Clinical Laboratory - Haematology - Hematopoietic Organs

Hematopoietic Organs

  • Sites of Hematopoiesis:

    • Embryo: Yolk sac (primitive erythropoiesis)

    • Fetus: Aorta-gonad-mesonephros (AGM) region (definitive erythropoiesis)

    • Adult: Bone marrow

  • Primary Lymphoid Tissues: Bone marrow and thymus; site of T and B cell development.

  • Secondary Lymphoid Tissues: Spleen and lymph nodes; site of further differentiation and division of immunocompetent T and B cells.

Bone Marrow

  • Location: Between trabeculae of spongy bone.

  • Compartments:

    • Vascular: Arteries, veins, stromal cells, hematopoietic cells.

    • Endosteal: Bone remodeling, hematopoietic stem cells.

  • Vasculature:

    • Nutrient and periosteal arteries supply blood.

    • Central vein drains blood.

  • Stroma:

    • Supporting tissue with macrophages, reticular cells (fibroblasts), and adipocytes.

    • Produces cytokines for hematopoiesis regulation.

    • Osteoblasts: Form bone, regulate HSC activity, alkaline phosphatase positive.

    • Osteoclasts: Bone resorption and remodeling, multinucleated.

  • Hematopoietic Cells:

    • Erythroblasts: 25-30% of marrow cells, near venous sinuses, develop in erythroblastic islands.

    • Granulocytes: Produced in nests near trabeculae and arterioles.

    • Megakaryocytes: Large, polyploid cells near vascular sinus, produce platelets.

    • Lymphocytes: Produced in lymphoid aggregates near arterioles.

  • Hematopoiesis:

    • Extramedullary hematopoiesis: Blood cell production outside bone marrow (e.g., liver, spleen).

Thymus

  • Location: Upper part of anterior mediastinum.

  • Structure:

    • Cortex: Densely packed lymphocytes (thymocytes), cortical epithelial cells, few macrophages.

    • Medulla: More mature thymocytes, medullary epithelial cells, dendritic cells, macrophages.

  • Function: Maturation of T lymphocytes.

Spleen

  • Location: Upper-left quadrant of abdomen.

  • Structure:

    • Capsule of connective tissue.

    • White pulp: Lymphocytes around central artery, periarterial lymphatic sheath (PALS) with T cells, B cell follicles, germinal centers.

    • Red pulp: Sinuses (vascular spaces with erythrocytes), cords (reticular tissue and macrophages).

    • Marginal zone: Reticular meshwork with blood vessels, macrophages, specialized B cells.

  • Function:

    • Filtration of foreign substances and old RBCs.

    • Platelet storage.

    • Immune defense.

    • Culling: Removal of senescent or damaged RBCs.

    • Pitting: Removal of particles from intact RBCs.

Lymph Nodes

  • Structure:

    • Cortex: B-cell follicles, T lymphocytes, macrophages, germinal centers.

    • Medulla: Cords of plasma cells, B cells, macrophages, sinusoids.

  • Function:

    • Filtration of lymph.

    • Immune defense.

Mucosa-Associated Lymphoid Tissue (MALT)

  • Location: Mucosal surfaces (e.g., Peyer's patches in intestine, tonsils).

  • Function:

    • Trap antigens crossing mucosal surfaces.

    • Initiate immune responses.