Haematopoiesis Lecture 4

Question 1

Descirbe Haemopoietic stem cells (HSCs)

Answer 1

HSCs are multipotent stem cells which can diffentiate into and give rise to different cell types via commited oligopotent progenitor cell intermediates.

HSCs are self renewing and proliferative and can replicate by symmetric (forms Long term HSCs, can self renew) or asymmetric divsion (produces Long term and short term HSC, which can self renew and commnit to formaing all blood cells)

Intrinsic and extrinsic factors regulate HSC numbers.

HSCs express different cell surface marker proteins (FACS to find sort out markers)

Question 2

Describe the site of haematopoesis

Answer 2

During early development, it occurs in the yolk sac (primitive haematopoesis).

Following onset of circulation, primitive HSCs go to the AGM region where they form HSC, these then populate the foeatal liver, thymus and spleen which take over haematopoesis (definitive haematopoesis)

Within adult haematopoesis occurs within bone marrow which takes over at birth.

Question 3

The HSC niche

Answer 3

the bone marrow niche supports self renewal and commitment to diffrentiation has several components.

Cellular components

Molecular components - secreted ligands thier recpeotrs, extracellular matrix, adhesion molecules and chemical gradients

When HSCs are closer to bone, they are dormant as they move into the bone marrow they come into contact with other molecules which signals and then makes this more active.

Question 4

Control of Megakaryocyte diffrentiation

Answer 4

The Megakaryocyte is highly specilised cell that is responsible for the prodctuojn of platlets and their release into the blood.

Megakaryocytes devleop in blone marrow from HSCs and undergo characteristic changes as they develop into mature Megakaryocytes.

Megakaryocyte diffrentaition is controlled by cytokines (Tpo), transcrption factors (GATA-1 , FOG-1, FLI-1) and signalling molecules (c-MPL). Also signals from bone marrow niche are important for regulating when and where Megakaryocytes mature.

Many common transcription factors in Megakaryocyte and erythroid diffrentaition but some are specific which allow them to go down each lineage.

Question 5

Describe regulation of GATA1/FOG1 activity

Answer 5

Within Megakaryocytes, Fli-1 is the Megakaryocyte lineage determining factor. Binding of Fli-1 via DNA binding domain to Megakaryocyte (meg) promotor activates transcription of GATA/FOG. But binding of Fli-1 to FOG in erythroid represses transcpritoin.

Within erythroid promotoers, EKLF is the erythroid lineage determining factor. Binding of EKLF via DNA bindnig domain to erythroid promotor activates transcription of GATA/FOG but represses transcrption in Megakaryocytes.

GAT1/FOG1 is involved in platelet surface receptors, cytoskeletal proteins in platelet formation.

Question 6

Describe regulation of Thrombopoietin (TPO)

Answer 6

TPO is added to HSC and it drives it down the Megakaryocyte lineage.

Binding of TPO to cMPL drives signlling pathway which leads to the proliferation and diffrentiation of Megakaryocyte and platelet production.

In regulation of TPO, TPO is made by liver/kidney at constant rate. Circulatory platelets bind to TPO and degrade it to allow control of platelet numbers.

As platelet number increases, there is more degraded TPO due to more platelts, if less TPO present, there are less Megakaryocytes. If platelts decrease, more TPO is released so more Megakaryocytes which can make more platelets.

Question 7

Describe maturation events in Megakaryocyte formation

Answer 7

The TPO induced signalling and gene regulation that drives Meg/platelet formation causes mature Megakaryocyte to become polyploid occuring by endomitsis (the replication of DNA without cell division) causing cells to become very large.

NExt causes cytoplasmic maturation, cytoplasm fills up with membranes, organelles and platelet specific proteisns ready for packaging into platelets.

Proplatlet formation then occurs

Question 8

Describe proplatelet formation from megakaryocytes

Answer 8

After reorganisation of Megakaryocyte cytoskeleton, Microtubles provide force for proplatelet extension. This is achieved via microtubles sliding past eachother due to the action of dynein. Delivery of vesicles, protein and organelles to the newly forming platelets occurs via kinesins and microtubule tracks. Protrusion of proplatelets throgh blood vessel wall requires podosomes and the activty of proteases to degrade extra-cellular matrix components. Proplatelets are then relased into the blood stream where the final stages of platelet formation occur