Bone marrow transplants containing blood stem cells are used routinely for blood diseases such as leukemia. Almost all stem cells currently in clinical trials are from the blood and bone marrow.
Many so-called stem cell therapies from bone marrow are actually mixtures of cells rather than pure stem cells. However, at least one company hopes to begin a clinical trial in spinal cord injury using neural cells grown from embryonic stem cells. Another company recently started a clinical trial using neural stem cells from fetal tissue.
Hematopoietic stem cells (HSCs) are found in bone marrow and create all types of blood cells: red blood cells, B and T lymphocytes, natural killer cells, neutrophils, platelets and more.
This type of stem cell was the first stem cell to be used medically. When a patient receives a bone marrow transplant during treatment for leukemia or other diseases, it is these cells that give rise to the new blood supply. Partly because samples containing these cells can be readily (albeit painfully) collected from individual patients, they are being explored for a variety of potential therapies. Therapies use a patient's own cells or cells from donors that, from a white blood cell's perspective, resemble the patient. Otherwise, the patient's immune system may attack the transplanted cells. Experimental therapies are trying to use purified populations of HSCs or looking for drugs that can cause a patient's own HSCs to proliferate without a transplant.
Mesenchymal cells (also called bone marrow stromal cells) are also found in bone marrow. They make many types of cells including bone cells, cartilage cells and fat cells. Like hematopoietic cells, mesenchymal cells are one of the few adult stem cells that can be obtained reliably. Mesenchymal stem cells are being explored in clinical trials for liver disease, heart disease, Crohn's disease and others. Like HSCs, these trials typically use a patient's own cells.
By adding hormones and other ingredients to the lab dishes, mesenchymal stem cells are grown in them. Scientists have been able to make a variety of more specialized tissue from these cells, including cells that appear to be nerve cells, skin cells and muscle cells. However, these cells are not understood as well as HSCs.
Cord blood cells
Stem cells in a baby's umbilical cord can be collected right after birth. Cord blood mainly contains HSCs and can be used to treat leukemia and other blood disorders. HSCs from cord blood do not require painful bone marrow extraction. Because babies' immune systems are immature, the cells are less likely to be rejected by a recipient's immune system than those from an adult donor. However, recipients of mismatched cord blood can still reject the transplanted cells, and the amount of HSCs in cord blood is too small for widespread clinical applications. Treating an adult or large child would require cord blood from multiple births; alternatively, scientists must find a way to encourage the cord blood stem cells to expand in culture to form many more of themselves.
Nonetheless, several cord blood banking programs are in progress to match donated blood with unrelated recipients. In 2005, the U.S. National Academies issued a report on the topic. Although many doubt its utility, several private companies also store blood that came from a particular child for future use by that family.
Neural stem cells
The brain maintains its own supply of stem cells. These cells seem to be able to migrate within the brain and spinal cord and even integrate with existing neurons. In mice, treatments like antidepressants and physiological conditions like exercise can stimulate these stem cells to produce new neurons. Songbirds form new neurons from their stem cells when they are learning new songs to attract mates, suggesting there may be a connection between neurogenesis and learning in humans. Neural stem cells derived from fetal tissues were tested in a clinical trial for Batten's disease in December 2006. In this fatal hereditary disease children's brain cells poison themselves, and the hope is that the transplanted cells will help clear the toxins. As with other transplant trials, subjects are taking drugs to keep the implanted cells from being rejected.
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What kinds of stem cells are being tested in humans?. Nat Rep Stem Cells (2007). https://doi.org/10.1038/stemcells.2007.24