HEMOPOIESIS LECTURE

Hemopoiesis = Hematopoiesis = the formation and the development of blood cells

All the cells that circulate in the     peripheral blood are derived from primitive mesenchymal cells i.e. pluripotential hematopoietic stem cells

Embryology

Pluripotential hematopoietic stem cells =tutipotent haemal stem cells (1 per 10⁴-10⁵ cells in the bone marrow)

Origin: mesenchymal tissue in the yolk sac during the first 6 week of gestation.

In the 2nd month a number of intraembryonic sites appear and replace the yolk sac.

These sites succeed but overlap each other in time, each site gradually increasing in importance then waning: liver & spleen (extramedullary hemopoiesis) --> bone marrow (myeloid tissues) (all blood cells) (medullary hemopoiesis) --> peripheral lymphoid tissues (only lymphocytes)

Sites of hematopoiesis after birth

Birth: mostly bone marrow; spleen and liver when needed

Beginning at 4 years: hemopoietic activity move to axial skeleton (flat bones, skull, ribs, sternum, clavicle, vertebrae, pelvic bones) and proximal ends of long bones (humerus, femur) --> complete at 18 years

Adult: remaining marrow cavities are fill with fat (medulla osseum flavum). By age 40 marrow in sternum, ribs, vertebrae and pelvis is composed of equal amount of hemopoietic tissues and fat.

Sites of hematopoiesis after birth

In the adults, in times of great demand the marrow in the long bones shafts may become hemopoietic again.

Extramedullary hemopoiesis may occur under certain conditions:

- if the bone marrow is no longer functional

- when bone marrow is not able to keep up with the demand for blood.

--> liver & spleen will become enlarged

Characteristics of stem cells

1. The parent cells to all of the cells that circulate in the peripheral blood

2. Morphologically indistinguishable from small to medium sized lymphocytes

3. While reside primarily in the bone marrow, they migrate freely in the peripheral blood

4. They may:

a. "rest" for long interval without dividing (prolonged intermitotic interval),

b. divide without differentiating further,

c. differentiate down any one of the several myeloid or lymphoid pathways of development.

Hemopoietic differentiation

1. Commitment of the stem cell to either one of two larger pathways, myeloid or lymphoid     (committed progenitor cells)¹ (tutipotent ---> pluripotent)²

2. Progeny of cells committed to each of these     2 pathways --> obligated to specific sublines of development

3. A. Three branches of myelod pathway:

    erythroid, megakaryocytic, and phagocytic

B. lymphoid pathway subdivides into

    B cell, T cell and "null" cell pathway

Stages of blood cells development

To generate a complete set of blood cells from a single tutipotent cells a definite sequence of major stages can be recognized:

1. Final commitment to a particular line of differentiation

2. Early cell proliferation to form a large pool of dividing cells

3. Differentiation as specific protein characterizing the particular line are synthesized

4. Final maturation: close of protein synthesis

5. Release of the cells from bone marrow

Stem cell à late stemm cell à proerythroblast à basophilic erythroblast à polychromatophilic erythroblast à normoblast à extrosion of nucleus à reticulocyte

Normally it takes 7 - 10 days for erythrocytes to develop from their precursor

It takes 10 - 14 days for blood neutrophils to develop from precursors

Structure & Function of Bone Marrow

Bone marrow has a vascular compartment and an extravascular compartment. 

Hematopoiesis takes place in the extravascular compartment. 

The extravascular compartment consists of a stroma of reticular connective tissue and a parenchyma of developing blood cells, plasma cell, macrophages and fat cells. 

The high activity of the bone marrow is demonstrated by its daily output of mature blood cells: 2.5 billion erythrocytes, 2.5 billion platelets, 50-100 billion granulocytes. The numbers of lymphocytes and monocytes is also very large.

Structure & Function of Bone Marrow (cont.)

• Bone marrow also function as the site of the removal of aged and defective erythrocytes and the differentiation of B lymphocytes. 
  
• It is also the site of numerous plasma cells.
  

The process of hematopoiesis

- Involves a complex interplay between the intrinsic genetic processes of blood cells and their environment.

- The interplay determines whether HSCs, progenitors, and mature blood cells remain quiescent, proliferate, differentiate, self-renew, or undergo apoptosis.

- All of the genetic and environmental mechanisms that govern blood production operate by affecting the relative balance of these fundamental cellular processes.

- In normal conditions, most HSCs and many progenitors are quiescent in the G0 phase; many progenitors are proliferating --> producing mature offspring. In the absence of any stresses, this is balanced by the rate of apoptosis in progenitors and mature cells.

- In the event of a stress e.g. bleeding / infection, several processes occur.

- Stored pools of cells in the marrow are quickly released into the circulation in order to localize to the site of injury.

- Fewer progenitors and mature cells undergo apoptosis.

- Quiescent progenitors and HSCs are stimulated by a variety of growth factors to proliferate and differentiate into mature white cells, red blood cells, and platelets.

Control of hematopoiesis

Probably the best characterized environmental regulators of hematopoiesis are cytokines (Hemopoietic Growth Factors) (Hemopoietic inductive microenvironment)

Cytokines are a broad family of proteins that mediate positive and negative affects on cellular quiescence, apoptosis, proliferation, and differentiation. In general, cytokines function by engaging a specific receptor and activating a variety of signaling pathways.

Hemopoietic cytokines

The response to these cytokines develops only when specific receptors for each of the cytokines are acquired by the cells.

Hematopoietic cytokines: produced through both autocrine and paracrine mechanisms and in many cases are produced by nonhematopoietic cells including bone marrow stroma and endothelium.

- Erythropoietin is produced primarily by     cells in the kidney

- thrombopoietin is produced by the liver

- IL-1 & TNFa are produced by     macrophages eating bacteria.     Stimulated by these cytokines --> Tcells, endothelial cells or fibroblast -->    produce G-CSF, GM-CSF

Hematopoietic inductive microenvironment

Microenvironment of the bone marrow is essential for hematopoiesis

1. Endothelial cells, fibroblast, and macrophage in the stroma are sources of hemapoietic cytokines that act on stem cells and early committed progenitor cells

2. It provides sorting mechanism that permits only the most immature cells (stem cells,     committed progenitor cells) and the most mature cells (RBC, neutrophils, platelets)     to exit the marrow

Microenvironment, cytokines, & cytokines receptors

Dynamics / kinetics in the blood

- Blood half-live of neutrophil: 6 - 8 hr

--> new blood populations of neutrophil is formed every 24 hours

--> neutropenia is the most frequent hematologic consequence when bone marrow is damaged

 

- Erythrocyte life span 100 -120 days

--> transfusion of erythrocytes and platelets is feasible

Reference: Lecture from Djoko Prakosa