![]() Just as carotenoids are the precursors of retinal, retinal is the precursor of the other forms of vitamin A. Ĭatalyzed by a beta-carotene 15,15'-monooxygenase or a beta-carotene 15,15'-dioxygenase. Living organisms produce retinal by irreversible oxidative cleavage of carotenoids. Invertebrates such as insects and squid use hydroxylated forms of retinal in their visual systems, which derive from conversion from other xanthophylls. The other main forms of vitamin A - retinol and a partially active form, retinoic acid - may both be produced from retinal. Some carnivores cannot convert any carotenoids at all. No other carotenoids can be converted by animals to retinal. These carotenoids must be obtained from plants or other photosynthetic organisms. They also produce it from β-cryptoxanthin, a type of xanthophyll. Vertebrate animals ingest retinal directly from meat, or they produce retinal from carotenoids - either from α-carotene or β-carotene - both of which are carotenes. Retinal was originally called retinene, and was renamed after it was discovered to be vitamin A aldehyde. The number of different molecules that can be converted to retinal varies from species to species. Retinal itself is considered to be a form of vitamin A when eaten by an animal. There are many forms of vitamin A - all of which are converted to retinal, which cannot be made without them. Since Retinal absorbs mostly green light and transmits purple light, this gave rise to the Purple Earth Hypothesis. In fact, a recent study suggests most living organisms on our planet ~3 billion years ago used retinal to convert sunlight into energy rather than chlorophyll. Some microorganisms use retinal to convert light into metabolic energy. ![]() Retinal, bound to proteins called opsins, is the chemical basis of visual phototransduction, the light-detection stage of visual perception (vision). These results demonstrate that human retinal organoids provide a preclinical research system for cell replacement therapies.Retinal (also known as retinaldehyde) is a polyene chromophore. Histological and ultrastructural data of virally-labeled photoreceptor transplants show characteristic morphological and structural features of polarized photoreceptors: inner segments and ribbon synapses, and donor-host cell contacts develop contributing to the retinal outer limiting membrane. Transplanted clusters frequently are located within or across the host photoreceptor layer, include cone and rod photoreceptors, and become infiltrated by cell processes of host Müller glia, indicative of structural integration. Using postmitotic retinal organoids (age >170-days) as a source for donor cells and as hosts, we show that six weeks after subretinal-like transplantation, large clusters of photoreceptors reproducibly incorporate into the host retina. Alternatively, donor cells were transplanted in large numbers by placing them in subretinal-like contact to the apical organoid surface. Donor cells were precisely transplanted by microinjection into the retina of host organoids, but high cell numbers might require multiple injections posing potential damage. ![]() Models for intra- and subretinal cell transplantation strategies were explored: Photoreceptor donor cells carrying a transgenic fluorescent reporter were enriched from acutely dissociated human retinal organoids. Here, we sought to determine if human retina organoids generated from pluripotent stem cells might assist cell replacement therapy development in a human-to-human setting. Whether retinal cell transplants also integrate into human retina, and how to optimize this for different pathologies are still unknown. So far, human photoreceptor transplants restored some visual function in degenerating mouse retina. Cell transplantation is a promising therapeutic approach to recover loss of neurons and vision in patient retinas. ![]()
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