L2 - Hematopoiesis

What is Hematopoiesis?

Production of all blood elements, including erythrocytes, leukocytes, and platelets

What is the Difference between Myelopoiesis and Lymphopoiesis?

• Myelopoiesis produces non-lymphoid blood cells

• Lymphopoiesis produces lymphocytes

What are Erythropoiesis, Granulopoiesis, and Thrombopoiesis?

Myelopoiesis produces non-lymphoid blood cells:

• Erythropoiesis: Production of erythrocytes

• Granulopoiesis: Production of neutrophils, eosinophils, and basophils

• Thrombopoiesis: Production of platelets

What is the Difference between Stem Cells, Progenitor Cells, and Precursor Cells?

• Stem cells: Can produce all blood cell lineages and self-renew

• Progenitor cells: Have restricted differentiation potential and limited self-renewal

• Precursor cells: Morphologically recognizable immature cells without self-renewal capacity

What does the Hematopoietic Pyramid illustrate?

A small number of stem cells generate increasingly larger populations of progenitors, precursors, and mature blood cells.

Can Stem Cells and Progenitor Cells be distinguished by Morphology Alone?

No

• Their identity depends on functional or molecular characteristics

What Morphologic Changes occur as Bone Marrow Cells mature?

1. Cell and nuclear size decrease

2. Nucleus-to-cytoplasm ratio decreases

3. Nucleoli disappear

4. Chromatin condenses

5. Cytoplasmic basophilia decreases as RNA content falls

Why does Cytoplasmic Basophilia decrease during Maturation?

RNA content decreases as lineage-specific products accumulate

Which Hematopoietic Lineage is the Exception to decreasing Cell Size during Maturation?

Megakaryocytes increase in size as they mature

What is the Developmental Sequence of Erythropoiesis?

Rubriblast → Prorubricyte → Basophilic rubricyte →

Polychromatic rubricyte → Metarubricyte → Reticulocyte → Mature erythrocyte

How does Erythroid Cell Morphology change during Maturation?

• Cells become smaller

• Chromatin condenses

• Cytoplasm becomes progressively pinker as hemoglobin accumulates

What is the Last Nucleated Stage of Erythropoiesis?

Metarubricyte

What is the Difference between Reticulocytes and Mature Erythrocytes?

Reticulocytes retain residual RNA → while mature mammalian erythrocytes lack a nucleus and have minimal residual RNA

What is the Developmental Sequence of Granulopoiesis?

Myeloblast → Progranulocyte → Myelocyte →

Metamyelocyte → Band neutrophil → Segmented neutrophil

When do Primary and Secondary Granules develop during Granulopoiesis?

• Primary granules: Progranulocyte stage

• Secondary granules: Myelocyte stage

How does Nuclear Shape change during Granulocyte Maturation?

Round → Indented → Band-shaped → Segmented

What is the Developmental Sequence of Megakaryocytes?

Megakaryoblast → Early promegakaryocyte →

Late promegakaryocyte → Megakaryocyte

What Morphologic Changes occur during Megakaryocyte Maturation?

• Cells become larger

• Develop multilobulated nuclei

• Accumulate abundant granular cytoplasm

• Platelets are released from their cytoplasm

What is Endomitosis?

DNA replication without normal cell division, increasing megakaryocyte ploidy and size

What is the Proliferative Pool?

Mitotically active precursor cells that divide to increase blood cell numbers

Which Cells belong to the Proliferative Pool?

• Erythroid: Rubriblasts, prorubricytes, and basophilic rubricytes

• Granulocytic: Myeloblasts, progranulocytes, and myelocytes

What is the Maturative Pool?

Postmitotic cells that complete differentiation without further cell division

Which Granulocytic Cells belong to the Maturative Pool?

Metamyelocytes, band neutrophils, and segmented neutrophils

What is the Bone Marrow Storage Pool?

A reserve of mature neutrophils that can be rapidly released into circulation when needed

Which Species have Large vs. Small Neutrophil Storage Pools?

• Large: Dogs and cats

• Small: Horses and cattle

What is the Primary Stimulus for Erythropoiesis?

Hypoxia (decreased oxygen availability)

Where is Erythropoietin (EPO) primarily produced?

1. The kidneys, specifically renal cortical peritubular interstitial fibroblast-like cells

2. The liver is a secondary source

What is the Function of PHD2 under Normal Oxygen Conditions?

PHD2 uses oxygen to hydroxylate HIF-2α, targeting it for degradation

What is the Function of HIF-2α?

A transcription factor that promotes EPO gene transcription

How does Hypoxia stimulate EPO Production through the PHD2–HIF-2α Pathway?

1. Hypoxia

2. Decreased PHD2 activity

3. HIF-2α stabilization

4. Increased EPO transcription

5. Increased erythropoiesis

What are the Major Effects of EPO on Erythropoiesis?

1. Proliferation of erythroid progenitor cells

2. Erythroid cell maturation

3. Hemoglobin synthesis

4. Earlier release of reticulocytes (*species variation)

Why is the Regenerative Response delayed after Acute Blood Loss?

Erythroid precursors require approximately 3–6 days to proliferate and mature before regeneration becomes detectable

How Many RBCs can One Rubriblast produce?

Approximately 16–32 RBCs after 4–5 mitoses

What is RBC Senescence?

Aging-associated oxidative and membrane damage that reduces erythrocyte deformability

Where are Senescent RBCs primarily removed?

By macrophages in the spleen

• Liver macrophages also contribute

What are Erythroblastic Islands?

Erythroblastic island consists of a central macrophage surrounded by erythroid precursors

What is the Function of the Central Macrophage in an Erythroblastic Island?

Supports erythroid proliferation and maturation, including iron supply

QUESTION 1: Which of the following cell types has the highest hemoglobin concentration?

a.) Rubricyte

b.) Rubriblast

c.) Erythrocyte

d.) Metarubricyte

ANSWER → c.) Erythrocyte 

• Hemoglobin accumulates as erythroid cells mature

• While the strong basophilia of early precursors reflects abundant RNA

What is the Primary Regulator of Granulopoiesis?

Granulocyte colony-stimulating factor (G-CSF)

Which Cells produce G-CSF?

• Primarily endothelial cells

• Fibroblasts

• Macrophages

What is the IL-23–IL-17–G-CSF Pathway?

1. IL-23 stimulates IL-17 production

2. IL-17 stimulates tissue cells to produce G-CSF

3. G-CSF increases granulopoiesis

How does Neutrophil Clearance regulate Granulopoiesis?

Macrophage clearance of apoptotic neutrophils decreases IL-23 production → reducing IL-17 and G-CSF signaling

What happens to the IL-23–IL-17–G-CSF Pathway when Neutrophil Numbers decrease?

Reduced neutrophil clearance → Increased IL-23 → Increased IL-17 → Increased G-CSF → Increased neutrophil production

What is the Function of the CXCL12–CXCR4 Axis?

Promotes neutrophil retention within bone marrow

What is the Function of CXCR2 Signaling?

Promotes neutrophil mobilization from bone marrow into circulation

Which Chemokines are associated with Neutrophil Mobilization?

CXCL1, CXCL2, and CXCL8

• Their relative roles vary by species

How do Corticosteroids affect the Neutrophil Storage Pool?

They promote the release of stored neutrophils into the peripheral circulation

Which Inflammatory Mediators increase Granulopoiesis during Infection?

• G-CSF

• GM-CSF

• IL-1

• IL-6

• TNF-α

What is a Left Shift?

Increased immature granulocytes in circulation

• Interpret with neutrophil count and clinical context

Which Cytokine promotes Eosinophil Production, Maturation, and Survival?

IL-5

What is the Monocyte Differentiation Sequence?

CFU-M → monoblast → promonocyte → monocyte

What is CFU-GM?

A shared progenitor of neutrophils and monocytes

Do Monocytes have a Bone Marrow Storage Pool?

No

• They leave marrow and can differentiate into tissue macrophages

QUESTION 2: Which of the following mediators is produced by tissue (i.e. non-hematopoietic) cells and directly stimulates granulopoiesis?

a.) IL-17

b.) CXCL8

c.) G-CSF

d.) IL-23

ANSWER → c.) G-CSF 

What are Mammalian Platelets?

Anucleate cytoplasmic fragments of megakaryocytes containing granules important in hemostasis

What is the Approximate Circulating Lifespan of Platelets?

Approximately 6–10 days

How do Non-mammalian Thrombocytes differ from Mammalian Platelets?

• Thrombocytes are nucleated cells

• Mammalian platelets are anucleate fragments

How are Mammalian Platelets released into Circulation?

Megakaryocyte proplatelets extend into sinusoids and fragment with cytoskeletal remodeling and blood flow

How do Avian Thrombocytes enter Circulation?

They develop within marrow sinusoids and enter blood without crossing vessel walls

What is Thrombopoietin (TPO), and Where is it produced?

A hormone produced primarily by the liver, with additional production by the kidneys, that regulates megakaryocyte development

What are the Major Effects of TPO?

Supports megakaryocyte progenitor survival, expansion, and maturation

How is Free TPO regulated by Circulating Platelets?

TPO is produced continuously, while circulating platelets bind and remove it

How do Thrombocytopenia and Thrombocytosis affect TPO Levels?

• Thrombocytopenia: Decreased platelet mass → Increased free TPO → Increased megakaryopoiesis

• Thrombocytosis: Increased platelet mass → Decreased free TPO → Decreased megakaryopoiesis

What is the Main Difference between EPO, G-CSF, and TPO Regulation?

• EPO: Regulated by oxygen availability

• G-CSF: Regulated by neutrophil turnover and inflammation

• TPO: Free levels are regulated by circulating platelet mass

QUESTION 3: Which of the following statements regarding thrombopoiesis is correct?

a.) Megakaryocyte maturation involves progressive reduction in cell size

b.) Macrophages provide the main source of circulating TPO

c.) Mature mammalian platelets retain a nucleus

d.) TPO supports megakaryocyte progenitor survival, expansion, and maturation

ANSWER → d.) TPO supports megakaryocyte progenitor survival, expansion, and maturation 

How can you distinguish the Proliferative Pool from the Maturative Pool using Cell Morphology?

• Proliferative pool:

  • Immature cells with larger nuclei

  • Higher nucleus-to-cytoplasm ratios

  • More basophilic cytoplasm

• Maturative pool:

  • More differentiated cells with condensed chromatin

  • Lower nucleus-to-cytoplasm ratios

  • Less basophilic cytoplasm

What are the Key Differences between Erythroid, Granulocytic, and Megakaryocytic Maturation?

• Erythroid:

  • Cells become smaller,

  • Accumulate hemoglobin

  • Lose their nuclei

• Granulocytic:

  • Cells develop specific granules and segmented nuclei

• Megakaryocytic:

  • Cells become larger

  • Develop multilobulated nuclei

  • Release platelets

Stem Cells vs. Progenitor Cells vs. Precursor Cells SUMMARY

Characteristic	Stem Cells	Progenitor Cells	Precursor Cells
Differentiation potential	All blood lineages	Restricted lineages	Committed lineage
Self-renewal	Long-term	Limited	None
Morphologically recognizable	No	No	Yes
Main function	Maintain hematopoiesis	Expand specific lineages	Mature into functional cells

Key concept: Stem cell → Progenitor cell → Precursor cell → Mature blood cell

Morphological Characteristics of Hematopoietic Cell Lineages SUMMARY

Characteristic	Erythroid Lineage	Granulocytic Lineage	Megakaryocytic Lineage
Cell size	Decreases	Decreases	Increases
Nuclear changes	Condenses → nucleus lost	Round → indented → band → segmented	Becomes multilobulated
Cytoplasmic changes	Blue → pink (hemoglobin accumulation)	Primary → secondary granules	Abundant granular cytoplasm
Key identifying stage	Metarubricyte = last nucleated stage	Myelocyte = first secondary granules	Mature megakaryocyte = very large cell
Final product	Erythrocyte	Mature granulocyte	Platelets

Important: Megakaryocytes are the exception to the general rule that hematopoietic cells decrease in size during maturation

Proliferative vs. Maturative vs. Storage Bone Marrow Pools SUMMARY

Characteristic	Proliferative Pool	Maturative Pool	Storage Pool
Main function	Cell division	Cell maturation	Reserve for rapid release
Mitotic activity	Yes	No	No
Erythroid examples	Rubriblast, prorubricyte, basophilic rubricyte	Later rubricytes, metarubricyte, reticulocyte	Not emphasized
Granulocytic examples	Myeloblast, progranulocyte, myelocyte	Metamyelocyte, band neutrophil	Mature segmented neutrophils
Key concept	Proliferation	Differentiation	Storage

Remember:

• Proliferative = Division

• Maturative = Differentiation

• Storage = Reserve

Erythropoiesis — PHD2–HIF-2α–EPO Regulation SUMMARY

Characteristic	Normoxia	Hypoxia
Oxygen availability	Normal	Low
PHD2 activity	Active	Decreased
HIF-2α	Degraded	Stabilized
EPO production	Baseline	Increased
Erythropoiesis	Baseline	Increased

Pathway to memorize: Hypoxia → ↓ PHD2 → ↑ HIF-2α → ↑ EPO → ↑ RBC production

Important: EPO is primarily produced by renal cortical peritubular interstitial cells

Granulopoiesis — Major Regulatory Pathways SUMMARY

Pathway/Mediator	Main Function
G-CSF	Primary regulator of granulopoiesis
IL-23 → IL-17 → G-CSF	Feedback regulation of neutrophil production
CXCL12–CXCR4	Retains neutrophils in bone marrow
CXCR2 signaling	Promotes neutrophil release into circulation
IL-5	Promotes eosinophil development
GM-CSF, IL-1, IL-6, TNF-α	Support increased granulopoiesis during inflammation

Key distinction:

• CXCR4 = Retention

• CXCR2 = Release

Thrombopoiesis — TPO Regulation SUMMARY

Characteristic	Thrombocytopenia	Thrombocytosis
Platelet mass	Decreased	Increased
Free TPO	Increased	Decreased
Megakaryopoiesis	Increased	Decreased
Result	Stimulates platelet production	Reduces stimulation of platelet production

Pathway to memorize: ↓ Platelets → ↑ Free TPO → ↑ Megakaryopoiesis → ↑ Platelet production

Important: TPO is primarily produced by the liver and removed from circulation through binding to platelets.