Haematopoiesis Flashcards
What is haematopoiesis?
The production of new blood cells.
Around 1 million produced per second.
If needed, production can increase 5-10 fold in times of need e.g. blood loss.
What is homeostasis?
Blood cell homeostasis requires balance between production and destruction.
How does the body maintain the high output of blood cells?
Through haematopoietic stem cells (HSCs).
These are multipotent stem cells - have the potential to differentiate into multiple, but limited cell types.
Through processes become oligopotent stem cells, then differentiated blood cells.
How were HSCs identified?
Bone marrow was isolate from a mouse, then the donor cells were injected into a mouse that had undergone radiotherapy which killed the bone marrow.
The donor cells can reconstitute the bone marrow and regenerate blood cells, which showed the HSCs were in the bone marrow.
How were HSCs classified after experimenting?
HSCs are bone marrow cells that are able to reconstitute all blood cell formation in irradiated animals.
It gives rise to all blood cell types.
How do HSCs give rise to all blood cell types?
Via committed oligopotent progenitor cell intermediates.
Oligopotent stem cells are myeloid and lymphoid progenitor cells.
What are myeloid progenitor cells?
Produce differentiated cells:
Megakaryocytes - which form platelets.
Eosinophils.
Basophils
Erythrocytes (RBCs)
Neutrophil
Monocytes - which form dendritic cells and macrophages.
What are lymphoid progenitor cells?
Produce differentiated cells:
T cell
B cell - which forms plasma cells.
NK cells
How do HSCs produce blood cells and maintain their population?
HSCs are self-renewing and proliferative, by symmetric and asymmetric division.
Long term HSCs (LT-HSCs) and short term HSCs (ST-HSCs) are in the population - heterogenous.
What is symmetric division?
The mother LT-HSC divides to form 2 identical LT-HSCs.
LT-HSCs can self-renew.
Ensures the population of HSCs in bone marrow remains high.
What is asymmetric division?
The mother LT-HSC produces 1 LT-HSC, and 1 ST-HSC.
This is due to the environment and epigenetic changes.
ST-HSCs have limited self-renewal, and have more capacity to move down the differentiation pathway.
So ST-HSCs form new blood cells.
How are HSC numbers controlled?
Intrinsic and extrinsic factors regulate the balance of symmetric to asymmetric cell division to maintain HSC number and blood cell production.
They convert quiescent LT-HSCs to active LT-HSCs, then control the type of division.
How do HSCs divide in a steady state situation?
When there are plenty of blood cells, they undergo asymmetric division.
This generates both LT-HSCs and ST-HSCs, maintaining HSC number and producing new blood cells.
How do HSCs divide when stem cells are lost?
The divisions become more symmetric so that both daughter cells produced are LT-HSCs.
However, this is at the expense of downstream differentiation into blood cells.
How do HSCs divide when blood cells are lost?
The divisions can become more symmetrical, so that both daughter cells produced are ST-HSCs, and differentiate into more blood cells.
How are HSCs defined?
Defined by the expression of different cell surface marker proteins.
LT-HSCs have surface markers that define their population e.g.
Kit and Sca1 show their stem cell property.
Have low markers that drive differentiation e.g. CD34, Flk2.
What markers do other HSCs have?
ST-HSCs again have Kit and Sca1.
But they have high CD34 expression, which is a marker of the differentiation pathway.
MPP-HSCs have high expression of CD34 and Flk2, shows it is a committed multipotent progenitor.
What markers do differentiated blood cells have?
No Kit or Sca1 markers, shows it has lost its stem cell property.
Neutrophil has expression of other cell markers, shows its differentiated.
What is primitive haematopoiesis?
During early development the yolk sac is responsible for any haematopoiesis, at about 6-8 weeks after fertilisation.
The first blood cells - primitive - are produced from the mesoderm layer of the yolk sac.
What is definitive haematopoiesis?
At 8 weeks onwards, the developing embryo develops blood and blood vessels, and the primitive blood cells migrate to the aorta-gonad-mesonephros (AGM) region.
Definitive HSCs are then formed - have the properties of adult stem cells and go on to make blood.
How does definitive haematopoiesis continue?
The embryo continues to develop organs.
HSCs move from the AGM region to the liver, spleen and thymus.
8 week to 7 month, majority of blood production comes from HSCs located here.
At about 7 months onwards, HSCs move and populate the bone marrow.
How does the site of haematopoiesis change from foetus to adult?
After birth, the bone marrow in all bones becomes the primary site of haematopoiesis in infants.
As the infant develops, only certain bones produce blood cells - ribs, sternum, vertebrae, skull and pelvis.
What is the HSC environment?
The environment of the HSC at all stages of haematopoiesis is important for its function and behaviour.
e.g. in the transition from primary to definitive haematopoiesis, the environment od the AGM determines its phenotype of LT-HSC.
What is the bone marrow niche?
The niche supports self-renewal and commitment to differentiation.
e.g. the HSC being in contact with bone marrow components.
There are cellular components and molecular components, as well as inflammation, extracellular matrix, hypoxia and metabolism, and physical factors.
Where are the HSCs in the bone marrow?
There is an endosteal niche, the hard bony part, that contains osteoblasts and osteoclasts.
The perivascular niche, is the bone marrow, and contains other cells - stromal cells, adipocytes, blood vessels, and HSCs.
How do the cellular components of the endosteal niche affect HSCs?
HSCs in contact with pro-osteoblasts will be receiving specific signals and interactions with the cells, which drives the HSCs to be quiescent.
T reg cells and macrophages feed into this process.
Stem cells in contact with osteoblasts, will receive different signals from the cells, which make the HSCs active to proliferate or differentiate.
How do the cellular components of the perivascular niche affect HSCs?
HSCs that are no longer in contact with the endosteal niche will receive different signals, from blood vessels, neuronal cells, stromal cells and other cells.
These signals will drive differentiation of the HSCs into blood cells.
How do molecular components affect HSCs?
Lots of secreted factors - made from other cells and transported to the bone marrow:
Chemokines
Hormones
Small signalling molecules
Bind to cell receptors and drive phenotype.
Location in the bone marrow will change the combinations.
What metabolic components affect HSCs?
Metabolites - glucose, oxygen concentration, lipid make-up of environment, which are different in relation to blood vessel.
Signalling molecules like Calcium and intercellular Ca2+.
How does the extracellular matrix affect HSCs?
The makeup of the bone marrow:
ECM proteins - collagens, fibronectins
Basement membrane - collagen rich.
Provide signals and structural information to HSC.
What physical factors affect HSCs?
Topography of cells (shape)
Shear forces - blood flow past the cells.
Elasticity and stiffness of proteins in environment impact how cells grow.
How do HSCs differentiate?
HSCs become common myeloid progenitor, which can then become megakaryocyte /erythrocyte progenitor.
This progenitor can then become either platelets or erythrocytes.
What is the Megakaryocyte pathway?
Burst forming unit or colony forming unit, can generate lots of megakaryocytes.
As differentiation carries on, megakaryoblasts are produced, which become megakaryocytes.
Megakaryocytes are the progenitor for platelets
What is the erythrocyte pathway?
Colony forming cells generate lots of progenitor
Erythroblasts. which these become reticulocytes - nucleus free progenitor cells, then become erythrocytes.
What molecules are required for control of blood cell differentiation?
Cytokines
Transcription factors
Signalling molecules
And signals from the bone marrow niche to regulate when and where blood cells mature.
Enables 2 different cells to be produced from the same progenitor.
What are cytokines?
Small specific proteins that control the behaviour of other cell types.
They bind to the receptor on HSCs and start the signalling and gene expression pathways down a cell lineage.
What are transcription factors?
When cytokines are bound to the HSCs, transcription factors bind to specific DNA sequences and regulate gene expression.
This makes other proteins needed to produce the cell.
What are signalling molecules?
Cell specific proteins and pathways which change or regulate cell behaviour.
What are the types of cytokines?
Very complex and many different types.
IL-3 is very common.
EPO is specifically driving erythrocyte differentiation.
TPO is specifically driving megakaryocyte differentiation.
The combination of different factors to produce signalling molecules drive differentiation down particular pathways.
How do cytokines impact differentiation?
IL-1, IL-3, IL-6 are common cytokines for differentiation of blood cells.
TPO or EPO drives differentiation down the megakaryocyte or erythrocyte pathway.
What is the interplay of transcription factors?
Common ones are GATA and Runx-1.
But the combinations of other transcription factors drives differentiation pathways.
e.g. Megakaryocytes and erythrocytes both have GATA-1, but the combination of transcription factors with GATA-1 help drive differentiation.
How do cytokine signalling pathways impact differentiation?
Cytokines bind to cells depending on the receptor type, and activate signalling pathways.
The differences in the pathways drives different gene expression pathways by transcription factors and phenotypes.
It can lead to cell survival, prevents apoptosis, inhibits stages of the cell cycle to allow differentiation.
What is the function of red blood cells?
Red blood cell differentiation drives the formation of cells adapted to carry oxygen around the body.
So the common progenitor cell differentiates to become more and more adapted for oxygen carrying.
What is red blood cell differentiation?
Increased haem synthesis so can carry oxygen.
Increased anaerobic glycolytic pathway, as RBCs have no mitochondria - makes enzymes needed to generate energy.
RBCs have no nucleus, chromatin condense and nuclei decrease in size, nucleus ejected from the cell.
RBCs need to be deformable - develop spectrin membrane skeleton so can get through capillaries, and produces large surface area.
What is the function of platelets?
Megakaryocyte differentiation drives the formation of cells adapted to carry out haemostasis and stop bleeding.
What is megakaryocyte differentiation?
Endomitosis, polyploidisation - cell replicates without dividing so increases the nuclei material, can make thousands of platelets.
Increase in internal membrane system - produces pro-platelets.
Formation of granules, containing agonists to activate clotting factors.
Increased expression of platelet surface receptors - enhance adhesion and aggregation.
Increase in cell size to produce many platelets.
Reorganisation of the cytoskeleton - increased surface area for granule secretion.
How are platelets released?
The megakaryocytes have projections called pro-platelets.
These fragment off and form thousands of platelets.
What are the characteristics of red blood cells?
Lots per blood (compared to platelets)
Flattened biconcave disc, no nucleus.
Lifespan of 120 days.
Transport of O2 and CO2 between tissues and lungs
What are the characteristics of platelets?
Less per blood (than RBCs).
Cellular fragments surrounded by plasma membrane, contains granules and cytoskeleton.
Lifespan of 7 days.
Haemostasis and release of growth factors for tissue repair.
