Although several previous experimental studies have demonstrated the presence of ADSC in the brain up to several weeks after being administered , the scarcity of stem cells in brain tissue could not account for the observed therapeutic outcomes (Leu et al. 2010; Jiang et al. 2014). The finding of stem cells not yet embedded into the brain tissue (Gutierrez-Fernandez et al. 2011; Ikegame et al. 2011) also precludes the possibility of their direct participation as fully functional neurons, implying their help through other mechanisms in the recovery process (Gutierrez-Fernandez et al. 2011). In concert with that finding, ADSC has been reported to produce a number of trophic factors including VEGF, angiopoietin-1, and HGF (Ikegame et al. 2011), insulin-like growth factor-1 (IGF-1) (Wei et al. 2009), TGF-β1 (Melief et al. 2013b), bone morphogenetic protein 2 (BMP2) and fibroblast growth factor 2 (FGF2) (Moriyama et al. 2012) as well as nervous system-related molecules including nerve growth factor (Banas et al. 2008; Ikegame et al. 2011), brain-derived neurotrophic factor (BDNF) (Iadecola and Anrather 2011; Liu et al. 2014), GFAP, nestin, and microtubule-associated protein 2 (MAP2) (Yang et al. 2011). Therefore, based on the actions of these trophic factors, it is rational to attribute the observed enhancement of angiogenesis and neurogenesis as well as the abatement of apoptosis to the paracrine effects of the administered ADSC (Leu et al. 2010; Gutierrez-Fernandez et al. 2012) .

Consistently, another intriguing finding is the discovery of therapeutic effects against stroke using cell-free ADSC culture medium (Cho et al. 2012; Egashira et al. 2012). One study applying human adipose-derived stem cell-conditioned medium to the lateral ventricle of a rat model of ischemic stroke 8 h after MCAO continuously for 7 days demonstrated not only a reduction of infarction volume and preservation of motor function, but also enhanced endothelial cell proliferation , reduced neural cell apoptosis, and suppressed astrogliosis in the penumbra regions (Cho et al. 2012). Another similar study using intracerebroventricular administration of concentrated murine adipose-derived stem cell-conditioned medium in a murine model of MCAO-induced ischemic stroke shed some light on the importance of the timing of treatment (Egashira et al. 2012). The result of that study showed that, while administration of conditioned medium prior to MACO exhibited a dose-dependent reduction in infarction volume of the brain and administration 5 min after MACO was still effective, the therapeutic effect vanished if conditioned medium was administered 2 h after MCAO (Egashira et al. 2012). By contrast, the former study reported effectiveness up to 8 h after MCAO before starting conditioned medium treatment (Cho et al. 2012). Other than the possible variations arising from the differences in the source of conditioned medium and the animal model used, the discrepancy in therapeutic effects between the two studies appears to be due to the way of conditioned medium administration. While the former adopted the approach of continuous intracerebroventricular infusion (Cho et al. 2012), the latter used single intracerebroventricular injection (Egashira et al. 2012) . Again, consistent with the results of previous experimental studies using ADSC transplantation for ischemic stroke (Leu et al. 2010; Gutierrez-Fernandez et al. 2012), it appears that early timing and repeated (if not continuous) treatment are of therapeutic advantage for both ADSC transplantation and conditioned medium therapy. In vitro, murine ADSC-derived conditioned medium has also been demonstrated to reduce glutamate-induced excitotoxicity in human neuroblastoma cells (Egashira et al. 2012).

The role of direct cell participation regarding the use of ADSC for the treatment of ischemic stroke remains controversial. Previous studies using bone marrow-derived mesenchymal stem cells demonstrated that physical presence of the infused stem cells depends on the route of administration. Implantation of stem cells in the injured brain was evident when the cells were given through the carotid artery (Gutierrez-Fernandez et al. 2011; Jiang et al. 2014) but not through the intravenous route (Gutierrez-Fernandez et al. 2011) . Neurological deficits, however, were improved regardless of the presence of implanted stem cells in the brain (Gutierrez-Fernandez et al. 2011; Jiang et al. 2014), raising the question regarding the therapeutic significance of stem cell implantation in stroke. Indeed, it has been shown that only a small fraction (around 0.02 %) of intravenously administered bone marrow-derived hematopoietic stem cells migrate to the ischemic brain, and most of the transplanted cells express microglial but not neural protein markers (Schwarting et al. 2008). For ADSC, while a study failed to identify evidence of migration or implantation of cells into the damaged brain after their intravenous injection in an animal model of stroke despite significant functional recovery (Gutierrez-Fernandez et al. 2013b), other experimental studies (Kim et al. 2007; Leu et al. 2010; Yang et al. 2012) have demonstrated presence of the transplanted ADSC several weeks after intravenous administration with the expression of von Willebran factor, a marker of endothelial cell (Kim et al. 2007; Leu et al. 2010) . Another study using ADSC to treat a rat model of hemorrhagic stroke through right lateral cerebral ventricular injection demonstrated the differentiation of the infused ADSC into neuron-like (NeuN+) and glial-like cells (GFAP+) in region surrounding the hematoma (Chen et al. 2012). Despite the relatively small number of ADSC to explain the overall functional recovery in the reported studies, their presence signifies “transdifferentitation” as a possible mechanism underlying the positive therapeutic impact (Gutierrez-Fernandez et al. 2013a). Indeed, the capacity of neural differentiation for ADSC has been extensively investigated (Cardozo et al. 2010; Kompisch et al. 2010; Liao et al. 2010; Qian et al. 2010; Abdanipour et al. 2011; Yu et al. 2011; Ahmadi et al. 2012). It has also been reported that, compared with bone marrow-derived mesenchymal stem cells , ADSC have superior neurogenic potential (Kang et al. 2004) . Consistently, previous studies using ADSC after induced neural differentiation for treating experimental ischemic stroke were also found to be effective in improving functional recovery (Kang et al. 2003b; Yang et al. 2011). On the other hand, another finding of interest is the requirement for direct physical contact between human ADSC and murine neural stem cells in vitro for induction of neuronal differentiation of the latter, further emphasizing the existence of a mechanism that involves cell-cell interaction other than that of transdifferentiation and paracrine effects in promoting neurogenesis (Kang et al. 2003a) .

Taking into account the immunological nature of stroke-elicited damage and the subsequent repairing process (Iadecola and Anrather 2011) , it is not surprising to find that ADSC exert their therapeutic actions at least partly through immunomodulation. Indeed, ADSC have been reported to produce a variety of immunomodulatory cytokines, including IL-1R, IL-6, IL-8, IL-18, toll-like receptor (TLR)-4, TGF-β1, plasminogen activator inhibitor-1 (PAI-1), G-CSF, GM-CSF, and monocyte chemotactic protein 1 (Banas et al. 2008; Leu et al. 2010; Ikegame et al. 2011; Melief et al. 2013b). Moreover, ADSC have been shown to suppress the differentiation of monocytes towards antigen-presenting immunogenic cells and promote differentiation towards an anti-inflammatory IL-10-producing cell type through the production of IL-6 (Melief et al. 2013a). Consistently, coculturing ADSC with allogeneic dendritic cells revealed that ADSC could negatively modulate immunity and induce immune tolerance through downregulating costimulatory molecules (i.e., CD80, CD83, CD86, and secretion of IL-12 and tumor necrosis factor (TNF)-alpha), while induce dendritic cell tolerance through upregulating indoleamine-2,3-dioxygenase (IDO). Cocultured dendritic cells were also found to inhibit CD4+ T cell activation and naive T cells toward Th1 helper cell polarization (Peng et al. 2012). Again, another credit given to ADSC as compared with bone marrow-derived mesenchymal stem cells in the aspect of immunomodulation in stroke treatment is the finding of a higher immunomodulatory capacity in the former than that in the latter (Melief et al. 2013b) .