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There are hundred of different types of cells present within the human body. In mammalian development these are structured and organised into the tissues and organs that make up an organism (Watt et al., 2010) The starting point of all this are stem cells, which are derived during early development, from the blastocyst, and have the ability to go on and form all the cells that make up an adult organism (Thomson et al.,1998). General characteristics of all stem cells are that they are unspecialised, and can renew themselves through the processes of cellular division. Also, under conditions which are found to be specific to organs or tissues, they go on to become specific to that environment, and have special functions. Stem cells also act, within the adult body, as a repair system. When these resident cells divide, each new daughter cell goes on to either remain in that tissue as a stem cell, or differentiate into any other type of cell required (Lathija 1979), for example a red blood cell, heart cell, or hepatocyte. (Figure 1)
Scientists have classically worked on the two different isolated classifications, embryonic stem cells or adult stem cells. Embryonic stem cells, as the name suggests, are derived from embryos that are the product of in vitro fertilisation techniques. The actual cells themselves are sourced from the blastocyst, more precisely the inner cell mass, one of three layers that make up this structure. Adult stem cells, on the other hand, are defined as being an undifferentiated cell, that is present and resides within adult differentiated tissue. These cells are thought of as being limited to produce only the cells of that tissue, albeit any of the varieties of specialised cell present. One important way in which these cells differ from classical embryonic stem cells, is that there origin is not known in all tissues. This is still a target of research in many cases. It is thought that they do tend to have specific â€œstem cell nichesâ€Â within tissues and organs though. Later, a third classification of stem cells was derived, due to the development of techniques in various labs which allowed the induction of a stem cell like state in previously specialised adult cells. These are classified as induced pluripotent stem cells.
The science behind the isolation of these cells was originally adapted from mouse models. Stem cells were first isolated from mouse embryos in laboratories in 1981. Detailed studies of these cells and the way they are derived from the blastocyst layers, led to the discovery and development of techniques to isolate human embryonic stem cells, from human embryos (Thomson et al, 1998) Research of adult stem cells intensified after the discovery in the late 1950's that bone marrow contained at least two types of cells displaying the characteristics of stem cells. These were titled haemopoetic stem cells, which differentiate to form all the cells present in the blood, and mesenchymal stem cells. These cells were to go on to be of great significance and interest, due to there ability to differentiate into the cells that make up bones, cartilage, fat, and the fibrous connective tissues of the body (Leichty et al, 2000)
Figure 1 summarising the lineage and classification of all the major types of stem cells, developed from Bongso and Lee (2005)
One of the most interesting areas of research in recent years has been regenerative medicine. This can be defined as the science of replacing damaged tissues, or organs with new healthy ones, which go one to have the same functional characteristics as the originals. This area of medicine has been fraught with hurdles, but stem cells look to play a very important role, once again, due to their defining characteristics of development to any cell, and being unspecialised. One area of intense research is type 1 diabetes. This disease is characterised by the chronic destruction of pancreatic beta cells through an autoimmune reaction (Atkinson and Eisenbarth, 2001). The current treatment for type 1 diabetes is daily therapeutic injections of insulin. However there is current promise being shown in studies where pancreatic islets have been replaced with new tissue (Ryan et al., 2002). These studies have again been hit with the two main restrictions of transplantation surgery, lack of donor tissue, and immune rejection of tissue that is transplanted. One possible solution to this is to grow islets from induced pluripotent stem cells, taken from a patient. It is hoped that co-transplantation of these cells may help aid immuno-acceptance of transplants, or even promote growth of new islets themselves, being that in vitro, or in vivo. The focus of this discussion is looking at the breakthroughs in stem cell research relevant to diabetes, and how these have bettered our understanding of disease phenotypes, and ways to treat type 1 diabetes in future.
The transplantation of islets from cadavers was first described in the early 1980's (Lacy, 1982). Although this was, at the time, seen as a breakthrough in diabetes treatment, it was soon characterised by a disappointly low success rate. However, since the turn of the millenium, there has been widespread implementation of the Edmonton Islet Transplantation protocol, which has seen a normalisation of techniques used, and the immunosuppresive drugs used to avoid immune rejection (Shapiro et al., 2000).
To better understand how to regulate, and reverse the effects of type 1 diabetes, it is important to understand which cells are vital in the pancreas, more importantly which of these cells aid in regeneration of damaged tissue. It has been shown previously that pancreatic cells can be replaced when exposed to particular stresses (Holland et al.,2004). This lead to the many research groups hypothesising that progenitor cells did indeed exist, and that if the regenerative ability of these cells could be harnessed, type 1 diabetes could be ameliorated significantly. One cell type which receives a great deal of interest from researchers is the pancreatic ductal cell. These cells were analysed to for transcription markers, and it was found that Pdx-1, a key transcription factor for pancreatic development, especially important for growth of islets, was present in all ductal cells (Heimberg et al., 2000).