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Understanding Blood Cell Formation

Apr 9, 2025

18.2 Production of the Formed Elements - Anatomy and Physiology 2e

Learning Objectives

  • Understand how formed elements of blood are generated from bone marrow stem cells.
  • Discuss the role of hemopoietic growth factors in blood cell production.

Overview

  • Formed elements have brief lifespans.
    • Most erythrocytes, leukocytes, and platelets live only a few hours to weeks.
    • Memory cells can survive for years.
  • Hemopoiesis: the process of forming new blood cells and platelets.
  • Blood donation:
    • Plasma replaced in 24 hours.
    • Blood cells require 4-6 weeks for replacement.

Sites of Hemopoiesis

  • Before Birth:
    • Begins in the yolk sac, then moves to fetal liver, spleen, lymphatic tissue, and eventually red bone marrow.
  • After Birth:
    • Primarily occurs in red bone marrow within spongy bone.
    • In adults, mainly in cranial/pelvic bones, vertebrae, sternum, and proximal epiphyses of femur/humerus.
  • Extramedullary Hemopoiesis:
    • Liver and spleen can generate formed elements if bone marrow fails.

Differentiation of Formed Elements from Stem Cells

  • All formed elements arise from red bone marrow stem cells.
  • Stem cells divide into a daughter stem cell and a specialized cell.
  • Hierarchy of Stem Cells:
    • Totipotent Stem Cell: All body cells.
    • Pluripotent Stem Cell: Multiple cell types and supporting fetal membranes.
    • Mesenchymal Cell: Connective tissues like bone, cartilage, blood.
    • Hematopoietic Stem Cell (Hemocytoblast): Origin of all blood cells.
  • Lymphoid Stem Cells:
    • Produce lymphocytes (T cells, B cells, NK cells).
    • Migrate to lymphatic tissues for maturation.
    • B cells mature in bone marrow; T cells mature in thymus.
  • Myeloid Stem Cells:
    • Produce erythrocytes, megakaryocytes, monocytes, and granular leukocytes.

Hemopoietic Growth Factors

  • Erythropoietin (EPO):
    • Produced in kidneys, stimulates erythrocyte production.
    • Used medically and as a performance-enhancing drug.
  • Thrombopoietin:
    • Produced in liver/kidneys, stimulates platelet formation.
  • Cytokines:
    • Include colony-stimulating factors (CSFs) and interleukins.
    • CSFs stimulate leukocyte differentiation.
    • Interleukins involved in immunity and cell differentiation.

Everyday Connection - Blood Doping

  • Enhances performance by increasing RBCs, thus aerobic capacity.
  • Risks: Increased blood viscosity, heart strain, infection.
  • Methods: Transfusion of RBCs or use of synthetic EPO.
  • Legal Status: Banned in sports, but synthetic EPO used in medical treatments.

Bone Marrow Sampling and Transplants

  • Biopsy: Diagnostic test for severe anemias and cancers.
  • Transplants: Donor bone marrow replaces faulty marrow.
    • Requires matching donor to avoid tissue rejection.
  • Procedure:
    • Traditional method is painful, involving iliac crest region.
    • Modern methods involve stem cell isolation from blood samples.
    • Stem cells are cultured, analyzed, or frozen for future use.

References

  • This content is based on "Anatomy and Physiology 2e" by OpenStax, available at OpenStax