The embryos from which the embryonic stem cells are derived are typically four or five days old and are a hollow microscopic ball of cells called the blastocyst. The blastocyst includes three structures the trophoblast, which is the layer of cells that surrounds the blastocyst; the blastocoel, which is the hollow cavity inside the blastocyst; and the inner cell mass, which is a group of approximately 30 cells at one end of the blastocoel. The embryonic stem cells which may be either totipotent (having full potential to make any cells) or pluripotent cells (having potential for most but not all cells of the body) are collected from the inner cell mass, and are transferred to in vitro dishes for further expansion/manipulation.
Currently, the creation of new embryonic stem cells is regulated in many countries. Most notably the USA has severe restrictions on new cell line creations whilst countries like India, Australia, and the UK have no restrictions.
What are adult stem cells?
An adult stem cell (ASC) is an undifferentiated cell found among differentiated cells in a tissue or organ., can renew itself, and can differentiate to yield the major specialized cell types of the tissue or organ. The primary roles of adult stem cells in a living organism are to maintain and repair the tissue in which they are found. Another term for adult stem cells are somatic cells, and the origin of the ASCs in a tissue is sill unknown.
Where will researchers or clinicians collect adult stem cells?
ASCs can be found in blood, bone marrow, muscle tissue, adipose tissue, and umbilical cord blood, however the available concentration of stem cells usually dictates the preferred donor tissue.
Two decision points determine the preferred donor tissue: (1) will the cells be expanded in culture, or (2) will they simply be concentrated and used without further replication. Today, most clinicians use bone marrow, cord blood or adipose derived stem cells without doing expansion, but methods, reagents, and proteins are under development in research labs to allow controlled cell expansion and differentiation resulting in high cell copy numbers. Ultimately, expansion may become the preferred approach since high cell dose appears important to positive and effective clinical outcome.
How do stem cells impact a repair or healing process?
This is the big question cell biologists are trying to answer today. It is believed that damaged tissues or cells send out chemical signals which begin the recruitment of stem cells to the site of damage. This process is often referred to as homing, but it is not well understood. Furthermore, once a stem cell has reached a damaged site, it is believed that growth factors are upregulated which drive further expansion of the cells at the local site. This entire field of homing and differentiation regulation is driven by proteins commonly referred to as growth factors, cytokines, chemokines or ligands.
Clinical Uses for treatment of disease
Presently, only adult stem cells are being used to treat patients. Most notably is the use of umbilical cord blood stem cells which are capable of giving rise to hematopoietic, epithelial, endothelial and neural tissues. Therefore, they are amenable to treatment of a wide variety of diseases, and this promising source has fueled the growth of cord blood banks worldwide. Specifically, the doctor will collect the residual blood in the placental cord after childbirth, and this blood is further processed for the young and highly potent cells prior to cryopreservation. The cryopreserved cells can be available for the donor, the donor’s family or even an unrelated recipient at a later date so long as an adequate HLA match is found to reduce the risk of host versus graft disease.
Other sources of adult stem cells used in clinical practice today is bone marrow, which may be harvested from either the patient themselves (referred to as an autologous or self matching tissue) or a related (referred to as allogenic or non-self tissue) HLA matched sample; muscle tissue (autologous) which is harvested from a large muscle in the patients body and stem cells are isolated/purified and expanded prior to transplant; and, adipose (autologous fat tissue) which the stems cells harvested and purified prior to transplant. Each of these tissue sources of stem cells hold considerable promise for use in ischemic tissue repair specific for revascularization in cardiac repair post a myocardial infarction or in the legs of diabetics which are facing loss of blood flow from limb ischemia. Both of these treatments are in Phase I / Phase II studies around the globe. |
