## 2 Biological Signalling Process of Retina  
The retina is used to study neurosensory processes because it is seen as the organ that gives access to the brain. It is considered a good experimental system because of its anatomical organization (layers of different cell types and extracellular compartments), as well as its easy visualization [6; 54]. PR cells detect light that passes through the internal retina and reaches the PRs of the OR. The internal retina has bipolar cells, horizontal cells, Mueller glial cells, and retinal ganglion cells, whose function is to process visual information and to transmit it to the brain. Light is focused onto the photoreceptors that are present on the surface of the retina, then retinal circuits process these signals and convert them into a neural pulse code.  
The different photoreceptors contact each other through electrical synapses and contact the bipolar cells through chemical synapses, which in turn communicate with amacrine cells or divert them to ganglion cells for transmission to the occipital lobe of the brain, which finally interprets the neural impulses as a flow of images. In the retina, the information flow traverses a direct path (from photoreceptors to bipolar cells to ganglion cells) and an indirect path (where the AII amacrine cells are interposed between bipolar and ganglion cells). Furthermore, the horizontal and amacrine cells stimulate lateral connectivity among the different pathways [35].  
RPE, which is a support cell layer, is next to the PRs. There is a link between the physiology of RPEs and RPs, and therefore vision depends on suitable communication between these two cell types [55]. RPEs are responsible for recycling the visual cycle components, cleaning the PR membrane fragments that were generated in the daily PR renewal, transporting nutrients from the circulation to the subretinal space, and expelling retinal debris [8]. This epithelium belongs to the Bruch basement membrane  
occur due to these receptors, which allows their actions to be delimited by the emergence of receptors through the availability and subtype of ligands. In addition, the same receptor initiates signals that may have different effects on different endothelial cells [72]. Thus, only one growth factor sculpts the developing retinal vasculature through spatiotemporal changes in VEGF and VEGFR distributions.  
Differential effects of VEGF isoforms, a distinct spatial range of action, and orientation functions of VEGF isoforms produce alternative transduction of VEGFR activation in different endothelial cells. The relationship between VEGF and other angiogenic or angioinhibitory factors also contributes to the creation of complex vascular networks [73; 74; 75; 76].  
The study of retinal disorders improves the research and development of new models for retinal angiogenesis, as well as improving the investigation of neurovascular relationships and the development of new antiangiogenic therapies. These studies have determined oxygen to be the main \"actor\" in retinal angiogenesis, together with metabolic, immune, glial, neuronal, gene expression, and perivascular and progenitor cell factors [54].  
Angiogenesis is known to be responsible for tissue growth, development, and repair. Thus, understanding how the various cell types interact to control angiogenesis makes it possible to understand the occurrence of proliferative vascular diseases, such as diabetic retinopathy [77]. Therefore, the retina is seen as a model of the neurovascular system. In recent years, there has been growing interest from scientists, with considerable progress being made in determining the mechanisms that induce and standardize the retinal vascular system, as well as understanding disease processes leading to its disintegration [54].