📝 STANDARD DESCRIPTION:
Bone Marrow — soft tissue inside bone cavities; main haematopoietic organ in the adult.
2 types:
• Red marrow — active haematopoiesis. In flat bones (sternum, ribs, ilium, vertebrae) of adult; in all bones of young animals.
• Yellow marrow — fat-filled; fills shafts of long bones in adult.
Cells produced:
• Erythrocytes, all leukocytes, platelets — all from a single haematopoietic stem cell (HSC).
Sites for marrow aspiration: sternum, wing of ilium (large animals); iliac crest, proximal humerus (small animals).
📚 DETAILED DESCRIPTION:
BONE MARROW & HAEMATOPOIESIS
Bone marrow is the principal haematopoietic organ of the adult. It is housed in cavities of cancellous (spongy) bone and the medullary cavity of long bones.
I. STRUCTURE OF MARROW:
• Highly cellular tissue suspended in a network of reticular fibres + adipocytes + sinusoidal capillaries.
• Sinusoids — large thin-walled vessels lined by discontinuous endothelium; mature blood cells squeeze between endothelial cells to enter circulation.
• Stromal cells + reticular cells + macrophages support haematopoiesis (the 'haematopoietic niche').
II. RED VS YELLOW MARROW:
• Red Marrow (Medulla ossium rubra): Active haematopoiesis. Highly cellular; reticular meshwork; sinusoids; clusters of developing erythroid + myeloid + megakaryocytic cells.
– Locations in adult: vertebrae, sternum, ribs, ilium, skull, proximal ends of humerus + femur.
• Yellow Marrow (Medulla ossium flava): Mostly adipocytes; can revert to red marrow in chronic blood demand (anaemia, blood loss).
– Locations: shafts of long bones in adults.
III. HAEMATOPOIESIS — Cell Lineages from HSC:
The Multipotent Haematopoietic Stem Cell (HSC) gives rise to two main progenitor lines:
1. Common Myeloid Progenitor:
• Erythroid line: Proerythroblast → Basophilic → Polychromatic → Orthochromatic → Reticulocyte → mature RBC. (Loses nucleus in mammals.)
• Granulocyte-monocyte line:
– Myeloblast → Promyelocyte → Myelocyte (specialises into neutrophil, eosinophil, or basophil) → Metamyelocyte → Band cell → Mature granulocyte.
– Monoblast → Promonocyte → Monocyte.
• Megakaryocytic line: Megakaryoblast → Megakaryocyte (huge cell with one highly polyploid, multilobulated nucleus) → fragments cytoplasm into thousands of platelets.
2. Common Lymphoid Progenitor:
• Lymphoblast → Lymphocyte → matures in:
– Bursa of Fabricius (in birds) → B-cell.
– Bone marrow (in mammals) → B-cell.
– Thymus → T-cell.
IV. SITES OF HAEMATOPOIESIS THROUGH LIFE:
• Embryo: yolk sac.
• Foetus: liver + spleen.
• Newborn: marrow of all bones.
• Adult: marrow of axial skeleton + proximal long bones (red); rest = yellow.
V. REGULATION:
• Erythropoietin (EPO) — from kidney; ↑ RBC production in hypoxia.
• Thrombopoietin (TPO) — from liver; ↑ platelets.
• G-CSF, GM-CSF — granulocyte/macrophage colony-stimulating factors.
• Interleukins — modulate lymphopoiesis.
VI. MARROW ASPIRATION SITES (Clinical):
• Cattle / Horse: Sternum (manubrium), Wing of ilium (tuber coxae or sacrale).
• Dog / Cat: Iliac crest, Proximal humerus (greater tubercle), Trochanteric fossa of femur.
• Bird: Proximal tibiotarsus.
// COMPARATIVE ANALYSIS
| BIRD |
Bursa of Fabricius at cloaca — site of B-lymphocyte maturation; mammals don't have it (B-cells mature in marrow itself). |
| MAMMAL |
Anucleate RBC because the nucleus is extruded by the orthochromatic erythroblast late in maturation — increases haemoglobin loading. |
CLINICAL RELEVANCE
Bone Marrow Aspiration: For diagnosis of anaemia, leukaemia, lymphoma, immune-mediated thrombocytopenia. Aplastic Anaemia: Marrow failure → pancytopenia. Erythroleukaemia / Myeloid Leukaemia: Cancer of marrow lineages.
