Overview
This lecture explains how blood's formed elements are produced, the sites and processes of hemopoiesis, the role of growth factors, and clinical relevance such as blood doping and bone marrow transplants.
Hemopoiesis and Lifespan of Formed Elements
- Hemopoiesis (hematopoiesis) is the production of blood's formed elements (RBCs, WBCs, platelets).
- Most formed elements live only a few hours to weeks, except memory cells.
- Blood cell replacement is rapid and continuous; plasma is replaced faster than cells after blood donation.
Sites of Hemopoiesis
- Before birth, hemopoiesis occurs in the yolk sac, liver, spleen, lymphatic tissue, and red bone marrow.
- After birth, it primarily occurs in red marrow in cranial, pelvic bones, vertebrae, sternum, and proximal femur/humerus.
- Extramedullary hemopoiesis occurs in the liver and spleen if bone marrow fails (e.g., due to disease).
Differentiation of Blood Cells from Stem Cells
- All blood cells originate from hematopoietic stem cells (hemocytoblasts) in red bone marrow.
- Stem cells undergo mitosis; one remains a stem cell, the other differentiates.
- Hierarchy: totipotent → pluripotent → mesenchymal → hematopoietic stem cell.
- Hematopoietic stem cells become lymphoid or myeloid stem cells.
- Lymphoid stem cells produce lymphocytes (T cells, B cells, NK cells).
- Myeloid stem cells produce erythrocytes, megakaryocytes (platelets), monocytes, and other leukocytes.
- Precursor cells (e.g., -blasts) develop before maturing into functional blood cells.
Hemopoietic Growth Factors
- Erythropoietin (EPO): kidney hormone that stimulates RBC production in response to low oxygen.
- Thrombopoietin: produced by liver and kidneys, triggers platelet production.
- Cytokines: glycoproteins stimulating blood cell progenitors; include colony-stimulating factors (CSFs) and interleukins (ILs).
- CSFs guide differentiation into specific leukocytes and other cells.
- Interleukins, made by various cells, regulate immunity and inflammation.
Clinical Connections: Blood Doping & Bone Marrow Transplants
- Blood doping involves increasing RBC count (via transfusion or synthetic EPO) to enhance oxygen delivery for sports; raises health risks.
- Bone marrow biopsies and transplants diagnose/treat anemia and leukemia; stem cells now often obtained from blood samples.
- Transplants require donor-recipient matching to avoid tissue rejection; patient's diseased marrow is destroyed before transplant.
Key Terms & Definitions
- Hemopoiesis — production of blood's formed elements.
- Hematopoietic stem cell (hemocytoblast) — stem cell giving rise to all blood cells.
- Lymphoid stem cell — forms lymphocytes.
- Myeloid stem cell — forms RBCs, platelets, and most leukocytes.
- Erythropoietin (EPO) — hormone stimulating RBC production.
- Thrombopoietin — hormone stimulating platelet formation.
- Cytokines — signaling proteins stimulating blood cell growth.
- Colony-stimulating factors (CSFs) — cytokines directing WBC production.
- Interleukins (ILs) — cytokines involved in immune regulation.
Action Items / Next Steps
- Watch the linked video on the dangers and side effects of blood doping.
- Review figure 18.4 for cell differentiation pathways.
- Study key growth factors and their functions for upcoming assessments.