Stem cell therapy has opened a new era of tissue regeneration. MSCs may provide an alternative source for retinal regeneration. In addition to the established multipotential of MSCs to differentiate into osteoblasts, adipocytes, and chondrocytes, they were seen to differentiate into retinal cells in early studies. Bone-marrow-derived MSCs (BMSCs) and adipose-tissue-derived MSCs (AMSCs) are shown to express RPE and photoreceptor cell markers by exposure to conditioned medium in vitro ^[1][2][35,36]. Intravenous injection of MSCs into diabetic rats resulted in the targeted migration of the transplanted MSCs to the damaged retina and the expression of astrocytes and photoreceptor cell markers. However, from a dialectical point of view, we should be aware that ectopic expression of some photoreceptor markers does not signify that cells differentiate into photoreceptors and does not mean these cells are functional. Researchers tried to replace damaged retinal cells by directional differentiation of MSCs in order to reconstruct the retinal neural network and restore the activity of nerve conduction ^[3][4][37,38]. In animal models of retinal laser injury, almost all MSCs migrated and integrated into the layers of the retina by intravitreal MSC injection ^[5][6][31,39]. Recently, Ripolles-Garcia A et al. showed that human donor photoreceptor precursor cells were generated in vitro from hESCs and were integrated and differentiated into the canine retina. Notably, these generated photoreceptors have synaptic pedicle-like structures which could establish contact with second-order neurons. Thus, their findings set the stage for evaluating functional vision restoration following photoreceptor replacement in canine models of inherited retinal degeneration ^[7][40].

In contrast, intravitreal administration of MSCs remained in the vitreous cavity and did not differentiate into neural or perivascular-like cells while preventing retinal ganglion cell loss by triggering an effective cytoprotective microenvironment in the models of diabetic and experimental ocular hypertension ^[8][9][41,42]. However, undifferentiated MSCs are able to express many mature neuronal and glial markers, such as βIII-tubulin and GFAP, which make them more controversial for early discoveries ^[10][11][43,44]. Conversely, they might be overly simplistic and optimistic for the concept of direct cell replacement, since retinal degeneration diseases usually involve several layers and cell types of retina. Instead, administrated MSCs with paracrine trophic effects and some capable of direct engraftment might allow the stem cells to maximize their potential in retinal regeneration ^{[12][13][14][15]}[45,46,47,48].

Chronic and recurrent uveitis involving the choroid and retina are usually treated by corticosteroids and immunosuppressants with many serious potential side effects, including myelosuppression, tumor formation, and liver and kidney damage. By targeting different parts of the immune system, MSCs are supposed to inhibit the autoimmune response, allograft rejection, and graft-versus-host disease ^[16][50]. Theoretically, MSCs are superior to corticosteroids and immunosuppressants in that they also possess neurotrophic and antimicrobial effects ^[17][18][19][51,52,53].

MSCs are currently being tested in preclinical studies for the treatment of experimental autoimmune uveitis (EAU) owing to their immunosuppressive properties ^[20][54]. Yang et al. reported four cases of refractory uveitis resistant to traditional systemic administration, which resolved after intravenous injection with human-umbilical-cord-derived MSCs (hUMSCs) ^[21][55]. A single course of MSC-based therapy at the onset of the disease suffices to protect against ocular inflammation in EAU ^[22][56]. Moreover, double treatments of MSCs with longer intervals have a significant curing effect ^[23][57]. The Ouresearchers' studies indicated that MSC-based therapy is more effective in controlling inflammation, reducing relapses, and protecting the retina in recurrent EAU than dexamethasone. The beneficial effects of MSCs in recurrent EAU were attributed to a significant decrease in Th1/Th17-mediated inflammation, to regulation of the balance between Th17 and Tregs, and to suppression of the function of antigen-presenting cells ^[19][23][53,57]. Further study demonstrated that the immunomodulatory function of MSCs might be mediated through the CD73/adenosine pathway in human and mouse T cells. CD73 on MSCs, upregulated by transforming growth factor-β1 (TGF-β1), cooperated with CD39 and CD73 on activated T cells to produce adenosine, resulting in inhibition of T-cell proliferation ^[24][58]. Injection of MSCs regulated STAT1 and STAT6 phosphorylation to reduce the levels of migration-related proteins in dendritic cells (DCs) and inhibit the proliferation of DCs ^[25][59]. Recent research shows that MSCs continue to play a vital role in EAU, as they inhibit the activation of CD4+T cells by direct intercellular contact and activation of prostaglandin signaling ^[26][60].

MSCs are able to regulate the toll-like receptor 4 signaling pathway and pro-inflammatory factors such as tumor necrosis factor-α, interleukin (IL)-1β, and reactive oxygen species to reduce retinal cell apoptosis, increase retinal inner layer thickness, and reduce neuroinflammation ^[27][61]. Furthermore, plenty of studies have shown that MSCs are capable of secreting a variety of neurotrophic factors, such as basic fibroblast growth factors (bFGF), ciliary neurotrophic factors, ganglion-cell-derived neurotrophic factors (GDNF), nerve growth factors (NGF), and brain-derived neurotrophic factors in the treatment of retinal diseases ^[28][29][62,63]. Under the effect of neurotrophic factors secreted by MSCs, conditioned medium of MSCs can promote the photoreceptor cell proliferation in vitro and inhibit their death ^[30][64].

In the animal model of retinal degeneration, rats suffering from retinitis pigmentosa were given subretinal injection or intravenous injection with MSCs, which successfully inhibits retinal degeneration progress and protects the function of the retina, but transplanted MSCs did not migrate or integrate into the retina ^[4][31][32][38,65,66]. As for animal models of retinal ischemia reperfusion injury and glaucoma, intravitreal injection of MSCs could significantly improve the retinal ganglion cell survival ^{[9][33][34][35]}[42,67,68,69]. MSCs distributed along the inner limiting membrane expressed a variety of neurotrophic factors, but only a handful of stem cells could migrate into the retina. Coculture of MSCs with retinal explants also confirmed the reduction in apoptosis and increase in the nerve fiber layer and inner plexiform layer thicknesses. Cell secretome demonstrated that MSCs secrete a number of neuroprotective proteins and suggest that platelet-derived growth factor secretion in particular may play an important role in MSC-mediated retinal ganglion cell neuroprotection ^[36][70]. MSCs might display their therapeutic effect in a paracrine fashion by secreting neuroprotective and anti-inflammatory factors to preserve the homeostasis of the ECM and regulate the intraocular microenvironment, which is beneficial for the integrity of the retina and tissue repair ^[37][71]. This experimental evidence all points to the speculation that the paracrine effect of MSCs may play a key role in the method of cell protection on the retina instead of migrating into the retina or differentiating into retina cells. In these diseases, MSCs give a neurotrophic effect, mainly by secreting neurotrophic factors, in addition to other possible mechanisms of MSCs, which include regulating the inflammatory process, repairing the blood vessel damage, and promoting synaptic regeneration by adjusting the inhibiting signal to activate the intrinsic repair mechanisms.

Pathological process of inflammatory reaction and tissue damage are involved in the above diseases such as uveitis, laser-induced retinal damage, ischemia injury, and diabetic retinopathy. MSCs play a role in anti-inflammation and promoting restoration of the retinal tissue damage. MSCs have the capacity to home into damaged tissue with inflammation, probably in response to chemokines, adhesion molecules, and matrix metalloproteinases, following intravenous injection ^[5][38][31,72]. However, the specific mechanism still remains a mystery ^[39][73]. In vitro results suggested that the stromal-derived factor 1/C-X-C chemokine receptor type 4 (SDF-1/CXCR4) and hepatocyte growth factor/c-met (HGF/c-met) axes, along with MMPs, act as important signals for migration and homing of mesenchymal stem cells ^{[40][41][42][43]}[74,75,76,77]. An in vivo study also demonstrated that src family protein kinases are activated by SDF-1/CXCR4 signaling and play an essential role in SDF-1/CXCR4-mediated MSCs’ chemotactic response and ischemic cardiac recruitment ^[44][78].

ROS levels increase dramatically under environmental stress, which causes serious damage to cellular structures in retinal disease. As hMSCs were able to scavenge free radicals, promote endogenous antioxidant defenses, alter mitochondrial bioenergetics, transfer mitochondria to impaired cells, and effectively regulate oxidative stress, they have been studied as a treatment for oxidative injury. Ohkouchi et al. showed that MSCs elevated the survival rate of A549 cells through Stanniocalcin 1 upregulation ^[45][79]. Injection of AMSCs into the subretinal space of mice under oxidative stress protected and rescued RPE and photosensor cells ^[46][80].

Abnormal angiogenesis is one of the main reasons for many ocular diseases, including DR, retinopathy of prematurity, and AMD. As MSCs can secrete angiogenesis-related factors and proteins, such as VEGF, fibroblast growth factor, HGF, TGF-β1, and insulin-like growth factor 1, they can promote angiogenesis and repair retinal ischemic injury ^[47][48][49][81,82,83].

Studies have shown that adult stem cells ^[50][84], AMSCs ^[51][85], iPSCs ^[52][25], and autologous BMSCs ^[53][17] are promising treatment options for animal models of DR and its complications. Elevated blood glucose levels in patients with DR lead to increased levels of reactive oxygen species (ROS), damaged pericytes and endothelial cells, vascular degeneration, and formation of new vessels. MSCs can not only differentiate into pericytes and reverse the changes in extracellular matrix proteins ^[54][86], but also inhibit the inflammatory response caused by oxidative stress through reducing the levels of pro-inflammatory factors, calcium, and ROS ^[55][87]. At the same time, the expression of intraocular nerve growth factor, bFGF, GDNF, and other nutritional factors, such as NGF and NT-3, can be induced to reduce nerve cell apoptosis ^[56][88].

In addition, MSCs also have anti-angiogenic effects in proliferative retinopathy. High levels of TGF-β1 secreted from human placental amniotic membrane derived MSCs demonstrated rescue potential on suppressing retinal neovascularization in a mouse model of oxygen-induced retinopathy ^[48][82].

Mitochondria create chemical energy for biochemical activities. A majority of mitochondrial proteins are encoded from nucleus DNA, maintaining mitochondrial functions, while mitochondria also contains its own DNA, known as mitochondrial DNA, which encodes 13 proteins ^[57][89]. Dysfunction of mitochondria is a signal indicating cellular senescence, and mitochondrial injury finally results in RPE cell death and degenerative retinal disease ^[58][90]. Both aging and hyperglycemia can lead to oxidative stress, damaging mitochondria and accelerating AMD and RD development ^[59][91]. In glaucoma, mitochondrial dysfunction is accompanied with retinal ganglion cell (RGC) degeneration ^[60][92], which is difficult to repair.

A fascinating and creative way that MSCs rescue impaired neural cells has provoked profound thought. Through different approaches, MSCs can deliver their own mitochondria to injured cells, in order to promote their repair and regeneration. This phenomenon was first described by Spees et al., who cocultured mitochondrial gene-depleted cells with MSCs. The mutant cells with enhanced mitochondria showed expression of mitochondrial proteins, and significantly increased ATP production and decreased lactate levels, a byproduct of anerobic respiration ^[61][93]. Previous studies have revealed several ways that mitochondria can be transferred, such as via tunneling nanotubes, gap junctions, or exosomes ^[62][94]. This kind of donation pathway is proven in ocular cells, including the corneal endothelium, RPE, and photoreceptors ^[63][95]. Intravitreal iPSC-derived MSC transplantation can significantly transfer mitochondria to damaged RGCs and improve retinal function ^[64][96]. 

The adult visual pathway can possibly rebuild new synaptic interactions and guide new axons in certain circumstances, which provide an opportunity for cellular therapy. However, being similar to the central nervous system, the retina is not easily influenced by the outside factors, for example, stem cells, due to the retinal barriers. The blood–retinal barrier is part of the blood–ocular barrier that consists of retinal vascular endothelium and RPE ^[65][99]. The physiological barrier of retinal blood vessels comprises a single layer of non-fenestrated endothelial cells which maintain the inner blood–retinal barrier. The tight junctions between retinal epithelial cells, which form the outer blood–retinal barrier, prevent the passage of large molecules from choriocapillaris into the retina. In addition, internal and external limiting membrane, extracellular matrix components (such as chondroitin sulfate proteoglycans), active RGC synapses, glial scars produced by reactive gliosis in the injury, and pathological conditions are also important parts of the retina barriers ^[66][100].

Whether using local or systemic administration, MSCs were not seen to have migrated or integrated into the retina on an ideal scale while the retinal barriers were relatively integrated ^[4][33][34][38,67,68]. Thus, these retinal barriers remain the major obstacle for directional differentiation of stem cells to replace damaged retinal cells. A previous study delineated that retinal MSCs’ migration correlated positively with the amount of peeled internal limiting membrane, and targeted disruption of glial reactivity, with α-aminoadipic acid treatment, dramatically improved the structural integration of intravitreally transplanted cells ^[67][101]. By the manipulation of mechanical injury, incorporation of grafted stem cells was seen in 50% of the injured retinas, as well as subsequent differentiation into the neuronal and glial lineage, and formation of synapse-like structures were implied in the adult rat retina ^[68][102]. Another study showed that matrix metalloproteinase-2 can promote host–donor integration by degrading CD44 and neurocan at the outer surface of the degenerative retina without disruption of the host retinal architecture ^[69][103]. In addition, subretinal injection with chondroitinase ABC combined with enhanced immune suppression caused a dramatic increase in the migration of stem cells into all the retinal cell layers ^[66][100].

MSCs also have limitations for human retinal disease treatment due to the vulnerability of their expression. Some new technologies, such as gene therapy, retinal organoids in vitro, and bio-printing technology, have demonstrated prospective therapeutic capabilities to repair damaged retinal cells.

Erythropoietin-engineered human MSCs enhance differentiation into retinal photoreceptors in retinal degenerative diseases ^[70][104]. HiPSC-derived RGCs are seeded on a biodegradable poly (lactic-co-glycolic acid) scaffold to create an engineered biomaterial ^[71][105]. Retinal-ganglion-like cells differentiated from dental pulp stem cells using 3D networks to replace the lost and damaged RGCs ^[72][106].

Exosomes are the tiniest extracellular vesicles with bi-lipid membranes shuttling active cargoes (for example genetic material, proteins, and other biologically active molecules) involved in the complex intercellular communication system ^[73][107]. Being released by various cell types (such as B and T cells, DCs, cancer cells, stem cells, and endothelial cells), the main traits of exosomes are in accordance with the function of their original cells.

Recent discoveries noticed that MSC-derived exosomes, 50–150 nm microvesicles, could inherit the multiple functions from MSCs and might be the key mediators of MSC paracrine activity ^[74][75][108,109]. They have been studied in various disease models with many encouraging results. In cardiovascular disease, the infarct size and cardiac function were ameliorated in myocardial ischemia/reperfusion injury and acute myocardial infarction following administration of exosomes derived from MSCs ^[76][110]. MSC-derived exosomes also play a role in inflammation regulation and ischemia/reperfusion-induced renal injury repair, partially by suppressing the recruitment and activation of macrophages related to the C-C motif chemokine receptor-2 expression ^[77][111]. Extracellular vesicles exhibit immunomodulatory properties similar to their original MSCs through inhibitory activity on B-cell proliferation, intervention of shifting T cells from an activated status to a T regulatory phenotype, reduction in interferon-γ production, and increased release of immunosuppressive cytokines (such as prostaglandin E2, TGF-β, IL-10, and IL-6) ^{[78][79][80][81]}[112,113,114,115]. In addition, the protective effect of MSCs on acute lung injury can be potentiated by ischemic preconditioning through the secretion of exosomes ^[82][116]. In neurological diseases, a recent study demonstrated that systemic administration of cell-free exosomes generated by MSCs improves functional recovery in rats that suffered from traumatic brain injury, while another group proposed that combined MSCs and MSC-derived exosome therapy displayed the best result in reducing the brain infarct volume and preserving neurological function in rats after acute ischemic stroke ^[83][84][117,118].

Although preclinical studies of exosomes derived from stem cells have been applied to the treatment for a variety of diseases, very few results came out in ophthalmology. ThWe researchers first observed the therapeutic effect of exosomes derived from MSCs in retinal dysfunction ^[85][119]. MSC-derived exosomes were able to pass retinal barriers and diffuse throughout the retina after intravitreal injection. After laser injury, MSC/exosome-treated groups showed smaller lesioned areas, less TUNEL-positive cells, and better ERG responses. Further in vivo and in vitro experiments suggested that their suppression of injury-induced inflammation may be via the down-regulation of monocyte chemotactic protein-1.

While extant obstacles limit the clinical applications of simple stem cell treatment for retinal diseases, such as retinal barriers, the alternative application of MSC-derived exosomes remains more promising, primarily due to their inherited abilities, low immunogenicity, long half-life in circulation, and other cell-free advantages. Additionally, MSCs can produce a higher number of exosomes compared to other cells. However, whether MSC-derived exosomes preserve the same effective properties compared to the cells themselves remains controversial, as MSCs may act by releasing other active soluble factors and the biological features of exosomes may vary with different extracellular environments. Therefore, further studies are required to understand the full dimension of this complex intercellular communication system and how it can be optimized to take full advantage of its therapeutic effects.