Haematopoeisis and hscs Flashcards
(12 cards)
haemapotieiss
Notes: Haemopoiesis: production of blood under the control of growth factors, inhibitors and microenvironment of the bone marrow playing a role in regulation.
lifelong production that decreases with age. unchecked growth of hscs and immature blood cells results in leukemaia.
in utero: production of blood cells begins in the embyronic yolk sac (14-q9 days up to 2 months)
predominant site during second trimester is the liver and spleen (3-7 months)
from 7 months- bone marrow
bone marrow is in meducally cavity of long and axial bones. in children-present in all bones.
primary site of adult haematopoiesis: periosteul layer covers outer surface. cortical bone is the outer layer of compact bone. trabecular bone is the inner layer of spongy bone. endosteal layer is at the interface of bone and basement membrane.
HSCS ARE MOSTLY DORMAT PROTECTS THEM FROM GENOMIC DAMAGE
components of blood
observed; some data suggested a negative impact of the CCL on outcomes. This data raises questions about development and validation of potency assays for clinical testing of cell products, and lack of US FDA requirements for in vivo testing of intended clinical cell lines. Also used for treating Alzheimer’s and found no clinical efficacy; reinforce the notion that individual human NSC lines need to be carefully assessed for efficacy and safety in appropriate long-term models. - The company had not done a pre-clinical assessment on the CCL (clinical cell line) - The failure of the CCL was reported by Cummings et al to the Company prior to the trial - The data were not shared with patients (the company cited ‘commercial sensitivity’) - The data from an earlier trial have not been published Lecture 8: Haematopoiesis and haematopoietic stem cells Learning outcomes: * Explain the difference between stem cells and progenitor cells * Describe the components of haematopoietic system. * Critically evaluate the important mechanisms of haematopoietic stem cells differentiation. * Critically evaluate the potential therapeutic use of de-differentiation of effector cells. Notes: Haemopoiesis: production of blood under the control of growth factors, inhibitors and microenvironment of the bone marrow playing a role in regulation. What are the Components of the Blood System? Bone marrow and lymphatic system produce red blood cells, white blood cells and coagulation factors - Cells within bone marrow are all at different stages of differentiation. - Pluripotent stem cells can differentiate into common myeloid progenitor cells or common lymphoid progenitor cells - Myeloid = RBC’s, monocytes, neutrophils, eosinophils and basophils (non-specific immune response cells) and platelets - Lymphoid = lymphocytes i.e. T/B cells and NK cells (specific immune response cells) - Red cells make up most of the blood content, due to the presence of the haem-group thus making the blood appear red - Platelets are important in coagulation of the blood during injury and RBCs transport oxygen throughout the body Stem cell: unspecialised cell that can self-renew by mitosis whilst in the undifferentiated state and can give rise to various cell types; they proliferate and differentiate into progenitor cells, committed to one specific cell line. All blood cells are derived from pluripotent stem cells which are supported by stromal cells. Stem cells differentiate into multipotent progenitor cells i.e. myeloid or lymphoid. They then finally differentiate into mature cells that are recognisable, committed marrow precursors e.g. eosinophil.
telomeres
Telomeres Telomere: repeating sequence of double-stranded DNA located at the ends of chromosomes that protects chromosome ends from degradation activities. 1. Greater telomere length is associated with immortalised cell lines such as embryonic stem cells and cancer cells 2. As cells divide and differentiate throughout the lifespan of an organism or cell line, the telomeres become progressively shortened and lose the ability to maintain their length. 3. Telomerase is an enzyme that binds to the end of telomeres and lengthens them by adding repeating sequences of DNA via an RNA template 4. Telomerase adds several repeated DNA sequences then releases second enzyme, DNA polymerase, that attaches to the complimentary strand of DNA to complete the double stranded extension of the chromosome ends 5. High levels of telomerase activity are detected in embryonic SC and cancer cells, whereas little or no telomerase activity is present in most mature, differentiated cell types 6. Telomerase gives us the ability to have a good support for our adult stem cells because we can replace existing dying cells through the differentiation of stem cells that have an infinite lifetime
3 mechanisms of stem cell differentiation
3 Mechanisms of Stem Cell Differentiation: - Microenvironment - Divisional asymmetry - MicroRNAs Specialised Niche Niche: microenvironment that provides signals needed to repress stem cell differentiation and division. Non-niche microenvironment provides signals that induce differentiation and division.
Divisional Asymmetry a. Divisional Asymmetry: one of the daughter cells retain stem cell status and other differentiates depending on microenvironment b. Environmental asymmetry: after division, one of the two identical daughter cells remains in the self-renewing niche microenvironment while the other relocates outside the niche to a different, differentiation-promoting microenvironment
- MicroRNA (miRNA) Regulation
MicroRNAs are short, non-coding RNA molecules that regulate gene expression after transcription.
They can turn off specific genes by degrading mRNA or blocking translation.
In stem cells, certain microRNAs can promote or inhibit differentiation into specific lineages.
🧠 Example: miR-145 represses pluripotency genes like OCT4, pushing stem cells to differentiate.
hae
Haematopoiesis - Requires nutrients, haemoglobin synthesis and packaging i.e. RBC formation - Bone marrow provides a sufficient microenvironment for haematopoiesis to occur by providing a suitable stromal matrix on which the stem cells can grow and divide. Haemoglobin Synthesis Ultimately, protoporphyrin combines with iron in the ferrous (Fe2+) state to form haem, each molecule of which combines with a globin chain. A tetramer of four globin chains each with its own
haem group is then formed to make up a haemoglobin molecule. Transferrin takes Fe inside and so are important for the transformation. Red Cell Progenitors: nucleus starts to become scattered and disappears. Final Red Cells are nucleated. - Lack of iron leads to different types of anaemia - Inflammation or malignancy prevents iron transport to body causing deficiency - Lack of protoporphyrin or different structure = sideroblastic anaemia - Alpha or beta chain globin in a different form = thalassaemia
transferrin, iron. forms haem.
haematpoiesis microenvironment
Microenvironment in Haematopoiesis - There is an environment where CSF available controls how the cells differentiation from myeloid. - Differentiation happens at different levels. In bone marrow, differentiate into neutrophil to defend body against bacteria. Other cells e.g. monocytes move from bone marrow to tissue and become macrophages. - Myeloid= differentiation in bone marrow only - Monocytes = have differentiation in bone marrow, then in blood and then finally in the tissue to become activated. This is because they are pro-inflammatory. Differentiation location is dependent on the function of the cell. Q What organ do myeloid cells differentiate? A: It depends on the type of cell - some in bone marrow and some in the tissue e.g. macrophage There are specific factors that differentiate them e.g. G-CSF. The effect of the factor is dependent on what level of differentiation you are at; the same factor can have different effects. G-CSF at earlier stage leads to proliferation and in later stages causes maturation.
Myeloid Differentiation CSF is not the only factor. There are also cytokines like IL3 and IL5 which combine to differentiate a cell. Lymphoid Differentiation A combination of different cytokines and then migration to lymphoid organs results in differentiation. B and T cells go to thymus and become specialised. Majority of differentiation happens in the lymphoid system.
therapeutic use
What are the Therapeutic Uses? There are 3 routes to regeneration: Dedifferentiation, transdifferentiation and reprogramming. Can be reprogrammed into pluripotent stem cells. Fibroblast to neuroblasts. X Very expensive. Side effects.
Induction of Pluripotency Terminally differentiation cells become primed cells and then partially reprogrammed cells which can be turned into iPSCs. The primed cells can also undergo somatic programme downregulation. Some animals already have this mechanism e.g. in Newt eyes. Newt lens regeneration. Pigmented epithelial cells (PECs) dedifferentiate, losing their pigment and changing their morphology, before entering a new lineage and differentiating into mature lens cells. But they are far less complex than humans and the method is not fully understoo
CD34
transmembrane phosphoglycoprotein. adhesion molecule. used in selection and enrichment of hscs for bone marrow transplants. a marker for hsc. possess colony-forming potential. but expressed by lots of non haematopoetic cell types.
etheropoiesis
Erythropoiesis Overview
Erythropoiesis is the process by which new red blood cells (erythrocytes) are produced in the bone marrow.
🔗 Progenitor Cells (Lineage Progression)
Hematopoietic Stem Cell (HSC)
→ Common Myeloid Progenitor (CMP)
→ Megakaryocyte-Erythroid Progenitor (MEP)
→ Burst-Forming Unit-Erythroid (BFU-E)
→ Colony-Forming Unit-Erythroid (CFU-E) 🔑 Highly EPO-responsive
→ Proerythroblast
→ Basophilic → Polychromatic → Orthochromatic Erythroblasts
→ Reticulocyte (immature RBC)
→ Erythrocyte (mature RBC)
🧬 Key Transcription Factors
These control gene expression needed for red cell development:
GATA-1: Master regulator of erythropoiesis; promotes erythroid gene expression and suppresses apoptosis.
KLF1 (EKLF): Regulates β-globin gene expression and other erythroid-specific genes.
TAL1 (SCL): Essential for early erythroid commitment.
FOG-1: Works with GATA-1 to stabilize its activity.
RUNX1 and PU.1: Involved earlier in lineage decisions (e.g., myeloid vs lymphoid fate).
🧪 Erythropoietin (EPO)
EPO is a glycoprotein hormone produced mainly by the interstitial cells of the kidney.
It binds to EPO receptors (EPOR) on CFU-Es and proerythroblasts, promoting survival, proliferation, and differentiation.
🔺 When is EPO released?
Triggered by hypoxia (low oxygen levels).
Detected by oxygen-sensing HIF (hypoxia-inducible factor) pathway in kidneys.
In normoxia, HIF is degraded.
In hypoxia, HIF is stabilized → increases EPO gene expression.
🩸 Polycythaemia Rubra Vera (PRV)
A myeloproliferative neoplasm.
Characterized by uncontrolled RBC production, often independent of EPO.
Caused by JAK2 V617F mutation, which activates EPO signaling without EPO.
Symptoms: headaches, ruddy complexion, itching, increased risk of clots.
EPO levels are often low due to negative feedback.
🦠 Renal Cell Carcinoma (RCC) and EPO
Certain kidney tumors like RCC can abnormally produce EPO.
This can lead to secondary polycythaemia (↑ RBC count due to excess EPO).
EPO levels are high in this case.
Example of a paraneoplastic syndrome.
eosinophils vs basophil appearance
basophils- histamine release, abundant purple cytoplasmic granules. stained by BASIC dyes
eosinophils- stained by ACIDIC dyes. bilobed nucleus and orange/red cytoplasmic granules. allergic reactions and parasites.
lymphopoiesis
Lymphopoiesis is the formation of lymphocytes (B cells, T cells, NK cells) from hematopoietic stem cells (HSCs) in the bone marrow.
🔁 Lineage Pathway:
HSC → Common Lymphoid Progenitor (CLP) →
→ B cells (mature in bone marrow)
→ T cells (mature in thymus)
→ NK cells
🧬 Acute Lymphoblastic Leukaemia (ALL)
Cancer of immature lymphoid cells (usually B or T cell precursors).
Rapid progression, common in children.
Bone marrow is crowded with blasts, suppressing normal blood cell production.
🧬 Chronic Lymphocytic Leukaemia (CLL)
Cancer of mature B cells.
Slow progression, more common in older adults.
Cells accumulate in blood, marrow, lymph nodes, but don’t function properly.
thrombopoiesis
hrombopoiesis (Platelet Production)
Thrombopoiesis is the process by which platelets (thrombocytes) are produced from megakaryocytes.
🔁 Lineage:
HSC → CMP → Megakaryocyte-Erythroid Progenitor →
→ Megakaryoblast → Megakaryocyte → Platelets
🧠 Key Concepts:
🔸 Endomitosis
A special cell cycle where megakaryocytes replicate their DNA without dividing.
Result: very large polyploid cells (up to 64N), packed with cytoplasm to make platelets.
🔸 Cytoplasmic Restructuring
Megakaryocytes undergo massive cytoplasmic expansion and restructuring into proplatelets.
🔸 Proplatelets and Shear Forces
Proplatelets are long extensions from megakaryocytes.
Shear stress from blood flow (especially in lung vasculature) breaks off platelets from the tips.
🧪 Thrombopoietin (TPO) and c-MPL
TPO is a hormone (made mainly by the liver) that regulates platelet production.
Acts on c-MPL receptor on:
Megakaryocytes
Platelets
Megakaryocyte-biased HSCs
🔄 TPO Regulation: Internalisation
When platelet levels are high, TPO binds to c-MPL on platelets, is internalised and degraded → less TPO available to stimulate megakaryocytes.
When platelet levels are low → more free TPO → stimulates megakaryopoiesis.
⚠️ Clinical Links
🧬 Essential Thrombocythaemia (ET)
Mutation in c-MPL or JAK2 can cause excessive platelet production.
TPO signalling becomes abnormally activated, leading to thrombocytosis.
🧬 Megakaryocyte-biased HSCs
Some HSCs are primed to become megakaryocytes.
These express CD41 and vWF, especially under stress conditions (e.g. bone marrow failure).
💊 Aplastic Anaemia Treatment
In aplastic anaemia, bone marrow fails to produce enough blood cells.
TPO receptor agonists can help:
Eltrombopag (oral)
Romiplostim (injectable)
These stimulate c-MPL to enhance platelet (and even stem cell) recovery.Activators of megakaryopoiesis.
