🦴 Bone Marrow Aspirate and Biopsy — Explained Simply

What Is Bone Marrow?

• Skull
• Sternum (breastbone)
• Scapulae (shoulder blades)
• Ribs
• Pelvic bones
• Upper ends of long bones (like the thigh bone)

Structure of Bone (Simplified)

1. Cortex — the hard outer shell (like the crust of a loaf of bread). Made of compact bone with tiny canals (called Haversian canals) running through it.
2. Medulla — the inner spongy part, made of a lattice of thin struts called trabeculae (like a sponge). The spaces between trabeculae hold the marrow.

• Osteoblasts — build new bone
• Osteocytes — maintain bone
• Osteoclasts — break down old bone

What's Inside the Marrow?

1. Parenchyma (the working cells)

• Haematopoietic stem cells — the "mother cells" that give rise to all blood cells
• Developing red blood cells (erythroid cells), white blood cells (myeloid cells), and platelet-producing cells (megakaryocytes)

2. Stroma (the supporting tissue)

• Fat cells, fibroblasts, histiocytes (scavenger cells), blood vessels, and the gel-like matrix that holds everything together

Why Is Bone Marrow Examined? (Purpose)

The Key Principles:

1. Bone marrow has an organised structure — cells are arranged in predictable patterns
2. In a healthy person, blood cells have specific numerical and spatial relationships with each other
3. Each cell type has a distinct appearance (cytological features)
4. Cell appearance reflects its lineage (what type) and maturity (how developed it is)
5. Any of these can be disrupted in disease

When Is Bone Marrow Examination Ordered? (Indications)

Anaemia (Low blood count):

• Microcytic anaemia (small red cells): to check iron stores and look for sideroblasts (iron-loaded cells)
• Macrocytic anaemia (large red cells): to confirm if it's megaloblastic (due to B12/folate deficiency)
• Normocytic anaemia (normal-sized red cells): when reticulocyte count is not increased, to look for production or maturation problems

Abnormal Blood Counts:

• Neutropenia (low neutrophils), thrombocytopenia (low platelets), pancytopenia (all cells low): to check if precursor cells are present and functioning normally, or if there's leukaemia

Cancer Diagnosis and Staging:

• Non-Hodgkin's lymphoma
• Hodgkin's lymphoma
• Metastatic carcinoma (cancer spread from another organ)
• Small round cell tumours (common in children)

Bone Marrow Structural Problems:

• Fibrosis (scar tissue replacing marrow)
• Necrosis (dead marrow tissue)
• Gelatinous marrow transformation (marrow replaced by gel-like substance)

Other Reasons:

• Unexplained leukoerythroblastic blood picture (immature cells appearing in blood)
• Suspected multiple myeloma (plasma cell cancer)
• Pyrexia of unknown origin (unexplained fever)
• Granuloma or metastatic focal lesions
• Amyloidosis (protein deposits in tissues)
• Metabolic bone diseases

Contraindications (When NOT to Do It)

• Biopsy in clotting disorders (coagulopathies): Aspiration can still be done, but clotting factor replacement and 24–48 hours of observation are needed first
• Sternal aspirate in osteoporosis or children: Risk of the needle going too deep and perforating underlying major blood vessels or the right atrium

Where Is It Done? (Sites)

The Procedure — Step by Step

1. Consent: A written informed consent is obtained from the patient
2. Relevant history is collected — clinical impression, lab results (iron, B12, folate), transfusion history, chemotherapy history
3. Lignocaine sensitivity test is done (lignocaine = local anaesthetic)
4. Local anaesthesia is applied to numb the area
5. A special needle is inserted through the skin and bone cortex into the marrow space
6. For aspiration: marrow fluid is sucked out using a syringe
7. For biopsy (trephine): a core of bone and marrow tissue is removed

Two Types of Bone Marrow Procedures

Handling the Sample

Aspirate:

• Smears should be made immediately at the bedside (cells deteriorate fast)
• Remaining material goes into an EDTA bottle (anticoagulant)
• If immunophenotyping or cytogenetics are needed, preservative-free heparin is used instead
• Some material can be fixed in fixative for histological (tissue) sections
• Films (smears) are fixed in absolute methanol
• Too much anticoagulant causes a pink-staining amorphous material and clumping of erythroblasts — this must be avoided

Centrifugation (spinning):

Direct Films:

• A drop of marrow is placed on a slide
• Spread into a film 3–5 cm long, not wider than 2 cm
• Marrow particles are dragged (not squashed) behind the spreader, leaving a trail of cells

Trephine Biopsy — Processing

What Makes a Good Biopsy?

• Length: 1.5–2.5 cm (optimal ~1.6 cm)
• Note: bone shrinks ~25% during processing
• Should contain 5–6 trabecular spaces
• Good quality staining throughout

Complications

• Haemorrhage (bleeding)
• Pain
• Infection
• Perforation of major vessels (especially with sternal approach)
• Risks of general anaesthesia or sedation if used

Summary in One Paragraph