • Neutrophils (Granulocyte Family)

    • Neutrophil (PMN, Polymorphonuclear Neutrophil):
      • Size: 10-15 μm
      • Nucleus: Segmented (3-5 lobes), condensed chromatin, generally no nucleoli
      • Cytoplasm: Pale pink or lilac color, fine, faint granules
      • Function: Phagocytosis and killing of bacteria and fungi (primary defense against infection)
      • Key Features: Segmented nucleus, small pinkish granules
    • Band Neutrophil:
      • Also known as “stab cell”
      • Size: 10-15 μm
      • Nucleus: Horseshoe-shaped (banded, not segmented)
      • Cytoplasm: Same as neutrophil
      • Significance: Immature neutrophil; increased numbers indicate a “left shift,” often seen in infection
      • Key Features: Banded nucleus, similar to a neutrophil
    • Metamyelocyte:
      • Size: 10-18 μm
      • Nucleus: Kidney bean-shaped (indented), coarser chromatin than band
      • Cytoplasm: Pale blue to pink, many granules
      • Significance: Less mature neutrophil precursor than a band
      • Key Features: Indented nucleus, not segmented like a neutrophil
    • Myelocyte:
      • Size: 12-20 μm
      • Nucleus: Round to oval, eccentric (off-center), coarser chromatin, may have visible nucleoli
      • Cytoplasm: More abundant than in promyelocyte, distinct granules
      • Significance: Immature neutrophil precursor
      • Key Features: More cytoplasm and distinct granules compared to more immature precursors
    • Promyelocyte:
      • Size: 14-24 μm
      • Nucleus: Round to oval, prominent nucleoli
      • Cytoplasm: Basophilic, many large, azurophilic (purple) primary granules
      • Significance: Immature neutrophil precursor
      • Key Features: Large size, prominent nucleoli, many large granules
    • Myeloblast:
      • Size: 14-20 μm
      • Nucleus: Round to oval, fine chromatin, prominent nucleoli
      • Cytoplasm: Scant, basophilic, few or no granules
      • Significance: The most immature granulocyte precursor; increased numbers suggest acute myeloid leukemia (AML)
      • Key Features: Immature chromatin, prominent nucleoli, lack of abundant granules

  • Eosinophils (Granulocyte Family)

    • Eosinophil:
      • Size: 12-17 μm
      • Nucleus: Usually bilobed (2 lobes)
      • Cytoplasm: Packed with large, uniform, refractile granules that stain bright orange-red with eosin dye
      • Function: Defense against parasites, allergic reactions
      • Key Features: Bilobed nucleus, large orange-red granules

  • Basophils (Granulocyte Family)

    • Basophil:
      • Size: 10-14 μm
      • Nucleus: Lobulated, but often obscured by granules
      • Cytoplasm: Packed with large, irregular granules that stain dark blue or purple with basic dyes
      • Function: Release of histamine and other mediators involved in allergic and inflammatory responses
      • Key Features: Dark blue/purple granules obscuring the nucleus

  • Monocytes

    • Monocyte:
      • Size: 12-20 μm (largest of the normal WBCs)
      • Nucleus: Kidney-shaped, horseshoe-shaped, or convoluted; lacy, less condensed chromatin; may have nucleoli
      • Cytoplasm: Gray-blue, abundant, fine granules and vacuoles (“ground glass” appearance)
      • Function: Phagocytosis, antigen presentation, cytokine production
      • Key Features: Large size, kidney-shaped nucleus, “ground glass” cytoplasm

  • Lymphocytes

    • Small Lymphocyte:
      • Size: 7-10 μm (similar to RBC size)
      • Nucleus: Round or slightly indented, dense and darkly stained chromatin, usually no nucleoli
      • Cytoplasm: Scant, light blue
      • Function: Involved in adaptive immunity (T cells and B cells)
      • Key Features: High N:C ratio, round nucleus, scant cytoplasm
    • Large Lymphocyte:
      • Size: 12-16 μm (larger than RBCs)
      • Nucleus: Round or slightly indented, may have more open chromatin
      • Cytoplasm: More abundant than in small lymphocytes, may contain a few granules
      • Function: Can include activated T cells, B cells, or NK cells
      • Key Features: More cytoplasm than small lymphocytes, but still a high N:C ratio
    • Reactive (Atypical) Lymphocyte:
      • Size: Variable, often larger than normal lymphocytes
      • Nucleus: Variable shape (may be indented, folded, or irregular), less condensed chromatin, may have prominent nucleoli
      • Cytoplasm: Abundant, may be basophilic (dark blue) or vacuolated, may indent around RBCs
      • Function: Lymphocytes responding to an antigen (e.g., viral infection)
      • Key Features: Abundant, often dark-staining cytoplasm, irregular nucleus
    • Plasma Cell:
      • Size: 10-20 μm
      • Nucleus: Eccentric (off-center), round, “clock-face” or “cartwheel” chromatin pattern
      • Cytoplasm: Deeply basophilic, often with a perinuclear halo (area near the nucleus that is less stained)
      • Function: Antibody production
      • Key Features: Eccentric nucleus, “clock-face” chromatin, deeply basophilic cytoplasm

  • Additional Cells (Less Common)

    • Blasts:
      • General Features:
        • Large size
        • High N:C ratio
        • Fine, dispersed chromatin
        • Prominent nucleoli
        • May have scant cytoplasm
      • Myeloblast:
        • Granules may be present
        • May have Auer rods (rod-shaped inclusions) - Pathognomonic for AML (but not always present)
        • Positive for myeloperoxidase (MPO) stain
      • Lymphoblast:
        • Usually lack granules and Auer rods
        • Negative for MPO stain
    • Nucleated Red Blood Cells (nRBCs):
      • Also known as erythroblasts
      • Immature RBCs with a nucleus
      • Normally found in the bone marrow, but can appear in the peripheral blood in certain conditions
      • Stages of nRBCs:
        • Pronormoblast (Proerythroblast)
        • Basophilic Normoblast
        • Polychromatic Normoblast
        • Orthochromic Normoblast

Blood

  • Normal Red Blood Cells

    • Normocytes:
      • Description: Normal-sized RBCs
      • Size: Approximately 6-8 μm in diameter (roughly the size of a lymphocyte nucleus)
      • Shape: Biconcave disc (round with central pallor)
      • Color: Pink to orange-red with a central pallor that occupies about 1/3 of the cell diameter
      • Clinical Significance: The predominant cell type in healthy individuals

  • Abnormal Red Blood Cells (Poikilocytes)

    • Anisocytosis = General term for variation in RBC size

      • Microcytes:
        • Description: Small RBCs
        • Size: <6 μm in diameter (MCV <80 fL)
        • Associated Conditions: Iron deficiency anemia, thalassemia, sideroblastic anemia, anemia of chronic disease
      • Macrocytes:
        • Description: Large RBCs
        • Size: >9 μm in diameter (MCV >100 fL)
        • Shape: Round (not oval, unless megaloblastic)
        • Associated Conditions: Vitamin B12 deficiency, folate deficiency, alcoholism, liver disease, hypothyroidism, myelodysplastic syndromes (MDS), reticulocytosis

    • Hypochromia:

      • Description: RBCs with decreased hemoglobin content
      • Appearance: Pale with an increased central pallor (>1/3 of the cell diameter)
      • Associated Conditions: Iron deficiency anemia, thalassemia, sideroblastic anemia

    • Polychromasia:

      • Description: RBCs with a bluish tint due to residual RNA
      • Appearance: Larger than mature RBCs, lack central pallor, stain blue-gray with Wright stain
      • Significance: Indicates increased erythropoiesis (reticulocytes)
      • Associated Conditions: Hemolytic anemia, acute blood loss, response to therapy

    • Spherocytes:

      • Description: Small, spherical RBCs with no central pallor
      • Appearance: Small, round, intensely stained (lack central pallor)
      • Associated Conditions: Hereditary spherocytosis, autoimmune hemolytic anemia

    • Elliptocytes (Ovalocytes):

      • Description: Elongated or oval-shaped RBCs
      • Appearance: Elliptical or oval shape
      • Associated Conditions: Hereditary elliptocytosis, iron deficiency anemia, thalassemia

    • Target Cells (Codocytes):

      • Description: RBCs with a bullseye appearance
      • Appearance: Central area of hemoglobin surrounded by a clear ring and then an outer ring of hemoglobin
      • Associated Conditions: Liver disease, thalassemia, hemoglobinopathies (e.g., HbC disease), iron deficiency anemia, post-splenectomy

    • Sickle Cells (Drepanocytes):

      • Description: Crescent-shaped or sickle-shaped RBCs
      • Appearance: Elongated, crescent or sickle shape
      • Associated Conditions: Sickle cell anemia (HbSS), sickle cell trait (HbAS)

    • Schistocytes (Fragmented Cells):

      • Description: Fragments of RBCs with irregular shapes and pointed ends
      • Appearance: Small, irregularly shaped fragments
      • Associated Conditions: Microangiopathic hemolytic anemia (MAHA) - thrombotic thrombocytopenic purpura (TTP), hemolytic uremic syndrome (HUS), disseminated intravascular coagulation (DIC), mechanical heart valves

    • Acanthocytes (Spur Cells):

      • Description: RBCs with irregularly spaced, thorny projections
      • Appearance: Irregular, spiky projections of varying length
      • Associated Conditions: Liver disease, abetalipoproteinemia

    • Echinocytes (Burr Cells):

      • Description: RBCs with evenly spaced, blunt projections
      • Appearance: Small, uniform, spiky projections
      • Associated Conditions: Uremia (kidney failure), pyruvate kinase deficiency, artifact (crenation)

    • Teardrop Cells (Dacrocytes):

      • Description: RBCs shaped like teardrops or pears
      • Appearance: One end is rounded, and the other end is elongated or pointed
      • Associated Conditions: Primary myelofibrosis, thalassemia, myelodysplastic syndromes, extramedullary hematopoiesis

    • Stomatocytes:

      • Description: RBCs with a slit-like area of central pallor (mouth-like appearance)
      • Appearance: Slit-like central pallor
      • Associated Conditions: Hereditary stomatocytosis, alcoholism, liver disease

    • Agglutination

      • Definition: The clumping or aggregation of red blood cells due to antibody- or complement-mediated interactions
      • Appearance:
        • Irregular Clumps: RBCs form irregular, three-dimensional clumps or clusters
        • Grape-like Clusters: Agglutinates often appear as grape-like clusters of RBCs
        • Random Distribution: The clumps are randomly distributed throughout the smear
        • Macroscopic Agglutination: In some cases, the agglutination may be visible to the naked eye as clumping in the EDTA tube
      • Associated Conditions:
        • Cold Agglutinin Disease (Cold AIHA): IgM autoantibodies bind to RBCs at low temperatures
        • Autoimmune Hemolytic Anemia (AIHA): IgG or IgM autoantibodies against RBCs
        • Transfusion Reactions: Antibodies against donor RBC antigens
        • Paroxysmal Cold Hemoglobinuria (PCH): Donath-Landsteiner antibody (IgG) binds to RBCs at low temperatures

    • Rouleaux Formation

      • Definition: The stacking of red blood cells in a linear arrangement, resembling stacks of coins or poker chips
      • Appearance:*
        • Stacks of Coins: RBCs are arranged in linear stacks, resembling stacks of coins or poker chips
        • Even Distribution: The stacks are relatively uniform and evenly distributed
        • Reversible: Rouleaux formation is reversible; the RBCs can be easily dispersed by adding saline
      • Associated Conditions:
        • Multiple Myeloma: Increased levels of monoclonal immunoglobulins (paraproteins)
        • Waldenström Macroglobulinemia: Increased levels of IgM
        • Chronic Inflammatory Conditions: Increased fibrinogen and acute phase reactants
        • Hypergammaglobulinemia: Increased levels of immunoglobulins
        • Artifact: Can be an artifact if the counting area of the smear is too thick

    • Basophilic Stippling:

      • Description: Presence of small, dark-blue granules scattered throughout the RBC cytoplasm
      • Composition: Ribosomal RNA precipitates
      • Associated Conditions: Thalassemia, lead poisoning, myelodysplastic syndromes (MDS)

    • Howell-Jolly Bodies:

      • Description: Round, dark-purple nuclear remnants in RBCs
      • Composition: DNA fragments
      • Associated Conditions: Splenectomy or splenic dysfunction, hemolytic anemia, megaloblastic anemia

    • Pappenheimer Bodies:

      • Description: Small, irregular, iron-containing granules in RBCs
      • Composition: Iron (non-heme iron)
      • Require Prussian blue stain to visualize
      • Associated Conditions: Sideroblastic anemia, post-splenectomy, myelodysplastic syndromes

    • Heinz Bodies:

      • Description: Inclusions composed of denatured hemoglobin
      • Require supravital stain (e.g., crystal violet or brilliant cresyl blue) to visualize
      • Associated Conditions: G6PD deficiency, unstable hemoglobin variants

Bone Marrow

  • Hematopoietic Stem Cells and Progenitor Cells (Blasts)

    • Hematopoietic Stem Cells (HSCs):
      • Description: Rare, self-renewing cells that give rise to all blood cell lineages
      • Morphology: Difficult to identify morphologically
      • Identification: Best identified by immunophenotyping using flow cytometry (e.g., CD34+, CD38-)
      • Significance: The source of all blood cells; mutations in HSCs can lead to hematologic malignancies
    • Multipotent Progenitors (MPPs):
      • Description: Cells derived from HSCs that have lost some self-renewal capacity but can still differentiate into multiple lineages
      • Morphology: Difficult to distinguish morphologically from HSCs
      • Identification: Identified by immunophenotyping
    • Common Myeloid Progenitor (CMP):
      • Description: A progenitor cell that gives rise to granulocytes, monocytes, macrophages, and megakaryocytes
      • Morphology: Difficult to distinguish morphologically from other progenitor cells
      • Identification: Identified by immunophenotyping
    • Common Lymphoid Progenitor (CLP):
      • Description: A progenitor cell that gives rise to lymphocytes (T cells, B cells, and NK cells)
      • Morphology: Difficult to distinguish morphologically from other progenitor cells
      • Identification: Identified by immunophenotyping

  • Myeloid Lineage

These cells are part of the granulocytic and monocytic series * Myeloblast: * Description: Immature granulocyte precursor * Size: 14-20 μm * Nucleus: Large, round to oval, fine chromatin, 1-5 distinct nucleoli * Cytoplasm: Scant to moderate amount, deeply basophilic, lacks granules * Distinguishing Features: * High nucleus-to-cytoplasm (N:C) ratio * Prominent nucleoli * Lack of granules * Clinical Significance: Increased in acute myeloid leukemia (AML)

*   **Promyelocyte:**
    *   Description: More mature than myeloblast
    *   Size: 15-25 μm
    *   Nucleus: Round to oval, slightly coarser chromatin than myeloblast, 1-3 nucleoli
    *   Cytoplasm: More abundant than myeloblast, basophilic, contains primary (azurophilic) granules
    *   Distinguishing Features:
        *   Presence of primary (azurophilic) granules
        *   Nucleoli may still be visible
    *   Clinical Significance: Increased in AML, especially acute promyelocytic leukemia (APL)

*   **Myelocyte:**
    *   Description: Intermediate stage in granulocyte maturation
    *   Size: 12-18 μm
    *   Nucleus: Round and eccentric (off-center), condensed chromatin, nucleoli usually absent
    *   Cytoplasm: Less basophilic than promyelocyte, contains both primary and secondary (specific) granules
    *   Distinguishing Features:
        *   Smaller size than promyelocyte
        *   Eccentric nucleus
        *   "Dawn of neutrophilia": The cytoplasm appears pinkish due to the development of secondary granules (in neutrophilic myelocytes)
    *   Types:
        *   Neutrophilic myelocyte: Produces neutrophilic granules (stain pink or lavender)
        *   Eosinophilic myelocyte: Produces eosinophilic granules (stain orange-red)
        *   Basophilic myelocyte: Produces basophilic granules (stain dark blue or purple)

*   **Metamyelocyte:**
    *   Description: A stage in granulocyte maturation intermediate between myelocyte and band neutrophil
    *   Size: 10-15 μm
    *   Nucleus: Indented (kidney bean-shaped), condensed chromatin, no nucleoli
    *   Cytoplasm: Abundant, contains secondary granules
    *   Distinguishing Features:
        *   Indented nucleus
        *   No nucleoli
    *   Types:
        *   Neutrophilic metamyelocyte
        *   Eosinophilic metamyelocyte
        *   Basophilic metamyelocyte

*   **Band Neutrophil:**
    *   Description: An immature neutrophil with a horseshoe-shaped (band-shaped) nucleus
    *   Size: 10-12 μm
    *   Nucleus: Band-shaped, condensed chromatin, no nucleoli
    *   Cytoplasm: Pinkish-lilac, fine granules
    *   Distinguishing Features:
        *   Band-shaped nucleus with parallel sides and no segmentation
        *   Absence of a thin filament connecting the nuclear lobes

*   **Monoblast:**
    *   Description: Earliest recognizable monocyte precursor
    *   Size: 12-20 μm
    *   Nucleus: Large, round to oval, fine chromatin, prominent nucleoli
    *   Cytoplasm: Abundant, basophilic, lacks granules
    *   Distinguishing Features:
        *   High N:C ratio
        *   Prominent nucleoli
        *   Lack of granules

*   **Promonocyte:**
    *   Description: More mature than monoblast
    *   Size: 14-18 μm
    *   Nucleus: Large, indented or folded, lacy chromatin, may have nucleoli
    *   Cytoplasm: Gray-blue, abundant, fine granules
    *   Distinguishing Features:
        *   Indented or folded nucleus
        *   Cytoplasmic granules

*   **Megakaryoblast:**
    *   Description: Immature megakaryocyte precursor
    *   Size: 20-50 μm
    *   Nucleus: Large, single nucleus with multiple lobes, prominent nucleoli
    *   Cytoplasm: Deeply basophilic, lacks granules
    *   Distinguishing Features:
        *   Large size
        *   Multi-lobated nucleus
        *   Lacks granules

*   **Promegakaryocyte:**
    *   Description: More mature than megakaryoblast
    *   Size: 25-80 μm
    *   Nucleus: Large, multi-lobated, less distinct nucleoli
    *   Cytoplasm: More abundant than megakaryoblast, less basophilic, contains some granules
    *   Distinguishing Features:
        *   Large size
        *   Granular cytoplasm

*   **Megakaryocyte:**
    *   Description: Mature platelet-producing cell
    *   Size: 30-100 μm (largest cell in the bone marrow)
    *   Nucleus: Large, multi-lobated, condensed chromatin
    *   Cytoplasm: Abundant, pinkish-blue, contains numerous granules
    *   Distinguishing Features:
        *   Extremely large size
        *   Multi-lobated nucleus
        *   Abundant granular cytoplasm
    *   Function: Produces platelets by shedding portions of its cytoplasm

  • Erythroid Lineage

    • Pronormoblast (Proerythroblast):
      • Description: The earliest recognizable erythroid precursor
      • Size: 14-20 μm
      • Nucleus: Large, round, fine chromatin, 1-2 distinct nucleoli
      • Cytoplasm: Deeply basophilic (due to high RNA content)
      • Distinguishing Features:
        • High N:C ratio
        • Intense basophilia of the cytoplasm
    • Basophilic Normoblast:
      • Description: More mature than pronormoblast
      • Size: 12-17 μm
      • Nucleus: Round, slightly condensed chromatin, no nucleoli
      • Cytoplasm: Intensely basophilic
      • Distinguishing Features:
        • Smaller size than pronormoblast
        • More condensed chromatin
    • Polychromatic Normoblast:
      • Description: As hemoglobin synthesis increases, the cytoplasm becomes less basophilic
      • Size: 10-15 μm
      • Nucleus: Round, more condensed chromatin
      • Cytoplasm: Gray-blue (polychromatic) due to the presence of both hemoglobin and ribosomes
      • Distinguishing Features:
        • Gray-blue cytoplasm
        • More condensed chromatin
    • Orthochromic Normoblast:
      • Description: The final nucleated stage of erythroid maturation
      • Size: 8-10 μm
      • Nucleus: Small, dense, pyknotic (about to be extruded)
      • Cytoplasm: Pink (due to predominant hemoglobin)
      • Distinguishing Features:
        • Small, dense nucleus
        • Mostly pink cytoplasm
    • Reticulocyte:
      • Anucleated red blood cell with residual RNA, not found in the bone marrow

  • Key Considerations

    • Mast Cells: Although present in the bone marrow, mast cells are technically tissue cells and not part of the hematopoietic lineages. They are granulated cells important in allergic responses.
    • Plasma Cells: Although derived from lymphocytes, plasma cells can also reside in the bone marrow. They have distinct eccentric nuclei and deeply basophilic cytoplasm.

Body Fluids

  • Neutrophils:

    • Morphology: Similar to neutrophils in peripheral blood (segmented nucleus, pink-lilac granules)
    • Significance: Increased numbers often indicate bacterial infection or inflammation
      • Degenerated Neutrophils: In older fluids, neutrophils may appear pyknotic (shrunken, dark nucleus) or may be lysed

  • Lymphocytes:

    • Morphology: Similar to lymphocytes in peripheral blood (round nucleus, scant cytoplasm)
    • Significance: Increased numbers may suggest viral infection, tuberculosis, fungal infection, or chronic inflammation
    • Reactive Lymphocytes: (Atypical lymphocytes)
      • More abundant cytoplasm
      • Basophilic cytoplasm
      • May have nucleoli

  • Monocytes/Macrophages:

    • Morphology: Large cells with a kidney bean-shaped or horseshoe-shaped nucleus, abundant gray-blue cytoplasm, and vacuoles
    • Significance: Found in various inflammatory conditions and infections; can also indicate hemorrhage (erythrophagocytosis)
    • Distinguishing Features: Cytoplasmic vacuoles and/or phagocytized material can be diagnostic of specific processes

  • Eosinophils:

    • Morphology: Similar to eosinophils in peripheral blood (bilobed nucleus, large, orange-red granules)
    • Significance: Often associated with parasitic infections, allergic reactions, or hypersensitivity reactions

  • Basophils:

    • Morphology: Similar to basophils in peripheral blood (irregular, lobulated nucleus obscured by dark blue/purple granules)
    • Significance: Less common than other cell types; may be seen in allergic reactions or certain malignancies

  • Mesothelial Cells (Serous Fluids - Pleural, Peritoneal, Pericardial):

    • Morphology:
      • Round to oval cells
      • Eccentric nucleus (off-center)
      • Abundant cytoplasm (often blue-gray or pink)
      • May have a “skirt” or “ruffled border”
      • May be multinucleated
    • Significance: Normal lining cells of the serous membranes
      • Increased numbers can indicate inflammation or irritation of the serous membranes
    • Reactive Mesothelial Cells: May have more prominent nucleoli, increased cytoplasm, and mitotic figures

  • Synovial Lining Cells (Synovial Fluid):

    • Morphology:
      • Round to oval cells
      • Eccentric nucleus
      • Moderate amount of cytoplasm
    • Significance: Normal cells found in synovial fluid
      • Increased numbers may indicate inflammation or joint damage

  • Additional Cells (Less Common)

  • Blast Cells:

    • Morphology:
      • Large cells with a high nuclear-to-cytoplasmic (N:C) ratio
      • Fine chromatin
      • Prominent nucleoli
      • May have scant cytoplasm
    • Significance: Suggestive of leukemia or lymphoma with involvement of the body fluid
    • Classification: Requires flow cytometry and other specialized tests to determine lineage (myeloid vs. lymphoid)

  • Malignant Cells (Non-Hematologic):

    • Morphology:
      • Highly variable, depending on the type of cancer
      • May have abnormal nuclear features (e.g., irregular shape, coarse chromatin, prominent nucleoli)
      • May have abundant cytoplasm with vacuoles or other inclusions
    • Significance: Indicates metastatic cancer in the body fluid
    • Origin: Usually represent metastatic carcinoma from a solid tumor
    • Identification: Requires careful cytological evaluation and correlation with clinical history and imaging studies

  • Ependymal Cells (Cerebrospinal Fluid - CSF):

    • Morphology:
      • Columnar or cuboidal cells
      • Round to oval nucleus
      • Finely granular cytoplasm
    • Significance: Normal cells lining the ventricles of the brain and the central canal of the spinal cord
      • Increased numbers may indicate disruption of the ventricular system

  • Hemosiderin-Laden Macrophages (All Fluid Types):

    • Morphology:
      • Phagocytes that contain pigment granules of broken-down red blood cells with hemosiderin (iron storage complex).
    • Significance: Indicate prior hemorrhage (bleeding) into the body fluid

  • Lipophages (Fat-Laden Macrophages, Gitter Cells):

    • Morphology:
      • Macrophages with numerous vacuoles containing lipids
    • Significance: Indicate tissue destruction and fat necrosis
      • Commonly seen in CSF after stroke or traumatic brain injury

  • Crystals (Synovial Fluid):

    • Urate Crystals: Needle-shaped, negatively birefringent crystals seen in gout
    • Calcium Pyrophosphate Dihydrate (CPPD) Crystals: Rhomboid or rod-shaped, weakly positively birefringent crystals seen in pseudogout
    • Cholesterol Crystals: Flat, notched, rhomboid-shaped crystals seen in chronic effusions

  • Organisms:

    • Bacteria: Small, round, or rod-shaped organisms that may be seen within or outside of cells
    • Fungi: May appear as yeast or hyphae
    • Parasites: Rare, but may be seen in certain infections

  • Other Artifacts and Contaminants:

    • Air Bubbles
    • Starch Granules (from gloves)
    • Contaminating Cells from Adjacent Tissues