Stem Cell Standardization

S tem cells are difficult to isolate and differentiate into specialized cell types with high yield and accuracy, and the procedures to efficiently implant them into injured organs remain to be defined. The failure thus far experienced in applying stem cell technologies to repair parenchymal organs can be ascribed not only to the insufficient knowledge of basic mechanisms, but also to the lack of standardized criteria and protocols. In the absence of automated or standardized tools and procedures to handle stem cells, each laboratory has followed its own “recipe” often using inconsistent nomenclature and noncomparable, if not confusing, experimental protocols. All this has made difficult to learn from the others and, ultimately, has hampered the progression of knowledge on stem cell behavior and the implementation of concepts, doctrines, procedures, and designs that could allow efficient, safe, and more cost-effective advancement of cell treatments based on higher levels of compatibility and commonality.

Stem cells are inherently unstable and, in fact, metastable. Their state is thought to be maintained by the neighboring microenvironment niche delivering a dynamically symmetric array of signals finely orchestrated along the time axis [1] (Fig. 1) to prevent ageing and to preserve their multipotency [2]. The modulation of the niche signals therefore sustains progenitor cell self-renewal or induces their commitment on the basis of tissue-specific needs [3]. De facto, the whole differentiation process can be seen as a continuum of niche states (Fig. 2). Among the many factors involved in regulating stem cell features and fate within the niche are stem cell–stem cell and stem cell–differentiated cell interactions as well as interactions between progenitor cells and adhesion molecules, extracellular matrix components, growth factors, and cytokines. Microenvironmental metabolites and chemical agents, including pH, ionic strength (eg, Ca²⁺ concentration), and oxygen tension are also considered fundamental. Additionally, segregation within niches is modulated by environment-controlled sensors, that is, cell membrane and intracellular proteins, so that only those specifically correlated with a specific differentiating step are active.

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FIG. 1.

The tripartite axes of stimuli, which interact dynamically to characterize a stem cell symmetric environment throughout its differentiation. Note that the features of the symmetric environment (depicted by colored arrows) change with time.

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FIG. 2.

The differentiation pathway of a metastable (inherently unstable) stem/progenitor cell. Differentiation entails the restriction of stem/progenitor cell potency associated with energetic variations (E on the y-axis) and should be conceived as a continuum of different metastable states (small blue and purple cells at the bottom of the graph) along the time axis (t on the x-axis). From the starting condition of pluri/multipotency, the undifferentiated stem cell as well as the intermediate cell populations must overcome the energetic transitions (dotted red lines and the above arrows) inherent to each metastable state, to progress along the differentiation path. Once a very small modification in local or long-range extracellular signaling occurs, the environmental equilibrium is perturbed and the metastable progenitor cell is attracted toward a different differentiated state. The progression along the differentiation pathway toward one of many possible stable states is driven by the modulation of the symmetry among different extracellular signals (mechanostructural, physicochemical, and biochemical factors).

This intricate arrangement of elements enjoys near countless degrees of freedom, all changing with time, defying our desire to understand the complexity of the whole system and hence predict progenitor cell fate from a given initial state. In this context, current protocols on isolation, purification, expansion, and implantation into damaged organs, adopting conventional techniques and strategies for their culture and manipulation ex vivo, neglect stem cell idiosyncrasies. Conversely, the replication in vitro of the continuum of microenvironmental niches to safely and precisely govern progenitor cell differentiation and homing into the target organ requires to take into account the critical symmetry of myriad of signal-releasing factors [1]. Therefore, the emphasis on stem/progenitor cell technology must be shifted from a trial-and-error–based biochemical tuning of the desired phenotype toward more rigorous and well-defined engineering and modeling techniques. The evolution of stem cell research from an artisanal to a clinical enterprise requires a commitment toward industrial “standardization” of the different aspects composing the mosaic of materials, procedures, and expertise contributing to the successful exploitation of this complex endeavor.

Standardization is a fundamental achievement of the industrial society that has made available safe and cost-effective tools. A continued process of standardization has spurred research into innovative solutions and affected all aspects (materials, procedures, regulations, etc.) of technological and the associated business development. In contemporary biology and medicine, except for a few technological fields, materials, procedures, and indices are still far from being standardized.

To repair rather than replace a damaged organ, exhausted or injured specialized cells must be replaced. This might be achievable by introducing a population of cells with the appropriate potential to home and thence supply the necessary number of cells that are capable of locally differentiating to the required phenotype. Alternatively, prior to their introduction into the body, stem cell fate must be appropriately directed to generate all or most of the specialized cells constituting a particular tissue in vitro. The development of robust and generalizable stem cell techniques can only be achieved by adopting standardized materials and procedures in a long-term process actuated by merging the knowledge derived from different disciplines across the world.

There is, therefore, a strong impetus for change. Initially, this requires a cross-fertilization to encourage a positive and expanding melting pot of ideas, methods, and expertise to benefit from stem cell technology. Similarly, we must coordinate our understanding of what definitions and standards will be mutually understood and applied to achieve standardization of stem cell research. This cannot be a concern of the biomedical field alone, but must involve the knowledge accumulated in a multiplicity of fields, such as mathematics, information and communication technology, material science, and engineering. The convergence between biomedicine and engineering holds promise to be highly beneficial. Synergistic application of information technology, advanced materials, imaging, nanotechnology, and sophisticated modeling and simulation are now being incorporated into the clinical approach. Some scientists and analysts believe that the transformation of biology into an “information” science from a “discovery” science could be the pinnacle of all innovations. The compliance of stem cell products with widely recognized and respected standards will maximize their clinical efficiency and compatibility with those manufactured or offered by others, thereby increasing potential applications and widespread acceptance. To this end, stem cell standardization requires the availability of complex numerical models of cell/cell, cell/biomaterials, and cell/bioactive factors relationship to help investigators to wade through the diversity and disparity of knowledge now beginning to emerge as well as of large dedicated data banks and communication networks to share and categorize the huge quantity of new information published. However, beyond its biomedical relevance, effective standardization is a factor of economical development, because it promotes forceful competition and enhances profitability, shaping the industry itself and allowing all parties to realize mutual gains, but only by making mutually consistent decisions. Finally, standardization cannot entail strictly technical issues, clinical aspects, and economic interest, only. In fact, the use of stem cells for technological purposes must occur in a widely consented regulatory context to avoid offence to the different cultural and religious mentalities.

In this way only, stem cell researchers can move forward with a true shared understanding of the materials, methods, and values of our research, and how best that work can be translated in an industrialized process able to supply clinicians and patients with efficient, safe, and more cost-effective cell treatments. Just as the Engineering Standards Committee developed much needed assessment and certification of production that evolved into the international ISO system, stem cell research needs to decide how best and under whose auspices the standardization of stem cell research should proceed. Stem Cells and Development invites comments and indeed commentaries to the editor on this subject for future publication on how best to advance our field.