Haemopoiesis

Question 1

Define haemopoiesis?

Answer 1

Formation of blood cells

Question 2

Function of red cells?

Answer 2

Erythrocytes (no nucleus) - O2 / CO2 transport

Question 3

Function of platelets?

Answer 3

No nucleus

Primary haemostasis

Question 4

Functions of white cells?

Answer 4

Granulocytes:
• Neutrophils - phagocytosis and mediate acute inflammation
• Eosinophils - destroy parasites and modulate hypersensitivity reactions
• Basophils - modulate hypersensitivity reactions

Monocytes (differentiate into macrophages):
• Modulate immune reactions
• Phagocytic clearance
• Regulatory functions

Lymphocytes:
• B cells - humoral immunity (antibodies)
• T cells - cell-mediated immunity and regulatory functions
• Natural Killer (NK) cells - anti-viral / tumour

Question 5

Various processes by which the blood cells are formed?

Answer 5

Haemopoiesis - production of blood cells; umbrella term for the rest

Erythropoiesis

Thrombopoiesis

Myelopoiesis or granulopoiesis

Lymphopoiesis

Question 6

Level of productivity of red cells?

Answer 6

x 10(12) /L - red cells are the most abundant blood cell

Lifespan is ~120 days

Question 7

Level of productivity of neutrophils?

Answer 7

x 10(9) /L

Lifespan is ~7-8 hours

Question 8

Level of productivity of platelets?

Answer 8

x 10(9) /L

Lifespan if ~7-10 days

Question 9

Ancestry of rbcs?

Answer 9

1. Pronormoblast
2. Early normoblast
3. Intermediate normoblast
4. Late normoblast
5. Reticulocyte - immediate red cell precursors that cause polychromasia; they lose their RNA over 1-7 days
6. Mature rbc

Question 10

How do platelets form?

Answer 10

Cytoplasmic fragments of megakaryocytes

NOTE - megakaryocytes are polypoid cells that are platelet precursors

Question 11

What are ‘blast’ cells?

Answer 11

Nucleated precursor cell:
• Erythroblast - rbc precursor
• Myeloblast - neutrophil lineage

Question 12

What are myelocytes?

Answer 12

Nucleated precursor between neutrophils and blasts

Question 13

Ancestry of blood cells, i.e: precursors of the precursors?

Answer 13

Haemopoietic progenitor cell; progenitors and, ultimately, all haemopoietic cells come from haemopoietic stem cells

All of the haemopoietic stem progenitor cells look the same and, on microscopy, cannot differentiate those that will give rise to one type of cell from another

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Question 14

Properties of the pluripotent stem cells?

Answer 14

Self-renewal - a property of stem cells that is lost in its descendants

Proliferation - increase in numbers

Differentiation - the descendants go through lineage-commitment, where they choose a certain lineage:
• Myeloid (granulocytes, monocytes, megakaryocytes, and dendritic cells)
• Lymphoid (T cells, B cells, NK cells)

Maturation - descendants acquire functional properties and may stop proliferation; e.g: reticulocytes, mature rbcs, neutrophils cannot proliferate

Apoptosis - descendants undergo cell death

NOTE - there is an overlap between developmental event, e.g: proliferation and maturation can occur simultaneously

Question 15

Define myeloid?

Answer 15

May refer to bone marrow, e.g: myeloid/marrow malignancy

May refer to lineage, e.g: non-lymphoid

Granulocytes and precursors - myeloid : erythroid ratio

Question 16

Sites from which haemopoietic stem cells are derived?

Answer 16

From the mesoderm

Circulating committed progenitors are detectable as early as week 5

Question 17

Sites of haemopoiesis throughout life?

Answer 17

Yolk sac - 1st site of erythroid activity; stops by week 10

Liver - starts by week 6

Spleen - starts by week 12; it likely only contributes a small amount in humans

Bone marrow - starts by week 16 and continues throughout life

Question 18

Location of bone marrow in adults?

Answer 18

Mainly the axial skeleton but also the proximal ends of long bones, e.g: humerus and femur

Question 19

Where are bone marrow biopsies taken from?

Answer 19

In adults, from the posterior iliac crest

In children,

NOTE - specific injections are administered in order to mobilise stem cells from the bone marrow into the blood, for collection and transplantation

Question 20

Structure of bone marrow?

Answer 20

Complex organ surrounded by a shell of bone, with a neurovascular supply

Trabecular bone projects into the bone marrow and this form the endosteum, which is the interface between bone and bone marrow

Question 21

Compartments in the bone marrow?

Answer 21

Cellular:
• Haemopoietic cells
• Non-haemopoietic cells, e.g: adipocytes, fibroblasts, osteoclasts, osteoblasts

Connective tissue matrix

Vascular elements

Question 22

Describe bone marrow vasculature

Answer 22

Supplied by a nutrient artery and a periosteal network’ arterioles drain into sinuses; these venous sinuses drain into larger central sinuses

NOTE - compared to capillaries, sinuses are larger and have a discontinuous basement membrane, with open pores, increasing the permeability

Question 23

Structure of the bone marrow sinus walls?

Answer 23

Endothelial cells have gaps between then called fenestrations

At some regions, the endothelial cells form tight junctions, for tighter control of entry/exit into the sinus

Adventitial cells can contract to change become smaller

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Question 24

How are mature blood cell released from marrow?

Answer 24

Can pass through fenestrations in endothelial cells, to enter the circulation

Question 25

Changes that occur due to the release of red cells?

Answer 25

Assoc. with sinusoidal dilatation and increased blood flow

Question 26

Where do neutrophils go once released from the marrow?

Answer 26

Actively migrate towards the sinusoid

Question 27

What do megakaryocytes do once released?

Answer 27

Extend long branching processes, called proplatelets, into the sinusoidal blood vessels

Question 28

Difference between red and yellow marrow?

Answer 28

Red marrow - haemopoietically active Yellow marrow - fatty, inactive marrow; increases with age and this results in reduced marrow cellularity in older individuals

Question 29

Calculating cellularity?

Answer 29

100 - age = cellularity (%)

Question 30

What is the myeloid : erythroid ratio?

Answer 30

Relationship of neutrophils and precursors to proportion of nucleated red cell precursors

Question 31

Normal myeloid : erythroid ratio?

Answer 31

Ranges from 1.5 : 1 to 3.3 : 1 This can change, e.g: reversal in haemolysis as a compensatory response

Question 32

Regulation of haemopoiesis?

Answer 32

1. Intrinsic properties of cells, e.g: stem cells vs progenitor cells vs mature cells; different cells have different transcription factors 2. Signals from immediate surroundings and the periphery, i.e: microenvironmental factors

Question 33

Examples of regulation of haemopoiesis?

Answer 33

Erythroid maturation occurs around 'nurse' macrophages in the form of islands Neutrophil and its precursor maturation is regulated by G-CSF (granulocyte-colony stimulating factor) Thrombopoietin regulates growth and development of megakaryocytes from their precursors

Question 34

Microenvironmental regulation of haemopoiesis (for stem cells)?

Answer 34

Haemopoietic stem cells occupy a niche (anatomical site) that provides signals for expansion, differentiation or dormancy These niche areas are around vasculature (arteriole or sinusoid) and provide access to cytokines NOTE - the niche can be altered in disease states or with therapy

Question 35

Investigations used to assess haemopoiesis?

Answer 35

Routinely undertaken: • Blood count • Cell indices • Morphology (blood film) ``` Less common (specialist tests): • Bone marrow examination ```

Question 36

Appearance of blood cells on blood film?

Answer 36

i.e: ID cells using their morphology ADD IMAGE NOTE - monocytes and lymphocytes are mononucleocytes, i.e: they do not have segmented nucleus

Question 37

Investigating mature cells?

Answer 37

Blood count and morphological assessment are often sufficient Expression of antigens indicating the lineage or stage of development can be studied but not usually required Study of antigen expression using specific antibodies (immunophenotyping)

Question 38

How do we assess haemopoiesis?

Answer 38

Haemopoietic progenitor/stem cells are morphologically indistinguishable from one another 1. Immunophenotyping 2. Cytochemistry 3. Clonogenic assays 4. Animal models

Question 39

What is immunophenotyping?

Answer 39

Identifies pattern of antigen expression unique to a cell lineage; it uses combinations of antibodies that are specific to different antigens

Question 40

Process of immunophenotyping?

Answer 40

Antibodies are used to target various antigens on cells; these antibodies have fluorochrome on them, which emits lights of different colours when hit with a laser Software (flow-cytometry) then detects the colour of light emitted E.g: when a specific antibody binds to CD3 on T-lymphocytes, green light is emitted When a specific antibody binds to CD34 on haemopoietic stem cells, red light is emitted ADD IMAGE