Background: All combinations of harvesting, processing, and injection have been attempted to maximize fat graft take following transplantation. Two theories behind fat transplantation have been proposed: cell survival and host replacement. Although the cell survival theory states that fat cells survive and undergo neovascularization following transfer, host replacement theory predicts adipocyte necrosis and replacement of cells by host tissues. Whether or not transferred fat survives, proliferates, or is replaced by fibrous tissue is relevant for the investment of future resources into this thriving field of research. Methods: A literature search of the MEDLINE and Cochrane databases was performed for studies focusing on the histology of grafted fat after transplantation up to December of 2013. Histologic examinations of grafted fat were reviewed and compared in humans and animals. Results: Sixty-six articles met inclusion criteria, and eight of them were human studies. There was widespread diversity in the method of fat harvest and transfer among the studies, and the date of examination after transfer. Many studies reported the presence of viable adipocytes, although an extensive amount of fibrosis and inflammatory infiltration was also seen, depending on the period of examination. Conclusions: Free fat grafts show a variable response following transplantation, with significant disagreement in the reported evidence. Although neovascularization and preservation of adipocyte architecture appear possible, other fat grafts are completely replaced by necrotic ghost cells and fibrotic ingrowth. Adipocyte survival likely contributes to volume maintenance, but fibrosis may also play a role.
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