L27 - Laboratory assessment of Bone marrow Pt.1

What is Hematopoiesis, and Where Does It Occur?

Blood cell formation

• Primarily in bone marrow in mammals and birds

What is Extramedullary Hematopoiesis?

Blood cell production OUTSIDE marrow, primarily in the spleen and liver

Which Cell Lineages are Myeloid versus Lymphoid?

Myeloid:

• Granulocytes

• Monocytes

• RBCs

• Megakaryocytes

Lymphoid:

• B cells

• T cells

• Plasma cells

QUESTION: When should a bone marrow evaluation be performed?

ANSWER → When clinical findings and CBC abnormalities suggest a hematologic disorder not fully explained by peripheral blood

Which Unexplained Cytopenias Indicate Marrow Evaluation?

1. Persistent nonregenerative anemia

2. Neutropenia without left shift

3. Thrombocytopenia

4. Bicytopenia

5. Pancytopenia

Why May Horses Require Marrow Evaluation for Anemia?

Horses release few circulating reticulocytes →making erythroid regeneration difficult to assess by CBC alone

Why is Neutropenia without a Left Shift Concerning?

It raises concern for inadequate marrow production

• A left shift more often suggests increased peripheral demand

When Do Increased Blood Cell Counts Indicate Marrow Evaluation?

Persistent, unexplained, or extreme elevations of WBCs, RBCs, or platelets after excluding common causes

When Do Atypical Circulating Cells Indicate Marrow Evaluation?

When unclassifiable cells are present, especially without enlarged lymph nodes or spleen to sample.

Why is Marrow Evaluated in Suspected Primary Leukemia?

To confirm leukemia, characterize cell lineage/phenotype, and assess remaining normal hematopoiesis

Which Neoplasms May Require Marrow Evaluation for Staging?

Marrow involvement helps determine disease extent and prognosis

1. Lymphoma(prognosis worse if marrow involvement)

2. Mast cell neoplasia

Which Other Conditions May Indicate Marrow Evaluation?

1. Unexplained hypercalcemia

2. Unexplained hyperglobulinemia (monoclonal gammopathy)

3. Selected infections

4. Storage diseases

5. Iron deficiency

Which Infections May Be Identified in Bone Marrow?

Especially histoplasmosis and leishmaniasis

Which Findings are Generally NOT Indications for Marrow Evaluation?

1. Regenerative anemia

2. Isolated lymphopenia

3. Non-extreme neutrophilia

4. Erythrocytosis

QUESTION: Why is Lymphopenia NOT a Routine Marrow Indication?

Lymphocyte counts depend mainly on secondary lymphoid tissues and lymphocyte distribution/kinetics

Why Must Marrow Findings Be Compared with a Current CBC?

Marrow production & maturation must be interpreted alongside the peripheral blood abnormalities

What are the Advantages and Disadvantages of Marrow Aspiration?

Advantages:

1. Best individual-cell detail and maturation

2. Rapid

3. Inexpensive

Disadvantages:

1. May miss focal disease or misrepresent cellularity

What are the Advantages and Disadvantages of Core Biopsy?

Advantages:

1. Better assessment of overall cellularity and tissue architecture

2. Better for diagnosis of fibrosis, necrosis, focal infiltrative disease

Disadvantages:

1. Slower

2. Costlier

3. Less individual-cell detail

What Does a Clinical Pathologist Assess in Marrow?

1. Sample adequacy

2. Overall marrow cellularity

3. Megakaryocytes

4. Adequacy, maturation and morphology of major cell lineages

5. Granulocytic to Erythroid (G:E) ratio

6. Other cells

7. Iron stores

QUESTION: Are unit particles present?

ANSWER → Adequate marrow aspirates should contain recognizable marrow particles (spicules), indicating that marrow tissue was collected

What Does Marrow Cellularity Mean?

Proportion of hematopoietic cells versus fat

• Adult ~50:50

• Young ~75:25

• Old ~25:75

How Do Hypercellularity, Hypocellularity, and Aplasia Differ?

• Hypercellular: more cells

• Hypocellular: fewer cells

• Aplasia: near-absence of hematopoietic cells

How Do Hyperplasia, Hypoplasia, and Neoplasia Differ?

• Hyperplasia: increased normal cells

• Hypoplasia: decreased production

• Neoplasia: abnormal proliferating cells

What Does Megakaryocytic Hyperplasia Suggest in Thrombocytopenia?

Marrow response to peripheral platelet destruction or consumption

• Severe cases in dogs suggest ITP (immune mediated thrombocytopenia)

QUESTION: Why esp. ITP ??

ANSWER → Severe thrombocytopenia with increased megakaryocytes suggests peripheral platelet destruction

• ITP is an important cause of extremely low platelet counts in dogs.

What is the G:E Ratio, and What is Normal?

Granulocytic-to-erythroid precursor ratio

• Normally about 1–1.5:1

• Interpret with cellularity and CBC.

What Does a Hypercellular Marrow with a High G:E Ratio Suggest?

Possible granulocytic hyperplasia and/or erythroid hypoplasia

• Assess each lineage separately.

What Do Maturation Arrest, Left Shift, and Dysplasia Suggest?

• Arrest: neoplasia/viral disease

• Left shift: demand or late-stage cell loss

• Dysplasia: drugs, viruses, neoplasia

What are Potential Causes of Increased Marrow Lymphocytes or Plasma Cells?

• Lymphocytes: infection, immune disease, neoplasia

• Plasma cells: age, immune stimulation, plasma-cell neoplasia

QUESTION Why do plasma cells increase with age?

ANSWER → Long-lived plasma cells accumulate in bone marrow over time as a result of previous immune responses 

What is Aplastic Pancytopenia, and What Can Cause It?

Pancytopenia with marrow aplasia

• Causes include:

  • Immune disease

  • Viruses

  • Drugs

  • Estrogen

  • Ehrlichia

  • Radiation

  • Toxins

Why Do Cytopenias Develop in a Particular Order with Aplasia?

Neutropenia → Thrombocytopenia → Anemia

• Reflecting progressively longer circulating cell lifespans

How Do Necrosis, Myelofibrosis, and Myelophthisis Differ?

• Necrosis: marrow cell death

• Myelofibrosis: fibrous replacement

• Myelophthisis: displacement by abnormal cells/tissue

TABLE 1. Indications for Bone Marrow Evaluation

QUESTION: When should you consider evaluating the bone marrow?

CBC / Clinical Finding	Why Evaluate Bone Marrow?
Persistent nonregenerative anemia	Investigate inadequate RBC production
Neutropenia without left shift	Investigate inadequate neutrophil production
Unexplained thrombocytopenia	Evaluate platelet production
Bicytopenia / pancytopenia	Investigate failure or replacement of multiple marrow lineages
Extreme or unexplained leukocytosis	Investigate possible leukemia
Persistent unexplained erythrocytosis	Investigate excessive RBC production
Extreme or unexplained thrombocytosis	Investigate abnormal platelet production
Unclassifiable circulating cells	Investigate possible hematopoietic neoplasia
Lymphoma / mast cell neoplasia	Evaluate marrow involvement for staging
Unexplained hypercalcemia / monoclonal gammopathy	Investigate possible marrow-associated neoplasia
Selected infections / storage diseases	Identify organisms or abnormal cellular accumulation

• Generally NOT indications:

  • Regenerative anemia

  • Isolated lymphopenia

  • Non-extreme neutrophilia or erythrocytosis

• Exam tip: Persistent, unexplained cytopenias are particularly important because they can indicate inadequate bone marrow production

TABLE  2. Bone Marrow Aspirate vs. Core Biopsy

QUESTION: Which method is better for evaluating different marrow abnormalities?

Feature	Aspirate	Core Biopsy
Evaluates	Individual cell morphology	Tissue architecture
Cell maturation	Excellent	Limited
Cellularity	Less reliable	More reliable
Fibrosis	Difficult to assess	Better
Necrosis	May be missed	Better
Focal lesions	May be missed	Better
Speed	Faster	Slower
Cost	Less expensive	More expensive

Remember:

• Aspirate = What do the individual cells look like?

• Biopsy = What does the marrow tissue look like?

They are complementary rather than interchangeable

TABLE 3. Bone Marrow Terminology

QUESTION: What do the clinical pathologist's descriptions actually mean?

Term	Meaning
Hypercellularity	Increased proportion of hematopoietic cells
Hypocellularity	Decreased proportion of hematopoietic cells
Aplasia	Near absence of hematopoietic cells
Hyperplasia	Increased number of cells in a lineage
Hypoplasia	Decreased number of cells in a lineage
Neoplasia	Abnormal, uncontrolled cellular proliferation
Maturation arrest	Failure of cells to mature beyond a particular stage
Dysplasia	Abnormal cellular development or morphology
Myelofibrosis	Replacement of marrow with fibrous tissue
Myelophthisis	Displacement of normal marrow by abnormal cells or tissue
Necrosis	Death of marrow cells or tissue

Important distinction: Cellularity describes the marrow overall, while hyperplasia or hypoplasia may describe a particular lineage.

A marrow can be hypercellular overall but have erythroid hypoplasia if another cell population has increased substantially.

TABLE 4. Interpreting Marrow Findings and Differential Diagnoses

QUESTION: If the pathologist reports an abnormal marrow finding, what conditions should you consider?

Bone Marrow Finding	Potential Causes / Interpretation
Hypercellularity	Increased production in response to cell loss, destruction, or consumption; neoplastic infiltration
Hypocellularity	Toxicity, immune-mediated disease, infections, drugs
Aplasia	Aplastic pancytopenia, necrosis, myelophthisis
Megakaryocytic hyperplasia + thrombocytopenia	Peripheral platelet destruction or consumption, including ITP
Maturation arrest	Neoplasia, viral disease
Marrow left shift	Increased peripheral demand or consumption; loss of later-stage cells
Dysplasia	Drugs, viral disease, neoplasia
Increased lymphocytes	Infection, immune-mediated disease, neoplasia
Increased plasma cells	Age, immune stimulation, plasma-cell neoplasia
Myelofibrosis	Fibrous tissue replacing marrow
Myelophthisis	Abnormal cells or tissue displacing normal hematopoiesis

Remember: A marrow finding is not always a diagnosis. For example, hypercellularity can represent either an appropriate regenerative response or neoplastic infiltration.

TABLE 5. Interpreting Cellularity and the G:E Ratio

QUESTION: How do you interpret the marrow's overall cellularity and individual lineages?

Measurement	Normal / Expected Finding	Interpretation
Cellularity — young animals	~75% cells : 25% fat	More hematopoietic tissue
Cellularity — adults	~50% cells : 50% fat	Typical adult marrow
Cellularity — older animals	~25% cells : 75% fat	More fatty tissue
G:E ratio	Approximately 1–1.5:1	Granulocytic to erythroid precursors
Increased G:E ratio	More granulocytic relative to erythroid cells	Granulocytic hyperplasia and/or erythroid hypoplasia
Decreased G:E ratio	More erythroid relative to granulocytic cells	Erythroid hyperplasia and/or granulocytic hypoplasia