W4L7 - Haematopoiesis & Bone Marrow Flashcards
Haematopoiesis
The production of the cellular elements of the blood
Haematopoietic cells are derived from haematopoietic stem cells (HSC)
HSC must be able to:
- reproduce themselves in a process of self-renewal
- others differentiate to formed mature blood cells
Haematopoiesis can only occur in specialised sites
Haematopoietic Niche
Haematopoiesis can only occur in a protected environment known as the ‘haematopoietic niche’
The haematopoietic niche provides:
- soluble cytokines
- cell-cell contacts
- insoluble extra-cellular matrix
A lack of any of these components will impair the development of haematopoietic stem cells (HSC)
In the adult bone marrow there is a low frequency of HSCs, with two to five HSCs per 105 total bone marrow cells
Soluble Factors
The proliferation & differentiation of mature blood cells is controlled by many soluble haematopoietic growth factors - stem cell factor - colony stimulating factors (CSFs) - interleukins (IL-1 to 11) WBC - granulocyte (macrophage CSF) - granulocyte CSF - macrophage CSF RBC - erythropoietin Megakaryocytes/platelets - IL-6 - thrombopoietin
Cells Present in the Haematopoietic Niche
Mesenchymal stem/stromal cells Osteoblast, osteoclast Schwann-like cells Endothelial cells Nestin+ mesenchymal cells Adipocytes Macrophages
Haematopoietic Niche Role
Important role of niche cells to regulate
- survival, self renewal, migration and differentiation of HSC
Mechanisms
- cell contact interactions
- production of growth factors (cytokines, chemokines)
- production of extracellular matrix molecules (ECM)
Haematopoietic Stem Cells
LSK (Lin– SCA1+ KIT+) HSC
Three ‘sub-populations’:
1. Long-term repopulating (LTHSC)
2. Short-term repopulating (STHSC)
3. Multi-potential progenitor (MPP) => differentiate
All show different gene expression patterns
100 LSK HSC can provide protection from lethal irradiation
Can also circulate in peripheral blood at low concentration
Haematopoietic Stem Cells - How to Increase Circulating Numbers
Administration of exogenous growth factors
For example, granulocyte colony stimulating factor (G-CSF) can be used clinically as a mobiliser of HSC
Automated Haematology Analysers
E.g. Sysmex analysers
HPC: haematopoietic precursor cells = CD34+ HSC
Used to determine numbers of HPC in peripheral blood
Why is Apoptosis Required in Haematopoiesis?
Counter-balance cell proliferation, i.e. remove cells excess to requirement
Remove imperfect/damaged cells - potentially dangerous if clonal expansion
Active Haematopoiesis
Foetus - 0-2 months - yolk sac - 2-7 months - liver, spleen - 5-9 months - bone marrow, thymus Infants - widespread bone marrow - thymus Adults - vertebrae - ribs - sternum - skull - pelvis - femur
Extra-Medullary Haematopoiesis
Haematopoiesis that occurs ‘outside’ the bone marrow May occur in a range of tissues Typical sites include: - spleen - liver - lung - lymph node Atypical sites: - may occur in any location Detected by imaging and biopsy
Bone Marrow Components
Haematopoietic cells - erythroid - myeloid - lymphoid - megakaryocytic Fat cells (adipocytes) Stromal cells - mesenchymal population Endothelial cells - capillaries - venules - sinusoids Bone cells - osteoblasts - osteoclasts Macrophages
Bone Marrow - Normal Characteristics
Mixed adipose & haematopoietic tissue All cell lines represented - erythroid, myeloid, megakaryocytic Myeloid:Erythroid ratio 1.5 - 4.0 : 1 Orderly maturation sequence
Erythropoiesis in Bone Marrow
Occurs in within erythroblastic islands (E.I.) within the bone marrow
Range of stages of erythroid development present within E.I.
Central iron-containing macrophage (‘nurse’ cell) provides iron for haeme formation
Enucleation of RBC Precursors
Orthochromic normoblasts Occurs within bone marrow Mediated by microtubules & actin-myosin Rapid process (~10 minutes) Produces: - reticulocyte - pyrenocyte
Morphological Changes with Maturation - Granulocytopoiesis
Myeloid cells become smaller
N:C ratio decreases
Nucleus
- nucleus becomes smaller & segments form
- nucleolus disappears
- chromatin pattern becomes denser
Cytoplasm
- becomes lighter (less basophilic) in colour
- (specific/secondary) granules form & increase density
Haematopoiesis - Monocytopoiesis
Monocytes develop from the pluripotent stem cell, via the myeloid stem cell
Monocytic precursors not typically found in the peripheral blood
Developmental stages:
- monoblast
- promonocyte
- monocyte
Megakaryocytopoiesis / Thrombopoiesis
Stages of development
- megakaryoblast
- promegakaryocyte
- megakaryocyte (largest cell in bone marrow and have many nuclei)
- mature platelet (cytoplasmic fragments from megakaryocyte)
Megakaryocytes are resident in the bone marrow
From megakaryoblast to megakaryocyte, these are present in bone marrow, whereas mature platelets found in circulation
Thrombopoietin primarily influences the production & maturation
Megakaryoblast to platelets take ~5 days
Platelets Information
Average lifespan ~ 9.5 days
~2/3 circulate in the blood
~1/3 sequestered in the spleen
Early release platelets are larger than more mature platelets, often contain RNA
Haematopoiesis - Lymphopoiesis
Stages of lymphocyte development - lymphoblast - prolymphocyte - lymphocyte (small lymphocyte, large lymphocyte) B lymphocyte => plasma cell
Clinical Assessment of Bone Marrow
Must be interpreted with a concurrent FBC
Consider:
- cellularity
- cells present
- relative numbers of 3 cell lines; e.g. M:E
- maturation sequence (orderly?)
- cell morphology
Bone Marrow - Decreased Production
‘Hypoproliferative disorders’ - erythroid hypoplasia - myeloid hypoplasia - megakaryocytic hypoplasia Combined hypoplasia (2 or 3 lines) - bilineage hypoplasia - trilineage hypoplasia Reflected in the peripheral blood - anaemia - leukopenia - thrombocytopenia - bicytopenia - pancytopenia
Pancytopenia
Typical profile for a patient receiving myelosuppressive therapy
Severe reduction in the neutrophil count
Moderate normocytic anaemia
Thrombocytopenia
Bone Marrow - Reasons for Decreased Production
Congenital - fanconi anaemia Autoimmune disease - pure red cell aplasia - idiopathic aplastic anaemia Radiation, chemical, drugs - cytotoxic chemotherapeutic agents - lithium - choramphenicol
Bone Marrow - Increased Production
‘Hyperproliferative disorders’ - erythroid hyperplasia - myeloid hyperplasia - megakaryocytic hyperplasia Combined hyperplasia (2 or 3 lines) - bilineage hyperplasia - trilineage hyperplasia Reflected in the peripheral blood - erythrocytosis - leukocytosis - thrombocytosis
Bone Marrow - Increased Production due to Reactive Changes
Appropriate production i.e. in response to stimulus
E.g. myeloid hyperplasia in response to inflammation
- IL-1, IL-6 => increased myelopoiesis, increased M:E
E.g. erythroid hyperplasia in response to anaemia
- Epo => increased erythropoiesis, decreased M:E
Bone Marrow - Increased Production due to Neoplastic Changes
Inappropriate production Spectrum of malignancy Myelodysplasia Myeloproliferation Leukaemia
What do the Niche cells do with HSCs?
Niche cells act to control HSC
Effects
- cause cells to become quiescent/inactive and act as a storage
- promote self renewal
- promote asymmetrical division => differentiation
Stages of Erythropoiesis
- Pronormoblast
- Basophilic normoblast
- Polychromatophilic normoblast
- Orthochromic normoblast
- final stage with a nucleus - Reticulocyte (polychromatophilic RBC)
- Mature RBC
Stages of Granulocytopoiesis
- Myeloblast
- Promyelocyte
- myeloblast and promyelocyte don’t have distinguishing features that tells us whether it will form a neutrophil, eosinophil or basophil - Myelocyte
- Metamyelocyte
- Band nucleus
- Mature granulocytes
- neutrophils, eosinophils, basophils
