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