The Discovery of Haemopoietic Stem Cell
Blood was regarded as an important tissue, but till the mid 1800s it was not known how blood cells were made. Three discoveries lead to the modern concept of haemopoietic stem cell.
- Neumann and Bizzozero separately, in 1868, proposed that the bone marrow was the site of blood production throughout the postnatal life.
- Improvement in staining techniques lead to the discovery of a spectrum of cells in the bone marrow. Pappenheimer organized these cells into a tree with the most mature cells being at the leaves. He proposed that at the trunk was a cell that was so primitive that it could not be typed into any lineage. Ehrlich and Schilling differed from Pappenheimer in the existence of a single precursor for all blood cells. They proposed that there were two (Ehrlich) and three (Schilling) different precursor for blood cells respectively.
- The stem cell defies morphological definition. It is possible to define populations that are rich in stem cells but not all the cells thus defined are stem cells. The next chapter in the definition of haemopoietic stem cell was written when the focus shifted from what stem cells looked to what they could do. Two discoveries, both by scientists not intending to looking for the haemopoietic stem cell, laid the foundation for the modern concepts of haemopoietic stem cells
- Lineage is important in cattle. It provides assurance to a farmer that the calf he is rearing will give a good amount of milk. Calves of good lineage command a high price. In the 1930s and 40s blood groups seemed to be reliable way of determining paternity of an animal. In 1944 a breeder reported twin calves of different sexes but identical blood groups. In 1945 Ray Owen in an attempted to resolve why calves that appeared to have two fathers had identical groups. He discovered that the calves actually had two erythrocyte populations (Genetics 1996; 144:855-59). Each calf had blood from both the calves. The cattle uterine anatomy allows communication between extra-embryonic circulation of twins. The only explanation for two types of erythrocytes in each calf was that
i. Cells capable of producing both types of blood cells were present in the bone marrow of both the calves
ii. The bone marrow of each calf had been seeded by cells from calls that could produce blood cells and arose from the other calf.
iii. These cells could produce blood indefinitely
- Till and McCoulloch were interested in developing an assay for radiosensitivity of marrow cells. Lethally irradiated animals that survive acute radiation sickness succumb to bone marrow failure. Till and McCoulloch transplanted lethally irradiated mice with bone marrow cells exposed to different doses of radiation. They discovered the following
i. Animals survive the radiation and develop islands of hematopoiesis in the spleen in the form of nodules.
ii. The number of the nodules is related to the dose of bone marrow cell infused.
iii. The colonies contain cells of erythroid and myeloid. The cells show different degree of maturity
iv. Using chromosomal markers the colonies were shown to arising from a single cell
v. When cells from the colonies were transplanted to another lethally irradiated mouse cells some colonies were able to establish haemopoietic splenic colonies.
These studies eastablished that the bone marrow had cells that that could self-renew, have a high capacity to proliferate and have capacity to differentiate into multiple lineages defining a haemopoietic stem cell.
Identification of the Pleuripotent haemopoietic
Isolation of a pure population of stem cells has not been possible.
The definition of stem cell is a functional definition. The stem cell is defined as a cell that has the capacity to
- Self-renewal: Self-renewal is the phenomena of stem cells giving rise to more stem cells. Self-renewal is essential to maintain the stem cell pool and ensure adequate blood production. The stem cells population can be maintained at a constant levels only if half the cell produced as a result mitosis retain the characters of a stem cell and the other half differentiate into blood cells. If more than half the stem cells differentiate, the stem cell pool will eventually deplete. If less than half the stem cells differentiate there would be an unnecessary accumulation of stem cells. The details of the self-renewal and commitment processes are not completely understood.
- Indefinite proliferation: The bone marrow stem cells maintain haematopoiesis for a lifetime.
- Capacity to differentiate into many cell types: The haemopoietic stem cell gives rise to the erythrocytes, myelocytes, lymphocytes, monocytes, megakaryocytes and the dendritic cells.
Stem cells are identified by their ability to reconstitute haematopoiesis in a lethally irradiated animal.
Haemopoietic stem cell transplant became a reality in 1968. Initially the bone marrow was the source of stem cells. Today the peripheral blood and occasionally the cord blood serve as the source of stem cells. The number of stem cell infused determines the success of the transplant. Animal transplantation take too long to be clinically useful.
CD34 is a trans-membrane protein expressed on haemopoietic progenitors till the stage of committed progenitor cells. CD34 expression can rapidly be assessed by flow cytometery. CD34+ cells show the characters of haemopoietic stem cells
- Autologous CD34 enriched population has been shown to protect against myeloablative doses of radiation and chemotherapy. CD34- negative cell fail to do so.
- Allogenic CD34+ cell are able to establish haematopoiesis. This chemerism remains stable over a prolonged period of time.
Haemopoietic stem cell is a CD34+ cell that lacks lineage commitment markers. These include
- T cells: CD2
- B cells: CD19
- Monocytes: CD14
- Granulocytes: CD15
- NK Cells: CD16
- Erythroid cells: Glycophorin A
- Others: CD24, CD56 and CD66b, DR
Not all the CD34+ cells are pleuripotent stem cell but CD34 is a useful marker for estimating the number of stem cells infused during a haemopoietic stem cell transplant.