Erythrocytes and platelets Flashcards
How abundant are RBCs?
Blood makes up 7 – 8% of human body weight. RBCs are the most abundant blood cells and make up ¼ of ALL human cells
Erythropoiesis
The process of the maturation of blood cells, which occurs in the bone marrow. It lasts 7 days and follows hematopoiesis. RBCs go through various stages until they lose their organelles and their nucleus: erythroblasts, reticulocytes, mature, senescent. The cells circulate for 120 days, and then they enter senescence and the cells are removed
RBC function
Transports O2 to all cells & tissues of the body. Each circulation lasts about 20 secs
Deformability of RBCs
RBCs have a discoid shape. They are deformable, so they can exist in many different shapes. This allows them to squeeze through small blood vessels. The flexibility and resiliency of the spectrin cytoskeleton accounts for this property. The resiliency of spectrin allows the RBC to regain its discoid shape and distinguishes young from senescent RBCs. Deformability is especially important when RBCs are traveling through capillaries
How do RBCs transport oxygen?
Mature RBCs do not have organelles, their cytoplasm is filled with hemoglobin. Hemoglobin contains iron, which oxygen binds to
Why are RBCs considered unique cells?
They do not have a nucleus, mitochondria, or organelles. Therefore, they do not have any DNA or RNA and there is no DNA replication, transcription, or translation. However, this optimizes the cells for oxygen transport and carbon dioxide exchange, since there is lots of space to be filled with hemoglobin. RBCs use none of their oxygen, they transport the oxygen they carry. RBCs are considered end stage cells, there is no proliferation since there is no nucleus. The cells have a characteristic biconcave (discoid) shape, which increases the surface area for gas exchange. RBCs obtain energy through glycolysis (the only metabolic process they carry out)
Hemoglobin structure
A tetrameric protein, has 2 α and 2 β chains. Each of the subunits exhibits a tertiary globin fold. Hemoglobin contains a molecule called heme, which is a porphyrin molecule (heterocyclic ring) with a bound iron ion. When it is ferrous state (Fe2+) the iron is capable of binding oxygen. When in its ferric state (Fe3+) it does not bind oxygen until it can release the oxygen it is carrying
Oxygen binding to hemoglobin
Upon binding, oxygen oxidizes ferrous iron (Fe2+) to ferric iron (Fe3+), and the iron will not be able to bind any other oxygen until it goes back to the ferrous state. Carbon dioxide binds to hemoglobin, but not on iron- it binds to protein chains of hemoglobin and therefore does not compete with oxygen for binding sites
Discoid shape of RBCs
The shape is due to the spectrin cytoskeleton. Spectrin is the main structural protein here, but actin acts as a junctional protein. The α subunit of spectrin forms a dimer with the β subunit (end-on-end). The long strands of αβ spectrin form coiled tetramers. This forms a lattice-like cortical network which is strong yet flexible. The RBC membrane is anchored to the spectrin cytoskeleton
Connections of the RBC membrane to the spectrin cytoskeleton
- Band 3-ankyrin-spectrin- membrane protein
- Glycophorin C-protein 4.1-junctional complex. This part of the cytoskeleton contains actin & actin-binding proteins like adducin, dematin, tropomyosin, & tropomodulin
Blood type
ABO antigens determine blood type- this refers to the glycosylation of RBC membrane proteins. The band 3 protein contains these sugars. If someone is transfused with an incorrect blood type, there is a robust IgM response to non-self antigens- hemagglutination, complement lysis. A and B antigens are produced from core oligosaccharide (O). All blood types contain the core oligosaccharide, so the O blood type (universal donor) is not recognized as foreign.
A vs B blood types
The A blood type contains an extra N-acetyl-galactosamine which the O blood type lacks. Type B blood has an extra galactose that the O blood type lacks. Type AB blood has both of those extra sugar groups, so it is considered the universal acceptor
Hemolytic transfusion reactions
This is a host reaction to the transfusion of an incorrect blood type. IgM recognizes the blood cells as foreign and reacts w/ incoming RBCs. IgM recruits the complement, so the MAC forms. MAC kills transfused RBCs. When this process occurs, IgM binds to band 3 and recruits complement component C1q. Ultimately leads to the formation of C5 convertase and the MAC is produced
Bone marrow
All blood cells originate in the bone marrow- there is red and yellow marrow. Yellow marrow is primarily fat deposits, but red marrow contains developing blood cells
Hematopoiesis
The process starts with a multipotent hematopoietic stem cell, which is the model for all blood cells. The model is capable of proliferation and self-renewal. It differentiates to become a multipotent hematopoietic progenitor and goes through more development to become a common lymphoid progenitor. The lymphoid progenitor acts as a model for B cells, T cells, NK cells, and dendritic cells. A common myeloid progenitor can also form, which acts as a model for monocytes, neutrophils, eosinophils, basophils, mast cells, megakaryocytes, and erythrocytes
Hematopoiesis cell development stages (4)
- Starts off with a multipotent hematopoietic stem cell
- Develops into a multipotent hematopoietic progenitor
- This can develop into a common lymphoid progenitor OR a common myeloid progenitor
- These 2 progenitors act as models for specific cells. The myeloid progenitor develops into erythrocytes and platelets
Common myeloid progenitor
Precursor of RBCs. It develops into a megakaryocyte-erythroid progenitor, which will make either megakaryocytes or erythrocytes. To make erythrocytes, we go from hematopoiesis to pre-erythropoiesis.
Pre- erythropoiesis
The cell becomes an early erythroid progenitor. The cytokine IL-3 is a key growth factor here. It then becomes a late erythroid progenitor, where erythropoietin is a key growth factor
Erythropoietin
When more RBCs need to be produced, EPO production is triggered in the kidney. This stimulates late erythroid progenitor to undergo erythropoiesis and become mature RBCs
2 main situations that create a need for more RBCs
- Anemia
- Lack of O2- can be the result of anemia
Erythropoiesis cell development stages (6)
- Proerythroblast
- Early erythroblast
- Intermediate erythroblast
- Late erythroblast
- Reticulocyte
- Mature erythrocyte
Proerythroblasts
Normal-looking cells – nucleus, all organelles present. There is no hemoglobin production yet
Early erythroblasts
All the organelles are still there, but hemoglobin production begins
Intermediate erythroblasts
Accumulation of hemoglobin, organelles are still present
Late erythroblast
Undergo enucleation- the nucleus is removed. Lose some organelles
Reticulocyte
Have some mitochondria, nucleic acid (rRNA), and ribosomes, but they lack a nucleus. These cells circulate at low levels (1-3%) for about 1 day before becoming a mature RBC
Mature erythrocyte
Devoid of all organelles and nucleic acid, filled with hemoglobin
Enucleation process
Caspases 2, 3, and 8 have a role here. Caspase 3 targets nuclear lamins and acinus protein without inducing cell death. The cleavage of lamins leads to the breakdown of the nucleus. Acinus is activated to condense chromatin. DNase 2 is released from nearby macrophages and breaks down DNA. Once the nucleus is fully extruded, it/its pieces are removed by macrophages
RBC senescence
Hemoglobin (HGB) is modified as RBCs age. Modified HGB can begin to bind Band 3, which disrupts its anchorage to spectrin. Band 3 also breaks down, severely interfering with membrane anchoring. Vesiculation ensues, exposes phosphatidylserine and other antigens. Vesicles/cells removed
RBC senescence mechanism
Band 3 is located in the RBC membrane. It is bound to ankyrin in the cytoplasm, which is bound to the spectrin cytoskeleton. During senescence, Band 3 begins to break down, and vesicles form since the membrane is no longer bound to ankyrin and the cytoskeleton. RBCs undergo a shape change and become echinocytes (a cell covered in vesicles). As band 3 breaks down, its structure changes and it is recognized as a different protein. The change in structure is called the senescent cell antigen, which is a key marker of senescent RBCs
Reticuloendothelial system
The recognition and removal of senescent RBCs occurs in the reticuloendothelial system in the liver and spleen. Cords of Billroth are part of this system
Cords of Billroth
Part of the reticuloendothelial system in the spleen. There are small blood sinuses that permeate into this part of the organ. RBCs are delivered through the sinuses into the cords of Billroth. If they are senescent, macrophages in the cords will recognize this. RBCs squeeze through blood vessel endothelial cells to exit the cords and re-enter the blood circulation. Only young RBCs can do this, so senescent RBCs will not be able to re-enter circulation
2 situations where senescent RBCs cannot re-enter circulation
- Opsonized RBCs (with antibodies) due to modification of Band 3. The modified Band 3 is recognized as a senescent cell antigen. Once antibodies are bound, the cells can’t undergo a shape change to re-enter the circulation
- Echinocytes- the membrane has been severely disrupted, a shape change occurs
Sphero-echinocytes
An irreversible shape that occurs as the membrane of echinocytes becomes more severely disrupted
Megakaryocytes
Extraordinarily large cells, up to 60 μm diameter. They come from the same cells as RBCs- erythroid progenitor cells, and megakaryocytes are the precursors of platelets. These cells are polyploid- they have more than 2 paired sets of chromosomes. As the cytoplasmic volume increases, the chromosomes can multiply without cell division. These cells are located in the bone marrow, and full megakaryocytes will become permanent residents of the bone marrow
Platelet production (5)
- Megakaryocytes lie close to blood sinuses in the bone marrow
- They have pseudopod-like extensions that extend between endothelial cells of the blood sinuses. The extensions stick out into the flowing blood
- Due to the force of blood flow, pieces of these extensions break off and are swept away in the blood. These pieces are platelets
- Enough breaks off so that all that is left of the megakaryocyte is the nucleus
- The nucleus can be carried through the blood and removed by alveolar macrophages
Platelets
Anucleate cellular fragments of megakaryocytes that are 2-3 μm. They do have organelles- mitochondria, dense secretory granules, and membranous networks reminiscent of ER. However, they do lack a nucleus. Platelets have a cytoskeletal network. They are capable of typical cell metabolic processes (since they have mitochondria they also have an ETC), whereas RBCs only perform glycolysis
Platelet secretory granules
Secretory granules have a number of components- some are clotting factors that are released and some are molecules that recruit clotting factors- Von Willebrand factor is an example. Other components include cytokines and chemokines, as well as growth factors that will stimulate repair of blood vessels. All of these components are released when the platelets are activated
Primary hemostasis
Platelets play key role – they aggregate to form a platelet-rich clot (white clot). As platelets are activated, they degranulate- the secretory granules begin to release their components. Activated platelets have receptors that recruit a clotting factor called fibrinogen, which helps with the aggregation of platelets
Secondary hemostasis
This is the process where the full blood clot forms- clotting cascade. Fibrinogen & and clotting factors form a fibrin clot (red clot). The final thrombus is a mixture of platelets, fibrin, and RBCs
Primary hemostasis stages (3)
- Rolling adhesion
- Activation
- Thrombus formation
Rolling adhesion- primary hemostasis
As they are activated, platelets exhibit a rolling adhesion to the wall of the damaged blood vessel. A transmembrane glycoprotein called GPIB/IX/V. It is present on the platelets and
binds collagen & von Willebrand factor (vWf). Platelets will only be exposed to collagen if there is a break in the endothelial layer of the blood vessel. This is because collagen is only present in underlying connective tissue, not in the blood. Endothelial cells secrete vWf during injury
Activation- primary hemostasis
Platelets express an integrin called GPIIb/IIIa, which tightly binds to vWf & fibrinogen. This allows for platelet adhesion and aggregation, and the formation of a white clot. Platelets are activated via G-protein coupled receptors (GPCRs)- once they are activated, signaling steps occur that lead to the actual activation of the platelets. This results in degranulation and the platelets acquire a spiny appearance once they are activated
Thrombus formation- secondary hemostasis
Tight adhesion & aggregation. Activated platelets recruit clotting factors to form the strong fibrin clot
Reopro mechanism
Anti blood clotting, monoclonal antibody based medication. Reopro is an antibody that binds to the integrin GPIIb/IIIa so platelet aggregation cannot occur and blood clotting is prevented
Clotting cascade
Secondary hemostasis- what follows primary hemostasis. The stable clot forms during this step. Thrombin cleaves fibrinogen, creating the fibrous protein called fibrin
