ADULT STEM CELLS: SOURCES AND MANIPULATION

The body comprises different types of progenitor cells (stem cells - SCs) capable of giving rise to daughter cells with more restricted developmental potential. Stem cells occur as unspecialized cells, lacking tissue specific characteristics, maintaining the undifferentiated phenotype until they are exposed to appropriate signals. SCs have capacity for extensive self-renewal, and apparently maintain themselves throughout the entire life of an organism. Under the influence of specific signals, SCs can differentiate into specialized cells of different lineages.

Stem cells include embryonic stem cells (ESCs), isolated from the inner cell mass of the blastocyst and mesenchymal stromal cells (MSCs), isolated from fetal and adult tissues. ESCs are pluripotent cells derived from the re-implantation blastocyst inner cell mass, and they can give rise into virtually any cell type, except placenta and supporting tissue cells [1]. However, there are social and ethical constraints surrounding their use for research or therapy. Adult SCs, have the capacity to differentiate into cells of the mesodermal lineages (multipotent) . However, recent reports indicate that in fact they have a much broader differentiation potential [2]. Despite having a more restricted differentiation potential, these cells raise less ethical issues making them more suitable for therapeutic applications. Mesenchymal stromal cells are multipotent cells isolated from adult tissues, such as adipose tissue [4], bone marrow [5], trabecular bone [6], muscle [7], dermis [8], periosteum [9], blood [10], synovial membrane [11], periodontal ligament [12] , and deciduous teeth [13]. These cells can be also isolated from extra-embryonic tissues, including placenta, amniotic fluid, and umbilical cord [3].

The International Society for Cellular Therapy (ISCT) stated that MSCs are characterized by 3 criteria: plastic adherence to tissue culture flasks; 95% of the MSC population must express the surface markers CD105, CD73 and CD90 and lack expression of CD45, CD34, CD14 or CD11b, CD79a or CD19 and HLA class II; Finally, the cells must differentiate in vitro into osteoblasts, adipocytes and chondrocytes under standard differentiating conditions [14]. Another important assay is the colony forming unity fibroblasts (CFU-Fs) assays to assess the clonogenicity of MSCs, i.e, the ability to generate colonies of fibroblast-like cells from single cells, when plated in very low density [15]. The clonogenicity of MSCs reflects the ability of a cell to grow in a density-insensitive fashion.

The most well known and studied source of MSCs is bone marrow [16]. Human bone marrow stromal cells (hBMSCs) are typically isolated from the mononuclear layer of the bone marrow after separation by density gradient centrifugation [17, 18].
MSCs represent the most suitable source of cells for regenerative medicine and tissue engineering (TE) applications. Human bone marrow stromal cells seeded and cultured onto tri-dimensional (3D) biodegradable chitosan-polyester scaffolds in osteogenic conditions, differentiate into the osteogenic lineage [19]. Moreover, this in vitro TE strategy was studied in vivo using a critical size cranial defect in nude mice, and the results showed that the 3D constructs were able to promote bone regeneration in a superior level than scaffolds without cell [20].