Fat transplantation is extensively used as a postoperative reconstruction method such as craniofacial surgery and mastectomy; however, most of the transplanted adipose tissue is necrotic, engraftment is unstable, and developing more stable fat transplantation method is awaited. The engraftment instability of adipose tissue transplantation arises when the adipose tissue for transplantation is placed in an ischemic environment, and the transplanted adipose tissue developed necrosis [2, 7–9, 12, 13]. To improve adipose tissue engraftment, we believe that it is important to reduce the cell count with necrosis arising from such ischemic environment, i.e., neovascularization and cell proliferation within the transplanted adipose tissue is required [13–15]. This study’s results suggest that in rat adipose tissue, collagen impregnation to transplanted adipose tissue increases angiogenesis and improves the fat engraftment rate via the number of adipocytes and the proliferation of macrophages. In this study, adipose tissue transplanted into rats histologically promoted angiogenesis following collagen impregnation, thereby improving the engraftment rate. The tissue remodeling mechanism appeared to be involved in the increase in engraftment rate. After transplantation, the adipose tissue is placed in an ischemic environment, which causes tissue necrosis and tissue remodeling . Macrophage migration is strongly involved in the process of such remodeling [17, 18]. Macrophages are broadly divided into M1 and M2 macrophages. M1 macrophages induce the inflammatory reaction required for tissue engraftment, and they are involved in the scavenging of necrotic adipose tissue, whereas M2 macrophages are involved in tissue repair, and both M1 and M2 macrophages carry out an important role in the process of adipose tissue remodeling, and engraftment [18–23]. Furthermore, macrophages improve the survival of adipose tissue grafts by inducing neovascularization and activated stem cells [18, 24]. In this study, we reported that collagen impregnation to adipose tissue transplants significantly increased M1, and M2 macrophages in the transplanted tissue. In particular, M2 macrophages considerably increased, and we believe that this result reflects a state whereby impregnating transplanted adipose tissue with collagen induces the inflammatory reaction required for tissue remodeling and induces a potent tissue repair action [19, 24, 25].
In this study, no significant difference was observed in the expression level of vascular endothelial growth factor (VEGF) mRNA, which is a factor that promotes neovascularization at 1 week and 4 weeks after transplantation; moreover, the level tended to be higher in the collagen group. This result might reflect the fact that impregnating transplanted adipose tissue with collagen serves as a platform for neovascularization and helps give rise to abundant neovascularization in the transplanted tissue. It is possible that the ligation and clustering of integrin receptors α1β1/α2 β1 on the surface of endothelial cells (EC) via the GFPGER (502–507) sequence of the collagen fibers is important for neovascularization activity . While there is still room for debate, to summarize the results of the histological evaluation, we can draw the conclusion that collagen impregnation to transplanted adipose tissue produced additional neovascularization [25–27]. Recently, attention has been drawn to the effectiveness of adipose-derived stem cells (ASC) as a means to promote neovascularization in transplanted adipose tissue. ASC induce differentiation to adipocytes by co-culturing with mature adipocytes , and they contribute to neovascularization by differentiating into vascular EC [14, 29–32]. Furthermore, it has been reported that hypoxia results in the ASC secretion of neovascularization growth factors such as VEGF, and therefore induces neovascularization in an ischemic environment after transplantation . ASC presents in subcutaneous adipose tissue can differentiate into adipocytes ; it has been reported that when ASC are cultured from collagen, more lipids are produced, which is a marker of adipose differentiation . Therefore, collagen impregnation could support ASC differentiation into adipocytes. In this study, on comparing the level of adiponectin mRNA expression at 1 week and 4 weeks after transplantation; the expression level tended to be higher, although not significantly higher, in the control group. In the collagen group, a significant increase in the level of adiponectin mRNA expression was observed at 4 weeks. Furthermore, while there was no significant difference observed, at 4 weeks, the level of adiponectin mRNA expression tended to be higher in the collagen group than in the control group. Adiponectin is an adipocytokine produced particularly in adipose tissue and that produces and secretes various bioactive factors . In humans, it is the gene with the most abundant expression in adipose tissue . This study’s results show that when collagen was added, there was a greater viable adipocyte count at 4 weeks after transplantation, which supports the higher engraftment rate in the collagen group indicated by semi-quantitative evaluation. Moreover, in the collagen group, there was a greater change over time from 1 to 4 weeks after transplantation. This suggests that collagen impregnation leads to greater viable adipocyte proliferation. Our results suggested that collagen impregnation increases the adipocyte count and increases neovascularization in the transplanted adipose tissue. It is possible that a greater effect can be anticipated by combining ASC with collagen, and therefore this needs to be studied further in future.