Prospect of Stem Cells in Bone Tissue Engineering: A Review

Abstract

Mesenchymal stem cells (MSCs) have been the subject of many studies in recent years, ranging from basic science that looks into MSCs properties to studies that aim for developing bioengineered tissues and organs. Adult bone marrow-derived mesenchymal stem cells (BM-MSCs) have been the focus of most studies due to the inherent potential of these cells to differentiate into various cell types. Although, the discovery of induced pluripotent stem cells (iPSCs) represents a paradigm shift in our understanding of cellular differentiation. These cells are another attractive stem cell source because of their ability to be reprogramed, allowing the generation of multiple cell types from a single cell. This paper briefly covers various types of stem cell sources that have been used for tissue engineering applications, with a focus on bone regeneration. Then, an overview of some recent studies making use of MSC-seeded 3D scaffold systems for bone tissue engineering has been presented. The emphasis has been placed on the reported scaffold properties that tend to improve MSCs adhesion, proliferation, and osteogenic differentiation outcomes.

Figure 1

The ability of MSCs in the bone marrow cavity to self-renew (curved arrow) and to differentiate (straight, solid arrows) towards the mesodermal lineage (including bone cell). The reported ability to transdifferentiate into cells of other lineages (ectoderm and endoderm) is shown by dashed arrows, as transdifferentiation is controversial in vivo. Reprinted by permission from Macmillan Publishers Ltd., Nature Reviews Immunology, Uccelli et al. [32], copyright © 2008.

Figure 2

To generate iPSCs, fibroblasts (or another type of adult somatic cell) are transduced with retroviruses encoding four pluripotency factors (SOX2, KLF4, c-MYC, and OCT4). Fully reprogrammed iPSCs have similar properties to ESCs. They are competent to form teratomas on injection into mice and are capable of generating progeny. Patient's cells can be used to derive iPSCs, which can then be induced to undergo differentiation into various types of somatic cells, all with the same genetic information as the patient. Reprinted by permission from Macmillan Publishers Ltd., Nature, Yamanaka & Blau [42], copyright © 2010.

Figure 3

How MSCs make it to bone marrow. During development, the primitive bone marrow stroma includes skeletal progenitors that originate outside of the marrow cavity (primitive periosteum and perichondrium) and invade the forming cavity along blood vessels. Similar dynamic interactions with ingrowing blood vessels are reproduced in transplants of human MSCs and are probably the basis for the perisinusoidal position of MSCs in the intact postnatal bone marrow. Recruitment of mesenchymal cells to a mural cell fate (and a subendothelial position), a general phenomenon in development and organ growth, is mediated by endothelial cell (EC) derived PDGF-BB, which signals through PDGFR-β expressed on mesenchymal cells (and MSCs). Presumptive mural cells (as well as human and mouse bone marrow MSCs) produce Ang-1, which is crucial for the integrity, survival, and remodeling of vascular lattices. Ang-1 also induces quiescence of hematopoietic stem cells (HSCs). Both mural cells and endothelial cells are induced to mitotic quiescence by active TGF-β1, which is released through proteolytic cleavage of the latent form at sites of mural cell, endothelial cell contacts. Reprinted by permission from Macmillan Publishers Ltd., Nature Medicine, Bianco et al. [46], copyright © 2013.

Figure 4

Vascularization approaches for bone tissue engineering. (a) In vitro prevascularization techniques induce cell-seeded scaffolds to form vasculature via exogenous growth factors. Following implantation in the bone defect, these engineered capillaries will in theory rapidly anastomose to perfuse the entire graft. (b) In vivo ectopic prevascularization involves implantation of a cell-seeded scaffold into a highly vascularized bed, such as muscle or arteriovenous (AV) loop, to allow extensive vascular ingrowth. The graft is transplanted as a free flap to the bone defect and surgically anastomosed with the surrounding vessels to immediately perfuse the graft. (c) In vivo orthotopic vascularization involves direct implantation of scaffolds into the bone defect for in situ tissue development. Cells seeded into the scaffolds can be aggregated to improve cell survival and endogenous cell signaling. Scaffolds can be functionalized for the controlled release of growth factors (stars) that induce bone and vascular growth. Reprinted by permission from Elsevier Ltd., Current Opinion in Chemical Engineering, Hutton & Grayson [22], copyright © 2014.

Figure 5

The microstructure and nanostructure of bone and the nanostructured material used in bone regeneration. (a) At the macroscopic level, bone consists of a dense shell of cortical bone with porous cancellous bone at both ends. (b) Repeating osteon units within cortical bone. In the osteons, 20–30 concentric layers of collagen fibers, called lamellae, are arranged at 90° surrounding the central canal, containing blood vessels and nerves. (c) Collagen fibers (100–2000 nm) are composed of collagen fibrils. The tertiary structure of collagen fibrils includes a 67 nm periodicity and 40 nm gaps between collagen molecules. The hydroxyapatite (HA) crystals are embedded in these gaps between collagen molecules and increase the rigidity of the bone. Nanostructures with the features of nanopattern (d), nanofibers (e), nanotubers (f), nanopores (g), nanospheres (h), and nanocomposites (i) with structural components with a feature size in the nanoscale. Reprinted by permission from Macmillan Publishers Ltd., Bone Research, Gong et al. [56], copyright © 2015.

Figure 6

(a) Photomicrograph of a 3D-bioplotted PLGA/nHA scaffold; (b) DNA staining of the nuclei (DAPI) and anti-β-tubulin antibody staining of cell-seeded scaffolds on day 5 showing uniform cell adhesion onto the strands.

Figure 7

SEM micrographs of hMSC-seeded PLGA/nHA scaffolds: (a) day zero, no cells; (b, c) day zero, with cells; (d) day 5, no cells; (e, f) day 5, with cells.

Figure 8

Hybrid 3D-bioplotting/TIPS scaffold fabrication technique, (a) 3D-bioplotting of the PEG constructs and the adjustable bioplotting parameters, modified from [108]; (b) schematics of the scaffold fabrication process [82]. Figure 8(a) was reprinted by permission from John Wiley and Sons: Polymer Engineering & Science, Yousefi et al. [108], copyright © 2007. Figure 8(b) was reprinted with kind permission from Springer Science + Business Media: Journal of Materials Science: Materials in Medicine, 2015, 26:116, Akbarzadeh et al. [82], copyright © 2015.