Structure of the retina and retinal compartments involved in degenerative diseases. MGCs—Müller glial cells, AMD—age-related macular degeneration, PVR—proliferative vitreoretinopathy, RP—retinitis pigmentosa.
The general types of retinal degeneration include age-related macular degeneration (AMD), glaucoma, retinitis pigmentosa (RP), and proliferative vitreoretinopathy (PVR). All of these disorders, except glaucoma, are caused by the loss of cells and cell–cell interactions in the functional light perception system, RPE and NR (
Figure 2). AMD, affecting, according to approximate estimates, a quarter or more of the global population aged 65+, is accompanied by the loss of photoreceptors in the maculae region, where the light rays are focused on the retina. There are two forms of AMD: the “dry” (prevailing) and “wet” AMD
[4]. With the dry AMD in an atrophic form, extracellular matrix molecules accumulate in the space outside the RPE, which causes the formation of so-called druses, consisting of fats, vitronectin, amyloid proteins, and inflammatory proteins, accumulated inside the RPE layer (
Figure 2). These changes occur in RPE within the maculae region, causing partial cell death, layer disorganization, disruption of RPE functions, and para-inflammatory reaction, which inevitably results in the loss of photoreceptors
[5][6]. The wet AMD, also referred to as neovascular (exudative) form, is manifested as the proliferation of a network of blood vessels lining the RPE choroidal membrane in the maculae region. Vessels become dysfunctional and leaky, with fluid and blood accumulating in the maculae region
[7] (
Figure 2). This causes disjunction of RPE apical processes and photoreceptors, while the connections between them are mandatory for light perception. AMD treatment is diverse, involving neurotrophic factors, growth factors, cell viability factors, and also oxidative stress-preventing factors
[8][9][10][11]. With the wet AMD, vascular endothelial growth factor (VEGF) inhibitors and the photodynamic therapy are mainly used
[12][13]. Despite efforts aimed at developing adequate therapy, the challenges associated with the treatment of AMD to preserve vision are still substantial. In this regard, the idea of cell replacement with the help of cell sources, including intrinsic ones, to regenerate RPE and photoreceptors becomes highly relevant.