Impact of Microgravity and Other Spaceflight Factors on Retina of Vertebrates and Humans In Vivo and In Vitro

Spaceflight (SF) increases the risk of developmental, regenerative, and physiological disorders in animals and humans. Astronauts, besides bone loss, muscle atrophy, and cardiovascular and immune system alterations, undergo ocular disorders affecting posterior eye tissues, including the retina. Few studies revealed abnormalities in the development and changes in the regeneration of eye tissues in lower vertebrates after SF and simulated microgravity. Under microgravity conditions, mammals show disturbances in the retinal vascular system and increased risk of oxidative stress that can lead to cell death in the retina. Animal studies provided evidence of gene expression changes associated with cellular stress, inflammation, and aberrant signaling pathways. Experiments using retinal cells in microgravity-modeling systems in vitro additionally indicated micro-g-induced changes at the molecular level. Here, we provide an overview of the literature and the authors' own data to assess the predictive value of structural and functional alterations for developing countermeasures and mitigating the SF effects on the human retina. Further emphasis is given to the importance of animal studies on the retina and other eye tissues in vivo and retinal cells in vitro aboard spacecraft for understanding alterations in the vertebrate visual system in response to stress caused by gravity variations.

The Chick Embryo and Its Structures as a Model System for Experimental Ophthalmology

The possibilities of using the chick embryo and its individual structures as a model system in experimental ophthalmology are considered. Cultures of the retina and spinal ganglia from chick embryos are used in the development of new methods for the treatment of glaucomatous optic neuropathy and ischemic optic neuropathy. The chorioallantoic membrane is used for modelling vascular pathologies of the eye, screening of anti-VEGF drugs, and assessing biocompatibility of implants. Co-culturing of chick embryo nervous tissue and human corneal cells makes it possible to study the processes of corneal reinnervation. The use of chick embryo cells and tissues in the "organ-on-a-chip" system opens up wide opportunities for fundamental and applied ophthalmological studies.

[Application of the organ-on-a-chip technology in experimental ophthalmology]

Organ-on-chip is a microfluidic device that can reproduce in vitro the minimal functional unit of an organ or system of organs and model various physiological processes and body structures with high accuracy. This review covers the main approaches to the use of the organ-on-chip technology in modern experimental ophthalmology. The analysis of literature sources revealed the following main applications of the organ-on-chip technology in ophthalmology; the technology allows modeling the anterior eye surface and its diseases, such as dry eye syndrome, as well as disorders of the posterior segment of the eye such as age-related macular degeneration, diabetic macular edema, diabetic retinopathy, glaucoma. Culturing of eye tissues in microfluidic systems helps identify the toxic effects and pharmacological activity of new compounds, and provides an opportunity for deeper understanding of the normal physiology of the eye and the pathogenesis of ocular diseases. In addition, the technology can reduce the cost and duration of experiments. Thus, the organ-on-a-chip technology has a great potential in the field of experimental ophthalmology and preclinical trials of new ophthalmic drugs.