Bone reconstruction of large defects using bone marrow derived autologous stem cells

Abstract

Bone is a tissue that has the ability to heal itself when fractured. Occasionally, a critical defect can be formed when part of the bone is lost or excised, in this case the bone fails to heal and requires bone reconstruction to prevent a non-union defect. Autogenous cancellous bone is the current gold standard treatment in bone loss. Because the amount of autogenous cancellous bone that can be harvested is limited, the expanding need for bone reconstruction is paired by the growth of interest in the discipline of tissue engineering. Labs worldwide are working to provide the right carrier and the right set of cells that, once retransplanted, will ensure bone repair. Several investigators have focused their attention on a subset of autologous non-hematopoietic stem/progenitor cells contained in the adult bone marrow stroma, referred to as stromal stem cells (SSC), as the appropriate cells to be transplanted. The use of autologous cells is facilitated by less stringent ethical and regulatory issues and does not require the patient to be immunologically suppressed. In pre-clinical and clinical protocols of critical defects in which SSC are employed, two approaches are mainly used: in the first, SSC are derived from bone marrow and directly introduced at the lesion site, in the second, SSC are derived from several sites and are expanded ex vivo before being implanted. Both approaches, equally correct in principle, will have to demonstrate, with definitive evidence of their efficacy, their capability of solving a critical clinical problem such as non-union. In this report we outline the difficulties of working with SSC.

Introduction

The first piece of evidence that marrow can form bone dates back more than a hundred years [1]. From that time came the assumption that cells contained in marrow could be used to produce new bone. In spite of the age of this discovery and the scientific milestones achieved over all these years the field it is still vibrant, and the scientific community working on those cells is very involved and energetic. One negative consequence of this vital community is that it is difficult to find consensus. For instance, there is no agreement even on the name to be given to these cells. Names such as “mesenchymal stem cells” and “bone marrow stromal stem cells” are frequently used [2], [3].

Difficulties in giving a single name to these cells is also a consequence of their nature. In the case of these cells they have been shown to originate many specialised cell types and not only of mesenchymal lineage, such as bone, cartilage and fat [4], [5], but they have also been shown to be determined to became hepatic and neuronal cells [6], [7], making the name “mesenchymal stem cells” outdated. We prefer to name the cells by indicating their origin instead of their ability to form tissues, and in our paper we will describe them as “stromal stem cells” (SSC).

It is not just a matter of cell name, the confusion of having different names for the same cells would be easily overcome if there existed a way to determine that all the laboratories are working on the same cell type. Morphologically SSC have several phenotypes and in spite of several attempts none have been able so far to identify a protein, and originate an antibody, that would specifically identify these cells [8], [9], [10]. This makes it difficult to compare results from studies conducted in several institutions.

In addition, the nature of cultivated SSC is under debate. While some researchers regard them as stem cells, others consider them only precursors cells. A stem cell is by definition a cell that has the ability of indefinite self-renewal and is capable of forming at least one specialised cell type. Although these cells can be kept in culture for a long time, as achieved previously in several laboratories [11], [12], [13], there are no doubts that our bone marrow knows how to cultivate stem cells better than researchers do, and that, sooner or later, the stem cell fraction of the expanded cells will be lost in cultures. However, the researchers' duty is to obtain a number that would be useful in clinical practice and not to keep stem cells alive for the entire body life span like the bone marrow does. If it can be proven that they are stem cells for the time required to expand the cells, even just a few days, researchers may be allowed to call them “stem cells”.

Despite such a murky and difficult environment, the number of investigators that use SSC in pre-clinical or clinical studies is surprisingly increasing. This is because several investigators are focused more on proving the efficacy of SSC in bone tissue reconstruction, than on really understanding the biology and the exact role of these cells in bone reconstruction. The reason is simple, these investigators are pressed to solve orthopaedic clinical problems and do not want to invest time and resources studying SSC biology if these cells are not able to perform the task needed. This approach is risky, especially if the researchers are not sure which and how many stem cells are implanted, because, in case of failure, the results of the study would spread the knowledge that SSC are not effective at solving a particular clinical problem, even if no stem cells have been used in their studies.

There are two main approaches to SSC clinical application: the first, which is followed by most investigators, is to use unexpanded SSC. SSC are extracted from the bone marrow and can be reintroduced during surgery with or with out being concentrated. In the second approach, SSC are expanded ex vivo to reach a cell number relevant for the clinical application before being reintroduced during surgery. Both approaches are equally good in principle and possibly the efficacy of both approaches could be the same, however, both approaches have several advantages and disadvantages that will be described in this review.