Stem cells and tissue engineering

Transcription

1 Stem cells and tissue engineering S. Swaminathan Director Centre for Nanotechnology & Advanced Biomaterials School of Chemical & Biotechnology SASTRA University Thanjavur Tamil Nadu Joint Initiative of IITs and IISc Funded by MHRD Page 1 of 8

2 Table of Contents 1. CAN STEM CELLS BUILD TISSUES? Mesenchymal stem cells in tissue engineering Hematopoietic stem cells (HSC) Tissue derived stem cells... 6 Joint Initiative of IITs and IISc Funded by MHRD Page 2 of 8

3 1. Can stem cells build tissues? Stem cells have the plasticity to differentiate into many lineages. Hence it plays a vital role as a starting material in tissue engineering because of its in vivo regenerative potential. Generally, the main objective of tissue engineers is to develop two fundamental tissue types (i.e) either epithelial tissue or connective tissue. Each of this tissue possesses stem cells. Now the next specific question that arises is about the engraftment potential of the cultured stem cells. This is because; there are evidences in the literature to confirm the potential loss of stem cells in culture very quickly. In vivo, stem cells are undifferentiated cells, which are capable of differentiating at critical conditions. Hence, it is mandatory to maintain the microenvironment of stem cells in vivo. It was identified that the soluble growth factors, nutrients and signals from the supportive cells of mesenchymal origin improves the culture condition by which we can prolong the culture without affecting the differentiation and pluripotency. Though there are different types of stem cells available, stem cells with minimum or no ethical concern and patient friendly has received much attention in tissue engineering. Hence, stem cells such as mesenchymal stem cells, hematopoietic stem cells, and tissue-derived stem cells receive much attention. 1.1 Mesenchymal stem cells in tissue engineering Mesenchymal stem cells are the adult stem cells derived from the bone marrow. This cell type has the capacity to differentiate into many lineages namely osteocyte for bone; chondrocyte for cartilage; myoblast for muscle; fibroblast for tendon or ligament; and other connective tissue cell types under specific culture conditions. In addition to this, these cells can be isolated from the patients since they avoid the immune complications. Generally, various dimensional scaffold structures with porous characteristics have been evaluated for the proliferation as well as the differentiation of the MSC. This is because porous characteristics are very Joint Initiative of IITs and IISc Funded by MHRD Page 3 of 8

4 essential for the vascularization, cell infiltration and also for the diffusion of nutrients to and waste from the scaffold. Whatever the tissue may be to regenerate, design of proper ECM play a major role in the stem cells aided tissue engineering. This is because stem cells alone are not sufficient to construct the three-dimensional organization of tissues. In addition, stem cell delivery is mainly to replace the lost cells at that site of injury. However, delivering stem cells to the target tissue without any cell loss is really tough to achieve. Hence, targeted delivery of MSC is highly efficient to restore the function of lost tissues. This MSC delivery can be achieved using different carriers such as injectable gels, scaffold matrixes, films, or even immunoprotective microencapsulation. Though, the delivery is achieved by targeted carrier, the next important challenge is to provide the appropriate signal to achieve a desired differentiation. Number of polymeric candidates is widely explored for the tissue engineering applications such as natural materials like collagen, alginate, gelatin and synthetic polymers like polyethylene glycol, polycaprolactone, polyglycolic acid, polylactic acid, and even poly(lactide-co-glycolide). Bone marrow derived mesenchymal stem cells has found to have different positive markers such as STRO-1, CD29, CD44, CD71, CD90, CD106 and so on, which are required to isolate the cells from the heterogeneous population. These isolated stem cells will then be expanded in vitro without affecting its differentiation potential. Under certain conditions, some exogenous growth factors are required to stimulate the osteogenic differentiation of BM-MSCs for bone regeneration. It was also found that the osteogenic induction with the growth promoting signals improved the formation of alkaline phophatase positive colonies with a cuboidal morphology and finally progressed to mineralized bone formation. Generally osteogenic induction medium comprises essential elements such as β- glycerophosphate, ascorbic acid along with dexamethasone. Other exogenic factors such as recombinant bone morphogenic protein (rh-bmp) and parathyroid hormone related peptide will used for the osteogenic differentiation and Joint Initiative of IITs and IISc Funded by MHRD Page 4 of 8

5 proliferations. However, care should be taken always with the passage number of stem cells, which detemines the multilineage potential of the stem cells. Though the mesenchymal stem cells alone are used as therapeutic agents for bone formation, there are a few reports proved the efficiency of 3-D scaffold seeded along with the BM-MSCs. For example, BM-MSCs were seeded within collagen sponge scaffold and also self-assembled peptide nanofiber scaffold has been used for the in vitro and in vivo osteogenic differentiation, showing significant improvement in the bone formation. There are various reports indicated the use of BM-MSCs-hydroxyapatite hybrid particles in bone formation. MSCs have also been used in the cartilage regeneration, which can promote the chondrogenesis in the presence of transforming growth factor-β. Poly(L-lactide-coε-caprolactone) (PLCL) has been modified with chitosan and evaluated the adhesion, phenotypic change as well as proliferation of human bone marrow derived MSCs. Chitosan modified PLCL scaffold has shown improved cell compatibility with the adequate mechanical strength towards chondrogenesis as compared to unmodified PLCL scaffold. Hence, many attempts have been made to evaluate the material property with proper modification on MSCs proliferation as well as differentiation. Another key question is how well the choice of the cell source would affect the chondrogenesis potential in 3D scaffolds? The major elements present in the chondrogenic differentiation medium are TGF-β, insulin, dexamethasone along with BMP 6. The cell source that they have tested are human chondrocytes (HC), embryonic stem cells (ESC), embryonic stem cell derived mesenchymal stem cells (ESC-MSC) and also mesenchymal stem cells derived from bone marrow and adipose tissue (BM-MSC; ASC). This study has even made use of two different scaffolds namely chitosan and silk fibroin scaffolds with loaded BMP 6. The metabolic activity of the cells was found to be higher in the silk scaffolds as compared to chitosan scaffolds due to difference in pore size. This result confirms the role of scaffolds and its architecture on these cells metabolic activity. Among all cells, EMS-MSCs has shown better chondrogenic Joint Initiative of IITs and IISc Funded by MHRD Page 5 of 8

6 property in both the scaffolds and identified as a promising cell for chondrogenesis. This type of experiments has proven the role of cell source for regeneration. 1.2 Hematopoietic stem cells (HSC) HSC can self-renew and also differentiated into many lineages such as hematopoietic tissues such as blood and immune system, non-hematopoietic tissues such as cardiac myocytes, vascular endothelial cells and even hepatocytes. The positive cell surface markers are Sca-1 and c-kit, characterizing the HSC. The major advantage of such stem cells is the ready availability from different sources such as bone marrow, cord blood and even from mobilized peripheral blood. These cells can even be used for cell replacement therapy for treating acute renal failure and can self renew or develop into the progenitors, which are mainly precursor of various types of blood cells. However obtaining a pure HSC in culture and expansion of culture are two big barriers since impure HSCs promote the graft versus host rejections. 1.3 Tissue derived stem cells Like bone marrow derived mesenchymal stem cells, some tissue-derived stem cells have been exploited for regeneration. For example, liver stem cells are unipotent having excellent regenerative potential. The ideal characteristics of liver stem cells are the ready availability, which does not have any ethical concerns. Simple isolation technique with a unique surface marker, able to expand in vitro, can be cryopreserved with high viability, ability to retain the hepatocyte differentiating potential and no chances of malignancy over time exploits its use in regeneration. Under normal conditions, differentiated hepatocytes and bile duct cells can divide and self-renew. Under some pathological circumstances, oval cell or hepatocyte progenitor cell appears in the liver, which possesses bipotential plasticity (i.e.) this oval cells are being able to differentiate into hepatocytes as well as bilary cells, which are thought to be originated from the hematopoietic stem Joint Initiative of IITs and IISc Funded by MHRD Page 6 of 8

7 cells. This oval cell has positive markers such as albumin, fetoprotein, cytokeratins and some progenitor markers. Fig 1: Bipotential plasticity of hepatocyte progenitor cells Another example is neuronal stem cells which are originally isolated from the ventricular zone of human fetal brain. These neural stem cells can self renew and give rise to progenitor cells of neuroblast or glioblast. Neuroblast progenitor is Joint Initiative of IITs and IISc Funded by MHRD Page 7 of 8

8 intended for the development of neurons, whereas glioblast cell give rise to glial cells such as astrocytes, and oligodendrocytes. Fig 2: Lineage of Neural stem cells Joint Initiative of IITs and IISc Funded by MHRD Page 8 of 8