La Medicina Estetica - N° 4 - Anno 2007
Stem cells have attracted folk’s imagination on the great promise that they may be responsible for the repair of damaged tissues, or used in the treatment of degenerative diseases and also employed to mitigate the functional disorders associated with the ageing of some cell types.Adult stem cells reside in most tissues, but the actual role they play in cell homeostasis and tissue repair varies much depending on the type of tissue: their regenerative and repairing contribution is clear in the case of fast-turnover tissues while it is much less clear in tissues having a slow (or even absent) turnover.The possibility that stem cells might be used in therapies for the mitigation of ageing-associated degenerative processes inevitably poses a couple of fundamental questions. First, what is the effect of ageing on the number of stem cells residing in the various tissues and on their functionality? Second, how does the tissue environment contribute to the decline of stem cells’ functionality associated with ageing? The answer given to these two essential questions has certainly a major importance in the formulation of therapeutic approaches based on the use of stem cells and also in the development of a regenerative medicine pinned on solid scientific grounds. A few cases gave the chance to study the first issue. Having this in mind, we carried out researches based on the identification and isolation of stem cells from certain tissues so as to evaluate the relevant number and function across the different ontogenetic phases. For instance, with respect to hemopoietic and muscular stem cells (the satellite cells), it has been possible to demonstrate that their number does not decrease during ageing time, while instead their repopulating and regenerating functions decline with it. The survey research should therefore be targeted so as to cover the mechanisms that are responsible for the functional «ageing» of stem cells.The second ageing mechanism is far more difficult to be examined from an experimental point of view, as it depends on the complex interaction between stem cells and tissue microenvironment. The difficulty of the in-vitro reconstruction of the tissue microenvironment stands out and therefore there isn’t, at the moment, any experimental model allowing the evaluation and the definition of the exact role played by tissue microenvironment modifications that contribute to the decline of stem cells during ageing time.A further element of complexity is added by the type of tissue, as certain types of tissue have a high cell turnover rate and, as a consequence, a remarkable regenerative potential in response to tissue damages (such as, for instance, hemopoietic tissues, the intestinal epithelium, the skin). Other tissues have instead a low cell turnover rate and limited regenerative capacity (such as, for example, the myocardium tissue and nervous tissue). Of course, the role of stem cells is different in the above various types of tissues, and therefore also the role of the ageing process in the modification of stem cells’ environment is different in those tissues.It is clear that any progress we achieve in the basic knowledge of stem cell biology represents a fundamentally important factor that enables a therapeutic use of these cells aiming at the regeneration of tissues damaged by pathological processes or as a consequence of ageing. From a therapeutic point of view, it would be possible to apply two different approaches: (i) stimulation of endogenous stem cells; (ii) transplantation of stem cells expanded in vitro. The studies carried out so far have clearly shown that the use of stem cells for therapeutic purposes had a completely different outcome in the case of fast-turnover tissues with respect to results obtained with slow-turnover tissues.
a cura di: Ugo Testa