2 Haematopoiesis

Hematopoiesis (or hemopoiesis)

  • Definition: Refers to the formation of formed elements of blood, encompassing various cellular components.

  • Components include:

  • Red blood cells (erythrocytes)

  • White blood cells (leukocytes)

  • Platelets

  • Location: Occurs within the red bone marrow, specifically in the spongy middle part of the bone.

Differentiation of Hematopoietic Stem Cells

  • Definition of Hematopoietic Stem Cells: Undifferentiated stem cells located in bone marrow that give rise to all blood cell types.

  • Cell Line Differentiation:

    • Two main progenitor cell lines:

    • Myeloid progenitor cell line

      • Produces:

      • Erythrocytes (red blood cells)

      • Leukocytes (white blood cells)

      • Platelets

    • Lymphoid progenitor cell line

      • Produces:

      • Lymphocytes (e.g., T cells, B cells, natural killer cells)

Processes Resulting from Differentiation

  • Erythropoiesis

    • Definition: Process of red blood cell formation.

    • Importance: Ongoing process required for oxygen transport.

    • Contextual example: Climbing Mount Everest results in lower oxygen levels, prompting erythropoiesis.

    • Hormonal Regulation:

    • Erythropoietin (EPO)

      • Source: Secreted by the kidneys

      • Function: Stimulates red blood cell formation by acting on hematopoietic stem cells in red bone marrow.

    • Process Overview:

    1. Decrease in blood oxygen levels

    2. Kidneys secrete EPO into the bloodstream

    3. EPO signals stem cells in bone marrow to divide

    4. Formation of erythroblasts (immature red blood cells)

    • Characteristics of Erythroblasts:

    • Size and hemoglobin content determine classification.

    • Stages of Differentiation:

    • Erythroblasts develop structures and features characteristic of red blood cells.

    • Important transition: Loss of nucleus prior to entering circulation.

    • Final Maturation:

    • Complete maturity occurs 1-2 days post-circulation, involving organelle loss.

    • Timeline of Erythropoiesis:

    • Takes approximately 7 days for maturation; lifespan of mature red blood cells: 120 days.

    • Removal of Old Red Blood Cells:

    • Processed by macrophages in bone marrow, spleen, and liver after 120 days.

Leukopoiesis

  • Definition: Process of white blood cell formation.

  • Regulation: Initiated by growth factors and cytokines that stimulate differentiation of hematopoietic stem cells in red bone marrow.

  • Differentiation Process:

    • Stem cells differentiate into myeloid or lymphoid progenitor cells.

    • Subsequent maturation occurs either in bone marrow or lymphoid tissue.

  • Maturation Timeline and Lifespan Variability:

    • Neutrophils: Maturation within hours to days.

    • Lymphocytes: Maturation can range from weeks to years, with extended lifespans.

Comparison of Erythropoiesis and Leukopoiesis

  • Location: Both processes occur in red bone marrow.

  • Cell Types Produced:

    • Erythropoiesis produces red blood cells.

    • Leukopoiesis produces white blood cells.

  • Stimulating Factors:

    • Erythropoiesis is stimulated by erythropoietin.

    • Leukopoiesis is stimulated by various growth factors and cytokines.

  • Maturation Time:

    • Erythropoiesis: About 7 days for red blood cells.

    • Leukopoiesis: Highly variable duration for white blood cell maturation.

  • Lifespan of Cells:

    • Red blood cells: Approximately 120 days.

    • White blood cells: Lifespan varies from hours to years based on cell type.