Discussion: Biological Properties and Therapeutic Value of Cryopreserved Fat Tissue
The wider process of cryopreservation is of great importance in other areas of regenerative medicine, for the storage of both allogeneic and autologous cells to develop methods for the wider use of stem cells.7 These technologies are now developing into realities with the large-scale banking and preparation of mesenchymal stem cells suitable for human clinical trials.8 The preparation and storage of autologous cells has largely been carried out in other fields, such as in the storage of hemopoietic stem cells from bone marrow or the storage of eggs, sperm, and embryos to preserve fertility.9
In this article, Mashiko et al.3 continue to develop the discussion on how best to cryopreserve lipoaspirate fat samples, how many adipose-derived stromal cells could be subsequently cultured from these, and what the quality of fat grafts would be that could be generated in a murine model. They have sensibly evaluated whether two simple cryopreservation protocols for adipose tissue could be reliable techniques for clinical use: the first was simple freezing to −80°C, and the second was using an adipose tissue cryopreservation medium and slow freezing until −80°C was reached. Adipose cell viability is assessed using a proxy marker for cell viability, perilipin. What this article reminds us of is that the process of cryopreservation is toxic to cells, despite using a cryopreservant that is designed to preserve fat. Although it might be clinically attractive to simply freeze and store the lipoaspirate as evaluated in this article, it is clearly demonstrated that this approach leaves scant viable cells.
To date, studies have shown conflicting and contradicting reports regarding the viability of cryopreserved fat tissues.10–13 This can largely be accounted for by the large number of variables that can affect the survival and functionality of adipose cells.14 The challenge of cell preservation is influenced by cellular factors such as size, membrane permeability, morphology, ultrastructural complexity, and composition of subcellular organelles, in addition to the cell density and composition of the preservation medium.14 Specifically, for cryopreservation, differences in agents used and cooling/thawing protocols, storage temperature, and storage duration can contribute to different results. The added challenge of cryopreservation of lipoaspirate is the presence of cell clusters limiting the diffusion of cryoprotectant into all cells, thus exposing the tissue to increased risk of freezing injury through ice crystal formation.14
This limitation of cryopreservation of composite tissue as opposed to cell suspension is seen in other tissues.15 However, what is not answered is how the two cryopreservation methods used in the study compare to methods of cell storage used for the protection of other tissues or cell types. The authors conclude that regardless of cooling speed, temperature, and the presence of cryoprotective agents, cryopreservation produces adipose tissues unsuitable for clinical use.