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Comparative Anatomy and Physiology of the Mammalian Eye
Published in David W. Hobson, Dermal and Ocular Toxicology, 2020
The sensory retina is thickest near the optic nerve and thins toward the periphery. External to the sensory retina is the RPE and internally is the vitreous humor. The photoreceptors are in the outer portion of the sensory retina and can be divided into the cell body, inner segment, and outer segment. In addition, photoreceptors are divided into rods and cones based on their morphology, physiology, and sensitivity. As suggested by their name, the rods are long and slender, while the cones tend to have a shorter, wider appearance. The cones are further divided according to their spectral absorption characteristics into the red-sensitive (570 nm), green-sensitive (540 nm), and the blue-sensitive (440 nm) cones.130
Medical and Mathematical Background
Published in Arwa Ahmed Gasm Elseid, Alnazier Osman Mohammed Hamza, Computer-Aided Glaucoma Diagnosis System, 2020
Arwa Ahmed Gasm Elseid, Alnazier Osman Mohammed Hamza
The retina is a multi-layered sensory tissue that lies at the back of the eye, and contains millions of photoreceptors that capture light rays and convert them into electrical impulses that will in turn convert them into images in the brain. The main photoreceptors in the retina are rods and cones. Rod cells are very sensitive to changes in contrast, even at low light levels, hence they are able to detect movement, but they are imprecise and insensitive to color located in the periphery of the retina and are used for scotopic vision (night vision). Cones are high precision cells capable of detecting the colors that are concentrated in the macula, the area responsible for photopic vision (day vision). The very central portion of the macula is called the fovea, which is where the human eye is able to best distinguish visual details. The loss of peripheral vision may cause damage to the macula and can result in the loss of central vision.
The nervous system and the eye
Published in C. Simon Herrington, Muir's Textbook of Pathology, 2020
James A.R. Nicoll, William Stewart, Fiona Roberts
Age-related macular degeneration (ARMD) is the most important cause of untreatable visual loss in the ageing Western population. The pathogenesis is poorly understood but recent studies have implicated local inflammation and activation of complement among the processes involved. In particular, a specific polymorphism (Y402H) in the gene encoding complement factor H is strongly associated with disease susceptibility. The disease results in atrophy of photoreceptors at the macula and is accompanied by degenerative changes in the retinal pigment epithelium (RPE) (dry ARMD). This degeneration in the RPE may be complicated by haemorrhage and fibrosis (wet ARMD; disciform degeneration). The overlying photoreceptor tissue is destroyed with loss of central vision. Early disciform degeneration may now be successfully treated by intraocular injection of anti-VEGF antibodies.
Distribution of the Retinal Microcirculation Based on the Morphology of the Optic Nerve Head in High Myopia
Published in Seminars in Ophthalmology, 2023
Wenquan Tang, YuLin Luo, Xuanchu Duan
On the other hand, the choroidal capillary provides the main vascular supply for the DMVD, while the SMVD is derived from the central artery of the retina. We further discovered that reduced SFCT values were associated with higher odds of the tilted optic discs’ occurrence, but there were not as such observations as with FAZ and MVD. So FAZ remained stable during the realization of this study. Also, it contains photoreceptors responsible for visual sensitivity and central visual acuity. Its size and shape partly reflect the status of the macular microcirculation, which is most likely associated with maculopathy.16 Other reports have shown that altered retinal degeneration caused by HM can lead to FAZ enlargement, shedding capillaries, and reduction of macular microcirculation.17 As previously described, most of the oxygen supplied to the FAZ comes from the choroid rather than the retinal circulation. We found small changes in the SFCT. Perhaps FAZ significantly changes when the choroid occurs with extreme atrophy. Therefore, it can be inferred that the choroid’s structure and functional stability have a critical effect on FAZ. In that sense, Sun et al. also found that FAZ were not noticeable in the early phase of HM.18 In another report, Yilmaz et al. l discovered a significantly negative correlation between AI and age.19
Emerging gene therapy products for RPGR-associated X-linked retinitis pigmentosa
Published in Expert Opinion on Emerging Drugs, 2022
Cristina Martinez-Fernandez de la Camara, Jasmina Cehajic-Kapetanovic, Robert E. MacLaren
IRDs are excellent candidates for gene and cell therapies – the accessibility of the retina allows relatively noninvasive procedures, the effect of the treatment can be easily monitored and the blood-retinal barrier limits the immunological response to the treatment by reducing systemic spread. Cell therapy aims to inject or implant viable cells (usually progenitor stem cells) that can replace the affected cells in the eye. Photoreceptor replacement therapy involves the differentiation of stem cells or progenitor cells into photoreceptors, the integration, and survival of these cells in the retina and the establishment of synaptic connectivity to bipolar cells. Over the last years, numerous preclinical studies have been developed to establish protocols to overcome the obstacles and advance toward the generation and characterization of transplantable photoreceptors [21,22]. Several cell therapy clinical trials to treat RP are ongoing (Table 1).
Circadian genes in major depressive disorder
Published in The World Journal of Biological Psychiatry, 2020
Lindsay Melhuish Beaupre, Gregory M. Brown, James L. Kennedy
Sleep–wake cycles are largely determined by circadian rhythms (Mongrain et al. 2004). These rhythms are controlled by a set of genes known as the circadian or clock genes (Reppert and Weaver 2002). In brief, there is a transcriptional-translational feedback loop in mammals that is controlled by two transcription factors, Circadian Locomotor Output Cycles Kaput (CLOCK) and Aryl Hydrocarbon Receptor Nuclear Translocator Like (BMAL1/ARNTL). CLOCK and BMAL1 act on other clock genes, such as Cryptochrome 1 and 2 and Period 1, 2 and 3 to increase their expression, which, in turn, will lead to a feedback mechanism whereby cryptochrome and period regulate their own expression (Jagannath, Peirson and Foster 2013). In its entirety, this cycle takes approximately 24 h (King and Takahashi 2000). The majority of the cells in the body follow this same pattern of oscillation because of the suprachiasmatic nucleus (SCN), the pacemaker of the brain, which is located in the hypothalamus (for a complete review see Partch et al. 2014). Light is perceived by three types of photoreceptors: rods, cones and a tiny group of retinal ganglion cells that are uniquely photosensitive (ipRGCs). Only the SCN receives input from the ipRGCs, while the rods and cones provide visual input for perception of images. The ipRGCs contain melanopsin, a photopigment that is especially sensitive to blue light which recently has been shown to be the wavelength effective in regulating SCN activity (Bailes and Lucas 2013; Jagannath, Peirson and Foster 2013).