Blood formation is a complex process that occurs primarily in bone marrow. Hemopoietic stem cells give rise to all types of blood cells through differentiation. This process is crucial for maintaining a healthy blood supply and immune system.

Regulation of blood cell production involves various growth factors and cytokines. These molecules stimulate the production of specific blood cell types in response to the body's needs. The balance between cell production and removal is essential for blood homeostasis.

Hemopoiesis and Blood Cell Formation

Process of hemopoiesis

• Hemopoiesis involves the formation of blood cells primarily in the red bone marrow of certain bones (pelvis, sternum, ribs, and vertebrae)
• Hemopoietic stem cells (HSCs) serve as the multipotent precursors capable of differentiating into any type of blood cell
  • HSCs divide and differentiate into myeloid and lymphoid lineages
  • HSCs reside in specialized microenvironments called stem cell niches
• Myeloid lineage differentiates into erythrocytes (red blood cells), platelets, and myeloblasts
  • Myeloblasts further differentiate into granulocytes (neutrophils, eosinophils, and basophils) and monocytes (white blood cells)
• Lymphoid lineage differentiates into lymphoblasts
  • Lymphoblasts further differentiate into T lymphocytes, B lymphocytes, and natural killer cells (white blood cells)
• Precursor cells represent intermediate stages between HSCs and mature blood cells
  • Each blood cell type has a specific precursor cell (erythroblasts for erythrocytes, megakaryoblasts for platelets)
• Cell differentiation involves the progressive specialization of cells into specific blood cell types

Regulation of blood cell production

• Hemopoietic growth factors, cytokines that stimulate blood cell proliferation and differentiation, regulate blood cell production
• Erythropoietin (EPO) stimulates erythrocyte production
  • Produced by the kidneys in response to low oxygen levels (hypoxia)
• Thrombopoietin (TPO) stimulates platelet production
  • Produced by the liver and kidneys
• Granulocyte colony-stimulating factor (G-CSF) and granulocyte-macrophage colony-stimulating factor (GM-CSF) stimulate granulocyte and monocyte production
  • Produced by various tissues (endothelial cells, fibroblasts)
• Interleukins (IL-3, IL-7) and other cytokines regulate lymphocyte production
  • Produced by immune cells (T lymphocytes, macrophages)
• The hematopoietic microenvironment, including stromal cells and extracellular matrix, supports blood cell development

Sites of blood cell formation

• Before birth, hemopoiesis occurs in the yolk sac, liver, and spleen
  • Yolk sac serves as the primary site during the first few weeks of embryonic development
  • Liver becomes the main site from the second trimester until shortly before birth
  • Spleen plays a minor role during fetal development
• After birth, hemopoiesis occurs primarily in the red bone marrow
  • Red bone marrow is found in the spongy bone of the skull, ribs, sternum, vertebrae, and ends of long bones
  • In adults, the main sites are the pelvis, sternum, ribs, and vertebrae
• In times of increased demand, the liver and spleen may resume their hemopoietic function (extramedullary hemopoiesis)
• Hematopoietic organs, such as bone marrow, thymus, and lymph nodes, play crucial roles in blood cell production and maturation

Cell turnover and homeostasis

• Blood cell production is balanced with cell removal to maintain homeostasis
• Apoptosis, or programmed cell death, helps regulate blood cell populations and remove damaged or unnecessary cells