Progenitor cells: role and usage in bone tissue engineering approaches for spinal fusion

Advancement of in vitro osteogenesis, or the production of bone, is a complex process that has significant clinical implications. Surgical intervention of several spinal disorders entails decompression of the spinal cord and nerves which can lead to subsequent biomechanical instability of the spine. Spinal arthrodesis (fusion) is often required to correct this instability and necessary to eliminate the resulting pathological motion of vertebral segments. Therefore, the achievement of proper spinal fusion, is a critical determinant of treatment efficacy. This chapter focuses on the molecular and cellular components that are involved in bone growth and healing. Mesenchymal stem cells (MSCs) and hematopoietic stem cells (HSCs) are the precursor cells essential for the formation of the five different types of bone cells: osteoprogenitor cells, osteoblasts, osteoclasts, osteocytes and lining cells. Similarly, endothelial progenitor cells (EPCs) differentiate into endothelial cells, which are essential in angiogenesis and neovascularization. MSCs tri-lineage potential (osteogenic, chondrogenic and adipogenic lineages) have made them the focus of most experimental approaches. Here, we describe their individual roles, as well as pose novel concepts on how their collective role may be the optimal strategy to improve upon in vitro osteogenesis and whether this could also be translated to improved bone formation in vivo. Further, we discuss the various molecular markers that are available for cell identification and the tissue engineering strategies that could replicate the osteoinductive, osteoconductive and osteoproductive milieuthat is available in autograft. Finally, we present a broad primer on the possible integration of cellular, molecular and tissue engineering strategies to improve osteogenesis and the future trends that may bring the promise seen in the laboratory to fruition in preclinical animal models.