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1. Hematopoiesis
Usman Ali Shams
MLS M-Phil HaematoTechnology
University of Health Sciences Lahore

2. Learning Outcomes
At the end of session, students will be able to
• Understand about hematopoiesis and hematopoietic organ
• Describe the hematopoietic stem cells
• Explain about regulation of hemopoiesis
• Understand about the apoptosis

3. Contents
• What is meant by haematopoiesis.
• Sites of haematopoiesis.
• BM microenvironment.
• Haematopoietic stem/Progenitor cells.
• Regulation of haematopoiesis.
• Haematpoietic Growth Factors.
• Growth factor Receptor and Signal Transduction
• Adhesion molecule
• Cell Cycle
• Transcription Factor
• Apoptosis
• Kahoot Play

4. Is the process involved in the production of all blood cells from the
pluripotent stem cell; The hematopoietic stem cells (HSC).
What is meant by Hematopoiesis?

5. • Yolk Sac: In the first few weeks of gestation, the yolk sac is the main site of
hematopoiesis, however, definitive hematopoiesis derives from a population of stem
cells first observed on the dorsal aorta termed the aorta-gonads-mesonephros (AGM)
region.
• These Common Precursor of Endothelial and hematopoietic cells will proceed the
process to Liver, Spleen and Bone marrow.
• This initial hematopoiesis is termed primitive hematopoiesis and serves a supportive role
to rapidly produce erythroid cells, platelets, and macrophages prior to the formation of
the circulatory system.
Hematopoietic Organs

6. • Fetal liver and spleen: from 6 weeks until 6 – 7 months of fetal life they
act as Hematopoietic organ and continue until about 2 weeks after
delivery. And placenta also contribute to fetal hematopoiesis.
• Moreover, the liver and spleen can resume their fetal haemopoietic role
(‘extramedullary hemopoiesis’).
Hematopoietic Organs CONT….

7. • Bone marrow: BM becomes the main site of hematopoiesis from the 6 – 7
months of fetal life, which is distributed through the human skeleton but
gradually recedes with time so that in normal adult life, hematopoiesis is found
mainly in flat bones (sternum, pelvis, vertebrae, ribs and proximal ends of long
femur.
• During normal childhood and adult life the marrow is the only source of new
blood cells.
• The developing cells are situated outside the bone marrow sinuses; mature
cells are released into the sinus spaces, the marrow microcirculation and so
into the general circulation.
Hematopoietic Organs CONT….

8. • In Infancy period all of the bone marrow is Haemopoietic but with passage of
time there is a progressive conversion of fats in marrow occur at long bone so
that in adult healthy life Hemopoiesis is confined to Central Skeleton and
Proximal Ends of Femurs and Humeri.
• Even in these haemopoietic areas, approximately 50% of the marrow consists
of fat.
• And these have capability to reverse to the normal Hematopoiesis Process.
Hematopoietic Organs CONT….

9. Hematopoietic Organs CONT….

11. Who are the HSCs?
• All hematopoiesis derives from a pool of undifferentiated cells,
hematopoietic stem cells (HSCs), which give rise to all bone marrow cells
by the processes of proliferation and differentiation
• HSC is the mother cell for all types of blood cells. It is a pluripotent cell
capable of self renewal and differentiation.
• The stem cell compartment is the smallest of the hematopoietic precursor
compartments, comprising only ~0.5% of the total marrow nucleated cells.
• HSCs are capable to traffic around the body and are found in low number in
the peripheral blood.

12. • However, these rare cells are capable of regenerating the entire hematopoietic system. Thus,
they are defined as multipotential precursors because they maintain the capacity to give rise to
all lineages of blood cells.
• The other defining characteristic of stem cells is their high self renewal capacity The property
of self renewal keeps the marrow cellularity (the stem cell pool) constant in a steady state in
healthy individuals
• one stem cell is capable of producing about 106 mature blood cells after 20 cell divisions.
Hematopoietic stem Cells (HSCs) CONT…

14. • Although its exact phenotype is unknown, on immunological testing
the HSC is CD34+ CD38− and negative for lineage markers (Lin−)
and has the appearance of a small or medium‐sized lymphocyte.
• HSCs are localized at the bone–BM interface (Osteoblastic niche) and
around the blood vessels (vascular niche).
Hematopoietic stem Cells (HSCs)

15. Haemopoietic Progenitor Cells.
• To gain the cell demand impose on Hematopoietic system, some stem cells from the HSC
compartment initiate differentiation.
• As HSCs divide, they generate populations of differentiating cells that
have an increasingly limited capacity to self-renew and are gradually
more restricted in differentiation options.
• The Haemopoietic Progenitor Cells compartment is larger than the HSC compartment, constituting
~3% of the total nucleated hematopoietic cells.
• Haemopoietic Progenitor Cells do not possess self-renewal ability; in general, their process of cell
division is linked to differentiation. They are, in essence, transit cells said to be on a “suicide”
maturation pathway (because full maturation and differentiation result in a terminally differentiated
cell with a finite life span e.g RBC 120 days).
• The primary role of progenitor cells is to replace dead or damaged cells.

16. Bone marrow architecture/microenvironment:
• Bone marrow (BM) is a hematopoietic organ that resides within the protected confines of the bones
and is the major location for hematopoiesis.
• The bone marrow forms a suitable environment for stem cell survival, self‐renewal and formation of
differentiated progenitor cells.
• In adults, active BM is distributed throughout the vertebrae, ribs, pelvis, skull and proximal ends of
the long bones.
• It is composed of stromal cells and a microvascular network.

17. • Stromal Cells: The stromal cells include mesenchymal stem cells, adipocytes, fibroblasts,
osteoblasts, endothelial cells and macrophages
• Stromal cells secrete extra-cellular molecules as collagen, glycoproteins (fibronectin and
thrombospondin) and glycosaminoglycans (hyaluronic acid) to form the extracellular matrix.
• Additionally, stromal cells secrete several growth factors necessary for stem cell survival and
function.
• Mesenchymal stem cells are critical in stromal cell formation.
• Together with osteoblasts or endothelial cells they form niches and provide the growth
factors, adhesion molecules and cytokines which support stem cells,
• e.g. the protein jagged, on stromal cells, binds to a receptor NOTCH1 on stem cells
which then becomes a transcription factor involved in the cell cycle.
Bone marrow architecture/microenvironment:

19. • Hemopoiesis begin with stem cell division in which one cell replaces the stem cell
(self‐renewal ) and the other is committed to differentiation.
• Which cell lineage is selected for differentiation may depend both on chance and on the
external signals received by progenitor cells.
• Several transcription factors regulate survival of stem cells (e.g. SCL, GATA‐2,
NOTCH‐1), whereas others are involved in differentiation along the major cell lineages.
• For instance,PU.1 and the CEBP family commit cells to the myeloid lineage, whereas
GATA‐2 and then GATA‐1 and FOG‐1 have essential roles in erythropoietic and
megakaryocytic differentiation.
• The transcription factors induce synthesis of proteins specific to a cell lineage.
• For example, the erythroid‐ specific genes for globin and haem synthesis have binding motifs for
GATA‐1.
Regulation of Hemopoiesis

20. Regulation of Hemopoiesis

22. Regulation of Hemopoiesis
There should be a balance between cell production and cell death except at the times of requirement
Controlled cell
death
Controlled cell
production

23. • The haemopoietic growth factors are glycoprotein hormones that regulate the
proliferation and differentiation of haemopoietic progenitor cells and the function of
mature blood cells.
• They may act locally at the site where they are produced by cell–cell contact or circulate
in plasma.
• They also bind to the extracellular matrix to form niches to which stem and progenitor
cells adhere.
• The growth factors may cause cell proliferation but can also stimulate differentiation,
maturation, prevent apoptosis and affect the function of mature cells.
Hemopoietic Growth Factor

24. Hemopoietic Growth Factor Functions

25. • They share a number of common properties and act at different stages of hemopoiesis.
• Stromal cells are the major source of growth factors except for erythropoietin, 90% of which
is synthesized in the kidney, and thrombopoietin, made largely in the liver.
• Two or more factors can synergize in stimulating a particular cell to proliferate or
differentiate.
• i-e SCF and FLT3 ligand (FLT3‐L) act locally on the pluripotential stem cells and on early myeloid
and lymphoid progenitors
• Action of one growth factor on cell may stimulate production of another growth factor or
growth factor receptor.
• G‐CSF and thrombopoietin enhance the effects of SCF, FLT‐L, IL‐3 and GM‐CSF on survival and
differentiation of the early haemopoietic cells
Hemopoietic Growth Factor CONT……
FLT. Fms like tyrosine kinase, SCF. Stem cell factor, GM-CSF Grnanulocyte Monocyte colony stimulating factor, IL-3 Interleukin 3

27. • These factors maintain a pool of haemopoietic stem and progenitor cells on which later‐acting factors,
erythropoietin, G‐CSF, macrophage colony‐stimulating factor (M‐CSF), IL‐5 and thrombopoietin, act to
increase production of one or other cell lineage in response to the body’s need.
• Granulocyte and monocyte formation, for example, can be stimulated by infection or inflammation through
release of IL‐1 and tumor necrosis factor (TNF) which then stimulate stromal cells to produce growth
factors in an interacting network.
Hemopoietic Growth Factor CONT……

28. Hemopoietic Growth Factors

29. Growth Factor Receptors And Signal Transduction
• The biological effects of growth factors are mediated through specific receptors on target
cells.
• Many receptors (e.g. erythropoietin (epo) receptor (R), GM-CSF‐R) are from the
hematopoietic receptor superfamily which dimerize after binding their ligand.
• Dimerization of the receptor leads to activation of a complex series of intracellular signal
transduction pathways, of which the three major ones.
• JAK/STAT (Janus-associated Kinase / Singal Transducer and Activation Of Transcription)
• The Mitogen‐activated Protein (MAP) Kinase.
• The Phosphatidylinositol 3 (Pi3) Kinase

30. Mechanism of Pathways
• A growth factor molecule binds simultaneously to the
extracellular domains of two or three receptor molecules,
resulting in their aggregation.
• Receptor aggregation induces activation of the JAKs
which now phosphorylate members of the signal
transducer and activator of transcription (STAT) family of
transcription factors.
• This results in their dimerization and translocation from
the cell cytoplasm across the nuclear membrane to the
cell nucleus.
• Within the nucleus STAT dimers activate transcription of
specific genes expression at G2 site.
• Importance: The clinical importance of this pathway is
revealed by the finding of an activating mutation of the

31. Mechanism of Pathways
• JAK can also activate the MAPK pathway, which is
regulated by RAS and controls proliferation.
• PI3 kinases phosphorylate inositol lipids which have a
wide range of downstream effects including activation of
AKT leading to block of apoptosis and other actions.

32. • Family of Glyco-Protein.
• Enhance attachment of marrow precursor, leucocytes & platelets to various components of the
extracellular matrix, to endothelium, to other surfaces and to each other
• The adhesion molecules on the surface of leucocytes are termed receptors and these interact
with proteins termed ligands on the surface of target cells, e.g. endothelium.
• Importance: The adhesion molecules are important in the development and
inflammatory and immune responses, and in platelet–vessel wall and leucocyte–vessel wall
interactions.
• The adhesion molecules may also determine whether or not cells circulate in the bloodstream
or remain fixed in tissues.
• They may also partly determine whether or not tumor cells are susceptible to the body’s
immune defenses.
Adhesion Molecules

33. • The cell cycle, is a complex process that lies at the heart of hemopoiesis.
• The duration of the cell cycle is variable between different tissues, but the basic
principles remain constant.
• The cycle is divided in the following two way.
Cell Cycle

34. Cell Cycle M Phase

35. Cell Cycle Interphase
• The number of cells at each stage of the cell cycle can be assessed by exposing
cells to a chemical or radiolabel that gets incorporated into newly generated DNA
or by flow cytometry.

36. Cell Cycle Controlling
• The Cell cycle is controlled by two checkpoints which exist at
the End of G1 and G2 phase.
• Two major class of molecules control these checkpoints
1. Cyclin‐dependent Protein Kinases (Cdk), which phosphorylate
downstream protein targets,
2. Cyclins, which bind to Cdks and regulate their activity.

37. Transcription factors
• Transcription factors regulate gene expression by controlling the transcription
of specific genes or gene families.
• They contain two major Domain.
• 1. DNA‐Binding Domain, such as a leucine zipper or helix–loop–helix motif which binds to
a specific DNA sequence.
• 2. Activation Domain: which contributes to assembly of the transcription complex at a gene
promoter.
• Mutation, deletion or translocation of transcription factors underlie many cases of
hematological neoplasms

38. Apoptosis
• Apoptosis (programmed cell death) is a regulated process of physiological cell death in
which individual cells are triggered to activate intracellular proteins that lead to the death
of the cell.
• Apoptosis is an active process which involves the activation, expression, and regulation
of a series of genes.
• Apoptosis is a highly regulated and controlled process that confers advantages during an
organism’s life- cycle.
• Apoptosis have the Following steps
• 1) Cell shrinkage
• 2) Organelle reduction
• 3) Mitochondrial leakage
• 4) Chromatin condensation
• 5) Nuclear fragmentation
• 6) Membrane blebbing & changes

39. • Apoptosis results from the action of intracellular cysteine proteases called CASPASES
which are activated following cleavage and lead to endonuclease digestion of DNA and
disintegration of the cell skeleton.
• There are two major pathways by which CASPASES can be activated.
 The first is by signaling through membrane proteins such as Fas or TNF receptor via their
intracellular death domain.
 An example of this mechanism is shown by activated cytotoxic T cells expressing Fas
ligand which induces apoptosis in target cells.
 The second pathway is via the release of cytochrome c from mitochondria. Cytochrome c
binds to APAF‐1 which then activates caspases.
 DNA damage induced by irradiation or chemotherapy may act through this pathway.
Apoptosis CONT……

40. Apoptosis CONT……
• The protein p53 has an important role in sensing DNA damage.
• It activates apoptosis by raising the cell level of BAX which then increases cytochrome C release.
• P53 also shuts down the cell cycle to stop the damaged cell from dividing.
• The cellular level of p53 is rigidly controlled by a second protein, MDM2.
• Apart from enhancing apoptosis there are several intracellular proteins that protect cells from
apoptosis.
• The best characterized example is BCL‐2.
• BCL‐2 is the prototype of a family of related proteins, some of which are anti‐apoptotic.

41. • Many of the genetic changes associated with malignant disease led to a reduced rate of
apoptosis and hence prolonged cell survival.
• The clearest example is the translocation of the BCL‐2 gene to the immunoglobulin heavy
chain locus in the t(14;18) translocation in follicular lymphoma.
• Overexpression of the BCL‐2 protein makes the malignant B cells less susceptible to
apoptosis.
Apoptosis CONT……

43. References
• Hoffbrand’s essential haematology / A. Victor Hoffbrand, Paul A. H. Moss. —
eight edition.
• Wintrobe's clinical hematology / editors, John P. Greer, Daniel A. Arber,Bertil
Glader, Alan F. List, Robert T. Means Jr., Frixos Paraskevas, George M. Rodgers
; editor emeritus, John Foerster. —13th edition.
• Clinical laboratory hematology / Shirlyn B. McKenzie, J. Lynne Williams,
Kristin Landis-Piwowar. — Third edition.