Stem cells have received significant attention as an ideal source of regenerative-capable cells because of their multipotentiality and ability to replicate. Furthermore, stem cells have been used in a limited fashion for decades with great clinical success.

This article reviews some of the recent literature about stem cells and their use as a promising filler, and will discuss potentials and limitations.

The literature regarding stem cells were reviewed and summarized to include reported indications for use of stem cells as a filler, either a single agent or in combination with hyaluronic acid gel or acellular dermal matrix. We will briefly use the fat graft model and discuss its limitations compared with stem cells.

EMBRYONIC STEM CELLS

Embryonic stem cells are derived from the inner cell mass of the blastocyst and have the capacity to differentiate into all tissues of the body.1

Both mouse and human embryonic stem cells have demonstrated an in vitro capacity to form cardiomyocytes, hematopoietic progenitors, neurons, skeletal myocytes, adipocytes, osteocytes, chondrocytes, endothelial cells, and pancreatic islet cells when cultured under specific growth factor conditions.2,3

Multiple limitations, however, currently exist regarding the use of human embryonic stem cells in regenerative medicine. This includes unregulated differentiation and formation of teratomas and teratocarcinomas, especially in undifferentiated states and the potential immune response to an embryonic stem cell-derived tissue graft. Concerns have been raised over the acquisition of immunogenic residues secondary to culture on mouse feeder cells.4

Somatic nuclear cell transfer, also referred to as therapeutic cloning, involves the transfer of nuclei from postnatal somatic cells into an enucleated ovum. Mitotic divisions of this cell in culture lead to the generation of a blastocyst capable of yielding a whole new organism.5

A 51-year-old female patient before (top) and after 6 months of fat grafting to the face (bottom).

ADULT STEM CELLS

In contrast with embryonic-derived tissues, adult stem cell sources avoid the ethical concerns regarding fetal tissue harvesting for tissue-engineering purposes.

For tissue-engineering purposes, a well-studied adult stem cell population includes mesenchymal stem cells. Mesenchymal stem cells have been isolated from bone marrow, umbilical cord blood, and adipose tissue. The tissue origin of mesenchymal stem cells seems to be a major determinant of progenitor characteristics.

Adipose tissue-derived stem cells, in particular, fulfill several requirements proposed for successful clinical use in regenerative applications6: They can be readily harvested during a minor liposuction procedure under local anesthesia, and they have been successfully used in regenerative applications in numerous animal models. During this procedure, they demonstrated the capacity to differentiate into cartilage, bone, muscle, and adipose tissue.

TISSUE-SPECIFIC STEM CELLS

In addition to embryonic stem cells and mesenchymal stem cells, tissue-specific “resident” stem cells have been identified in almost all postnatal tissues and organs.7 They are capable of both self-renewal and differentiation throughout an individual’s life span and utilize both mechanisms to maintain a steady state and regenerate injured tissue.8

Induced Pluripotent Stem Cells

Takahashi and Yamanaka published a landmark article in 2006 that defined a specific set of transcription factors capable of reverting differentiated cells back into a pluripotent state, thus creating “induced” pluripotent stem cells.9

With the ease and reproducibility of generating induced pluripotent stem cells, compared with somatic nuclear cell transfer, experts have raised the hope that induced pluripotent stem cells might fulfill much of the promise of human embryonic stem cells in regenerative medicine.10

Stem Cells as Fillers

For years, both physicians and scientists have been searching for the perfect tissue filler—one that can be injected into different body areas and tissue planes, can survive in ischaemic tissues, and produces a long-lasting reproducible augmentation, without the drawbacks of current technology.

A 37-year-old patient before (top) and 8 months after fat grafting to the midface and an upper-lip lift (bottom).

Autologous fat grafting technique suffers from the drawbacks of donor-site morbidity and, more importantly, significant resorption of the grafted fat. Adipose tissue engineering using adult human stem cells has been found to overcome the shortcomings of autologous fat grafting in reconstructing facial defects. Mesenchymal stem cells that can self-renew and differentiate into mature adipocytes have been used to generate adipose tissue through both in vitro and in vivo cell-transplantation studies.11

Neuber was the first to publish findings regarding the use of autologous fat transplantation in 1893. He filled scars with autologous fat and found a reduction of transplant resorption by decreasing graft particle size.12 Unfortunately, despite more than 100 years of clinical use since then, little has been developed to improve free fat graft performance, and clinical experience has been lackluster.13,14 Specifically, the clinical longevity of the graft is highly variable and the volume of large grafts in particular decreases significantly over time.

Histologically, progressive loss of transplanted adipocytes is noted along with a conversion of the graft to fibrous tissue, oftentimes with cyst formation. The presumed mechanism of tissue loss appears to be primarily insufficient vascularity and cell death. However, scientific confirmation of this as the only mechanism involved is limited.

Other mechanisms—such as mechanical disruption of cells, lipid-induced membrane damage, apoptosis, or perhaps other potential mechanisms—are possible but have not been well studied.15 It has been suggested that the augmentation effect of fat grafting is attributed to an active role and differentiation of adult mesenchymal stem cells from the stromal fraction of the transplant.16,17,18,19

ADIPOSE STEM CELL-DERIVED AGENTS

In this article, the author has reviewed recent studies that have demonstrated the use of adult stem cells as a single agent or in combination with other filler materials.

In one study, Yoshimura and colleagues treated 23 patients with either demonstrable soft-tissue defects or breast augmentation using stem cell-supplemented fat transplantation for soft-tissue fill. This data has been presented in both the United States and Japan. It suggests that this approach may be feasible and effective.20

Despite the promising results of the trial, the final determination of the success and potential complications must be predicated on the complete study and its peer-reviewed evaluation.

In animal studies and trials undertaken to improve the results outcome, adipose-derived stem cells (autologous) were injected in combination with non-animal stabilized hyaluronic acid to improve graft vascularization and survival. Acellular dermal matrix was injected to improve the graft durability and Alginate powder mass transfer, thus allowing for subcutaneous injection and protecting cells from shearing forces.

A 42-year-old patient before (top) and 12 months after fat grafting to the midface (bottom).

Non-animal stabilized hyaluronic acid and adipose tissue-derived stem cells hold promise as a tool to achieve lasting volume fill in reconstructive surgical soft-tissue augmentation.21 In the proposed combination, the gel functions as a temporary scaffold, providing structural stability to the cells.

The combination of the filler with the autologous cells could improve the clinical outcome, preventing migration and resorption of the fat graft in this challenging host area of great mobility. While the synthetic matrix is slowly resorbed, differentiating or existing adipose-derived stem cells can synthesize new extracellular matrix and reorganize the matrix into the mature form found in fat.22

In a comparative study, one group was injected with mesenchymal stem cells and hyaluronic acid while the second and third group was injected with acellular dermal matrix (ADM) and hyaluronic acid, respectively.

In the first group, the mesenchymal stem cells, when combined with hyaluronic acid, were able to fill in deep folds with progressive improvement of skin tone and decreasing lines of expression more than the second and third group, where either hyaluronic or mesenchymal stem cells were injected as a single agent. When combined with hyaluronic acid, mesenchymal stem cells were able to fill in deep folds, with progressive improvement of skin tone and decreasing lines of expression.23,24

To obtain more durable soft-tissue filler, ADM was seeded with adipose-derived stem cells in one study where histologic analysis showed that adipose-derived stem cells were successfully seeded onto ADM. The thickness of the implanted material and the vascular density were highest 8 weeks postoperatively.25

For maximizing and facilitating adipose-derived stem cells, mass transfer is undertaken, ultimately allowing for subcutaneous injection to protect cells from shearing forces.

Alongside this process, Alginate powder was dissolved in saline, and adipose-derived stem cells were encapsulated (1 million cells/mL) in alginate using an electrostatic bead generator. The study shows that adipose-derived stem cells can be readily cultured, encapsulated, and injected in alginate microspheres. Stem cells suspended in alginate microspheres survive in vivo and are seen to replicate in vitro.26

CONCLUSION

Stem cell technology has flourished as an exciting field, encompassing almost every organ and tissue system. Current concepts of stem cell biology have provided much insight into the physiological and pathological states of tissue regeneration.

These previous reports and early clinical series show that adipose-derived stem cells offer the possibility of providing durable and autologous filler without the drawbacks of current technology.

Most of the reviewed studies are animal studies, and it is still too early to claim adipose-derived stem cells as the perfect tissue filler. Longer-term studies will allow greater understanding whether adipose-derived stem cells have the potential for a clinically relevant benefit in soft-tissue augmentation.


Mark Attala, MBBS, is a cosmetic surgeon and training registrar of Australasian College of Cosmetic Surgery based in Melbourne, Australia. This article was also published in the Journal of Cosmetic Surgery and Medicine. Attala can be reached at [email protected]

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