Night vision inverts chromatin

Researchers have discovered a cellular mechanism that helps nocturnal mammals see in the dark. Mice, cats, deer, lemurs, and other mammals that are active at night remodel the DNA within their eyes to turn photoreceptor cells into light-collecting lenses, according to a linkurl:study; published today (Apr. 16) in__ Cell__. Image: striatic and Animal Photos!

By | April 16, 2009

Researchers have discovered a cellular mechanism that helps nocturnal mammals see in the dark. Mice, cats, deer, lemurs, and other mammals that are active at night remodel the DNA within their eyes to turn photoreceptor cells into light-collecting lenses, according to a linkurl:study; published today (Apr. 16) in__ Cell__.
Image: striatic and Animal Photos!
In nearly all eukaryotic nuclei, chromatin -- the structural building block of chromosomes -- is spatially separated into distinct compartments. Condensed, non-coding heterochromatin is usually localized to the periphery of the nucleus, while extended, active euchromatin typically resides in the nuclear interior. This "conventional" pattern is nearly universal, and probably helps cells regulate essential nuclear functions such as how and when genes are expressed. But some nuclei are special. In 2006, a team led by linkurl:Didier Devys,; a molecular biologist at the Institute of Genetics and Molecular and Cellular Biology in Illkirch, France, linkurl:showed; that mouse rod photoreceptor cells have a different arrangement in which the chromatin is "inverted." In these cells, but not in other mouse cell types, heterochromatin is shunted to the interior, where it is enveloped by a thin ring of euchromatin. With this layout, all transcription takes place at the nuclear margins rather than at the core of nucleus as per usual. linkurl:Boris Joffe; and linkurl:Irina Solovei,; nuclear biologists working in linkurl:Thomas Cremer's; lab at the Ludwig-Maximilians University in Munich, Germany, independently observed the same phenomenon, and they set out to determine why this was so. In the new study, they investigated mouse retinas dissected at various ages, and showed that rod cells have the conventional chromatin structure at birth, but gradually the chromatin shuffles around the nucleus to create the inverted pattern after around four weeks. Looking beyond the mouse, Joffe and Solovei mapped the nuclear architecture of nearly 40 mammal species and found that all the nocturnal animals had the same inverted pattern as mice, also a nocturnal species, whereas all the diurnal animals had the conventional arrangement. The researchers also inspected a few bird and other vertebrate species, but found no inverted architecture outside of nocturnal mammals. The Munich researchers then teamed up with linkurl:Leo Peichl,; a neurobiologist at the Max Planck Institute for Brain Research in Frankfurt, and mapped the nuclear arrangements onto the phylogenetic tree of mammals. They showed that the inverted pattern arose early in the evolution of mammals, probably as an adaptation to nocturnal vision to help ancient mammals escape predation from dinosaurs and other large reptiles, which were mostly active in the daytime. The conventional pattern was then "reacquired" many times over in all the diurnal mammals, including humans. This indicates that the conventional architecture is under strong selective pressure in most well-lit environments, said Joffe. "We proved that there is some great advantage in the organization of the nuclei that we observe in most eukaryotic cells," he told __The Scientist__. Finally, Joffe and Solovei partnered with linkurl:Jochen Guck,; a biophysicist at the University of Cambridge, UK, to measure differences in how the two nuclear arrangements physically interact with light. Using live retinal cells from mice (a typical nocturnal species) and pigs (a typical diurnal species), they showed that inverted rod nuclei scattered light much less than rod nuclei with the conventional pattern. Computer simulations also indicated that when the inverted nuclei are stacked on end -- the organization found in the rod columns of nocturnal animals' retinas -- light is transmitted with very low scatter. Thus, nuclei with inverted chromatin act as converging lenses to focus light -- a necessary adaptation for seeing at dusk and dawn. "At very low light conditions [nocturnal mammals] need to be able to salvage every little photon that comes in," said Guck. The finding that physical properties can remodel chromatin is "striking," said Devys. But he noted that the researchers discovered only a correlation between the layout of the nucleus and the circadian lifestyle of different mammals. "They don't have conclusions about what this chromatin organization would do at the transcriptional level," he said.
**__Related stories:__***linkurl:Visual system surprise;
[7th July 2008]*linkurl:Seeing the light;
[6th September 2002]*linkurl:Photoreceptor biology;
[19th August 1996]


April 17, 2009

As an ex-scientist and now full time homemaker and mother of 2 children and with a son with COMPLETE ACHROMATOPSIA, a rare congenital recessive gene condition in which only his rod photoreceptor cells are functional in both his retinas, so he has excellent night vision but is blinded by daylight since the rod cells are soon 'blinded' by excess light and not having any functional cone cells(they may be there but they don't function) he is totally colour-blind-it occurred to me that in effect he is like a 'nocturnal animal' relying only on his rod cells for vision. In fact he wears darkened treatment lenses all the time and prefers to work in a dimly lit environment.Perhaps what I am suggesting is nonsense but it occurred to me that TOTAL ACHROMATS (also called Achromatopes) are the human equivalent of nocturnal creatures and it occurred to me whether Total Achromats, showing no functional cone activity, may also have adapted their rod cells in the same way as those of true nocturnal animals that you describe in your paper.Of course one could only study this idea if one had access to 'total achromat' corpses to dissect out the retinal cells-a large source of such people all bearing the same genetic defect can be found on the Pingelap islands in the pacific ocean ,as mentioned in the book entitled 'the island of the colourblind' by the famous Dr Oliver Sacks (a neurologist and neuroanthropolist)who went to that island with the late norwegian Dr Knut Nordby ( a vision scientist from Oslo university). I and my son had the priviledge of meeting the latter gentleman at the 1st Italian Achromatopsia organisation meeting. Dr Knut Nordby was the most studied total achromat of all time and I expect he gave hs eyes for medical research when he died.It is already known that there are genetic differences between European,pacific island and Amerian Achromats but the defects all seem to be in the signal transduction pathways into the cells( see Dr Lindsey Sharpe's work-also read his book 'Night Vision' in which Dr Knut b Nordby also writes a chapter on his condition. \nInterestingly,wheras my son has excellent night vision and daytime blindness, I am miopic and so tend to suffer the opposite i.e.I have night time blindness and good day time vision ,and a late uncle of mine, who was an excellent RAF pilot (partly because of his excellent day time vision )sadly had to leave the airforce due to suffering night time blindness.!!!

April 23, 2009

I DO REALISE that such an evolutionary adaption , if it occurred in achromats, would ONLY BE BENEFICIAL to them at night time or if they had access to darkened environments or dark glasses and since humans are NOT TRUE NOCTURNAL CREATURES and unlike true nocturnal animals , humans usually sleep at night and work at day, such an adaption would not prove very useful,particularly for the Pingelese people living in such a sunny environment, since although it may improve night time vision for them it would obviously worsen their already extreme difficulties of day -time blindness in extreme sunlight.So, in retrospet, my original idea was probably nonsense! since such an adaption if it occurred would only be beneficial to achromats who worked at night and slept at day!!!!!!

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