Erythropoiesis

• red blood cells (erythrocytes) are highly specialized cells with approximately 1012 new cells produced each day
• RBC lack a nucleus and most organelles and enzymes which other cells have. this allows them to be flexible to penetrate capillaries, however due to having no nucleus and therefore no DNA they are unable to proliferate and regenerate and so they have a short lifespan of ~120 days
• RBC are biconcave disc in shape. they have modified cell membranes to allow the free and easy passage of oxygen in and out of the cell. they also have haemoglobin which is the oxygen-carbon dioxide transporter to allow the transport of oxygen to the tissues and transport carbon dioxide from the tissues to the lungs for gaseous exchange
• erythropoiesis is the process of erythrocyte production - a highly regulated process occurring within the bone marrow by macrophages which provide cytokines and other signals (adhesion molecules and insulin-like growth factor 1) to promote maturation and proliferation of red blood cells
• differentiation from haematopoietic stem cells (HSC) to haematopoietic progenitor cells (HPC) (CFU-GEMM) to the mature erythrocyte. the erythroid progenitor precursors progressively decrease in size and progressively decrease in size and progressively accumulate haemoglobin as well as reducing basophilic material
• the basic substances for normal erythrocyte and haemoglobin production include amino acids (proteins), iron, vitamin B12, vitamin B6, folic acid (part of B2 complex) and trace minerals of cobalt and nickel - deficiencies in any of these substances will result in abnormal erythropoiesis
• erythropoietin (EPO) is an essential erythroid-specific factor. the glycoprotein is mainly produced by interstitial cells of the kidney (90%) and 10% in the liver although it does not get stored
• production of EPO is in response to low oxygen levels (hypoxia). hypoxia induces production of hypoxia-inducible factors (HIF-1a and HIF-1b) via oxygen sensing pathways
• in hypoxic conditions HIF-1 (hypoxia-induced factor 1) binds DNA as part of a protein complex and this induces activation of the EPO gene promotor which stimulate EPO production and new vessel formation and transferric receptor synthesis and reduce hepcidin synthesis. increasing iron absorption
• an increase in oxygen supply reduces EPO production
• EPO interacts with erythropoietin receptors (EPOR) on the cell membrane of its target cells in the circulation. the dimer span on the cell membrane with the enxyme Janus Kinase 2 (JAK2) attached to the intracellular tail of the receptor
• EPO binds with EPOR causing intracellular phosphorylation of JAK2 which in turn causes phosphorylation of tyrosine amino acid residues which promote phosphorylation of STAT protein messengers which migrate to the cell nucleus causing the activation of the transcription factors such as GATA transcription factors which are important in erythroid cell survival, differentiation and proliferation
• the development of RBC begin with BFU-E (burst forming unit erythroid) which is the earliest cell in the erythrocyte series but does not actively proliferate
• CFU-E (colony forming unit erythroid) actively proliferate and are influenced by EPO to undergo programmed series of cell division and cell maturation. they come after BFU-E in the development of RBC
• reticulocyte is the immature form of RBC but can be found in peripheral blood alongside mature RBC but in much smaller quantities
• erythroblasts are formed from pronormoblast (rubriblast) becoming basophiloc normoblasts (prorubicyte) becoming polychromatic normoblast (rubricyte) becoming orthochromic normoblast (metarubricyte) becoming erythrocyte
• Pronormoblast (Rubriblast) are the first recognizable erythrocyte precursor in the bone marrow, roughly 12 to 19μm in size with large round nucleus with zero to two nucleoli and dark blue cytoplasm and lack of granules
• Basophilic Normoblast (Prorubricyte) are 12 to 17μm ion size and when nuclear chromatin becomes more clumped (DNA condensed) so less likely to see nucleoli but the cytoplasm continues to appear basophilic
• Polychromatic normoblast (Rubricyte) are 11 to 15μm and where chromatin continues to become more clumped with grey cytoplasm (pink colour mixed with basophilia) due to the accumulation of haemoglobin
• Orthochromic Normoblast (metarubricyte) are 8 to 12μm with condensed chromatin, nucleus will be extruded from the cell and now acidophilic due to the presence of a large quantity of haemoglobin
• Reticulocyte (immature erythrocyte) are slightly larger than matured red blood cell, around 7 to 10μm and contain organelles such as mitochondria and ribosomes, but still contains RNA for haemoglobin production
• Part of this phase occurs in the bone marrow and part of the stage circulate in the peripheral blood, for 1 – 2 days. A reticulocyte count is performed in the clinical laboratory as an indicator of the rate of erythrocyte production (Normal range: 0.5% to 1.5% in adults (percentage of total erythrocytes))
• Red blood cell are 6 to 8μm, pink-staining due to haemoglobin, no nucleus, RNA or ribosomes which allows a high degree of structural flexibility. they are non-nucleated biconcave disc with modified membrane
• Red blood cell are highly specialized for oxygen transport, the lack of nucleus and organelles as well as their physical flexibility or deformability (can increase its length by 250%) allows it to pass along the smallest capillaries to deliver oxygen to the tissue
• Red blood cell membrane has three components:

  • Double layer consisting of equivalent amounts of phospholipids and cholesterol
  • Various proteins and glycoproteins embedded within the double phospholipid layer (The molecules which interface with the plasma are collectively called the glycocalyx)
  • An internal cytoskeletal scaffold or skeleton that gives the red blood cell its characteristic round but flattened shape – biconcave disc
• Functions of red cell membrane components include:

  • Transport of ions and molecules across the membrane (Band 3: anion exchange channel)
  • Adhesion (Band 3: linked to the intracellular skeletal components)
  • Receptor function (CD35: complement component receptor)
  • Structural function (Spectrin, protein 4.2, etc: cytoskeleton)
• the function of RBC is to transport oxygen which is achieved by the highly specialized protein haemoglobin. High oxygen concentration within the cell can be toxic as oxygen can form highly reactive chemical species (free radicals) which can damage proteins, fats, and carbohydrates
• RBC have antioxidants - peroxiredoxin 2 bound to Band 3, amino acid glutathione to defend the cell with high oxygen saturation
• RBC gereate ATP via anaerobic conversion of glucose to lactic acid as RBC have no mitochondria to maintain flexibility and make room for Hb
• Haemoglobin is a protein-based substance which contains iron and is synthesized by the blast cell precursors in the bone marrow
• Each red cell contains approximately 640 million haemoglobin molecules which account for 90% of the cell dry weight they are designed to absorb oxygen from areas of high oxygen content (e.g. lungs) (becomes oxyhaemoglobin) and release oxygen in areas where oxygen levels are low (e.g. body tissues) (Becomes deoxyhaemoglobin)
• Structure of haemoglobin:

  • Haem (1 part)
  • Non-protein part, contains the iron for the oxygen to attach
  • Globin (4 parts)
  • A protein part, consists of 4 individual subtypes of globin that come together to form a functional tetramer
• Haemoglobin has 4 subunits with each subunit carrying a heme group - each carrying a ferrous ion (Fe 2) which can carry one molecule of oxygen.
• each haemoglobin consists of 4 globin chains - 2 alpha and 2 beta
• biosynthesis of haem:

  1. Protoporphyrin IX binds with ferrous iron
  2. Synthesis occurs in the mitochondria and cytoplasm of pronormoblast through polychromatophilic erythrocyte
  3. Enzymes require vitamin B6, B12 and folate
  4. DNA synthesis: vitamin B12 and folate
  5. Heme biosynthesis: vitamin B6 and B12
• haem leaves the mitochondria and combined with globin chain to form haemoglobin
• matured RBC cannot produce haemoglobin as they have no mitochondria
• iron is an essential component of haemoglobin, and RBC contain about 2/3 of total body iron.
• erythropoiesis requires 20-25mg of iron a day - most of this comes from recycling RBC via macrophages and 1-2mg a day comes from dietary iron (meat and some veg)
• some iron is lost via intestinal tract the skin and minor blood loss but dietary iron is normally enough to compensate this