Biomedical Engineering Reference
In-Depth Information
Mesenchymal stem cells
(MSCs), which take part of the mesenchyme of varied tissues
such as the bone marrow, adipose tissue, brain, dental pulp and skin, and are capable of
differentiating into many cell lineages. MSCs present great potentials to the treatment of
several diseases due to their low immunogenicity, immunomodulatory properties, the
possibility of autologous transplantation, easy isolation and
in vitro
proliferation
possibility.
The bone marrow was the first source of MSCs described in the literature, and still remains
the more thoroughly studied stem cell type. Also present in the bone marrow, there are:
Hematopoietic stem
cells (HSCs), which differentiate into all the hematopoietic and
lymphoid cells from the blood. Therefore, HSCs are studied due to their roles in
leukemia and other blood diseases. Usually, the treatment of such diseases include the
substitution of the sick bone marrow to a healthy one, and in accordance to such fact,
studies involving HSCs are mainly focused on how HSCs behave in different live
organisms. Autologous grafts, or the implantation into a genetically similar live
organism, may be performed in order to treat blood related diseases, as well as
heterologous implantations, or grafting into genetically different live organisms.
Presently, these different graft types show paradox behaviours. Heterologous grafts
cause immune rejection in the host, requiring the host to be continuously submitted to
immunossuppression. This therapy can lead to patient death due to the absence of an
immune response to opportunist pathogens, however, this treatment is still commonly
used today. In cases where the patient's conditions are good, cells can be extracted from
the patient himself. This method is named autologous transplantation and is not
susceptible to host rejection (Friedenstein et al, 1966; Owen et al, 1988, Conrad et al,
2004; Davila et al, 2004 and Gregory et al, 2005).
Even though stem cells derived from bone marrow have been well studied, they do not
constitute the ideal mesenchymal stem cell source, due to the limited extent of MSC
isolation (low extractable quantity of tissue) and donor discomfort. Therefore, new
alternative sources have been proposed, including the adipose tissue. Adipose tissue is an
excellent tissue to obtain great quantities of mesenchymal stem cells and it presents low
discomfort when compared to bone marrow. According to Zuk, 2002, adipose tissue is a
viable source to obtain mesenchymal stem cells, and these cells present similar
characteristics to bone marrow MSCs.
Since the discovery of so many MSC sources, the International Society for Stem Cell Therapy
postulated that a cell will only be considered a MSC if it presents 3 characteristics: 1. Being
able to attach to cell culture surface; 2. Specific surface antigen expression; 3. Multipotent
differentiation potential (osteoblast, adipocytes, chondroblasts) (Dominici et al, 2006). MSCs
must then be capable of differentiating into cartilage, bone and muscular cell lineages, self-
renewing and proliferating
in vitro
.
Embryonic stem cells
(ESCs) are derived from the inner cell mass of the blastocist and present
great moral, religious and ethical barriers due to their isolation technique, which leads to
embryo destruction. They constitute a very promising stem cell type considering the tissue
engineering field, for their pluripotent differentiation potential and unlimited proliferation
capacity. Besides their ethical issues, ESCs also present the possibility of when injected
in vivo,
to produce teratomas. Thus, even though ESC present endless wonderful possibilities to be
used in several science fields, more studies are necessary to ensure their safety and efficiency
to be used in humans (Takahashi et al, 2006). In 2009, the biotech company Geron received the
FDA approval to start the first human clinical trial of embryonic stem cell-based therapy in
