Stem cells are able to remake the tissues they reside in, which include myosatellite cells in skeletal muscle, chondroblasts in the perichodondrium surrounding cartilage, oval cells in the liver, preductal stem cells in the pancreas, basal stem cells of epithelial tissues, and neuronal stem cells in the brain. Today's scientists are trying to use stem cells that occupy a certain area of one tissue to differentiate into cell types of another tissue. For example, stem cells from the dermis can differentiate into skeletal muscle, fat, cartilage and bone. On the other hand, we could have neural and muscle tissues could differentiate into blood.
The research is centered upon distinguishing the cell surface antigens and capacity for differentiation of the human reserve stem cells harvested from the connective tissues of skeletal muscle and dermis. Antibodies have been used to focus on differentiation of ‘cluster of differentiations', CD markers for short, to characterize and isolate hematopoietic cells based on the profiles of their cell surface antigens. Different CD markers each have their own functions, but we are going to use them as identifiers. Also, nonhematopietic, tumorigenic, and reserve stem cells present CD markers. As for some examples, CD51 and CD56 have been reported to be present on the endosteal cells of bone marrow, which are the layer of the vascular connective tissue lining the medullary cavities of the bone. CD34 is located on vascular endothelial cells, tumorigenic cells, thyroid interfollicular fibroblasts, and marrow stromal cells. CD44 has been located on periosteal stromal cells, which are situated around or produced external to bone and marrow, and mesenchymal progenitor cells, which are loosely organized undifferentiated mostly mesodermal cells that give rise to such structures as connective tissues, blood, lymphatics, and bone.
Six different stem cells were used. They were taken from fetal (both female and male), mature (both female), and geriatric (both female and male) donors. The stem cells were derived from connective tissues of skeletal muscle and dermis from fetal and mature donors. Geriatric cells were isolated from the endomysial, perimysial (the connective-tissue sheath that surrounds a muscle and forms sheaths for the bundles of muscle fibers)and epimysial (the external connective tissue sheath of a muscle) tissue compartments from a 77-year-old female patient and 67 year old male patient. The six sources of stem cells were labeled as such. Cells from the fetal female skeletal muscle connective tissue are "CF-SkM1" and for the male, they identified as "CM-SkM1". The mature female donors stem cells are identified as "NHDF1" for the 25-year-old and "NHDF2" for the 36-year-old. The 77-year-old female is labeled "PAL2" and the 67-year-old "PAL3".(1)
Cells only multiply a certain amount of times. They memorize the number of times they have divided. For example, if a human cell is frozen for years after 30 divisions, then it's allowed to multiply again, it will only divide 20 to 30 more times. This discovery from the year 1961 is called "Hayflick's Limit". He demonstrated that humans cells can divide about 50 to 70 times depending on the way they are cultured.(2) So between 50 times to 70 times, senescence occurs (which is the process of becoming old).
They decided to deal with progenitor cells and pluripotent cells. Progenitor cells are a biologically ancestral form of a cell. Pluripotent stem cells are cells that don't give rise to new organisms, just to new cell types. In the first group they had pluripotent stem cells that have divided 30 times, which is less than Hayflick's limit. In the other group, they chose cells that have divided beyond 70 times, which is more than Hayflick's limit, so that as the progenitor cells die around the Hayflick's limit. After the progenitor cells die, they should be limited to only the pluripotent stem cells.
To determine the existence of progenitor or pluripotent stem cells with the populations examined, they used the insulin/dexamethasone bioassay. Insulin and dexamethasone permit the identification of specific types of progenitor and pluripotent stem cells within an unknown population of cells. Insulin accelerates the phenotypic expression of progenitor stem cells but has no effect on the pluripotent stem cells. Dexamethasone induces expression in pluripotent stem cells, but doesn't alter phenotypic expression in progenitor stem cells. So if progenitor cells are present in the culture, there will be no difference in the quality or quantity of phenotypes expressing due to insulin.
These phenotypes were expressed at all concentrations of dexamethasone. Maximal expression of a particular phenotype differed with respect to both dexamethasone concentration and time in culture. Maximum expression of skeletal muscle, smooth muscle, cardiac muscle, fibroblasts, and endothelial cells, was induced with 10-8 M dexamethasone within the first two weeks of culture. For adipocytes, maximum expression occurred at 10-9 M dexamethasone in two weeks of culture. Maximum expression of cartilage occurred with 10-7 M dexamethasone in two weeks. Maximum expression of bone was induced with 10-9 M dexamethasone by the end of four weeks. These alterations occurred in all six human stem cell populations at 30 cell divisions, less than Hayflick's law. Above 70 cell divisions, insulin had no effect on the cells, but dexamethasone induced expression for fat, muscle, cartilage, bone, fibroblasts, and endothelial cells. So the data suggest that both progenitor cells (insulin accelerated morphologies) and pluripotent cells (dexamethasone-induced morphologies) were present in the populations of human stem cells at 30 cell divisions.
CM-SkM, CF-SkM, NHDF1, NHDF2, PAL#2, and PAL#3 cells were tested for the presence of CD3, CD4, CD8, CD11c, CD33, CD36, CD38, CD45, CD90, CD117, glycophorin-A, and HLA-II. Mature and geriatric donors exhibited positive staining for CD34 and CD90, but CD34 wasn't detected in the fetal donors, only CD90. For CD3, CD4, CD8, CD11c, CD33, CD36, CD38, CD45, CD90, CD117, and HLA-II the cells came out to be negative for these proteins by staining. Glycophorin-A, CD38, and CD4 were positive only in the NHDF2 group. For CD34, the mature and geriatric samples were greater than the fetal samples, and for CD90, the adult and geriatric samples were not significantly different then from the fetal tissue.
CD34 is known to be show up on cells that are committed to becoming blood and cells that are uncommitted to becoming blood even though they're both hematopoietic precursor cells. Neuronal tissue and small endothelial cells also present CD34. It is thought that CD34 is involved in the regulation of the differentiation of the hematopoietic stem cells. Clinicians utilize CD34 to purify hematopoietic stem cells and progenitor cells for use in autologous (derived from the same person) bone marrow transplantation.(3) CD90 also show up on hematopoietic cells, thymic nurse cells, neuronal tissue, renal mesangial cells, placenta, and some connective tissues. Human blood is positive for both CD34 and CD90. CD34 and CD90 don't belong only in the hematopoietic lineage. They have the ability to differentiate and then express phenotypic marker from multiple mesodermal lineage such as muscle, adipocytes, cartilage, bone, fibroblasts, and endothelial cells. So what this experiment shows is that CD34 and CD90 are not completely dedicated to only hematopoietic cells and they are utilized by the body not only for there functions, but also as identifiers.
In the hematopoietic system, bone marrow stromal cells that include fibroblasts, endothelial cells, macrophages, and adipocytes are recognized to play a role in normal blood formation. In vitro, it has been demonstrated that stromal cells regulate the proliferation and differentiation of hematopoietic CD34 via excreted cytokines and contact mediated interactions.(3)
Stem cell transplantation is also used to counteract myelosuppression. Myelosuppression is a reduction in the ability of the bone marrow to produce blood cells. This provides the patient to be able to withstand higher doses of chemotherapeutic agents, thus increasing the survival in many patients. After stem cell transplantation, most patients experience neutropenia, the lowering of the white blood cells that fight infections, and thrombocytopenia, which is an abnormally low blood platelet count.
Patients undergoing hematopoietic cells stem cell transplantation are better off when the stem cells are derived from peripheral blood progenitors compared to bone marrow progenitors. The peripheral blood stem cells join the original blood much more quickly than bone marrow stem cells because the peripheral blood progenitors seem to be set for a greater acceleration in mitotic activity upon cytokine exposure. It has been also demonstrated that peripheral blood are able to engraft faster with neutrophil and platelets because of CD34 interactions with cytokines.(4)
In this age of technology, we are able to grow stem cells from adults, but now the problem is finding a source in the body that will engraft more quickly, which can increase the rate of survival. Studying the actions of CD34 with cytokines in blood is helpful, but it is still not completely understood. More experiments need to be done with actual patients and not only vitro to see if the same mechanisms hold for other stem cell transplantation other than blood.
Copyright © 2002 Ashor Hiskail and Koni Stone
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