Most stem cell therapies include grafting some form of replacement tissue into the patient. There are three forms of grafts commonly considered: autograft, allograft and xenograft
An autograft is a transplant from one part of a person into a new position of the same individual. An allograft is a transplant from one person to another, where the donor is genetically different from the recipient, but still a member of the same species. A xenograft is a transplant from an animal for one species to a recipient of another species.
Since autograft most closely resembles the patients other tissues, it offers the least autoimmune rejection and is therefore often the most desirable of the three options, unless there are other considerations like autoimmunity.
An early stage of the development of an embryo is the organization of the ball of cells into a structure composed of three germ layers: ectoderm, mesoderm and endoderm.
Ectoderm is the outer later of cells in the embryo. After separation into the three germ layers, it is the ectoderm that is in contact with the amniotic sac. Ectoderm gives rise to external structures such as the skin and hair. Mesoderm is the middle layer that gives rise to body components such as connective tissues, the cardiovascular system, blood, lymphatics and the urogenital system. The endoderm layer is the innermost layer of the embryo. Endoderm eventually forms the gastrointestinal tract and epithelial aspect of other components such as the lower respiratory system.
Three groups of stem cells exist, each classified according to the range of types of cells that they can produce. Arranged in order of largest to smallest range of possible cells, these are: totipotent, pluripotent and multipotent.
Totipotent stem cells can form all the different cells of the entire organism. This includes not only all the tissues found in the body, such as bone, skin and muscle, but also the surrounding placenta during gestation. A fertilized egg, and the small cluster of cells produced as its few initial rounds of cell division are examples of totipotent cells.
Pluripotent stem cells have the ability to form cells from all three aforementioned germ layers (ectoderm, mesoderm, and endoderm). However, unlike totipotent stem cells, they cannot generate all the cells of the whole organism. Pluripotent stem cells can be further classified into four subtypes. These are: embryonic germ cells, embryonic carcinoma cells, the multipotent adult progenitor cells (MAPC) found in bone marrow, and embryonic stem cells. Embryonic germ cells are further along the path toward end tissue than embryonic stem cells, so have comparatively less possible fates. Embryonic carcinoma cells are primarily aneuploid, which makes them a suboptimal choice for therapeutics. Multipotent adult progenitor cells are a relatively new discovery (Jiang, 2002) that be clonally expanded and induced to differentiate into cells of all three germ lines. These multipotent adult progenitor cells appear to hold good promise, but their primary drawback is their rarity in the bone marrow and the accompanying difficulty in isolation (Jiang, 2002). Embryonic stem cells offer good therapeutic potential, but are hampered by ethical controversy.
Multipotent cells can differentiate into one of a small number of potential cells that are suitable to their relative location. Their role is often to replace cells that are constantly being turned over. For example, a blood stem cell could give rise to various types of leucocytes, erythrocytes, or platelets.
Differentiation refers to the mechanism in which the stem cell precursor takes on the features of the specialized cell of the desired tissue, such as a hepatocyte in the liver, a myocyte in muscle or a neuron in the brain. There is apparently no limit to the type of specialized cell that can be created through differentiation from a stem cell precursor.
In vitro, human embryonic stem cells differentiate spontaneously. The culture conditions help determine which cell types will be created from the embryonic stem cells.
Generally, the human embryonic stem cells are usually assisted to differentiate in vitro though the addition of various growth factors and cytokines (Schuldiner, 2000). However, in vitro, it is a challenge to produce pure amounts of a specific cell type. If the culture conditions are haphazard, a mixture of cell types is created instead.
Transdifferentiation the term given to the reported phenomenon of adult stem cells crossing from one tissue and differentiating into other cell tissue types. For example, an adult neural stem cell would be found to cross a boundary and make tissues that are not nerve-related tissue at all (Clarke, 2000). Transdifferentiation apparently flies in the face of classic embryology that states once a stem cell commits to a certain lineage, it is unable to switch to another lineage. It may be that adult stem cells, though still multipotent, may have the capacity to more types of cells that originally theorized.
“Plasticity” also known as “developmental plasticity” is a the term assigned to an adult stem cell's ability to develop into mature cells of a phenotype that is different from their tissue of origin (Wagers, 2004).
Bone marrow-derived stem cells have been found to be highly plastic. When bone marrow stem cells are transplanted into certain damaged tissue such as muscle, liver, or brain, they are not limited any more to developing into cells commonly found in bone marrow. Instead, they can form cells that are found in the damaged tissue into which they were transplanted (Grove, 2004).
Teratomas grossly display plasticity in that the histological cross-sections display many complex organ types, such as gut-like tissue, cartilage, and even secreting epithelium.
Cell fusion is rising in popularity as a possible underlying mechanism behind the effects seen in transdifferentiation. In the cell fusion process, a somatic stem cell “fuses” with an embryonic stem cell. The result is a different type of tissue or something similar to an embryonic stem cell.
In one experiment, neural stem cells cultured along with embryonic stem cells created non-neural tissues through cell fusion (Ying, 2002). Another experiment showed that when embryonic stem cells were grown with bone marrow cells, cells similar to embryonic stem cells were created, and they were aneuploid (Terada, 2002).
When marrow hematopoietic stem cells are transplanted from the bone marrow into damaged neural, cardiac or hepatic tissue, they cause production of replacement tissue (Wang, 2003; Alvarez-Dolado, 2003; Willenbring, 2004). It is thought that cell fusion is the reason (Wang, 2003; Alvarez-Dolado, 2003; Willenbring, 2004). There is evidence that this same cell fusion that is seen in the lab also takes place in vivo as well (Willenbring, 2004).
There are caveats, however. The cell fusion phenomenon only seems to happen only at a limited frequency in vivo (Lagasse, 2000; Wagers, 2002). Also, it requires both tissue damage and a survival advantage in order for the cells to propagate (Lagasse, 2000; Wagers, 2002). Furthermore, it seems that cell fusion is not the entire story. Studies using fluroscent markers have shown that bone marrow cells are able develop into epithelial cells of the liver, lung and the skin through a process other than cell fusion (Harris, 2004).
Most likely the final picture that will emerge is that cell fusion is a phenomenon that is more common in certain “fusogenic” organs like muscle and liver, the phenomenon can be induced by injury, and there are complementary alternative mechanisms at work such as angiogenesis induction and stem cell-mediated signaling pathways (Hess, 2003; Botta, 2004).
This branch of stem cell research began in the 1970s with the idea of an “inductive microenvironment” (Trentin, 1970), which was followed by the formal concept of the stem cell niche (Schofield, 1978).
Over the ensuing decades, the role and mechanism of stem cell niches was increasingly elucidated. A niche is a discrete cellular space. The microenvironment is the neighboring cellular elements and matrix that surrounds the stem cell. This neighboring milieu passes on chemical messengers at exact times and places to help shape the stem cell's development. An isolated stem cell, in the absence of its appropriate niche, might still replicate, but not towards the desired tissue type.
A teratoma is a usually a benign cancerous mass that is composed of multiple types of tissues, including tissues that are not typically found in the organ in which it is found. Often teratomas contain a mix of tissues of several different germ layers, such as grossly visible, fully formed teeth, hair, and secreting glands. Teratomas are most commonly found in the ovary (where they are usually benign).
A teratocarcinoma is a malignant teratoma, or a malignant epithelial tumor found inside a teratoma. Teratocarcinomas are most commonly found in the testicle.
Teratomas and teratocarcinomas are related to stem cell research in that the cells that create them, embryonic carcinoma cells, are closely linked to embryonic stem cells in many of their properties. Also, early embryonic cells can develop into a teratoma or teratocarcinoma when inserted into an area of the body other than the uterus.
There are several standard tools for genetically modifying stem cells as part of the research or therapeutic goals.
The first is random insertion, a process in which a desired piece of DNA is randomly inserted into the stem cell chromosome.
In mapping the structure function of the stem cell, this random insertion is accompanied by so-called “gene trapping”. In gene trapping, there is an antibiotic resistance gene added to the desired DNA segment, so that stem cells taking up the DNA can be selected. Since the gene could integrate anywhere, another addition to the desired DNA is a reporter gene, such as B-galactosidase. When B-galactosidase is inserted into a part of the DNA that is actually making proteins, it turns a stem cell colony blue under appropriate lab conditions. The antibiotic and B-galactosidase thus show that the stem cell has taken up the desired new DNA, and it has integrated into the chromosome into an area that can make the product (Durick, 1999).
An alternative to random insertion is placing the desired DNA into a predetermined site on the stem cell chromosome, a process called “homologous recombination”. In homologous recombination, the desired DNA segment replaces as segment of the chromosome that has a similar sequence.
Another common technique is a “gene knockout” in which a functional gene is knocked out of active expression. Gene knockouts are often done through homologous recombination. As an example usage, one can knockout the genes that produce the immunological markers for a stem cell, so that a graft recipient of the stem cells will be less likely to reject the transplanted cells.
As discussed earlier, there are different types of stem cells, but in order for a stem cell to be fully qualified as having all the requirements of a true embryonic stem cell, the following conditions need to be met (Bongso, 2004):
In research on human embryonic stem cells, all the criteria have been shown repeatedly for the first five criteria. However, creating actual chimera humans with stem cells is not carried out, due to the obvious ethical restrictions.