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Hematopoiesis
Jul 10, 2024
Hematopoiesis
Overview
Mature blood cells have a limited lifespan and must be continuously replaced.
This process is called hematopoiesis and occurs in the red bone marrow.
All blood cells derive from hematopoietic stem cells (HSCs).
Hematopoietic Stem Cells (HSCs)
HSCs are multipotent and can differentiate into all types of blood cells.
They maintain their numbers in the bone marrow through constant multiplication.
Blood cell formation from HSCs involves several intermediate progenitors and is regulated by cytokines.
Role of Cytokines
Cytokines are signaling molecules that control:
Proliferation
Differentiation
Survival or death of progenitors
Maintain steady-state levels of blood cells in normal situations.
Induce production of specific cell types in response to stimuli (e.g., blood loss).
Differentiation Process
Begins when progenitor cells develop surface receptors for specific stimulating factors.
Cells lose their potency and become committed to a certain cell type.
Production of Red Blood Cells (RBCs)
Stimulated by erythropoietin (EPO).
Differentiation involves:
Reduced size
Increased number
Hemoglobin production
Loss of nucleus
EPO is produced by the liver during fetal development and by the kidneys in adulthood.
Low levels of EPO compensate for normal RBC turnover.
Increased EPO secretion in response to hypoxemia (e.g., blood loss) leads to higher RBC production.
People at high altitudes and athletes have higher RBC counts due to lower oxygen levels and elevated oxygen demand, respectively.
Production of Granulocytes and Macrophages
Controlled by colony-stimulating factors (CSFs).
Maintained at constant levels by low CSFs, but increase greatly upon infection.
CSFs are secreted by lymphocytes and macrophages, and can be produced by various organ or cell types.
Can increase a thousand-fold in response to infection indicators like bacterial endotoxins.
Production of Platelets
Stimulated by thrombopoietin (TPO).
TPO is secreted by the kidneys and liver.
Leads to formation of megakaryocytes, which give rise to platelets.
Platelets are fragments of megakaryocyte cytoplasm.
Production regulated by negative feedback: low platelet levels promote production, while high levels inhibit it.
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