Term 2 Lecture 5: Haemopoiesis & Blood Components Flashcards
Humoural immunity
Soluble compounds of the immune system
Cytokines- A low molecular weight, secreted protein that stimulate/inhibit cell differentiation/proliferation
Interleukins - A group of cytokines that enable communication between leukocytes and particularly lymphocytes
Chemokines - structurally related substances that induce chemotaxis and activation of leukocytes
Haemopoiesis: blood cell formation
Haeme = blood
Poeisis = formation
- yolk sac is the primary site during embryological development
- the bone marrow is the major site of blood cell production (haemopoiesis) by 20th week of gestation, increased activity in third trimester of pregnancy
- at birth haematopoietic cells (red marrow) occupy all bone marrow space
- in adults bone marrow stem cells are used for haematopoiesis - regulated by cytokines and growth factors
Haemopoiesis timeline conception to birth
First trimester: yolk sac haemopoiesis (primitive wave) yields nucleated RBCs. Haemopoiesis is extraembryonic occuring in blood islands of yolk sac
Second trimester: HSCs then seem to migrate via blood stream to liver and spleen to seed these tissues, which then carry the burden of hemopoiesis during second trimester
7 months onwards: haemopoiesis in bone marrow
Primitive wave is prehepatic phase
Definitive wave is hematosplenothymic and medullolymphatic phase
Bone marrow
Red: contains stem cells involved in hematopoiesis - present in all bones in a child and in the ribs, cranium, vertebrae, sternum,pelvis and ends of long bones in adults (absent in arms and legs after shoulder/hip joint)
Yellow: contains adipose tissue and is inert, during childhood gradual replacement of red marrow w/yellow fatty marrow occurs
When there is continuous increased hematopoietic demand yellow marrow throughout the body may become red and active again. The spleen and liver are also capable of producing blood cells and are the main site of extramedullary haematopoiesis
Bone marrow, human evolution etc.
HSCs occupy a well protected niche
Bone marrow is a high calorie source providing fuel for expensive organs like the brain. Use of bone marrow fat may have allowed for enlargement of the brain.
Blood stem cells are protected from sunlight irradiation damage by being stored in bone marrow.
NOTE: bearded vultures eat only bone marrow and are not particularly smart so it is not as simple as eating bone marrow = intelligence
Protection from UV is an evolutionarily conserved feature of the haematopoietic niche
Melanocytes protect stem cells present in the kidneys of fish such as in zebra fish. This is viewable by labelling kidney tubule epithelia with green marker protein and labelling stem cells with red markers. In wild-type fish the melanocytes cover the stem cells whereas in mutants w/out melanocytes stem cells remain visible.
Terrestrial animals have their stem cells in bone marrow for increased UV protection
Stem cells
Self renew, differentiate into a range of lineages, slow replication
Types:
Totipotent - stem cell can develop into any tissue type, present in embryo/extraembryonic zone
Pluripotent - can become any of the three dermis layers - endo/ecto/mesoderm
Multipotent - can develop into only a specific lineage (can be manipulated in lab to become pluripotent)
Stem cells to T & B cells
Hematopoietic stem cell
Converts to lymphoid stem cell
Then to T or B precursor
To T cell/ T killer cell or B cell
B cell can further develop into a plasma cell
Stem cell to macrophage/neutrophil
Haematopoietic stem cell
Myeloid stem cell
Granulated macrophage CFU
Then: monoblast>promonocyte>monocyte> macrophage
Or
Myeloblast > promyelocyte > myelocyte>metamyelocyte>neutrophil
Stem cell to eosinophil, basophil or mast cell
Hematopoietic stem cell
Myeloid stem cell
Eosinophil CFU > myeloblast> promyelocyte > myelocyte>metamyelocyte> eosinophil
Or
Basophil CFU>”>”>”>”> Basophil
Or just before becoming basophil become mast precursor and then mast cell
Stem cell to platelets
Haematopoietic stem cell
Myeloid stem cell
Megakaryocyte CFU>megakaryoblast>megakaryocyte>platelets
Stem cell to RBC
Haematopoietic stem cell
Myeloid stem cell
Erythroid CFU > primitive/mature progenitor (react to EPO) > pro erythrocyte>basophilic erythroblast >polychromatiphilis erythroblast > orthochromatic erythroblast (stage where RBC loses it’s nucleus) > reticulocyte>erythrocyte (RBC)
Terminology for blood cell genesis from stem cells
Erythropoiesis - RBC genesis
Thrombopoiesis - platelet genesis
Granulopoiesis - neutrophil/basophil/eosinophil genesis
Lymphopoiesis - lymphocyte genesis
Monopoiesis - monocyte genesis
Erythroid lineage
Erythrocytes (RBCs) most abundant cell type in the blood
Contain haemoglobin (alpha2 and beta 2 chains in adults)
Contain no typical organelles or cytomembranes in cytoplasm IN HUMANS - reptiles amphibians and birds have nuclei in their RBCs
Average lifespan 120 days, senescent RBCs phagocytosed by macrophages in liver and spleen
Lack of O2 (hypoxia) or decrease of erythrocytes to circulating blood (anaemia) caused by excessive destruction of RBCs, bleeding, iron or B12 deficiency leads to stimulation of interstitial cells in renal cortex to synth and release glycoprotein erythroprotein (EPO) into the blood
EPO stimulates early stages of erythroid colony forming unit (CFU) to proliferate and differentiate to basophilic > polychromatiphilic > orthochromatic erythroblast.
The pro erythroblast is the first stage of RBC lineage and derives from mature progenitor (w/ nucleus and free ribosomes for Hb synth) on stimulation of EPO.
Synth of Hb proceeds and as it accumulated the nucleus is reduced in size. Chromatin condenses, free ribosomes decrease cell shrinks and nucleus of cell migrated to outer membrane where it is ejected. The cell then becomes a reticulocyte which finally matures to an RBC. The nuclei are taken up by macrophages.
Reticulocytes
Have no nuclei, methylene blue stain shows a network of strands in the cytoplasm (RNA), these cells are slightly larger than erythrocytes
Erythroblastic island
Most important structural element of erythropoiesis
Found in bone marrow
Composed of a central macrophage surrounded by erythrocyte precursors
Macrophage complexes iron with ferritin and transfers this to the reticulocytes where it becomes Hb
Red bone marrow
Consists of RBCs at diff stages of development and supporting tissue stroma rich in collagen and fibroblasts. Stem cells of white and red BCs & megakaryocytes divide to form platelets. Blood vessels running through the marrow are 1 epithelial cell thick through which mature RBCs can squeeze into the vessel
Large cells need a large nucles
Or multiple nuclei. A large cell cannot function with a small nucleuso
Control of thrombopoiesis
Thrombopoeitin (TPO) controls thrombopoiesis - platelet production
Megakaryocytes have TPO receptors on cell surface membrane as do the platelets as they are formed from this membrane. TPO is made in the liver and platelets bind to it in the blood system, sequester and inactivate it. So v. little TPO reaches the bone marrow and less megakaryocytes are converted to platelets. If there are not enough platelets less TPO is bound and so more reaches the bone marrow and more megakaryocytes are converted to platelets.
Control of erythropoeisis
Erythropoietin EPO controls erythropoiesis - RBC production
EPO is made by kidney cells primarily and also in the liver. Release is triggered by reduced oxygen levels (hypoxia)
Erythropoeisis is also affected by:
Interleukins (IL-1, IL-3, IL-6)
Protein (SF-E - colony stimulating factor)
B12 and folic acid for maturation
Fe, Cu, Zn,Co & vit c for Hb synth
More on erythroprotein EPO
34kd (v. small) glycoprotein produced mainly in kidney peritubular cells
Liver is secondary source
165 aa long and acts as a hormone
Glycosylation is critical for it’s function
As kidneys play a key role in RBC synth, kidney failure can lead to secondary anaemia
Renal tumours cause excess EPO secretion boosting erythropoeisis causing erythrocytosis
Too many RBC lead to infarcts (areas of tissue death) due to blood vessel blockages
EPO receptors are expressed in CFU-E & pronormoblasts.
EPO molecular control
EPO gene has several segments that can sense levels of O (sensor regions)
Under control of a protein of a transcription factor called HIF - hypoxia inducing factor (HIF 2 alpha and beta) and others
Response to hypoxia (in rats) shows increase in EPO mRNA in kidney and liver. Within 60 mins of onset mRNA levels can be amplified >1000 fold in kidney (93% of production is in kidneys)
Boosting performance/ cheating
Athletes can boost their performance by training at high altitudes where low O2 availability increases EPO production which goes to the bone marrow causing increase in RBC production
Cheating: Autologous, homologous transfusions, extract blood, freeze it then use it for transfusion before competition. However as blood is stored in plastic it is possible to test for residues (dioctyl phthalate) in blood
Other:
EPO injection (i.e. recombinant rhipo made in lab)
Prolyl hydroxylase inhibitor (PHD-I EG- 2216)
Erythropoeisis stimulation agents (ERAs) FG4592 (rodadustat)
Synthetic oxygen carriers
Breakdown of blood
Blood is plasma and cells
Cells include
platelets (megakaryocyte fragments)
& RBC/ WBC
WBC can be split into
Granulocytes :
neutrophils, eosinophils, basophils
Agranulocytes:
lymphocytes - B cells and T Cells
monocytes - macrophages
Neutrophils
Phagocytes and granulocytes
Make up 50-80% of circulating leukocytes
Have a single multilobed nucleus
Fine granules contain proteases & antimicrobial effector molecules such as defensins
Form the “front line of defence” the first layer of defense for an organism
Phagocytose bacteria in blood stream
Circulate for 7-10 hours in blood stream
Can leave blood stream by extravasation and infiltrate surrounding tissues remaining for a few days
Pliable nuclei, soft cells can squeeze through cell layers
12-15 micrometres in diameter
Migration to area of infection and ingestion of bacteria requires substances contained in granules
Primary granules contain: elastance & defensins & myeloperoxidase
Secondary granules contain: lysozyme, lactofferin, gelatinise and other proteases
Eosinophils
Stain with eosin hence name
12-15 micrometre diameter
Phagocytic- mainly responsible for killing indigestible parasites by degranulating and dissolving the cell surface.
About 1-4% of circulating leukocytes are eosinophils
Large specific granules appear bright red and clearly discernible
Bilobed nucleus
Granular content:
Eosinophil peroxidase (EP) - binds to microorganisms and facilitates their killing by macrophage
Major basic protein (MBP) - main component of granules, binds to & disrupts membranes of parasites. Causes basophils to release histamine by Ca ²+ mechanism
Eosinophil cationic protein (ECP) - neutralises heparin, causes fragmentation of parasites
Eosinophil driven neurotoxin (EDN) - secretory protein with micronucleus+ antiviral activity
Monocytes/macrophages
Make up 2-8% of leukocytes
Largest leukocyte 15-20 micrometres
Important in phagocytic defence
New monocytes circulate in blood for a few hours before migrating to tissues
In tissues become 5-10x larger and develop into active phagocytic cells - macrophages
Macrophage derived from monocytes are more efficient phagocytic cells than neutrophils
Lymphocytes
Make up 30-40% of leukocytes
Range from 7-12 micrometres
Normally small, similar to RBC size
97% small and 3% large
Nucleus stains densely and is normally round or slightly indented
Nucleus occupies most of the cell reducing cytoplasm to a thin basophilic rim which occassionally has a few lysosomes.
B lymphocytes are produced in bone marrow
T lymphocytes are produced in bone marrow but mature in the thymus.
Less abundant class it T killer cells.
In fetal development yolk sac, liver and spleen are sites where lymphocytes originate from.
In postnatal life the bone marrow and thymus are the primary lymphoid organs where lymphocytes develop before exposure to antigens.
Secondary lymphoid organs are lymph nodes - the spleen and lymphoid aggregates of the gastro-intestinal & respiratory tracts.
