Histology Lecture #18: Hemopoiesis

The process of blood cell (blood formed elements) production

What is hemopoiesis?

Stem cell multiplication and differentiation

What processes are involved in hemopoiesis?

To maintain a constant level of blood cells

What is the main objective of hemopoiesis?

Blood cells arise from the yolk sac (mesodermal origin)

Where does hemopoiesis occur in the early embryo (yolk sac phase)?

Mostly in the liver; spleen plays a minor role

Where does hemopoiesis occur in the second trimester (hepatic phase)?

Bone marrow becomes the major hematopoietic organ

Where does hemopoiesis occur from the third trimester onward (bone marrow phase)?

1. Erythropoiesis

2. Granulopoiesis

3. Monocytopoiesis

4. Thrombocytopoiesis

What Cell lineage differentiation occurs in bone marrow only? (4)***

Lymphopoiesis

What Cell lineage differentiation occurs in bone marrow and other lymphatic organs?

Hematopoietic stem cell (HSC) is pluripotent

What is the basic concept of the monophyletic theory of hemopoiesis?****

1) Self-renewal (yields more HSCs)

2) Differentiation into progenitor cells

What are the two possible fates of a hematopoietic stem cell (HSC)?

Two major colonies of multipotential progenitor cells:

1. Common myeloid progenitor (CMP) cells → differentiate into lineage-restricted progenitors

2. Common lymphoid progenitor (CLP) cells

What do pluripotent progenitor cells differentiate into? (2)

1) Megakaryocyte/erythrocyte progenitor (MEP) cells

2) Granulocyte/monocyte progenitor (GMP or CFU-GM) cells

What are the two lineage-restricted progenitors derived from common myeloid progenitor (CMP) cells?

1. Monopotent megakaryocyte-committed progenitor cells (MKP or CFU-Meg) → Megakaryocytes

2. Monopotent erythrocyte-committed progenitor cells (ErP or CFU-E) → Erythrocytes

What are the subtypes of Megakaryocyte/erythrocyte progenitor (MEP) cells? (2)

Requires high levels of PU.1 transcription factor:

1. Neutrophil progenitors (NoP or CFU-G) → Neutrophils

2. Eosinophil progenitors (EoP or CFU-Eo) → Eosinophils

3. Basophil/mast cell progenitors (BMCP):

• Basophil progenitors (BaP or CFU-Ba) in bone marrow → Basophils

• Mast progenitor cells (MCPs) in gastrointestinal mucosa → Mast cells

4. Monocyte progenitors (MoP or CFU-M) → Monocytes

What are the subtypes of Granulocyte/monocyte progenitor (GMP CFU-GM) cells?

1. B cells

2. T cells

3. NK cells

What cells are derived from common lymphoid progenitor (CLP) cells? (3)

No, they cannot be morphologically distinguished; they resemble large lymphocytes

Can stem and progenitor cells be morphologically distinguished?

Specific progenitor cells

From what do precursor cells (blasts) derive from?

Blasts gradually assume the morphologic characteristics of the mature, functional cell types they will become.

How do precursor cells (blasts) develop into mature hematological cells?

Morphological changes

What are the development of heatological cells marked by?

Colony-stimulating factors (CSF) or cytokines

What are hemopoietic growth factors also called?

Glycoproteins

What type of molecules are hemopoietic growth factors?

1. Stimulate proliferation of progenitor and precursor cells

2. Promote differentiation and maturation within specific lineages

What are the main functions of hemopoietic growth factors? (2)

Erythroid lineage

What lineage does erythropoietin (EPO) stimulate?

Thrombopoietic lineage

What lineage does thrombopoietin stimulate?

Granulocytic and monocytic lineages (all myeloid progenitor cells)

What lineages does granulocyte/monocyte-CSF stimulate?

Granulocytic lineage

What lineage does granulocyte-CSF stimulate?

Monocytic lineage

What lineage does monocyte-CSF stimulate?

1. Medullary canals of long bones (yellow bone marrow)

2. Cavities of cancellous bone (red bone marrow)

Where is bone marrow located? (2)

Adipose tissue

What does yellow bone marrow consist of?

Hematopoietic tissue

What does red bone marrow consist of?

All bone marrow in newborns is red

What type of bone marrow do newborns have?

Red bone marrow is gradually replaced by yellow bone marrow; can revert to red marrow under severe hypoxia

How does bone marrow change with age?

1. Reticular connective tissue

2. stromal cells (special fibroblastic cells)

3. reticular or adventitial cells

What forms the stroma of red bone marrow? (3)

1. Collagen type I

2. Proteoglycans

3. Fibronectin

4. Laminin (interact with integrin, binding cells to the matrix)

What components make up the matrix of red bone marrow? (4)

Hemopoietic cords (or islands) of cells

What forms the parenchyma of red bone marrow?

Sinusoids

What surrounds the hemopoietic cords in red bone marrow?

By reticular fibers

How are hemopoietic cells and macrophages supported in the parenchyma?

Both cell and nuclear sizes decrease

How do cell and nuclear sizes change during erythropoiesis?

Nucleoli decrease in size and eventually disappear

What happens to nucleoli during erythropoiesis?

Nucleus becomes pyknotic (chromatin density increases) and is eventually extruded from the cell

How does the nucleus change during erythropoiesis?

From basophilia (many polyribosomes initially) to acidophilia (increasing hemoglobin)

How does cytoplasmic staining change during erythropoiesis?

They disappear

What happens to mitochondria and other organelles during erythropoiesis?

Approximately one week; Three cell divisions

How long does erythropoiesis approximately take and how many cell divisions occur?

Erythropoietin (EPO)

What stimulates transcription of mRNA during erythropoiesis?

Common myeloid progenitor (CMP) → Megakaryocyte/erythrocyte progenitor (MEP) → Erythropoietin-sensitive erythrocyte-committed progenitors (ErPs or CFU-E)

From which progenitor cells do erythroid cells arise?

GATA-1

What transcription factor drives Megakaryocyte/erythrocyte progenitor (MEP) differentiation into the erythroid lineage?***

Proerythroblast

What is the first recognizable precursor cell of erythropoiesis?

Large cell with a large spherical nucleus, lacy chromatin, and one or two nucleoli

Describe the size and nucleus of a proerythroblast.

Mild basophilia with free polyribosomes

Describe the cytoplasm of a proerythroblast.

Smaller than a proerythroblast

How does the size of a basophilic erythroblast compare to a proerythroblast?

Cytoplasmic basophilia with many free polyribosomes

Describe the cytoplasm of a basophilic erythroblast.

Gradually increasing heterochromatin as rounds of mitosis occur

How does the nucleus of a basophilic erythroblast change?

Cellular size continues to reduce

How does the size of a polychromatophilic erythroblast change?

Mixture of basophilia and acidophilia

Describe the cytoplasm of a polychromatophilic erythroblast.

Heterochromatic nucleus with a checkerboard pattern

Describe the nucleus of a polychromatophilic erythroblast.

Normoblast

What is another name for the orthochromatophilic erythroblast?

Reduced cellular and nuclear sizes; slightly larger than a mature erythrocyte

How do the cell and nucleus sizes of an orthochromatophilic erythroblast compare to earlier stages?

Acidophilic with very few free polyribosomes

Describe the cytoplasm of an orthochromatophilic erythroblast

Very heterochromatic; incapable of cell division; nucleus is extruded late in this stage

Describe the nucleus*** of an orthochromatophilic erythroblast.

Anucleate (no nucleus)

What is the nucleus status of a reticulocyte?

Some polyribosomes and some mitochondria

What organelles are retained in reticulocytes?

Leave the bone marrow and enter the bloodstream

Where are reticulocytes ready to go?

Approximately 1%

What percentage of red blood cells in peripheral blood are reticulocytes?

Complete loss of ribosomes and mitochondria occurs while in the bloodstream

How do reticulocytes become mature erythrocytes?

Synthesis of proteins by RER and Golgi apparatus

What major cytoplasmic changes occur during granulopoiesis?

1) Azurophilic granules (promyelocyte stage) – lysosomal hydrolases, cytoplasmic basophilia, similar in all three granulocytes

2) Specific granules (myelocyte stage) – proteins packaged by Golgi, located closer to Golgi

What are the two stages of granule synthesis in granulopoiesis?

Multipotential common myeloid progenitor (CMP) stem cell

From which stem cell do granulocytes originate?

Granulocyte/monocyte progenitors (GMPs)

Into what progenitors do Multipotential common myeloid progenitor stem (CMP) cells differentiate into?

GM-CSF

Granulocyte colony-stimulating factor (G-CSF)

IL-3

What cytokines stimulate granulopoiesis? (3)

Six stages:

Myeloblast → Promyelocyte → Myelocyte → Metamyelocyte → Band (immature) cell → Mature neutrophil

How many morphologically identifiable stages are there in neutrophil development?***(Know order)

Five mitotic divisions

How many mitotic divisions occur during neutrophil development?

They undergo similar morphological changes as neutrophils

How do eosinophil and basophil granulopoiesis changes compare to neutrophils?

Eosinophil progenitor cells (EoP), induced by GM-CSF, IL-3, and IL-5

Into what progenitor do GMPs differentiate to form eosinophils?**

Basophil progenitor cells (BoP) in the absence of IL-5

Into what progenitor do GMPs differentiate to form basophils?

Differentiation is not possible until the myelocyte stage

At what stage is differentiation of granulocytes possible?

Myeloblast

What is the most immature recognizable cell of the myeloid series?

Large, spherical, euchromatic nucleus with 3-5 nucleoli

Describe the nucleus of a myeloblast.

Large nuclear-to-cytoplasm volume, few fine granules dispersed, intense basophilia, and an unstained Golgi area

How does the cytoplasm of a myeloblast appear?

Azurophilic granules containing

- lysosomal enzymes

- myeloperoxidase

What granules and enzymes are present in promyelocytes?

Intense basophilia

How does the cytoplasm of promyelocytes appear?

No, it is impossible to recognize subtypes at this stage

Can granulocyte subtypes be recognized at the promyelocyte stage?

Activation of different gene sets

What drives differentiation of promyelocytes into specific granulocyte lineages?

Appearance of specific granules in the myelocyte stage

What is the first sign of granulocyte differentiation?

From the convex surface of the Golgi apparatus

Where do specific granules emerge from in a myelocyte?

At the concave side of the Golgi apparatus

Where are azurophilic granules located in a myelocyte?

More or less spherical and becomes increasingly heterochromatic

Describe the nucleus of a myelocyte.

Subsequent cell divisions and increased production of specific granules

What leads to the next stage of granulopoiesis after the myelocyte?

Metamyelocyte stage

At what stage do neutrophils, eosinophils, and basophils become distinguishable?

Specific granules outnumber azurophilic granules

In metamyelocytes, which granules outnumber the other?

It is reduced

What happens to the Golgi apparatus in metamyelocytes?

Heterochromatic and indented

Describe the nucleus of a metamyelocyte.

1. Band stage (elongated nucleus, not yet polymorphic)

2. Segmented stage

What are the two stages of neutrophil development following the metamyelocyte stage?***

No, the band stage is rarely seen in eosinophils and basophils

Do eosinophils and basophils typically show a band stage?

Development of the first distinct nuclear lobes

What does the neutrophil band (stab) cell stage precede?

Elongated, nearly uniform in width, and horseshoe-shaped

Describe the nucleus of a neutrophil band cell.

Up to 3%

What percentage of circulating neutrophils are normally band cells?

During acute or chronic inflammations or infections

When do band cells increase in number?

Two to four lobules

How many nuclear lobules are present in a mature neutrophil?

Common myeloid progenitors (CMPs) differentiate into committed granulocyte/monocyte progenitors (GMPs) under the influence of IL-3

From which progenitor do monocytes originate?

Monocytes and dendritic cells

What cells do GMPs form in monopoiesis?