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Chapter 1: Formation of Blood Cells & Bone Marrow Biopsy - Made Easy

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🩸 1. General Aspects of Blood Cell Formation

• Red blood cells (RBCs) - carry oxygen
• White blood cells (WBCs/Leucocytes) - fight infections
• Platelets - help blood clot

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📍 2. Where Are Blood Cells Made? (Sites of Blood Formation)

Key point: Liver and spleen are backup sites. After birth, the bone marrow takes over as the only important site. If the body desperately needs more blood cells (like in severe anaemia), the liver and spleen can kick back in - this is called extramedullary haematopoiesis.

• Red marrow = active blood-making tissue (found in flat bones and the ends of long bones in adults). Volume = 1-2 litres.
• Yellow marrow = mostly fat, but can convert back to red marrow when demand increases.

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🌱 3. Stem Cells - The Master Cells

Stem Cells (self-replicating, totipotential)
        ↓
    Progenitor Cells (committed to one path)
        ↓
     ┌────────────────────────────────────────┐
     ↓          ↓           ↓         ↓       ↓
 Erythroid   Megakaryocytic  Myeloid  Monocyte  Lymphoid
 (RBCs)     (Platelets)     series   series    (T,B cells)

• Stem cells self-replicate (make copies of themselves) and also differentiate (transform into other types)
• As cells move down the pathway, they become increasingly committed - they can only become one type of blood cell
• Progenitor cells are the committed early cells - they can only produce one or a limited range of blood cell types

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🔬 4. Stem Cell Assays (Lab Tests for Stem Cells)

• Cells are grown in culture - a single progenitor cell divides and forms a colony of 40 or more cells
• CFU-GM = colony-forming unit for granulocytes and macrophages
• BFU-E = burst-forming unit for erythrocytes (very early, large burst of red cell colonies)
• About 1 in every 1,000 bone marrow cells can act as a precursor for one or more blood cell types

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❤️ 5. The Erythroid (Red Cell) Series

Steps in order (from earliest to most mature):

Simple memory trick: "Pro → Baso → Poly → Ortho → Reticulo → Mature RBC" - each step = less nucleus, more haemoglobin!

• Red/acidophilic = haemoglobin (pink/red)
• As cell matures: nucleus shrinks → gets pushed out → reticulocyte → full RBC

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⚪ 6. The Granulocyte Series (White Blood Cells with Granules)

Myeloblast → Promyelocyte → Myelocyte → Metamyelocyte → Band form → Segmented (mature)

Neutrophils (most common WBC - 42-74% of all WBCs):

• Kill bacteria; first responders to infection
• Mature cell has a segmented (lobulated) nucleus (2-5 lobes)
• Females have a "drumstick" appendage on one lobe (Barr body - inactive X chromosome)
• Band/stab form = one step before mature neutrophil (nucleus bent like a horseshoe)
• Bone marrow stores ~3x more neutrophils than are in the blood - a ready reserve pool

Eosinophils (1-3%):

• Fight parasites; involved in allergic reactions
• Have large, bright red-orange granules (love eosin stain)
• Slightly larger than neutrophils
• Increased in allergic conditions (e.g. asthma, hay fever) and parasitic infections

Basophils (0.4-2%):

• Contain large blue-purple granules that obscure the nucleus
• Involved in immediate allergic reactions (similar to mast cells)
• Least common granulocyte

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🟤 7. Monocyte-Macrophage Series

• Monocytes: kidney-shaped or horseshoe-shaped nucleus; pale grey cytoplasm
• Macrophages: found in lymph nodes, spleen, liver (called Kupffer cells), lungs - they engulf and destroy foreign material, dead cells, and pathogens
• Together they form the mononuclear phagocyte system
• Important in both innate immunity and antigen presentation to lymphocytes

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🫀 8. The Megakaryocytic Series (Platelet Formation)

1. Stem cell → Promegakaryocyte (undergoes DNA replication without dividing - becomes giant and polyploid)
2. → Megakaryocyte (huge cell in bone marrow, up to 90 µm)
3. Megakaryocyte protrudes extensions into bone marrow sinusoids
4. These extensions fragment into individual platelets (1-4 µm diameter)
5. Each megakaryocyte produces ~1,000+ platelets

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🔵 9. The Lymphoid Series (T and B Cells)

T Lymphocytes:

• Mature in the thymus
• CD4+ T helper cells, CD8+ cytotoxic T cells, suppressor T cells
• Cell-mediated immunity (attack infected cells directly)
• Can also detect foreign antigens and recruit other immune cells

B Lymphocytes:

• Mature in the bone marrow
• Produce antibodies (immunoglobulins)
• When stimulated → become plasma cells → secrete large amounts of antibody
• Humoral immunity (antibody-based protection)

Null Lymphocytes (neither T nor B):

• Include Natural Killer (NK) cells
• No specific antigen receptor but can kill tumour cells and virus-infected cells

Key T vs B cell differences:

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💊 10. Regulation of Haematopoiesis

Erythropoiesis Regulation:

• Controlled mainly by the hormone erythropoietin (EPO)
• Made primarily by the kidneys (also a little in the liver)
• When oxygen delivery to tissues falls (e.g. after blood loss, at high altitude, in anaemia) → kidneys sense low oxygen → release more EPO → stimulates bone marrow to make more RBCs
• This is a feedback loop: Low O₂ → more EPO → more RBCs → better O₂ delivery → EPO drops back down (see Fig. 1.2)

Granulopoiesis Regulation:

• Stimulated by colony-stimulating factors (CSFs) including G-CSF, GM-CSF
• Infections trigger monocytes/macrophages to release material that stimulates neutrophil production
• Bone marrow stores a reserve pool of neutrophils (~3x the circulating amount)

Thrombopoiesis (Platelet) Regulation:

• Feedback mechanism - when platelets decrease, production increases
• Thrombopoietin is the key hormone

Haematopoietic Reserve Capacity:

• Body normally needs ~2 × 10¹¹ red blood cells per day
• Under stress, it can increase to 4-6× normal
• Reticulocytes take ~7 days to go from stem cell to peripheral blood

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🦴 11. The Bone Marrow - Structure

• Fat cells (~50% of marrow volume)
• Haematopoietic cells (developing blood cells in all stages)
• A network of reticulin fibres (support framework)
• Sinusoids (special blood vessels) where mature cells enter the bloodstream
• Adventitial reticular cells, macrophages, lymphoid follicles

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🩺 12. Bone Marrow Biopsy

Aspiration (Fine needle):

• A wide-bore needle is inserted into the iliac crest (preferred in adults) or the sternum
• In children under 1 year: tibia is used
• Withdraws liquid marrow with a syringe
• Films are spread on glass slides for microscopy
• Can assess cell types and morphology

Trephine Biopsy:

• A larger needle takes a core of solid bone marrow
• Preserves the architecture (layout) of the marrow
• Better for assessing cellularity, fibrosis, infiltration (lymphoma, cancer spread, etc.)
• Stains used: Romanowsky (routine), iron stain, peroxidase, acid phosphatase, PAS, methyl methacrylate

When is a "dry tap" (failed aspiration)? When marrow is fibrotic (myelofibrosis) or packed with cells (leukaemia) - no liquid can be aspirated. You need a trephine biopsy in that case.

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🔑 Key Takeaways (Summary)

1. All blood cells come from one stem cell in the bone marrow
2. RBC production: Pro-erythroblast → Reticulocyte → Mature RBC (controlled by EPO from kidneys)
3. Granulocytes (neutrophils, eosinophils, basophils) fight infection and allergies
4. Monocytes circulate and become tissue macrophages
5. Platelets are made from giant megakaryocytes
6. T cells = cell-mediated immunity (via thymus); B cells = antibody production (via bone marrow)
7. Erythropoietin (EPO) is the key hormone for RBC production - released when oxygen is low
8. Red marrow is active; yellow marrow is fatty but can be converted back during demand
9. Bone marrow biopsy = aspiration (liquid) + trephine (core); iliac crest is preferred site in adults
10. Hairy cell leukaemia and myelofibrosis are classic causes of a "dry tap" on aspiration