Regenerative medicine encompasses all therapeutic approaches to enhance repair processes by replacing or regenerating cells, tissues, or organs to restore health. At its core, outcomes in regenerative medicine are influenced by the genome, the patient, and the cell (Fig. 95–1). Although diverse strategies have been examined, (e.g., stem cells from diverse sources, biomaterials, tissue engineering products, and neo-organogenesis), regenerative medicine is driven by the capacity of stem cells to acquire multiple and discrete fates, a property that can be exploited to repair and/or replace damaged tissue for improved therapeutic and functional outcome.

The discovery of cells that exhibited self-renewal and potency was the first identification of a new type of cells: embryonic stem cells.¹ However, the earliest conceptual understanding of the “epigenetic landscape” that is the basis for current understanding of stem cell plasticity was suggested in 1957 by CH Waddington.² Conceptually, Waddington likened embryonic stem cells to a marble sitting at the top of a hill poised to roll down and end up in any number of valleys. From this, it was understood that multiple paths could be taken by an embryonic stem cell to acquire its final phenotype. Although some tissues retain a pool of resident, semi-differentiated stem cells for limited repair and renewal, once an embryonic stem cell is committed to a specific cellular fate, it is considered terminally differentiated and the energy required to push it back up the hill to effect a “reset” is insurmountable (Fig. 95–2).

This initial framework to explain the concept of pluripotency was dramatically revised with the discovery of the Yamanaka factors,³ which demonstrated that the right combination of transcription factors could restore an embryonic phenotype from a fully differentiated adult fibroblast. This reprogrammed cell possesses an equivalent ability to that of embryonic counterparts, and is able to differentiate into multiple cell types.

This finding paved the way for a new era in developmental studies and disease modeling, as one could now obtain a patient cell and reprogram it to study pathological developmental from a pluripotent origin. In addition, the potential for novel and enhanced therapeutic modalities, such as autologous cellular repair, were now possible.

Several stem cell types have been used in regenerative medicine: embryonic stem cells (ESCs), induced pluripotent stem cells (iPSCs), and endogenous/adult stem cells. The former two of these are pluripotent and can acquire any cell fate given stimulation with the appropriate signaling cues. Endogenous and adult stem cells are more restricted in their potential, that is, they are considered multipotent or totipotent, and have a limited repertoire of fates compared to pluripotent cells. However, with these cell types, their final phenotypes tend to be more robust and the procedure for guiding them to their differentiated state is less complicated than it is for ESCs and iPSCs. Overall, each class of stem cell thus possesses advantages and limitations that may make them more or less suitable depending on application.

Embryonic Stem Cells

Derived from the inner cell mass of the blastocyst, these naïve cell types are self-renewing and undifferentiated, provided the proper growth factors and conditions are present. Morphologically, ESCs are small round structures that form characteristic stem cell colonies in culture (Fig. 95–3). Ultrastructurally, the most prominent features of ESCs are nuclei that can occupy as much as 95% of cellular volume (Fig. 95–3), and mitochondria that adjust their number and metabolic activity as the cell requires.⁴

As the nucleus accounts for the largest proportion of ESC structure and function, regulation of chromatin metabolism are heavily weighted in the ESC genome.^5,6 DNA replication, recombination, and repair are over represented, as are mechanisms of modification, that is methylation and acetylation, of DNA and histones.^6–9 Chromatin accessibility is essential in regulating pluripotency states of ESCs, thus nucleosome dynamics is a key determinant of the ESC ability to respond to stimulatory cues.^10,11 This capacity to regulate chromatin readability and accessibility contributes to ESC criticality,^12,13 a concept in which the pluripotent genome is maintained within a range of functionality that gives it the potential to form any cell type without sacrificing its capacity to self-renew in an undifferentiated form. In the transition from pluripotent to acquired cell type, these epigenomic mechanisms are tightly controlled, fully securing the committed fate of the embryonic stem cell.

Mitochondrial plasticity also plays a role in cell specialization.^14,15 The most salient metabolic characteristic of ESCs is their reliance on glycolysis to meet their energetic requirements. As ESCs begin to differentiate, mitochondria fully mature in support of the underlying metabolic switch from glycolysis to oxidative phosphorylation.⁴ This transition from a primitive metabolic pathway ensures that the growing energetic needs of the differentiated cell are met, as intricate ultrastructural remodeling, organelle biogenesis, and functional acquisition occurs with phenotype acquisition.

Induced Pluripotent Stem Cells

The discovery of cell reprogrammability started a revolution in the stem cell field, as it became possible to recapitulate embryonic pluripotency from adult cells.³ Taking an adult fibroblast cell, Shinya Yamanaka’s group strategically re-expressed transcription factors to define the minimum essential factors required to acquire an embryonic phenotype. They found that the combination of Oct4, Sox2, Klf4, and Myc reverted fibroblasts back to an embryonic-like phenotype. These induced pluripotency stem cells (iPSCs) exhibit hallmarks of an inner cell mass-derived ESC and are capable of differentiating into non-fibroblast cell types.¹⁶ This sparked the growth of an entire field of iPSC research with critical ongoing contributions to developmental and disease modeling in the areas of neurophysiology and cardiovascular biology, for example.^17–22

The iPSC phenomenon does raise interesting considerations with respect to transcriptional memory.^23–26 The notion of transcriptional memory suggests that although the phenotype of iPSCs is comparable to ESCs, there are epigenetic marks that regulate key elements of reprogramming that give rise to iPSCs with varying potency. In combination with varying reprogramming efficiencies, these specificity and yield issues present significant challenges for clinical applications beyond disease modeling to investigate patient-relevant pathological development.²⁷ While demonstration of robust ESC equivalency will be key to translational iPSC utility,^28,29 current implementation of human iPSCs as patient surrogates for pharmacological testing and cellular toxicity assays provides immediate demonstration of advantages in using iPSC technology.^30–32

Endogenous/Adult Stem Cells

The innate capacity for tissue repair is mediated by endogenous pools of totipotent stem cells that exhibit streamlined capacities to serve as progenitors for a limited subset of cell types. For example, hematopoietic stem cells and satellite cells in muscle are recognizable as the historically most characterized endogenous stem cell types,^33–35 and it is proposed that stem cell niches exist for each tissue in the body (Fig. 95–4).^36–38

Tissue specificities of these potential stem cell pools are limited to fewer cell fates owing to epigenomic variability between adult and embryonic stem cells. This is supported by distinct expression differences in key methyltransferases, demethylases, acetyltransferases, and deacetylases that regulate the methylation and acetylation processes on DNA and histones.^39,40 Altered DNA silencing and chromatin accessibility restrict programmatic elements that can be activated in endogenous/adult stem cells, focusing their potential to a subset of phenotypes and reinforcing the notion of transcriptional memory.

Mesenchymal stem cells (MSCs) have emerged as one of the most promising types of endogenous/adult stem cells due to a favorable ethical profile and better safety. They possess self-renewal ability and can differentiate into cells of multiple lineages, including mesoderm lineages, such as chondrocytes, osteocytes, and adipocytes, and ectodermic and endodermic cells. MSCs can be isolated from the bone marrow, adipose tissue, and umbilical cord, and are easily expanded in vitro. Experimental and early clinical data suggest that (MSCs) may be more effective than other cell types for diseases that are difficult to treat or untreatable (Fig. 95–5).

Brown adipocytes differentiate from another population of preadipocytes located in paraxial embryonic mesoderm and remain multilocular (having many small lipid droplets) with numerous mitochondria (not shown here). Mitochondrial metabolism of lipid in brown adipocytes releases heat rather than ATP. Cells functioning as brown adipocytes can also develop from beige adipocytes during adaptation to cold temperatures.

In the context of tissue repair and rehabilitation, translational applications can exploit the unique properties of the various stem cell types for multiple physiological systems. Based on data available in the clinical trials registry website from the National Institutes of Health (https://clinicaltrials.gov), at the time of this writing, there are around 1,000 active clinical trials associated with stem cell-based therapies in Phases 1 to 3. The majority of these clinical studies are based on endogenous/adult stem cells, with a number of therapy trials associated with active cardiovascular and neurological studies (Table 95–1). Notably, a majority of these trials utilize mesenchymal and hematopoietic stem cells. A few open trials are associated with pluripotent-based treatments, specifically human ESC (hESC). Of these, seven are associated with macular degeneration, one for Parkinson’s disease, and one for ischemic heart disease (Table 95–2). Transplantation of MSCs in animal models with spinal cord injury, have shown to promote repair, therefore, there is much interest in the clinical use of these cells in this condition as well.