2. Hemopoiesis

Hemopoiesis/Hematopoiesis

  • The process of generating all the formed elements in the blood.
  • Blood composition:
    • 45% formed elements (red blood cells, white blood cells, platelets).
    • 55% plasma.

Hematopoietic Stem Cells

  • All formed elements originate from a single hematopoietic stem cell.
  • Types of stem cells:
    • Totipotent: Can become any cell type (embryonic).
    • Pluripotent: Can become most cell types (blastocyst).
    • Multipotent: Can form a limited range of cell types (adult stem cells).
  • Hematopoietic stem cells are multipotent adult stem cells.

Importance of Hemopoiesis

  • Most formed elements have a short lifespan and do not divide in the bloodstream.
  • Hemopoiesis is a continual and rapid process to replenish blood cells.

Therapeutic Use of Hematopoietic Stem Cells

  • Hematopoietic stem cells can be reprogrammed to revert to a pluripotent state in the lab.
  • Cord Blood: Umbilical cord blood is a readily available source of hematopoietic stem cells.
    • Cord blood stem cells can be frozen and stored for later use.
    • Useful for patients undergoing chemotherapy or with diseases affecting blood cell production.
      • These stored cells are reintroduced to the recovering body to assist in reforming blood components.
    • Research is exploring the use of cord blood to generate other tissues, but this is not yet a common therapeutic option.

Location of Hemopoiesis

  • Adults: Occurs in red bone marrow found in:
    • Flat and irregular bones.
    • Ends of long bones.
    • Pelvis, vertebrae, ribs, and skull.
    • Sternum.
  • Young People (under 1 year): Occurs in most bones initially.
  • Red Bone Marrow: Responsible for hemopoiesis (Makes Red Blood cells, White Blood cells and platelets).
  • As we age, red bone marrow gets restricted to certain locations, and yellow marrow (fatty marrow for energy storage) takes over in other bones.

Process of Hemopoiesis

  • Starts with one hematopoietic stem cell.
  • The hematopoietic stem cell undergoes mitosis:
    • One cell remains a stem cell in the red bone marrow to maintain the stem cell pool.
    • The other cell differentiates based on signaling factors present in the body.
    • Differentiates into:
      • Myeloid stem cell, or
      • Lymphoid stem cell.

Myeloid vs. Lymphoid Lineage

  • These stem cells gives rise to different lineages.
  • Once a cell commits to a lineage, it cannot switch back.
  • Myeloid stem cells differentiate into:
    • Monocytes.
    • Granulocytes (basophils, eosinophils, neutrophils).
    • Red blood cells.
    • Platelets.
  • Lymphoid stem cells differentiate into:
    • Lymphocytes (B and T lymphocytes).
    • Natural killer cells.

Key Takeaways

  • Both myeloid and lymphoid lineages produce white blood cells.
  • Red blood cells and platelets are only produced by the myeloid lineage.
  • Each step involves multiple differentiation steps, amplifying the number of cells.
  • From one hematopoietic stem cell, thousands of specific cell types can be produced.
  • Understand the concept of myeloid vs. lymphoid lineages and their end products.
  • Remember that differentiation is a one-way process.
  • All occurs within the red bone marrow.
  • The regulation of these processes is important to maintain blood cell counts appropriately.