‘Fat chance’: a review of adipose tissue engineering and its role in plastic and reconstructive surgery

Abstract

Soft tissue reconstruction remains a continuing challenge for plastic and reconstructive surgeons. Standard methods of reconstruction such as local tissue transfer and free autologous tissue transfer are successful in addressing soft tissue cover, yet they do not come without the additional morbidity of donor sites. Autologous fat transfer has been used in reconstruction of soft tissue defects in different branches of plastic surgery, specifically breast and facial defect reconstruction, while further maintaining a role in body contouring procedures. Current autologous fat transfer techniques come with the drawbacks of donor-site morbidity and, more significantly, resorption of large amounts of fat. Advancement in tissue engineering has led to the use of engineered adipose tissue structures based on adipose-derived stem cells. This enables a mechanically similar reconstruct that is abundantly available.

Cosmetic and mechanical similarity with native tissue is the main clinical goal for engineered adipose tissue. Development of novel techniques in the availability of natural tissue is an exciting prospect; however, it is important to investigate the potential of cell sources and culture strategies for clinical applications. We review these techniques and their applications in plastic surgery.

Introduction

Plastic surgery has evolved as a specialty on the basis of a quest for reconstruction and the aim of achieving ‘like for like’ (Harold Gillies).¹ Each year a large number of procedures performed in plastic surgery units are for reconstruction of soft tissue defects due to trauma, congenital deformity and tumour resection. Soft tissue loss presents a continuing challenge in plastic and reconstructive surgery. Standard approaches to soft tissue reconstruction include autologous tissue flaps, autologous fat transplantation and alloplastic implants. These approaches have disadvantages including donor-site morbidity, implant migration, and absorption and foreign body reaction.²

Over the past decade, autologous fat transfer technique has progressively been used in replacement of volume deficit and contour in an array of plastic surgery branches, namely, in breast and facial reconstructive and aesthetic surgery.^3,4

Neuber first introduced autologous fat transfer in 1893 for facial defects; the technique was later popularised for breast reconstruction in 1895 by Czerny.⁵ During the 1950s, difficulty harvesting techniques and donor site morbidity due to fat block excisions resulted in decreased use of autologous fat transfer technique. Novel liposuction techniques have again elevated the use of autologous fat grafting as a means of reconstructing tissue defects. This does not come without disadvantages, most significantly resorption of large amounts of fat, which has been documented to range from 40% to 80%.⁶ Additionally, the success of using autologous fat tissue grafts to repair soft tissue defects has been limited, as free fat transplantation rarely attains an adequate tissue construct as a result of delayed neovascularisation and loss of graft due to cell necrosis.^7,8 Early studies by Mikus et al and Chachjir et al showed that 50–90% of fat graft was lost at the six-month follow-up.^9,10

Fat is an ideal filler because of its allogeneic nature; it is also a readily available source. Current literature suggests the development of two strategies to generate functional adipose tissue: transplantation of autologous tissue/cell and adipose tissue engineering. Stem cells, with their pluripotential and unlimited capacity for self-renewal, project great potential for tissue engineering and likely enable significant advances for distinct reconstructive procedures.

Methods

A literature search was performed using PubMed and MEDLINE. The search terms included ‘adipose-derived stem cell’, ‘fat transfer’ ‘fat harvest’, ‘adipose tissue processing’, ‘tissue engineering for adipose tissue’, ‘fat transfer in plastic surgery’ and ‘biomaterials for adipose-derived stem cells’. Backward chaining of reference lists from retrieved papers was also used to expand the search.

Findings

Adipose tissue and its components

The cellular components of adipose tissue include lipid-filled cells called adipocytes.⁸ The lipid content in the cytoplasm of these cells can be up to 90% of the cell volume; this lipid content is held in place with collagen fibres. Despite the fact that white fat cells do not have as high a vascularity as brown fat cells, each fat cell contains at least one capillary providing a vascular bed, enabling it to grow.⁸ Transplantation of adipocytes from autologous tissue to fill a soft tissue defect has been unsuccessful in restoring volume due to insufficient angiogenesis within the transplanted tissue and necrosis of the tissue due to delayed vascularity.^11,12

Lipoaspirate obtained from liposuction contains both adipocytes and preadipocytes, the latter forming only 10% of the concentrate. Approximately 400,000 liposuction procedures are carried out in the United States each year, leading to a collection of 100–3000ml of lipoaspirate.¹³ Most of this aspirate is discarded, but if it is suitably collected it can be a central source for adipose tissue harvesting.

Preadipocytes are found in enzymatically activated stromal vascular fraction – these are spindle-shaped cells which have the capability to become rounded due to differentiation with addition of growth factors. They are easily obtainable and readily expand, making them a good cell source.^14,15 Graft survival is hugely dependent on the proliferative ability of preadipocytes.¹⁶

Adipose-derived stem cells

In tissue engineering and regenerative medicine, stem cells are defined by their ability to renew and differentiate along multiple lineage pathways. For regenerative medicine purposes, donor tissue for stem cells should also be readily available in abundant quantities, easily harvestable with minimally invasive procedures, and should be safely transferable to an autologous or allogeneic host.¹³ There are several potential sources for obtaining stem cells for tissue regeneration or repair purposes (Table 1).

Table 1 .

Sources for stem cells include embryonic tissue, bone marrow, adipose tissue, and the brain. Each stem cell type has been shown to have the capacity for differentiating to cell types of multiple lineages

Type of stem cell	Source	Differentiation/lineage
Embryonic	Embryonic tissue	All types
Mesenchyme	Bone marrow, adipose tissue	Osteogenic, myogenic, chondrogenic, marrow,
Hematopoietic	Bone marrow	Blood cells, endothelial immune system lineage
Neural	Brain	Neurons, astrocytes, oligodendrocytes, blood cells

Embryonic stem cells are known to be pluripotent and give rise to all derivatives of the three germ cell layers (ectoderm, endoderm and mesoderm) making them an ideal source for reconstructs.¹⁷ However, the ethics surrounding the harvesting of embryonic stem cells has limited researchers in pursuing this vital source.¹⁸ This has further led to development of tissue engineering techniques for adult stem cell extraction.

Suck et al first described adipose-derived stem cells in 2001.¹⁹ Adult stem cells are derived from distinct somatic tissue and are referred to by their tissue origin; these cells have specialised characteristics in their ability to differentiate into multiple lineages. Over the years, adipose tissue engineering strategies have involved use of transplantation of preadipocytes and adipocytes to restore the volume of tissue lost at defect sites. The International Fat Applied Technology Society has termed these cells as ‘adipose-derived stem cells’ and has defined them as isolated, multipotent, plastic-adherent cell populations.¹³

The introduction of liposuction has markedly improved the harvest of adipose tissue, although the autologous fat obtained through liposuction can harm the lipid-filled cytoplasm of the adipocyte, resulting in inadequate angiogenesis of the transplanted tissue.²⁰ This procedure-induced damage results in a large cell population that will not retain the desired cell volume in vivo.^7,20 Mature adipocytes in culture are also limited in their proliferative abilities and are not readily expandable, their terminally differentiated state contributing to their failure in use for tissue engineering.^9,12,20

In contrast, adipose-derived stem cells possess the ability to be readily expandable in vivo and have the capacity to undergo adipogenic, osteogenic, chondrogenic and myogenic differentiation.¹⁹ Recent developments in tissue engineering have also confirmed a neurogenic, pancreatic endocrine phenotype expressing insulin, hepatic and endothelial differentiation.^21–24 The stromal vascular fraction of enzymatically digested tissue contains preadipocytes. These cells are fusiform or fibroblast-like in appearance before they are differentiated.²⁰ Differentiation of these cells results in morphological, and biochemical changes where the cells become rounded in shape and begin to accumulate triacylglycerol and lipid vacuoles.^9,12 Preadipocytes are considered more valuable as a cell source than mature adipocytes because they are easily cultured, easily expanded and easily obtained.

Tissue engineering methods for extracting adipose-derived stem cells

Adipose-derived stem cells can be extracted from the stromal vascular fraction by culturing on plastic due to their characteristic plastic-adherence property.²⁵ Figure 1 shows a schematic representation of harvest and extraction of adipose-derived stem cells. Studies show that the pellet at the bottom of centrifuged lipoaspirate has the highest volume of adipose-derived stem cells.²⁶ Literature suggests that the lipoaspirate from abdomen surrounding Scarpa’s fascia has cells that are significantly resistant to apoptosis compared with those from thighs and buttocks. Some surgeons have preferred the thigh as a donor site as it is shown to have resistance to weight fluctuation. The cells in younger patients have differentiating ability regardless of donor site, but in adults the arm and thigh are more productive. Further studies have confirmed these observations and the abdomen is now deemed a more preferable site for adipose tissue harvest.²⁷ Most patients have abundant tissue around the abdomen and a large amount of tissue can be harvested with minimal morbidity, although it is still important that standard procedures for tissue preparation, cell isolation and cell culture and nutrient support are followed to achieve the required results.^28–29

Figure 1 .

Schematic presentation of isolation and implantation of adipose tissue-derived stem cells

Recent advances have presented a very low concentration human serum culture and a serum-free environment. This serum-free environment allows the use of tissue-engineered adipose-derived stem cells in human clinical trials. Standard plastic as a biomaterial scaffold is used, as adipose-derived stem cells have a tendency to adhere to plastic, unlike other cells in the stromal vascular fraction obtained from the centrifuge. These cells are later identified by the surface markers CD73+, CD90+, CD105+, CD45–, CD34–, CD14 or CD11b and human leukocyte antigen – DR isotype with the use of fluorescence activated cell-sorting.³¹

Biomaterials in association with adipose-derived stem cells

The success of a reconstructive scaffold relies on a tissue-engineered substance that facilitates large volume regeneration. Adipose cells implanted on three-dimensional biomaterials are vital for reconstruction of three-dimensional tissues.³² Table 2 shows some favourable biomaterials with their respective properties, advantages and method of tissue engineering employed for their use.

Table 2 .

Biomaterials used for tissue engineering^{8,9,21,23–32}

Biomaterials	Properties	Strategies	Description
Collagenous microbeads	Allow ex-vivo proliferation	Injectable composite system	Injection micro carrier beads act as minimally invasive implant, stimulating host adipose cells
Type 1 collagen scaffolds	Allow in vivo replacement of damaged tissue	Scaffold-guided tissue regeneration	Preadipocytes cultured on bioabsorbable material implant in vivo causing cellular proliferation with scaffold resorption
Hyaluronic acid-based spongy scaffolds	Stable cell carriers generate volume retaining tissue	Scaffold-guided tissue regeneration
Omentum	Highly vascularised tissue of omentum, allows in vivo proliferation	Fragmented omentum combined with preadipocytes results in tissue high in triacylglycerol level.
Injectable poly(lactic-co-glycolic acid) spheres	Non-invasive soft tissue fillers	Injectable composite system	Formation of adipose tissue after 8 weeks in vivo.

Role of adipose-derived stem cell tissue engineering in plastic surgery

The use of adipose-derived stem cells has become vital in plastic and reconstructive surgery because of the increased demand for aesthetically natural tissue. To date, only 113 patients have been treated with adipose-derived stem cells around the world.^33–39 The main areas that have used adipose-derived stem cells in either clinical practice (trials) or laboratory research include wound healing, soft tissue augmentation and tissue engineering. Yoshimura et al have presented a series for soft tissue augmentation using cell-assisted lipotransfer for facial lipodystrophy and breast augmentation.^40–41

A meta-analysis by Wang and Wu has suggested better graft take in patients with cell-assisted lipotransfer compared with lipoinjection.⁴² Another study by Toyserkani et al concluded that cell-assisted lipotransfer is a safe and effective method for soft tissue augmentation.⁴³ Kim et al published results showing a 74.6% recovery in patients injected with ‘AdipoCell’ (adipose-derived stem cells harvested from liposuction and differentiated into mature adipocytes) also concluding that this is a safe technique.⁴⁴ Shukla et al published a study in 2015 where adipose-derived stem cells were used for wound healing in patients undergoing radiotherapy and report an improvement in the tissue nature.⁴⁵

Only five clinical trials currently taking place in the field of plastic surgery using adipose-derived stem-cell tissue, three have been completed but there are no published data concerning the complete results. The studies do not confirm a standard method of adipose-derived cell use and there does not seem to be a protocol for number of cells used. There is also no consensus on the number of treatments required. Engineered scaffold has yet had a minimal role in tissue regeneration. A single study by Stillaert et al used a hyaluronic acid scaffold seeded with adipose-derived stem cells but results did not show an increase in adipose tissue formation.⁴⁶ This was attributed to poor angiogenesis to sustain long adipose cell viability.

Conclusion

Soft tissue reconstruction will always remain the mainstay of plastic surgery. Cosmetic and mechanical resemblance with native tissue is the main clinical objective for engineered adipose tissue. The development of new techniques in the availability of natural tissue is an exciting prospect, but it is important to investigate the potential of the cell sources and culture strategies for clinical applications. Mechanical reliability and sustainability of tissue over time with the presence of metabolic activity will serve as a significant development over current reconstructive options. Autologous fat transplantation, with a minimally invasive cannula harvest, has lower donor-site morbidity than tissue flaps, but there is an unpredictable degree of resorption of the transplanted fat over engineering with adipose-derived stem cells.

As with all research, there are specific areas of scientific concern to consider for furthering the development of methodologies. With the use of stem cells for tissue engineering applications, there will be numerous concerns to address, including the standardisation of methods for tissue procurement, cell isolation and cell culture. Currently, adipose tissue-derived stem cells are obtained from liposuction aspirates or abdominoplasty procedures. The methods for harvesting the tissue may have an effect on the ability of the cells to proliferate and differentiate during in vitro culture, thereby introducing inconsistency into the process of cell retrieval and culture for each tissue sample. Variability in stem cell markers also limits the application of successful use.

Successfully engineering any tissue construct requires careful attention to all features of the construct. Adipose tissue-derived stem cells have a huge potential in renewal of tissue and restoration of tissue in respect to the field of plastic surgery. A comprehensive understanding of mechanisms of interaction among adipose stem cells, biomaterials and clinical applications is vital for successful outcomes. Further studies are required to ensure the safety and effectiveness of these procedures and assessment of durability of tissue-engineered constructs.