## 4 Discussion
However, most MSCs remained in the scaffold when tooth extraction was not performed. MSCs in the collagen gel seemed to have the ability to accumulate at a wound site via the systemic circulation. Therefore, we examined the localization of the MSCs after they migrated from the collagen scaffold, as shown in Figure 4.  
Regarding the effect of administration method, we previously conducted a study comparing systemic and topical administration, which suggested that subcutaneous administration of MSCs was ineffective when not applied with a scaffold [28]. Similar to previous findings [36; 37], the lung and liver accumulated many cells from the systemic circulation. In this experiment, many MSCs were observed in the lung following systemic administration without using scaffolds, but there was only a slight accumulation in the lung when the MSCs were injected within a collagen scaffold. After tooth extraction, fewer MSCs from the collagen scaffold were observed in the lung but could be seen in the wound area. Therefore, delivering MSCs within a scaffold may decrease the risk of lung or cardiac infarction. As MSCs are adherent cells, they readily form cell aggregates immediately after isolation from culture dishes [38]. Thus, when injected via the tail vein, most of the cells formed a mass. This might explain why intravascularly applied cells have higher risks for capture in capillary-like lung tissue. In contrast, cells migrating from the scaffold should move individually, making them less likely get clogged in the lung, which is supported by our results. Finally, we investigated the effect of MSCs with a collagen scaffold. As mentioned before, many reports suggest that MSCs could accumulate in the wound area after systemic administration [39]. As shown in Figure 5B, many cells were observed around the extraction site following conventional systemic injection, but a similar number of cells were observed around the site in the scaffold group. Both groups showed similar promotion of wound healing after tooth extraction, compared with both control groups (Figure 5C). On the contrary, we recently reported that the high density of MSCs at the injected site following local administration meant that it was hard for them to move from the administration site to the site of inflammation [28]. In the scaffold group, the cells present in a suitable space in the collagen apparently maintained their MSC properties. This indicates that side effects such as pulmonary embolism may be avoided by using collagen scaffolds, and that this process may also contribute to tissue healing. The amount of cytokines released from the wound site is reported to closely correlate with the degree of inflammation [40]. In addition, MSC migration in response to cytokines has been shown to occur in a dose-dependent manner [41]. This suggests that when severe damage occurs in a tissue, increased inflammation and cytokine release elicit migration of MSCs to that area. Based on this concept, our delivery method should control the number of MSCs migrating to the damaged site depending on the degree of damage, which may overcome some of the disadvantages of existing cell delivery methods.