Hematopoiesis Overview

Definition

• Hematopoiesis is the production of blood cells in the body, encompassing all formed elements (not just red blood cells).
• Platelets are cell fragments; thus, "formed elements" is a more accurate term.

Hematopoietic Stem Cells

• Hematopoietic Stem Cell: A multipotent stem cell that can differentiate into various blood cell lineages while maintaining a population of stem cells.
• Typically found in:
  • Fetal Development:
    • Yolk Sac: 2 weeks gestational.
    • Liver and Spleen: Around 3 months gestational.
  • Birth: Red bone marrow becomes the primary site for hematopoiesis.

Bone Marrow Types

• Red Bone Marrow: Active in hematopoiesis, found in long bones (e.g., femur, humerus) and flat bones (e.g., skull, sternum).
• Yellow Bone Marrow: Composed mainly of fat, replaces red marrow in about 50% of adult bone marrow.

Lineages from Hematopoietic Stem Cells

1. Lymphoid Lineage:

   • Produces lymphoid progenitor cells that develop into:
     • Natural Killer (NK) Cells: Part of the innate immune system, targets and kills infected cells.
     • Lymphoblasts (mature into T and B cells):
       • B Cells: Mature in bone marrow and produce antibodies.
       • T Cells: Mature in the thymus, develop into:
         • T Helper Cells (CD4+)
         • Cytotoxic T Cells (CD8+)

2. Myeloid Lineage:

   • Produces myeloid progenitor cells that develop into:
     • Erythrocytes (Red Blood Cells):
       • Proerythroblast → Erythroblast → Reticulocyte → Mature Erythrocyte.
       • Reticulocytes are immature red blood cells that enter the bloodstream and mature into erythrocytes in 1-2 days.
       • Importance in diagnosing anemia based on reticulocyte and erythrocyte levels.
     • Granulocytes:
       • Neutrophils: First responders to infection, involved in pus formation.
       • Eosinophils: Involved in allergic responses and parasitic infections.
       • Basophils: Release histamine in allergic reactions.
     • Monocytes → Macrophages: Phagocytic cells that gobble up pathogens and debris.
     • Dendritic Cells: Derived from monocytes, alert the immune system.
     • Megakaryocytes: Produce platelets (thrombocytes) through a process termed thrombopoiesis.

Regulation of Erythropoiesis

• Erythropoietin (EPO): A hormone primarily produced by kidneys in response to low oxygen levels in the blood.
  • Stimulates erythropoiesis by promoting the differentiation of progenitor cells into proerythroblasts.
  • Inhibits production when oxygen levels normalize, exemplifying negative feedback.

Conclusion

• Hematopoiesis is a complex process ensuring a steady supply of blood cells, regulated by hormones like EPO and involving multiple cell lineages.

Contact Information

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