September 28, 2009
In a cat's eye: the secret behind their nocturnal vision
Long story short: "DNA in the nuclei of their light-sensing cells — the rods at the back of the eye — has an unconventional structure that in effect turns each nucleus into a minute lens, collecting and concentrating light rays and projecting them deeper into the eye. This unusual structure is found only in nocturnal mammals and explains why they can see in light intensities a million times lower than daylight," according to an item about the research in The Financial Times.
The caption for the graphic above, which accompanied the original report in the April 17, 2009 issue of Cell: "DNA stains reveal that the nuclei of rod cells (top left) from a mouse’s retina have a different arrangement of DNA than ganglion cells (bottom left) or skin cells (right). Blue and red colors show where densely-packed inactive DNA called heterochromatin is located; green shows where active DNA is located. In most cells, heterochromatin is pushed to the outer edges of the nucleus. But rod cells in nocturnal mammals have heterochromatin concentrated in the center, where it acts like a lens to focus light, perhaps enhancing night vision."
Here is an April 22, 2009 Science Daily story with details about the findings.
Secret To Night Vision Found in DNA's Unconventional Architecture
Researchers have discovered an important element for making night vision possible in nocturnal mammals: the DNA within the photoreceptor rod cells responsible for low light vision is packaged in a very unconventional way, according to a report in the April 17th issue of Cell. That special DNA architecture turns the rod cell nuclei themselves into tiny light-collecting lenses, with millions of them in every nocturnal eye.
"The conventional architecture seen in almost all nuclei is invariably present in the rod cells of diurnal mammals, including primates, pigs and squirrels," said Boris Joffe of Ludwig-Maximilians University Munich. "On the other hand, the unique inverted architecture is universally present in nocturnal mammals," for instance, mice, cats and deer.
That architecture has important ramifications for the optical properties of those cells, added Jochen Guck of the University of Cambridge. "Diurnal nuclei are basically scattering obstacles," he said. "In nocturnal animals, they are little lenses. In one case, light is scattered in all directions and in the other it is focused in the forward direction," meaning that even at night, what little light there is can travel deeper into the eye where it can be perceived.
Coming to the realization that the structure of the nuclei of rod cells might have something to do with animals' behavior at night versus during the day took a great leap and an interdisciplinary team, Joffe added. That's because biologists typically think of DNA and its packaging into chromatin in terms of its effect on gene activity. "We tried every other possible explanation," Joffe added. "The idea that it had something to do with vision was a daring idea. People laughed at first."
In non-dividing cells, DNA is associated with proteins to form the so-called chromatin, with more condensed "heterochromatin" at the periphery and less condensed "euchromatin" in the interior. Although cell type-specific variants of nuclear architecture can differ notably in details, the researchers explained, the pattern described above is nearly universal and is conserved in both single-celled and multicellular organisms. The reason for the evolutionary stability of nuclear architecture is most probably the important role that the spatial arrangement of chromatin plays in regulating nuclear functions, they said.
Given that notion, the team took great interest in the fact that the nuclei of mouse rod cells shows essentially the opposite, inverted pattern. The central portion of their nuclei is occupied by a large mass of heterochromatin, while transcription factors that control the activity of genes are enriched at the nuclear periphery.
Mice aren't born with those unusual rod cells, they now report. Rather, the conventional nuclear architecture in their rod cells is completely transformed over the animals' first few weeks into the inverted pattern.
The inverted nuclear architecture found in mice is also present in other nocturnal animals, they found. "Our data revealed a wholly unexpected but very clear correlation between the rod nuclear architecture and lifestyle that was further supported by data on nearly forty animals," the researchers said. "Nocturnal mammals had the inverted pattern, while the diurnal ones showed the conventional one."
The correlation between the inverted nuclear architecture and night vision suggested that the inverted pattern might have an optical ramification, they said. After all, nocturnal mammals see at light intensities a million times lower than those available during the day, and their rod photoreceptors are known to possess a light sensitivity down to the level of a few photons. This high sensitivity demands a large number of rod cells, which increases the thickness of the retinas' outer nuclear layer (ONL). The optimization of light transmission through the ONL could therefore provide crucial advantages for nocturnal vision.
Indeed, measurements of how individual nuclei taken from the rod cells of nocturnal animals interact with light show that they act as collecting lenses. Computer simulations indicate that columns of such nuclei like those found in nocturnal animals' retinas would channel light efficiently toward the light-sensing rod outer segments.
The importance of this special nuclear arrangement comes only when you consider the columns, Guck said. "If each nuclei scattered light, it would all be over. The inversion in nocturnal animals makes sure that light is passed from one nucleus to the next. It is handed down so that it doesn't scatter."
The results show that despite the strong evolutionary conservation of the conventional pattern, nuclear architecture in rod cells was modified several times in the evolution of mammals, Joffe said.
"Taken together, paleontological, molecular, and morphological data strongly suggest (1) that the inverted pattern appeared very early in the evolution of mammals as an adaptation to nocturnal vision in this primarily nocturnal group of animals, (2) that, correspondingly, the conventional pattern was repeatedly reacquired in mammals that readopted a diurnal lifestyle, and (3) that restoration of the conventional architecture most likely demanded selective pressure for the conventional nuclear architecture," the researchers wrote. "Comparison of the inverted and conventional patterns can therefore highlight the advantageous features that predetermine the nearly universal prevalence of the conventional nuclear architecture."
Here's the Cell abstract.
Nuclear Architecture of Rod Photoreceptor Cells Adapts to Vision in Mammalian Evolution
We show that the nuclear architecture of rod photoreceptor cells differs fundamentally in nocturnal and diurnal mammals. The rods of diurnal retinas possess the conventional architecture found in nearly all eukaryotic cells, with most heterochromatin situated at the nuclear periphery and euchromatin residing toward the nuclear interior. The rods of nocturnal retinas have a unique inverted pattern, where heterochromatin localizes in the nuclear center, whereas euchromatin, as well as nascent transcripts and splicing machinery, line the nuclear border. The inverted pattern forms by remodeling of the conventional one during terminal differentiation of rods. The inverted rod nuclei act as collecting lenses, and computer simulations indicate that columns of such nuclei channel light efficiently toward the light-sensing rod outer segments. Comparison of the two patterns suggests that the conventional architecture prevails in eukaryotic nuclei because it results in more flexible chromosome arrangements, facilitating positional regulation of nuclear functions.
Motion-Activated Wisecracking GPS
From the website:
Motion-Activated Wisecracking GPS
This GPS unit will definitely tell you where to go — though you may not want to hear it.
Motion-activated gag unit says 20 different wisecracks (10 phrases per setting) with a backlit screen for nighttime use.
Features 2 settings (G- or R-rated for strong language), suction cup with clamp for windshield mount, and adjustable neck.
Requires 4 AA batteries (not included).
Have a listen here.
Experts' Expert: Harold McGee on how to prolong the life of berries
The term he used in his August 26, 2009 New York Times "The Curious Cook" column was "thermotherapy": a very hot bath for fruit.
Here's his most useful piece.
Prolonging the Life of Berries
One of summer’s great pleasures is eating berries of all kinds by the basketful. One of summer’s great frustrations is having baskets of berries go moldy overnight, or even by nightfall.
Over the years I’ve come up with various strategies for limiting my losses, but this summer I came across a surprising one, the most effective I’ve ever tried. Thermotherapy, it’s been called. A very hot fruit bath.
Fruits go moldy because mold spores are everywhere, readily germinate on the humid surfaces of actively respiring, moisture-exhaling fruits, and easily penetrate the smallest breach of their thin skins.
The first thing I do with a haul of berries, after eating my fill straight from the basket, is to unpack the rest and spread them out on kitchen or paper towels, so they’re not pressing against one another and trapping moisture.
If I want to keep them overnight or longer, I refrigerate them, because cold temperatures slow fruit metabolism and mold growth. I repack the berries as sparsely as possible, nest each basket in a second empty one to leave an air space at the bottom, and inflate and tie off a plastic produce bag around the baskets, so there’s room for the berries to breathe and the bag itself doesn’t cling to their surfaces.
Even with these precautions I’ve had baskets mold overnight in the refrigerator. So I followed up right away when I saw a reference in an agricultural journal on extending the shelf life of strawberries not with a chemical treatment or gamma irradiation, but with heat.
I gathered a dozen or so reports that hot-water treatments suppress mold growth on berries, grapes and stone fruits. The test temperatures ranged from 113 to 145 degrees, with exposure times of a few minutes at the lower temperatures, and 12 seconds at the highest.
I found it hard to believe that any part of a plant could tolerate 145-degree water. My finger in the same water would get a third-degree burn in less than 5 seconds, and eventually reach medium rare.
I bought pints of various berries, divided each batch into two samples, and heated one by immersing and swishing its plastic basket in a pot of hot water. I emptied the heated sample onto towels to cool down and dry. Then I repacked it, and encouraged both baskets to spoil by wrapping them airtight and letting them sweat on the kitchen counter. After 24 hours I counted the moldy berries in each basket.
The strawberries fared best when I heated them at 125 degrees for 30 seconds. In two samples from different sources, this treatment gave a total of 1 moldy berry out of 30, where the untreated baskets had 14. I also treated some bruised berries, including one with a moldy tip. After 24 hours none were moldy. The tip mold not only hadn’t spread, it had disappeared.
I tried the same treatment, 125 degrees for 30 seconds, on raspberries and blackberries, and got the same good results. There were many fewer moldy berries in the heated samples.
For thicker-skinned blueberries, a Canadian study recommended a 140-degree treatment for 30 seconds. I tested it twice, with samples of around 150 berries each time. That heat took the bloom off. It melted the natural wax that gives the berries their whitish cast, and left them midnight blue. It also cut the number of moldy berries from around 20 per sample to 2.
Research has also shown that exposure to hot air slows fruit spoilage. But hot air can take several hours, and I found it harder than hot water to apply precisely in the kitchen. I did spread some raspberries out on a sheet pan lined with towels, and put them in a 150-degree non-convection oven for 20 minutes. The berry bottoms got hotter than the tops, which were cooled by evaporation. Still, only 1 out of 48 heated berries became moldy, compared with 7 out of 52 in the unheated basket.
Why is it that delicate berries can survive heat high enough to kill mold and injure fingers? Probably because they have to do so in the field. One study of tomatoes found that intense sunlight raised their interiors to 122 degrees. Such heat hurts the quality of growing fruits, but I couldn’t taste much of an effect on briefly heated ripe fruits.
So if you find yourself plagued by quickly spoiling fruits, start giving them a brief hot bath before you spread them out or chill them. Thermotherapy can be healthy for all concerned.
Mutant Showerhead — go green: save water and shower with a friend
From the website:
Each adjustable showerhead can be individually set to: full body spray, pulsating massage, or a combination of both.
Fits any standard 0.5" bathroom pipe.
8" x 2.5" x 6".
Scala — by Horst Gläsker
"Holsteiner stairs in Wuppertal. Acrylic paint on 112 steps."
The German artist celebrates his 60th birthday this year.
[via What Alice Found]
What are they?
Answer here this time tomorrow.
From Damascus south to Jerusalem, 1932