Hematopoiesis

• Blood contains:

  1. Plasma (Liquid portion)
  2. Red blood cells
  3. White blood cells
  4. Platelets
• Blood is one of the largest organs in our body
• Blood is composed of

  • Plasma - Liquid portion (with ions, water, proteins, nutrients, wastes, O2 and CO2 = all soluble)
  • Red blood cells - Responsible for carrying O2 and CO2 have a life span of 120 days
  • White blood cells - Responsible for body defense
  • Platelets - Responsible for blood clotting (Stop bleeding)
• blood is formed through haematopoiesis which is the process of blood cell production, differentiation, and development7
• Red blood cells, white blood cells, and platelets are all produced through haematopoiesis and are regulated by cytokines, growth factors, hormones, and environmental factors to help stimulate cell development and differentiation. Ideally, number of cells produced = number of cell death
• Mature blood cells are released to the peripheral blood to perform specific functions
• Sites of haematopoiesis (Haematopoietic system) change at different stages. The haematopoietic system consists of:

  bone marrow,

  liver,

  spleen,

  lymph nodes,

  thymus
• Site of Haematopoeisis:

  • Fetus – 0-2 months @ yolk sac
  • Fetus – 2-7 months @ liver and spleen
  • Fetus – 5-9 months @ bone marrow
  • Infants – bone marrow through out the skeleton (practically all bones)
  • Adults – bone marrow (vertebrae, sternum, rib, skull, sacrum and pelvis, proximal ends of femur)
• Bone marrow is the spongy tissue found within the cavities of all bones

  is the most important organ for haematopoiesis (is the primary site of haematopoiesis after birth) and produces 5-10 x 1011 blood cells daily in a healthy individual7
• bone marrow consists of two compartments:

  • Yellow marrow - Usually inactive, mostly composed of adipose tissue
  • Red marrow - Contain the haemopoietic tissue and is usually active in the production of leukocytes, erythrocytes,and thrombocytes
• bone marrow forms a suitable environment around haemopoieticprogenitor cells for cell survival, self-renewal, and formation of differentiated progenitor cells
• red and yellow bone marrow are dynamic and in cases of large blood loss yellow bone marrow can become active and produce cells - why you loose fat cells when processing cells
• bone marrow is the main region of production but it can shift back to the liver in some situations
• Haematopoietic microenvironment within the hollow space in the bone, contains haematopoietic tissues (cords), Stem cells and their progeny. the stem cells can divide indefinitely
• Sinuses are the vascular spaces that are lined with endothelial cells. they regulate the release of mature and immature cells into the peripheral blood
• Non-haematopoietic cells within the bone marrow include:

  • Stromal cells (the major source of growth factors)
  • Mesenchymal stem cells, Osteoblasts (bone cells), and Endothelial cells. all form niches and provide growth factors, adhesion molecules and cytokines as well as help with cell viability and reproduction
  • Fibroblasts - Produce scaffolding and growth factors
  • Macrophages (WBC etc) - Produce growth factors and perform routine debris removal
  • Adipocytes - Store energy
• Haematopoiesis begins with the pluripotent stem cell = Haematopoietic Stem Cells (HSP). they can replicate, proliferate and differentiate into various lineage-specific stem cells - CD34+ (haematopoietic) and CD38- (surface marker) Stem cells are used for self-renewal and are usually dormant - only entering the cell cycle approximately once every 3 months to 3 years
• HSC are capable of about 50 cell divisions (Hayflick limit) and produce about 106 mature blood cells after 20 cell divisions generating mature blood cells. HSC are control by cytokines and haematopoietic growth factors and can be lineage-specific or can regulate cells in multiplelineages
• HSC are multipotential stem cells and can divide into many different blood cells
• Haematopoietic progenitor cells (HPC) are not capable of self-renewal but is actively dividing and committed to a single blood cell lineage
• blood cell lineages:

  • Mixed erythroid and megakaryocyte progenitor - Divide into separate erythroid and megakaryocyte progenitors
  • Mixed lymphoid, granulocyte and monocyte progenitor - Divide into separate granulocytes and monocytes and mixed lymphoid progenitors
  • Lymphoid progenitor cells - Differentiate into T lymphocytes, B lymphocytes and NK cells (The spleen, lymph nodes and thymus are secondary sites of lymphocyte production)
  • Myeloid progenitor cells - Differentiate into granulocytes, erythrocytes, monocytes, and megakaryocytes
• CFU-GEMM (colony forming unit - granulocyte erythrocyte monocyte megakaryocyte) is the earliest stem cells and produce red blood cells.

  Erythropoietin (EPO) is a lineage-specific glycoprotein thatinduces haemoglobin synthesis
• Leukopoiesis (white blood cell production) starts with CFU-GEMM

  • Myelopoiesis is controlled by GM-CSF, G-CSF, M-CSF, IL-3, IL-5, IL-11, and KIT ligands to promote the differentiation of CFU-GEMM into neutrophils, monocytes, eosinophils, and basophils
  • Lymphopoiesis is controlled by IL-2, IL-7, IL-12, and IL-15 to promote lymphoid differentiation
• Megakaryopoiesis/Thrombopoiesis (platelet production) starts with CFU-GEMM and is controlled by GM-CSF, IL-3, IL-6, IL-11, KIT ligand, and thrombopoietin(TPO) to promote megakaryopoiesis
• Cytokine growth factors and hormones control the maturation of blood cells - these can work at a low concentration and stimulate or inhibit blood cell production, differentiation, and trafficking. they can select which cell lineage for differentiation and can suppress apoptosis
• cytokines with positive influence include IL-1, IL-3, IL-6, FLT3, etc
• cytokines with a negative influence include TGF-β, TNF-α, etc
• The maturation of blood cells is controlled by cytokine growth factors and hormones such as:

  • Interleukins (ILs) are protein molecules that are produced by leucocytes and can work independently or in conjunction with other ILS. They act on other leucocytes and non-leucocytes
  • Colony-stimulating factors (CSFs) Act on precursors and are highly specific
• Stem cell factors (SCF) act on pluripotent stem cells and trigger cell differentiation
• erythropoietin (EPO) is mainly produced in the kidney and stimulate red blood cell production
• thrombopoietin (TPO) mainly produced in the liver and kidney and stimulate megakaryocyte maturation and production
• cytokines regulate target cells via:

  1. endocrine signals produced at great distances from target tissue and carried by the circulatory system to various sites
  2. paracrine signals releases locally and diffuse to act at short range on nearby cells
  3. autocrine are local mediators which act on the same cell that produce them
  4. juxtacrine signal specialized type of paracrine and requires direct cell-cell contact to achieve the desired effect by membrane bound cytokines
• endocrine signalling

  A) endocrine signaling

  B) autocrine

  C) paracrine

  D) juxtacrine

  
• Haematopoietic growth factors are a group of glycoprotein hormones mainly secreted by stromal cells (Except EPO in the kidney and TPO in the liver) and regulate haematopoietic progenitor cell proliferation, differentiation and prevent apoptosis. they can work locally, circulate in plasma, or bind to the ECM to form niches
• Growth factor binds to a specific receptor on the target cell’s membrane which causes dimerization. The binding causes changes to the intracellular domains of the receptor which activate intracellular signal transduction pathways through a series of phosphorylation events (E.g., JAK/STAT, MAPK)

  The signal may lead to cell proliferation, differentiation, maturation, functional stimulation, or halting apoptosis
• the cell cycle is a series of discrete stages to form two new cells

  • M phase (Division process)
  • Mitosis
  • Nuclear division
  • Cytokinesis
  • Cytoplasm division
  • G0 phase
  • G1 phase
  • S phase
  • G2 phase
• EPO stimulate red blood cell production and can be used to help patients with chronic renal disease as they often become anaemic as their kidney fails to produce the EPO

  TPO stimulate platelet production in the liver and kidney

  Growth factors used in stimulating stem cell expansion in vitro (lab) used for bone marrow transplantation (befbeforeoer insertion into patient)before
• Morphologic features of haematopoiesis cell during maturation:

  • Decrease in cell size - Decrease in the ratio of nucleus to cytoplasm (N:C ratio)
  • Decrease in the size of the nucleus, change in shape, condensation of nucleus chromatin leading to a loss of nucleoli and possible loss of the nucleus
  • Decrease in basophilia - specifically in immature cells with large amount of RNA which have an affinity for basic dye
  • Increase in the proportion of cytoplasm and possible appearance of granules in the cytoplasm
• Neutrophil cell line:

  • Myeloblast (10-20μm in diameter, The earliest identifiable precursor under light microscopy)
  • Promyelocyte (15-25μm in diameter, slightly larger)
  • Myelocyte (16-24μm in diameter, Round/oval nucleus and pink cytoplasmic granules)
  • Metamyelocyte (12-18μm in diameter, Kidney bean-shaped nucleus)
  • Band Cell (10-15μm in diameter, c-shaped nucleus)
  • Neutrophil (10-15μm in diameter, 2-5 lobed nucleus)
• neutrophiles have a dense nucleus, and consist of between two and five lobes with granules. they are the most numerous white blood cell in the peripheral blood (roughly 60% of the total)