In Vivo Imaging of Regenerated Tissue: State of Art and Future Perspectives

Abstract

Mesenchymal stem cells (MSCs) are multipotent cells which can give rise to mesenchymal and nonmesenchymal tissues in vitro and in vivo [1]. The distribution of resident MSCs throughout the post-natal organism is mainly related to their existence in perivascular niches [2]. They can differentiate into osteogenic, adipogenic, chondrogenic, myocardial, or neural lineages when exposed to specific stimuli, making them attractive for tissue regeneration [3, 4]. Emerging evidence has shown that MSC transplantation offers a means to stimulate tissue repair either by direct (exogenous) or indirect (endogenous) cell replacement or angiogenesis [5, 6]. In fact, exogenous MSCs have shown the ability to support a paracrine activation of endogenous stem cells for tissue repair by secreting chemokines, as stromal derived factor-1 alpha (SDF-1α), and/or growth factors, as vascular endothelial growth factor. Despite the rapid research advancement, possible tissue repair by adult stem cell therapy is currently hampered in vivo by poor cell viability and delivery efficiency, uncertain differentiating fate, and therefore the use of this approach has raised a number of bioethical questions [7]. Hence, the strong need for more effective therapeutic approaches emphasizing the physiological plasticity of postnatal organs following an injury [8, 9], and more accurate imaging methods to allow a long-term in vivo monitoring of tissue regeneration [10]. Indeed, one of the most important accomplishments of modern physiology is the development of imaging techniques able to explore biochemical/molecular processes in the intact organism, i.e. in the absence of confounding effects inevitably caused by invasive procedures or ex vivo experimental prepar