Adult Stem Cells and Bone: the What, Where, and When

There is a significant need for therapies to enhance fracture healing, especially in larger species like the horse. Traditional bone grafting techniques have limitations that may be avoided with stem cell tissue engineering strategies. While many of the early discoveries that serve as the foundation for the “stem” cells of today occurred within the last 70 years, sentinel findings by cell biologists and hematologists occurred in the 19 century. Hence, a long and convoluted history paired with an explosion of discovery over the last 20 years contributes to a plethora of definitions and standards within the stem cell field. For purposes of this review, the term multipotent stromal cells (MSCs) will be used to refer to cells that can be isolated by adherence to plastic and that possess the characteristics of self-renewal and the ability to differentiate into multiple tissue types including adipose, bone, and cartilage. Osteogenesis of two of the most common cell types harvested from adult mesenchyme, defined as originating from the embryonic mesoderm, will be covered. Hence, the key points will be specific to adipose(ASCs) and bone marrow-(BMSCs) derived multipotent cells to address the need for therapies to enhance fracture healing. An important consideration within MSC mediated bone formation is the difference between orthotopic and ectopic osteogenesis. Ectopic osteogenesis refers to ossification of tissue implanted outside of a normal site of osteogenesis (or outside of the implanted tissue origin). Orthotopic osteogenesis refers to bone formation in its correct anatomical location. Both orthopic and ectopic ossification models are used for many studies surrounding MSC osteogenesis. Implantation sites for experimental ectopic osteogenesis include subcutaneous, intramuscular, and less commonly, beneath the renal capsule. Ectopic sites allow a somewhat controlled environment for in vivo experimental bone formation with a relative lack of bone cytokine stimulation, cell-to-cell interaction with endogenous bone–forming cells, endongenous MSCs, and bone-stimulating mechanotransduction. However, these variables all contribute to bone regeneration in a normal fracture environment. Hence, while ectopic bone formation models contribute to optimization of MSC bone formation, the distinct biochemical and mechanical environment of orthotopic bone formation is perhaps most relevant to validation and implementation of MSC therapies. Translation of MSC technology to augment patient fracture healing requires biocompatible scaffolds to carry and then support the cells following implantation. As such, there is significant effort directed toward developing scaffolds that promote MSC osteogenesis based on principles largely derived from autograft bone function and remodeling. Scaffolds can be divided into three major classes, polymers, ceramics, and metals, all of which have distinct advantages and disadvantages. The ideal scenario is for MSC-loaded implants to initiate and support bone regeneration in parallel with complete resorption of the scaffold. Scaffold carriers should encourage MSC adhesion, proliferation, and osteogenic differentiation. Implanted