Tiger tiger, burning bright: the retinal specializations that cause glowing eyes
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Bobcats like to chase laser pointers. At least, Moses the bobcat, of Big Cat Rescue, does. Also up for midnight laser chases are Bailey the bobcat, the servals Santigo and Purrsonality, Rambo the Jungle Cat, and Diablo the Savannah Cat.
These preferences I learned while enjoying the latest video from Big Cat Rescue: Big Cats v.s. Laser Pointers, which asks the critical question of whether big cats, like house cats, enjoy chasing laser pointers (1).
As entertaining as I found the sight of a bobcat pouncing at a glowing red dot (which is to say, very), I was more struck by the sight of the eyes, luminous in the darkness. Like any proper biologist, I googled “feline eye glowing”.
1 Wikipedia page, and a pubmed search later…
Allow me to introduce the Tapetum lucidum (plural: tapeta lucida) a layer of highly reflective tissue found behind the retina of many vertebrate species, including: cats, dogs, elephants, and seals. Tapeta lucida (translation: bright tapestry) are composed of pigment cells that reflect light via organized arrays of crystalline purines, back along the light path and onto the photoreceptors, providing, essentially, a second chance at photon detection.
The effect of this light reflection is therefore to enable the animal to see in dimmer light, significantly enhancing night vision; in cats, the tapetum lucidum lowers the minimum threshold of vision by 6 times.
The general category of tapeta lucida can be broken down into subclasses based upon the structural characteristics of the reflective tissue, variations of which influence the efficacy of the reflection and the specific wavelengths of light sent back for re-collection by the retina (2).
- Retinal tapetum lucidum: the tissue is located within the retina (in the cytoplasm of the retinal epithelium, to be precise), as opposed to behind. Chemical nature and structural organization of the reflective material is variable. Seen in fruit bats, fish, reptiles and marsupials.
- Choroidal guanine tapetum: located next to the capillary network of the eye (the choriocapillaris), behind the retinal epithelium; the reflective material consists of crystals made of hexagonal stacks of guanine. Seen in the elasmobranches, a subclass of cartilaginous fish that include sharks and rays.
- Choroidal taptum cellulosum: located between the large branches of the choroid and the single-layered capillary bed of the choriocapillaris, immediately beneath the retinal epithelium. The reflective material consists of rectangular cells packed with organized refractive crystals of diverse shapes and makeup. Thickness of the tissue varies, from multilayered at the center, to a single layer at the periphery. Seen in many mammals
- Choroidal tapetum fibrosum: same location as above, the reflective material is made of a regularly arranged array of collagen fibers. Seen in all hoofed animals, plus whales.
The cat tapetum lucidum: an ideal reflector
Cats posses a Choroidal taptum cellulosum, one that is particularly thick at its center (15-20 layers), and composed of flattened, rectangular cells filled with membrane-bound reflective riboflavin rodlets (.1-.12 uM diameter) arranged in a hexagonal lattice, with precisely 0.15 uM separating each rodlet. This conformation yields a highly efficient light reflectance; theoretical calculations of ideal reflectance suggest that in most biological systems, 100% reflectance can be achieved using 10-20 layers of uniformly sized, orderly arranged, precisely spaced reflective material. Indeed the cat’s eye has been estimated to reflect 130 times more light than the human eye.
Extreme low-light adaptations: the Weddell Seal
Weddell seals, the most southern-dwelling mammals, live in the dim waters of Antarctica. Diving to depths of 750 meters (3), Weddell seals will preferentially use eyesight to hunt for prey. For this reason, their eyes are highly specialized for capturing as many photons as possible (4). These seals possess extremely thick tapetum lucidum, to maximally reflect light onto retinas chock-full of highly sensitive rods. Other specializations, including un-pigmented pigment epithelium, and high rod-to-RGC ratio (lots of convergence) also contribute to the Wendell seal’s extreme adaptation to low-light conditions.
No Tapetum Required
Many vertebrate animals are not equipped with a tapetum lucidum. These animals, which include squirrels, pigs, and both human and non-human primates, tend to be diurnal animals, with eyes that are adapted for visual acuity during bright light. The presence of a tapetum lucidum in a diurnal species would overwhelm the retina’s light-sensing capabilities during the brightest day; better to trade nighttime sensitivity (which is not hugely useful for an animal expected to be asleep at night), for the ability to see at noon. Thus, humans do not have tapetum lucidum, as we are primarily active during the day.
Could humans be implanted with a tapetum lucidum?
While spelunking ‘round the Internet, I chanced upon the question of whether humans could be implanted with a tapetum lucidum. With current technology, the answer is no, for a bunch of reasons, several of which are clearly laid out by Jeremy Wingard (ophthalmology resident and HHMI medical student fellow. Implanting the tapetum lucidum of an animal into a human is impractical given immune rejection. In addition, surgically integrating a reflective tissue behind the retina, is enormously difficult; the choroidal variants of tapeta lucida are intertwined with the vasculature of the eye. Inserting a tapetum lucidum would require separating a human’s retina from its choroid, which would necessarily disrupt blood flow to the retina, causing severe damage to the retina (5). One possibility would be to use genetic engineering, transforming tissue already within the human retina into reflective tissue capable of acting as a tapetum lucidum. This proposition is dependent on the availability of such tissue; a situation which would seem more likely if the tapetum lucidum represents a specialization of a common tissue within the retina. However, the origin of the tapetal cells that comprise the tapetum cellulosum, is unclear. One observation that may provide a hint as to the cellular origin of the tapetum, is that on the edges of the tapetum cellulosum, reflective tapetal cells are replaced by cells containing melanin, melanocytes. Indeed, cells that contain properties of both tapetal cells and melanocytes are not uncommon, suggesting that tapetal cells may be a specialized form of melanocyte. If this is the case, discovering a genetic basis for the specialization could allow selective reprogramming of human melanocytes to allow light reflection. However, the clear complexity of tapetal cells indicates a significant degree of specialization that is likely to be difficult to recapitulate. It is also worth reiterating that having a tapetum lucidum specializes an organism for low-light conditions; typical humans that live, love, and play in daylight would be ill served by a tapetum lucidum.
Translating the tapetum lucidum to technology
In a satisfying example of translating basic anatomical knowledge into technological achievement, the structure of the tapetum lucidum, and the luminescent glow that signifies it’s presence, inspired a common object; the raised pavement marker. These markers are retroreflectors; they gather light from the headlights of incoming cars, and reflect a portion of that light back at the source. The internal design of the reflectors determines the intensity of the reflected light, much as the structural specifics of tapeta lucida determines the efficacy of light reflectance back onto the retina. So the next time you’re driving along a dimly divided highway (Highway 280, in my case), wish for a pavement marker based upon a cat’s tapetum lucidum.
Sources
- Previous videos from the Big Cat Rescue have asked whether big cats like catnip (original, and with added science!), and if they purr.
- Ollivier et al (2004). Comparative morphology of the tapetum lucidum (among selected species). Veterinary Opthamology, 7(1): 11-22. (link)
- Wikipedia: Weddell Seal
- Wingard (2010). HHMI Bulletin: Ask A Scientist. (Link)
- Welsch et al (2001). Microscopic anatomy of the eye of the deep-diving Antarctic Weddell seal (Leptonychotes weddellii). J Morphol 248(2): 165-74. (Link)
