CONTROL OF HAEMOPOIESISDenise Pegnall

AIM

Demonstrate an understanding of the haemopoietic process

Where haemopoiesis takes place When haemopoiesis takes place Stem cells Stromal microenvironment Cell Cycle

HAEMOPOIESIS

The formation of blood cells High level turnover, need 1013 new cells

daily Bone Marrow contains only 1012

During 70 years, average 70kg human will produce 7 tons of blood cells

Blood cells for lifelong haemopoiesis cannot be preformed on the body

Renewable source

STEM CELLS

Mesenchymal osteoblasts, chondrocytes , adipocytes

Neural, Muscle, In the crypt of the gut, Hair follicle stem cells Haemopoietic stem cells (HSC)

Erythrocytes, platelets, monocytes, neutrophils, eosinophils, basophils, lymphocytes , natural killer cells

Transcription factors commit HSC’s to cell lineages PU-1 GATA-1

STEM CELLSo Important cells for haemopoietic productiono Haemopoietic stem cells, HSC’so Capable of self renewal

o maintain stem cell pool o Differentiation

o progenitor cells of each blood lineageo Regulation of Haemopoiesis starts with stem

cell divisiono One to self renew, One to differentiate

o Enormous proliferative capacityo One stem cell: 106 mature blood cells after

20 divisions o Rare: One stem cell per 20 million nucleated

cells

STROMAL MICROENVIRONMENT

o Stem cells require a suitable environment to grow and divide

o Stromal Matrixo Stromal cells

o Adipocytes, fibroblasts, endothelial cells, macrophages

o Microvascular networko Collagen, glycoproteins, glycosamines

o Stromal cells also secrete growth factors necessary for stem cell survival

o Mesenchymal cells, critical for stromal cell formationo osteoblasts, chondrocytes , adipocytes

BONE MARROW HUMAN STEM CELL

EMBRYONIC HAEMOPOIESIS

Earliest recognizable at 2 weeks Large nucleated red cell precursors

Haemoglobin Gower 1 (ζ2 ε2 ) Leucopoiesis/Thrombopoiesis @6 wks gestation Lymphocytes in lymph sacs@7 wks gestation Primary source foetal cells until @30 wks Liver

Ceases @ 40wks Haemoglobin F (α2 γ2 )

BONE MARROW

Foetal spleen produces blood cells @10 wks Continues through second trimester

Bone cavities from @20 wks gestation Humans Bone marrow sole source of blood

cells by 40 wks Gradual replacement of Hb F with Hb A (α2β2 ) Birth, haemopoietically active marrow fills

every available space

THALASSAEMIA PATIENT WITH FACIAL DEFORMITIES

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HAEMOPOEISIS TO ADULTHOOD

o Childhood marrow vol.increases parallel to increased marrow space made available by growth

o Average 3 year old, 1500ml active marrowo Entirely active & sufficient for needs of adulto As child grows, further expanding marrow

space filled with inactive marrowo Adult ¾ active marrow pelvis, vertebrae,

sternum o Adult, six fold reserve capacity o Extramedullary haemopoiesis

CELL CYCLE

Cell division cycle M. Phase, mitotic phase: division of cell Interphase: duplication of chromosomes

G1: cell begins to commit to replication S phase: DNA content doubles, chr. replicate G2: organelles copied, cytoplasmic vol. Increased

G0 State: resting stage Controlled by two checkpoints

CELL CYCLE

Co-ordinate division end of G1 & G2

Controlled by Cyclin dependent protein kinases (CDK) Cyclins

Majority of HSC’s in quiescent G0 stage Cell cycle dependent drugs, 5’Fluorouracil S phase specific agents

Cytosine arabinoside Hydroxyurea

P53

Quiescent state maintained by Transforming growth factor β (TGFβ) mediated by p53

Tumour suppressor gene Normally, short lived protein present at low

levels in unstressed mammalian cells Under stress, p53 accumulates in the nucleus

& binds to specific DNA sequences

P53

Induces or inhibits expression > 150 genes p21,GADD45,MDM2 ,IGFBP3 ,BAX4

DNA damage, p53 signalling network activated, induces cell cycle arrest, DNA repair and apoptosis

p53 targets enzyme p21, a cyclin dependent kinase inhibitor

p21 regulates activity of cyclin-CDK complexes

Inhibitors of c-CDK’s prevent phosphorylation of retinoblastoma proteins

CELL CYCLE

Retinoblastoma proteins remain bound to transcription factor of E2F family

Therefore genes required for progression of cell cycle not transcribed

Cells remain in quiescence

MAINTENANCE OF STEM CELL QUIESCENCE

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CONTROL OF HAEMOPOIESIS

o Intrinsic, extrinsic or both?o Extrinsic

o Cell-cell interaction in microenvironmento Cytokines

o Stem Cell Factor / receptor c-kito Flt3 ligand/ receptor Flt3

o Intrinsico SCL, Stem cell leukaemia haemopoietic

transcription factoro GATA-2o Both required for haemopoiesis in the yolk sac

E2F

Retinoblastoma