Question: What can stem cells be isolated by?
About the article "Stem cell", section "Identification".

Candidate passages:
In practice, stem cells are identified by whether they can regenerate tissue.
For example, the defining test for bone marrow or hematopoietic stem cells (HSCs) is the ability to transplant the cells and save an individual without HSCs.
This demonstrates that the cells can produce new blood cells over a long term.
It should also be possible to isolate stem cells from the transplanted individual, which can themselves be transplanted into another individual without HSCs, demonstrating that the stem cell was able to self-renew.
Properties of stem cells can be illustrated "in vitro", using methods such as clonogenic assays, in which single cells are assessed for their ability to differentiate and self-renew.
Stem cells can also be isolated by their possession of a distinctive set of cell surface markers.
However, "in vitro" culture conditions can alter the behavior of cells, making it unclear whether the cells will behave in a similar manner "in vivo".
There is considerable debate as to whether some proposed adult cell populations are truly stem cells.
Stem cells are cells found in all multi-cellular organisms.
They were isolated in mice in 1981, and in humans in 1998.
In humans there are many types of stem cells, each with varying levels of potency.
Potency is a measure of a cells differentiation potential, or the number of other cell types that can be made from that stem cell.
Embryonic stem cells are pluripotent stem cells derived from the inner cell mass of the blastocyst.
These stem cells can differentiate into all other cells in the human body and are the subject of much scientific research.
However, since they must be derived from early human embryos their production and use in research has been a hotly debated topic as the embryo most likely is destroyed in the process.
Stem cell treatments are a type of cell therapy that introduce new cells into adult bodies for possible treatment of cancer, diabetes, neurological disorders and other medical conditions.
Stem cells have been used to repair tissue damaged by disease or age.
Cloning also might be done with stem cells.
Pluripotent stem cells can also be derived from Somatic cell nuclear transfer which is a laboratory technique where a clone embryo is created from a donor nucleus.
Somatic cell nuclear transfer is also tightly regulated amongst various countries.
Until recently, the principal source of human embryonic stem cells has been donated embryos from fertility clinics.
In January 2007, researchers at Wake Forest University reported that "stem cells drawn from amniotic fluid donated by pregnant women hold much of the same promise as embryonic stem cells.
"In 2000, the NIH, under the administration of President Bill Clinton, issued "guidelines that allow federal funding of embryonic stem-cell research."
Most progenitors are described as oligopotent.
In this point of view, they may be compared to adult stem cells.
But progenitors are said to be in a further stage of cell differentiation.
They are in the “center” between stem cells and fully differentiated cells.
The kind of potency they have depends on the type of their "parent" stem cell and also on their niche.
Some progenitor cells were found during research, and were isolated.
After their marker was found, it was proven that these progenitor could move through the body and migrate towards the tissue where they are needed.
Many properties are shared by adult stem cells and progenitor cells.
But still, controversy remains because embryonic stem cells are true stem cells in that they are pluripotent and show unlimited capacity for self-renewal.
In contrast, many cells termed adult stem cells would be better defined as progenitor cells, as their capacities for unlimited self-renewal and plasticity have not been comprehensively demonstrated.
Progenitor cells are found in adult organisms and they act as a repair system for the body.
They replenish special cells, but also maintain the blood, skin and intestinal tissues.
They can also be found in developing embryonic pancreatic tissue. 
 Veterinary applications of stem cell therapy as a means of tissue regeneration have been largely shaped by research that began with the use of adult-derived mesenchymal stem cells to treat animals with injuries or defects affecting bone, cartilage, ligaments and/or tendons.
There are two main categories of stem cells used for treatments: allogeneic stem cells derived from a genetically different donor within the same species and autologous mesenchymal stem cells, derived from the patient prior to use in various treatments.
A third category, xenogenic stem cells, or stem cells derived from different species, are used primarily for research purposes, especially for human treatments.
Most stem cells intended for regenerative therapy are generally isolated either from the patient's bone marrow or from adipose tissue.
Mesenchymal stem cells can differentiate into the cells that make up bone, cartilage, tendons, and ligaments, as well as muscle, neural and other progenitor tissues, they have been the main type of stem cells studied in the treatment of diseases affecting these tissues.
The number of stem cells transplanted into damaged tissue may alter efficacy of treatment.
Accordingly, stem cells derived from bone marrow aspirates, for instance, are cultured in specialized laboratories for expansion to millions of cells.
Although adipose-derived tissue also requires processing prior to use, the culturing methodology for adipose-derived stem cells is not as extensive as that for bone marrow-derived cells.
While it is thought that bone-marrow derived stem cells are preferred for bone, cartilage, ligament, and tendon repair, others believe that the less challenging collection techniques and the multi-cellular microenvironment already present in adipose-derived stem cell fractions make the latter the preferred source for autologous transplantation.
New sources of mesenchymal stem cells are currently being researched, including stem cells present in the skin and dermis which are of interest because of the ease at which they can be harvested with minimal risk to the animal.
Hematopoetic stem cells have also been discovered to be travelling in the blood stream and possess equal differentiating ability as other mesenchymal stem cells, again with a very non-invasive harvesting technique.
There has been more recent interest in the use of extra embryonic mesenchymal stem cells.
Research is currently underway to examine the differentiating capabilities of stem cells found in the umbilical cord, yolk sac and placenta of different animals.
These stem cells are thought to have more differentiating ability than their adult counterparts, including the ability to more readily form tissues of endodermal and ectodermal origin.  
Adult stem cell lines isolated from mature tissues are commonly used in stem cell research, as are cells isolated from umbilical cord blood.
However, these cells have a genetic imprint of the host they were taken from, thus limiting their therapeutic use in genetic disorders.
Also, adult stem cells are not totipotent or pluripotent like embryonic stem cells, but rather more specialized cells that are multipotent.
Another source of stem cells is the iPS (induced pluripotent stem) cell.
This process involves reversing the differentiating cell signals that cause a stem cell to specialize.
In this way, a somatic cell can be worked backwards (induced) into a stem cell.
One added advantage of this type of stem cell is that the pluripotent cell has the same DNA as the donor and can be used therapeutically towards that end without painful bone marrow and spinal cord extraction techiques.
It also contributes to pain relief.