From Photons to Shapes and Colors: the Amazing Science behind Human Vision
The human eye is incredibly complex. It functions as a lens, a really sensitive light detector, and a transmitter which sends the information it gathers to the brain, where it is processed. But how does the eye work? What biochemical magic does it use to turn photons of light into the shapes and colors we see every day?
While the entire process is indeed fascinating, the most interesting part is definitely the way photoreceptor cells turn light into the electrical impulses the brain eventually interprets as vision, so that’s what I’m going to focus on.
Light passes through the cornea and the crystalline lens and gets focused onto a membrane which covers the back of the eye called the retina. Covering the retina are millions of tiny cells which due to their shape are called rods and cones. Most rods can be found along the outermost parts of the retina, while the cones are mostly concentrated in the middle, in a place called the fovea. It’s also worth mentioning that there are a lot more rods than there are cones, about 120 million to 6 or 7 million.
Each of these types of cells has its own specialty. Rods are really sensitive to light, but they only tell you how much of it there is and the general shape of things. This means they only show the world as black and white. Cones, on the other hand, are responsible for perceiving shapes and colors. Despite these differences, the underlying biochemistry of these structures is, nevertheless, quite similar. Rods are a bit simpler, so I’ll use them to explain the basic idea.
When you take a closer look at one of these rods, specifically at the top of the cell, you find that it is actually made up of membrane-bound discs which are basically like pancakes stacked up above one another. These discs contain a protein called rhodopsin, which in turn contains a chromophore (which is a molecule that can absorb light at a specific wavelength) called 11-cis-retinal. Initially, 11-cis-retinal has sort of a “bent” shape. But when light enters the eye and hits one of these molecules, it goes through something called isomerization, meaning it changes its configuration, basically straightening out and becoming all-trans-retinal (see below). This change in shape kicks off a process called the phototransduction cascade. It’s a bit convoluted, but what basically happens is that as the chromophore straightens, it doesn’t fit well into its rhodopsin “container” and changes configuration again, triggering a series of events which ends up closing the sodium channels (Na+) in the rod.
Sodium channels are like tiny faucets which allow sodium ions to enter the cell, keeping it electrically neutral. When these channels close, the cell becomes hyperpolarized, causing it to turn off. This activates the so-called bipolar cells, which ultimately leads to the sending of a signal to the brain through the optic nerve.
Cones basically work the same way. The difference is there are three types of cones, which are sensitive to red, green, or blue light. When we see something yellow, for example, some red and some green cones get activated. Depending on the intensity of the signal received from red and green cones, the brain interprets that information as color.
What this means is that there’s nothing inherently red, or blue, or yellow about anything. The so-called visible spectrum is just the wavelengths of light emitted or reflected by an object which we’ve evolved to perceive. Human vision is basically certain proteins responding to photons of certain wavelengths in a particular way, something which our brains find useful to interpret as colors. This may be hard to believe when seeing a spectacular landscape or a colorful work of art, but it certainly doesn’t take away any of the world’s beauty.
Chinese Dinosaur Might Have Been as Iridescent as a Hummingbird
Earlier this month, I wrote an article on a toy line of scientifically accurate Velociraptor action figures with plumage inspired by modern birds. I mused how impressive it would be if prehistoric raptors had been covered by feather patterns not unlike those in the toy line. Little did I know that two weeks later, researchers would reveal that some theropods had iridescent feathers that outshine David Silva’s velocifigures.
The Caihong juji, Mandarin for “rainbow with a big crest” (or just Caihong for short), was a “paravian theropod,” a clade commonly known for its winged forelimbs (even though many weren’t capable of flight) and enlarged sickle foot claws. In 2014, a farmer in the Qinlong County in the Hebei Province of Northeastern China gave a nearly complete Caihong fossil, feathers included, to The Paleontological Museum of Liaoning. Finding a complete skeleton is rare in paleontology and proved very helpful to the researchers. However, you might wonder just how scientists were able to determine the iridescent nature of the Caihong’s plumage. Two words: fossilized melanosomes.
Melanosomes are organelles that create, store, and transport melanin, which determines the pigments/colors of animal hair, fur, skin, scales, and feathers. Upon examining the Caihong’s head, crest, and tail feathers with an electron microscope, scientists discovered platelet-shaped structures similar in shape to the melanosomes that give hummingbirds their iridescent coloring. The rest of the body feathers had melanosome structures similar to those in the grey and black feathers of penguins, which would have made for an odd sight: a duck-sized dinosaur with body feathers as drab as a raven’s and head and neck feathers more colorful than a peacock’s.
The inferred feather coloration of the Caihong is not its only unusual feature, though. The dinosaur had longer arm and leg feathers than its relatives, and its tail feathers created a “tail surface area” that was larger than the famous proto-bird the Archaeopteryx. Furthermore, the Caihong had bony crests, which while common among most dinosaurs, are almost unheard of among paravian theropods. But, more importantly, it had proportionally long forearms, which is a feature of flight-capable theropods, even though scientists believe the Caihong didn’t fly. While this dinosaur apparently has the earliest examples of proportionally long forearms in the theropod fossil records, it still falls in line with the belief that the evolution of flight-capable feathers outpaced the evolution of flight-capable skeletons. The melanosomes, however, are the more intriguing discovery, since they are the earliest examples of “organized platlet-shaped nanostructures…in dinosaurian feathers.”
While paleontologists are confident the Caihong’s platelet structures are melanosomes, the researchers understand that their discovery is based partially on inference and could potentially be incorrect. If the structures aren’t melanosomes, well, that invalidates this entire article. But that’s what paleontology is all about: examining the evidence, creating inferences supported by that evidence, and changing those inferences when new information becomes available. Still, the concept of dinosaurs with iridescent feathers is pretty cool. If you want to learn more about the Caihong juji, you can read the original article on Nature.
Scientists Discover Velociraptor’s Cousin, and It Looks Like a Swan
When we hear the word Velociraptor, we usually think about the clever girls from Jurassic Park/World. In my case, I think about Dinobot from Beast Wars, but that’s neither here nor there, because after you’re done reading this article, you won’t be able to get the following thought out of your head: Velociraptors are related to a dinosaur best described as a prehistoric swan. Good luck ever seeing ol’ sickle claw the same way again.
Say hello to the Halszkaraptor essuilliei (Halszka for short), a dinosaur recently discovered at Ukhaa Tolgod (part of the Djadochta Formation in the Gobi Desert in Mongolia) by a team of paleontologists led by Andreau Cau of the Geological Museum Capellini in Bologna, Italy. Halszka’s around 75 million years old, which means he was alive during the late Cretaceous period during what is known as the Campanian age. And if you’re wondering why he looks like a swan, that’s because, according to a 3D synchrotron analysis (a process that uses x-rays so powerful they can only be produced in facilities the size of football stadiums), he’s semi-aquatic. Doesn’t he just look adorable?
Halszka belongs to the suborder therapod, known for specimens such as Tyrannosaurus Rex, the Velociraptor, and the ostrich. Therapods are known for their efficacy on land, but until the Halszkaraptor’s discovery, scientists thought they were exclusive to solid ground. Sure, some therapods might have eaten fish from time to time, but Halszka is the only known therapod with aquatic tendencies.
“The first time I examined the specimen, I even questioned whether it was a genuine fossil,” explained Cau. “This unexpected mix of traits makes it difficult to place Halszka within traditional classifications.” Given the long, swan-like neck, dinky flippers, and sickle-shaped claws reminiscent of the Velociraptor, I can’t blame him for being confused. Zoologists went through the same problem when they examined the first known platypus back in 1799; who wouldn’t be confused by such disparate features? “When we look beyond fossil dinosaurs, we find most of Halszkaraptor‘s unusual features among aquatic reptiles and swimming birds,” Cau continued. “The peculiar morphology of Halszkaraptor fits best with that of an amphibious predator that was adapted to a combined terrestrial and aquatic ecology: a peculiar lifestyle that was previously unreported in these dinosaurs. Thanks to synchrotron tomography, we now demonstrate that raptorial dinosaurs not only ran and flew, but also swam!”
Halszka now proudly sits as the first of a new genus of amphibious dinosaurs, capable of using its legs to walk on land and swim through the water and with a posture likely similar to those of modern day ducks and swans. Since the Gobi Desert, especially the Djadoctha Formation, appears to be a hotbed of important paleontological finds, many of which are therapods related to Halszka, who knows how many other amphibious dinosaurs are waiting to be discovered? Halszka could either be one of many previously undiscovered swimming raptors or as unique as the Mesonychid; nature’s first and only attempt at a hoofed predator.
If you are interested in reading Cau’s paper on the Halszkaraptor essuilliei, you can read the article on Nature. However, if you do not have a subscription, you should either read the Science Daily article or Andreau Cau’s personal blog (it is in Italian but an English translation is readily available on the page).
Researchers Use Stem Cells to Help Rats with Paraplegia Walk Again
Spinal cord injuries (SCIs) harm the nerves housed in the spinal cord, often causing irreversible damage to the body and its functions. Normally, physicians and other clinicians can only help people adapt to their SCIs instead of healing them, but researchers at Tel Aviv University and the Technion-Israel Institute of Technology might have discovered an essential key in the search for a possible cure. Just a word of warning: this article contains descriptions of animal experimentation, so some information might not be suitable for some readers.
Dr. Shulamit Levenberg of the Technion-Israel Institute of Technology recently led a multi-university study to determine if lab rats with simulated complete spinal cord injuries could regain the use of their hind legs with the introduction of stem cells and tissue engineered scaffolds. Some stem cells had been induced to differentiate (i.e., develop into different type of cells such as support cells), while others hadn’t been induced at all. And, the scaffolds were designed to “provide a 3D environment in which cells can attach, grow and differentiate, maintain cell distribution, and provide graft protection following transplantation.” In other words, the scaffolds made sure the stem cells grew as intended and weren’t accidentally damaged.
Researchers took lab rats and surgically removed a small portion of the lamina (the bony plates of the vertebrae that protect the spinal column and the vulnerable nerves) and cut through all of the nerves. Since incomplete spinal cord injuries only damage some nerves and can, for example, leave people unable to move their legs but capable of feeling through them or vice versa, the researchers had to sever all the nerves to simulate a complete spinal cord injury. Some rats were then implanted with the scaffold and stem cells (some of which were induced and some of which were not) to bridge the severed nerves. Other rats were implanted only with the scaffold, and a control group received neither scaffold nor stem cells.
After the scaffolds and stem cells were implanted, the researchers stitched up all the rats, including the control group, and observed them for any improvements. Rats that received both the scaffold and induced stem cells recovered better than the other groups; 42% of these rats were able to walk and support their body weight with their hind legs after three weeks. Furthermore 75% of this group reacted to stimuli in their hind legs and tail. Fewer rats with the non-induced stem cells recovered as fully as the rats with the induced stem cells. Furthermore, researchers found the scaffold-only group could not respond to any stimuli in their hind legs or tail, and rats in the control group did not improve at all.
While the study demonstrates that induced stem cells coupled with tissue engineered scaffolds could potentially help people with SCIs walk again, the number of rats who fully recovered was fairly low. Still, the results are promising and lay the groundwork for future studies that might one day develop a cure for SCIs. The full article on Levenberg’s study can be found on Frontiers.
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