Haematopoiesis

Question 1

What is haematopoiesis?

Answer 1

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.

Question 2

What is homeostasis?

Answer 2

Blood cell homeostasis requires balance between production and destruction.

Question 3

How does the body maintain the high output of blood cells?

Answer 3

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.

Question 4

How were HSCs identified?

Answer 4

Bone marrow was isolated 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.

Question 5

How were HSCs classified after experimenting?

Answer 5

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.

Question 6

How do HSCs give rise to all blood cell types?

Answer 6

Via committed oligopotent progenitor cell intermediates.
Oligopotent stem cells are myeloid and lymphoid progenitor cells.

Question 7

What are myeloid progenitor cells?

Answer 7

Produce differentiated cells:
Megakaryocytes - which form platelets.
Eosinophils.
Basophils
Erythrocytes (RBCs)
Neutrophil
Monocytes - which form dendritic cells and macrophages.

Question 8

What are lymphoid progenitor cells?

Answer 8

Produce differentiated cells:
T cell
B cell - which forms plasma cells.
NK cells

Question 9

How do HSCs produce blood cells and maintain their population?

Answer 9

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.

Question 10

What is symmetric division?

Answer 10

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.

Question 11

What is asymmetric division?

Answer 11

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.

Question 12

How are HSC numbers controlled?

Answer 12

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.

Question 13

How do HSCs divide in a steady state situation?

Answer 13

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.

Question 14

How do HSCs divide when stem cells are lost?

Answer 14

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.

Question 15

How do HSCs divide when blood cells are lost?

Answer 15

The divisions can become more symmetrical, so that both daughter cells produced are ST-HSCs, and differentiate into more blood cells.

Question 16

How are HSCs defined?

Answer 16

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.

Question 17

What markers do other HSCs have?

Answer 17

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.

Question 18

What markers do differentiated blood cells have?

Answer 18

No Kit or Sca1 markers, shows it has lost its stem cell property.
Neutrophil has expression of other cell markers, shows its differentiated.

Question 19

What is primitive haematopoiesis?

Answer 19

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.

Question 20

What is definitive haematopoiesis?

Answer 20

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.

Question 21

How does definitive haematopoiesis continue?

Answer 21

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.

Question 22

How does the site of haematopoiesis change from foetus to adult?

Answer 22

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.

Question 23

What is the HSC environment?

Answer 23

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 of the AGM determines its phenotype of LT-HSC.

Question 24

What is the bone marrow niche?

Answer 24

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.

Question 25

Where are the HSCs in the bone marrow?

Answer 25

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.

Question 26

How do the cellular components of the endosteal niche affect HSCs?

Answer 26

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.

Question 27

How do the cellular components of the perivascular niche affect HSCs?

Answer 27

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.

Question 28

How do molecular components affect HSCs?

Answer 28

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.

Question 29

What metabolic components affect HSCs?

Answer 29

Metabolites - glucose, oxygen concentration, lipid make-up of environment, which are different in relation to blood vessel. Signalling molecules like Calcium and intercellular Ca2+.

Question 30

How does the extracellular matrix affect HSCs?

Answer 30

The makeup of the bone marrow: ECM proteins - collagens, fibronectins Basement membrane - collagen rich. Provide signals and structural information to HSC.

Question 31

What physical factors affect HSCs?

Answer 31

Topography of cells (shape) Shear forces - blood flow past the cells. Elasticity and stiffness of proteins in environment impact how cells grow.

Question 32

How do HSCs differentiate?

Answer 32

HSCs become common myeloid progenitor, which can then become megakaryocyte /erythrocyte progenitor. This progenitor can then become either platelets or erythrocytes.

Question 33

What is the Megakaryocyte pathway?

Answer 33

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

Question 34

What is the erythrocyte pathway?

Answer 34

Colony forming cells generate lots of progenitor Erythroblasts. These become reticulocytes - nucleus free progenitor cells, then become erythrocytes.

Question 35

What molecules are required for control of blood cell differentiation?

Answer 35

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.

Question 36

What are cytokines?

Answer 36

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.

Question 37

What are transcription factors?

Answer 37

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.

Question 38

What are signalling molecules?

Answer 38

Cell specific proteins and pathways which change or regulate cell behaviour.

Question 39

What are the types of cytokines?

Answer 39

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.

Question 40

How do cytokines impact differentiation?

Answer 40

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.

Question 41

What is the interplay of transcription factors?

Answer 41

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.

Question 42

How do cytokine signalling pathways impact differentiation?

Answer 42

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.

Question 43

What is the function of red blood cells?

Answer 43

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.

Question 44

What is red blood cell differentiation?

Answer 44

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.

Question 45

What is the function of platelets?

Answer 45

Megakaryocyte differentiation drives the formation of cells adapted to carry out haemostasis and stop bleeding.

Question 46

What is megakaryocyte differentiation?

Answer 46

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.

Question 47

How are platelets released?

Answer 47

The megakaryocytes have projections called pro-platelets. These fragment off and form thousands of platelets.

Question 48

What are the characteristics of red blood cells?

Answer 48

Lots per blood (compared to platelets) Flattened biconcave disc, no nucleus. Lifespan of 120 days. Transport of O2 and CO2 between tissues and lungs

Question 49

What are the characteristics of platelets?

Answer 49

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.