Hematopoiesis: Blood Cell Formation

Terminology

• Hematopoiesis (or Hemopoiesis) refers to the formation of blood cells.
• Leukopoiesis is the formation of leukocytes.
• Erythropoiesis is the formation of erythrocytes.

Cell Origin

• All blood cells are produced in the bone marrow (medullary site).
  • This includes erythrocytes, platelets, neutrophils, and lymphocytes.
• Lymphocytes undergo further critical development in lymphoid tissues (extramedullary sites) such as lymph nodes, tonsils, spleen, and thymus.
• Eosinophils, basophils, and monocytes are also produced in bone marrow.
• Active ("red") bone marrow (medullary site) is primarily found in the ends of long bones like the femur, tibia, humerus, and ulna, and in flat bones like hip bones, sternum, and ribs.
• The liver and spleen (extramedullary sites) have a limited capacity to participate in hematopoiesis.

Stem Cells

• Pluripotent stem cells (PPSCs) are large primitive cells found in bone marrow and the umbilical cord.
• PPSCs are precursors to all blood cells, with the ability to become any type of blood cell.
• The final cell type is determined by a stimulus that acts on the stem cells.
• Each blood cell type requires a specific set of stimuli to activate PPSC differentiation.
• Once a stem cell begins to develop, it cannot revert or change into another cell type; hematopoiesis is a one-way street.
• The kidneys contain specialized cells that detect hypoxia and produce erythropoietin (EPO).
• EPO is carried by the bloodstream to the bone marrow, where it stimulates some stem cells to develop into erythrocytes.
• Production of blood cells involves numerous mitotic divisions, and one PPSC eventually becomes many cells of the same type it was originally stimulated to become.
• PPSCs self-perpetuate; during mitosis, half of the first division returns to the stem cell pool.

Maturation of Blood Cells

• Complete blood cell development occurs in bone marrow, except for some lymphocytes.
• The bone marrow and spleen have a reserve pool of mature blood cells that are released into circulation when needed.

Release of Blood Cells

• The size of the reserve pool varies by cell type, and a stimulus is needed for cells to be released.
• Hematopoiesis is continuous, involving constant stimulation and release of cells to replace those that die or are used.
• Pathological conditions can stimulate a large release of cells.
• A significant stimulus can deplete bone marrow reserves, leading to the release of immature cells, which are less effective than mature cells.

Blood Storage

• Bone marrow and spleen comprise the blood cell "storage pool".
• The spleen is a sponge-like organ that can hold a substantial amount of mature blood cells.
• When an animal is stimulated, the spleen contracts to release stored blood into the bloodstream.

Blood Volume

• Domestic mammalian species have approximately 75ml of blood per kilogram of lean body weight, ranging from 50 to 100ml/kg.
• The absolute limit for the maximum amount of blood that can be drawn is 25% of the total blood volume.

Leukocytes

• Leukopoiesis is the formation of white blood cells (WBCs).
• All WBCs originate from the same pool of PPSCs and initially appear identical.
• Specific stimuli determine the type of WBC that develops and each WBC has its own stimulus.

Leukocyte Function

• Leukocytes defend against foreign invaders such as viruses and bacteria.
• WBCs primarily function in tissues, using peripheral blood as their means of transport.

Leukocyte Classification

• Leukocytes are classified based on the presence or absence of specific staining cytoplasmic granules, resulting in granulocytes and agranulocytes.
  • Granulocytes (neutrophils, eosinophils, and basophils) have visible granules.
  • Agranulocytes (lymphocytes and monocytes) lack visible granules.
• Leukocyte nuclei vary in shape.
  • Polymorphonuclear cells (neutrophils, eosinophils, and basophils) have lobed, multilobed, or segmented nuclei.
  • Mononuclear cells (lymphocytes) have single, rounded nuclei.
  • Pleomorphic cells (monocytes) have varying, amorphous, non-segmented nuclei.
• Leukocytes are categorized by their type of defense.
  • Neutrophils, eosinophils, basophils, and monocytes use phagocytosis.
  • Lymphocytes use antibody production and cellular immunity, but have no phagocytic abilities.

Overview of hematopoiesis explaining the formation of blood cells, including leukocytes and erythrocytes. Covers cell origin in bone marrow and lymphoid tissues, highlighting the role of pluripotent stem cells (PPSCs). Discusses active bone marrow locations.