Also, studies that have compared decaf drinkers to those who drink regular brew have not shown an increased risk of cancer a.s.sociated with drinking decaffeinated coffee.
Gently down the stream There has been some coverage in the newspaper about a project of recycling wastewater in Southern California to make it drinkable again. If purifying wastewater is a lengthy and costly process, wouldn"t it be cheaper to do the same thing with seawater? Add to that the mental-health aspects a.s.sociated with the concept of drinking what used to be wastewater.
It may seem surprising, but according to the City of San Diego Water Department, it currently costs about twice as much to desalinate seawater as it costs to take the same quant.i.ty of water "from toilet to tap." The reason is that ocean water is about 25 times saltier than the starting point for recycled water.
Removing dissolved salts is the most energy-intensive step in producing drinking water. The more salt in the water, the more energy required to remove it. Salt removal is accomplished either via distillation or, more commonly, reverse osmosis. In reverse osmosis, water is pushed through a membrane that allows water molecules through, but not the dissolved salts.
According to a 2006 report by the Pacific Inst.i.tute for Studies in Development, Environment and Security, the cost of desalinating water in California and delivering it to users may be as high as 1 cent per gallon and is unlikely to fall below one-third cent per gallon. Although considerably cheaper than bottled water, even the lower estimate is more than the price paid by most urban water users and is about 10 times the price paid by farmers in the western United States.
San Diego imports about 90 percent of its drinking water from Northern California and the Colorado River. As the costs of alternative sources of water rise due to drought and increased demand, desalination will become a more viable option. It already has in the Middle East, which is home to more than half of the world"s desalination plants.
In the United States, direct recycling of wastewater to drinking water is not accepted practice, probably for the psychological reasons you mention. On the other hand, indirect recycling of wastewater into drinking water is common. For example, cities upstream discharge treated wastewater into rivers that serve as the drinking water supply for cities downstream. Also, in some places, including Los Angeles and Orange County, recycled water is used to top off underground aquifers that supply drinking water.
Recycled water does not contain levels of bacteria, heavy metals, or organic compounds that exceed drinking water standards. However, levels of dissolved salts are higher than those in the drinking water supply.
In San Diego, recycled water is used mainly for irrigation. Some new high-rises are being built with a dual plumbing system via which the city supplies recycled water for flushing toilets. The dual system adds around 10 percent to the cost of installing the plumbing.
Water, water everywhere In this age of scientific wonders, is there any chance of someone finding a method of converting seawater into freshwater economically? If and when that happens, what will we do with all the salt?
As population growth increases the demand on existing sources of freshwater, desalination of seawater is becoming increasingly economically viable. The worldwide market for water desalination is increasing about 15 percent annually.
Reverse osmosis, which involves forcing seawater through a membrane that allows only water molecules through, is the most popular desalination technology in the United States. Improvements in the membrane material that make it longer-lasting and less likely to clog are increasing its cost-effectiveness. The concentrated salt solution that remains behind typically is dumped back into the ocean.
Several new desalination technologies are being explored. Freeze separation involves freezing seawater to get ice crystals of pure water. In vacuum distillation, salt.w.a.ter is vaporized at low pressure, which requires less heat than distillation at atmospheric pressure. In electrodeionization, seawater pa.s.ses between two parallel membranes on the inside of oppositely charged plates. Because ions in the seawater are attracted to the plates, the sodium, chloride, and other ions are pulled out through the membranes, leaving behind pure water.
Reticent rubber Recently my neighbor remarked that he has always wondered why automobile tire wear dust, although it must add up to a significant amount, never builds up enough to be seen. I told him that I read somewhere that tire dust was eaten by bacteria specialized in that strange way. Is that a fact, or was it someone"s imagination?
Tire particles do acc.u.mulate on the roadside and are washed away by rain. But it is also true that many kinds of bacteria and fungi degrade rubber. Natural latex rubber, which is produced by more than 2,000 species of plants to protect their wounds while they heal, consists of long chains of carbon atoms. Rubber-degrading microbes break down the chains using specialized enzymes. Recently some of these enzymes have been identified and the DNA sequence of the corresponding genes determined.
Three-quarters of all natural rubber harvested is used to make automobile tires. Softness and flexibility are ideal for nature"s bandage for trees, as well as products made from it, such as rubber bands, gloves, and protection for other body parts. However, tires must be more durable. Therefore, tires are made using variations of the vulcanization process invented by Charles Goodyear in 1839.
Vulcanized rubber is produced by heating natural or man-made rubber with sulfur and other chemicals. During the process, sulfur bridges form between the rubber"s carbon chains. Microbes have difficulty gaining access to the linked chains in vulcanized rubber. Vulcanized rubber is also a less hospitable environment to microbes, because it is less permeable to gases and water than natural rubber. Furthermore, some of the chemicals added during the vulcanization process are toxic. As a result, biodegradation of rubber, especially vulcanized rubber, is a very slow process.
Because sc.r.a.p tires represent 12 percent of all solid waste, there is considerable interest in optimizing rubber biodegradation. Currently about half of waste rubber is combusted to generate electricity, or ground up and used in asphalt mixtures for road resurfacing. Vulcanized rubber can also be recycled by mixing fine particles of it with newly produced, nonvulcanized rubber, but the performance characteristics of recycled rubber are not optimal.
Recent research has shown that higher-quality recycled rubber can be produced by pretreating vulcanized rubber with bacteria that break sulfur bridges between carbon chains, which frees the carbon chains to form new links. Because researchers have discovered microbes specialized to cut carbon chains, microbes specialized to break sulfur bridges, and microbes that can detoxify vulcanized rubber, they are now exploring multistep approaches to bioremediation of tires.
On top of Old Smoky Inside casinos I can see no cigarette smoke, but when I leave, my clothes smell like smoke. I then conclude I"ve been exposed to secondhand smoke. Am I wrong?
You are probably correct, because the thousands of chemicals in cigarette smoke give it a very recognizable smell. a.s.suming that no one is smoking inside the building, it is possible that people are smoking outside, near the air intake openings.
It would not take much smoke for you to be able to detect it. One study showed that about 3,000 cubic meters of fresh air (equivalent to the volume of about 10 s.p.a.cious living rooms) is needed to sufficiently dilute the smoke from one cigarette to protect against eye and nose irritation. Also, the many fibers in fabric provide a large surface area to which smoke molecules can cling, so clothes tend to pick up the smell of smoke quite easily.
Odor eater Baking soda does a remarkable job of neutralizing foul odors. How does it work?
Many unpleasant odors, such as those a.s.sociated with vinegar, sour milk, and rotten eggs, are acidic molecules. Baking soda-sodium bicarbonate-is a weak base that can react chemically with acids to neutralize them. It can also react with a stronger base, so baking soda also neutralizes the basic molecules that cause fishy or ammonia smells.
The reaction between the baking soda and odor molecules is not visible, because not many molecules react at once. However, you can see the reaction if you mix vinegar and baking soda. The gas that bubbles off is carbon dioxide, which is formed during the reaction. This is the reason you may burp when you take antacids. Antacids are usually made of calcium carbonate, but their reaction with stomach acids is similar to the vinegar/baking soda reaction.
Exquisite earth What exactly is terra preta, and what does it have to do with reducing global warming?
Terra preta is Portuguese for "dark earth." It is a carbon-rich, highly fertile soil that covers as much as 10 percent of the Amazon Basin, an area as large as France. is Portuguese for "dark earth." It is a carbon-rich, highly fertile soil that covers as much as 10 percent of the Amazon Basin, an area as large as France. Terra preta Terra preta is also found in other, mostly tropical, regions. Archaeologists used to think that this black soil was deposits from ancient volcanoes or pond bottoms. However, chemical a.n.a.lyses of the soil, as well as the consistent presence of broken pottery, have led most researchers to conclude that the soil is the result of human activity. is also found in other, mostly tropical, regions. Archaeologists used to think that this black soil was deposits from ancient volcanoes or pond bottoms. However, chemical a.n.a.lyses of the soil, as well as the consistent presence of broken pottery, have led most researchers to conclude that the soil is the result of human activity.
The indigenous people of the Amazon Basin, who must have been more numerous than once supposed, began the deposition of terra preta terra preta nearly 2,500 years ago, according to carbon dating. The darkest soils seem to contain a mixture of waste from human settlements. Incorporated into the slightly lighter surrounding soils are large quant.i.ties of charred organic matter, or char. Good char is produced not by slash-and-burn agriculture, but by plant matter smoldered slowly in a low-oxygen environment. nearly 2,500 years ago, according to carbon dating. The darkest soils seem to contain a mixture of waste from human settlements. Incorporated into the slightly lighter surrounding soils are large quant.i.ties of charred organic matter, or char. Good char is produced not by slash-and-burn agriculture, but by plant matter smoldered slowly in a low-oxygen environment.
Crops grown in terra preta terra preta are twice as productive as those grown in nearby unaltered soil. The soil in the Amazon region typically is too poor to support sustainable agriculture. It is acidic, low in nutrients, and high in aluminum, which makes it toxic to soil microbes. Char reduces the soil"s acidity, makes the aluminum ions less reactive, and increases the soil"s capacity to retain nutrients. One study found that the bacterial diversity in are twice as productive as those grown in nearby unaltered soil. The soil in the Amazon region typically is too poor to support sustainable agriculture. It is acidic, low in nutrients, and high in aluminum, which makes it toxic to soil microbes. Char reduces the soil"s acidity, makes the aluminum ions less reactive, and increases the soil"s capacity to retain nutrients. One study found that the bacterial diversity in terra preta terra preta was 25 percent greater than that in adjacent unaltered soil. was 25 percent greater than that in adjacent unaltered soil.
Ancient farmers were surely not thinking about global warming when they incorporated char into the soil, but it is a remarkably effective method to sequester carbon dioxide. Plants sequester carbon dioxide as they grow because they use it as a building block for the molecules that const.i.tute wood. Unfortunately, when plants die and decompose, the carbon dioxide is released back into the atmosphere. In contrast, the black carbon from the char added by ancient farmers has remained in the soil for millennia.
Calculations by some researchers suggest that the creation of new terra preta terra preta could store more carbon each year than is emitted by all of current fossil fuel use. Efforts to incorporate char into large-scale farming are under way. One company has developed a contraption that converts farm waste into biofuel while producing char. But scientists do not yet have all the dirt on the ancient farmers" tricks, such as what is the right type of char for each soil type. could store more carbon each year than is emitted by all of current fossil fuel use. Efforts to incorporate char into large-scale farming are under way. One company has developed a contraption that converts farm waste into biofuel while producing char. But scientists do not yet have all the dirt on the ancient farmers" tricks, such as what is the right type of char for each soil type.
Drug disintegration Why does the effectiveness of vitamins, minerals, and medicines degrade over time? According to a pharmacist I asked, freezing does not keep them viable. Why not?
Freezing preserves food by interfering with microbial activity. But decay due to microbes is not the main problem for most medicines and nutritional supplements. Instead, over time, chemical reactions cause the degradation of the drug substance, the nondrug ingredients, or even the container, which may then leach chemicals into the medicine.
Reaction with oxygen in air (oxidation) and reaction with water (hydrolysis) are especially common modes of breakdown. Exposure to light, heat, and high humidity can increase the rate of drug decomposition. Accordingly, the bathroom medicine cabinet is a bad place to store medicine.
At least 90 percent of a medicine"s original potency must remain prior to its expiration date. Estimates of a medicine"s shelf life are based on standard conditions, but breakdown may be faster or slower, depending on the storage conditions. Breakdown products can be toxic, and their ident.i.ty can vary depending on how the medicine is stored.
Proper disposal of unused medicines is critical because drugs are now widespread in waterways. Even if their concentrations are too low to affect humans, they may affect fish and other wildlife, and residues of antibiotics might encourage the development of drug resistance in bacteria. Unless your munic.i.p.ality has a pharmaceutical take-back location, the Environmental Protection Agency recommends disposing of medicines in the trash after mixing them with an undesirable substance such as kitty litter.
Lone nutrients How are individual vitamins extracted or manufactured to put into pills or food supplements?
The first vitamin discovered, thiamin, was isolated at the beginning of the 20th century by soaking brown rice in water and separating the compound that dissolved. Nutritional compounds are still extracted from plant parts by bathing them with different liquids, such as alcohol, hydrocarbons, or water, and then distilling the resulting solution. The type of liquid selected for the extraction depends on the vitamin"s structure and whether it is water-soluble or fat-soluble.
It is cheaper to make vitamins than to extract them. Therefore, nearly all vitamins available commercially are manufactured. They are produced either through a series of complex chemical reactions perfected by chemists or by microorganisms that have been engineered to churn out large quant.i.ties of individual vitamins. They may also be made by a partnership of chemists and microbes, as in the case of vitamin C.
Vitamin C is produced in larger quant.i.ties than any other vitamin because, in addition to its use as a supplement, it is added to some cosmetics and is employed by the food industry to prevent the discoloration of food pigments. More than 100,000 tons of vitamin C is made annually. Most vitamin C producers use the Reichstein process, which dates to 1933. It transforms glucose into vitamin C in four steps, the first of which is accomplished by bacteria, and the subsequent three by chemists.
Vitamin B12 has a considerably larger and more complicated structure than vitamin C. The chemical synthesis of vitamin B12 involves about 70 steps, which makes it too technically challenging and expensive for industrial-scale production. Vitamin B12 cannot be extracted from plants either, because plants do not make it. However, many bacteria make vitamin B12 to catalyze reactions involved in fermentation. Genetically engineered bacterial strains that provide high yields of vitamin B12 are responsible for the more than 10 tons of vitamin B12 produced commercially each year.
Vitamin deficiencies were a pervasive health problem worldwide at the turn of the 20th century. In developed nations, programs established in the mid-20th century to fortify processed foods with vitamins have largely alleviated vitamin deficiencies. However, in developing nations, food processing and distribution are limited, and many staple crops, including corn, rice, and ca.s.sava, lack several vitamins. One approach to this problem is biofortification-breeding or engineering plants to produce additional vitamins. Progress has been made with biofortification of vitamin E, folate (vitamin B9), and beta-carotene (which our bodies can convert to vitamin A).
Rub-a-dub-dub I know that soap in the 19th century was made from animal fat. How does a bar of soap made today differ from that made with animal fat? And how did early people bathe before they had soaps made from animal fat?
The earliest accounts of soap-making date from 2800 B.C., but the process seems to have been independently discovered by many different civilizations and may have been known even in prehistoric times. However, soap was initially used for purposes other than bathing, such as to prepare wool for dyeing.
Some early cultures did emphasize personal cleanliness, including the Romans and the Greeks, who rubbed themselves with fine sand and oil and removed the mixture with a metal instrument called a strigil.
Soap consists of linear molecules with an electric charge on one end, which can be created by treating animal fat with a strong base (such as lye). The charged end of the soap molecule is attracted to water. The neutral end is repelled by water and combines with oil. These properties allow soap (old-fashioned and modern) to help oil and water mix.
Soap-making was an established craft in Europe by the late 17th century. Soaps from southern France, Spain, and Italy, made from olive oil, were particularly renowned for their quality. However, soap was heavily taxed and thus considered a luxury item.
Until the mid-19th century, most American colonists, particularly those in rural areas, made their own soap by boiling lye extracted from wood ash with tallow-animal fat-which they saved all year. Because it was difficult to get the concentration of the lye just right, this method produced inconsistent results.
The first key advance in soap-making was the invention, in the late 18th century, of a more reliable method of producing a strong base.
The second key advance was the development, during World War I, of detergents. Detergents are made from petroleum, and detergent molecules can be tailored to have specific properties. For example, although soap binds with the calcium ions found in hard water and produces soap sc.u.m, detergents can be made that do not bind with calcium.
Many of today"s soaps are actually detergents, or a mix of detergents with soap derived from vegetable oils or animal fats, with added fragrances, moisturizers, and vitamins.
The plethora of cleansing products available and our modern obsession with personal cleanliness can be traced back to an advertising campaign for Lifebuoy soap, begun in the 1930s, that coined the term B.O.
Ch.o.r.e tech How come I can use cold water in my washing machine but I have to use hot water in my dishwasher?
The reformulation of laundry detergents over the last few decades has made it possible to wash clothes in cold water and still get them clean. People traditionally have washed clothes in hot or warm water, but the increasing popularity of synthetic fabrics and the desire to reduce household energy consumption have fueled a trend toward cooler wash temperatures.
Fabrics get stained or dirty in three ways: Dirt gets physically trapped between the fibers, electrical attractions hold together the dirt and fabric molecules, or a chemical reaction occurs between a dirt compound and the fabric to form a new compound. In the latter situation, hot water may make a stain permanent by stimulating the chemical reaction. Otherwise, hot water makes it easier to separate dirt molecules from the fabric, because molecules jiggle around more when they are warmer.
Critical to the cold-water effectiveness of modern detergents is the inclusion of enzymes that chop up dirt molecules. The four major cla.s.ses of detergent enzymes are proteases, lipases, amylases, and cellulases. Proteases work on protein-based stains such as gra.s.s, egg, and blood. Lipases work on fats and oils. Amylases remove starch-based stains such as potatoes, gravy, and baby food. Cellulases smooth the surface of cotton fabrics by cutting off the fuzz and tiny fibers generated by wear and washing.
Surfactants have also been modified to enhance cold-water laundering. Surfactants loosen dirt from fabric and suspend it in the wash. Like soap, they are molecules with a hydrophilic (water-loving) tail and a hydrophobic (water-hating) head. When the hydrophobic ends of a bunch of surfactant molecules glom on to greasy grime, they form micelles-tiny oil blobs dissolved in water thanks to the outward-pointing hydrophilic tails of the surfactant. Cold-water surfactants have heads that are extra-hydrophobic to help them better interact with oil, which dissolves more poorly in cold water.
Dishwasher detergent has also been reformulated to enhance cleaning and make it less harsh on dishes. Of course, dishwashers do not rely on agitation to help remove cooked-on food. Even more important, hot water helps destroy bacteria and viruses that could cause food-borne illnesses.
Hot water is still used for laundry when there is a risk of disease transmission, such as for hospital and hotel linens, but for the most part, high-tech suds allow us to save energy and preserve our favorite duds.
3. Body parts
Toe the line
Why do some people, like me, have second toes that are longer than their big toes? Is it a genetic specificity? Is it a characteristic that occurs more in women than in men, or more in particular ethnic groups?
You are in good company. The Statue of Liberty has short big toes, or what is referred to as the Greek foot. Lady Liberty"s sculptor, Frederic Auguste Bartholdi, was trained in the cla.s.sical tradition, and Greek and Roman statues often have short big toes. Leonardo da Vinci drew skeletons with Greek feet, rather than so-called Egyptian feet, in which the big toe is longest.
Some cultures have considered short big toes to be a sign of intelligence. (Disclaimer: I have Greek feet.) Unfortunately, Greek feet got a bad rap when, in 1927, a doctor named Dudley Morton published a paper describing a foot disorder a.s.sociated with short big toes.
According to Morton"s findings, the head of a short big toe cannot readily reach the ground and therefore does not carry its full share of the body"s weight. As a result, the second toe carries extra weight. Calluses develop on the ball of the feet beneath the second and third toes, and tenderness may develop in this area.
However, a study of more than 3,500 men enlisted in the Canadian Army during World War II revealed absolutely no relationship between toe length and distribution of weight on the foot or foot pain. The researchers concluded that feet develop ways to compensate for variants in structure.
The reason for the discrepancy between these findings and Morton"s may be that the vast majority of patients who came to see Morton complaining of foot pain caused by "Morton"s Toe" were women. Women are more likely to wear high-heeled shoes, which compound the problem by forcing the weight toward the front of the foot.
Researchers have reported varying incidences of Greek feet in different populations, ranging from 3 percent to 40 percent. Extreme cases, where the big toes are less than two-thirds the length of the second toes, are rare. The trait is thought to be genetic, with Greek feet being recessive and Egyptian feet being dominant.
Gender differences in the relative lengths of fingers and toes are small. Nearly all of the research on gender differences in digit length has focused on the ratio of the lengths of the index and ring fingers. Studies indicate that minor differences in this ratio, possibly caused by exposure to hormones in the womb, are a.s.sociated with certain personality traits, susceptibility to disease, and even s.e.xual orientation. These claims are controversial, because research in many areas of human health and behavior has shown that most traits are a result of complex interactions between nature and nurture.
Surgeons" favorite organ If the appendix is a relatively useless organ in our bodies, why do we have it? Did the appendix used to have a purpose in the bodies of early man?
Someone once said that the only function of the appendix is financial support of the surgical profession. About 7 percent of the population in developed countries will suffer from appendicitis in their lifetime, but appendicitis seems to be rare in undeveloped countries. It is unclear if diet or some other factor contributes to this difference.
In humans, the appendix is a wormlike pouch, three-and-a-half inches long on average, attached to the first part of the large intestine. In herbivorous mammals, such as rabbits, a much larger a.n.a.logous structure houses bacteria that help break down cellulose, a large plant molecule. The appendix is present in many vertebrates, including other primates.
The human appendix does not contain cellulose-digesting bacteria, so humans cannot digest cellulose (which is why lettuce is roughage). Therefore, the appendix is often called a vestigial organ-a structure that has become diminished in size and lost its original physiological function.
That does not mean the human appendix has no function. Of the many functions hypothesized, a role in immunity is considered the most likely, although this remains controversial. The appendix, along with other parts of the digestive system, produces immune system cells, which can respond to ingested, disease-causing microbes. Whether the appendix contributes significantly to the immune response is unknown, since the lack of an appendix does not cause any obvious health problems.
Longer and longer How can your nails continue growing throughout your life? How are they formed?
The first sign of nail formation occurs at week 10 of prenatal development, when a thickened area of skin called the primary nail field appears at the tip of each finger. The nail fields burrow into the skin, and the side and lower borders become thickened to form nail folds. The cells in the bottom nail fold continue to divide to produce the nail.
The fingernails reach the tips of the fingers by the end of the eighth month of pregnancy. The toenails, which begin development later than the fingernails, reach the tips of the toes just before birth. The extent of nail growth can be used as an indicator of how prematurely a baby has been born.
Most of what we can see of our nails is tightly packed layers of dead cells that are full of a tough protein called keratin. Keratin is also an important component of hair, feathers, beaks, horns, hooves, and the outermost layer of skin.
As new cells are produced in the nail"s germinal matrix (base or root), which is located under the skin behind the fingernail, they are pressed forward and upward into the nail. They die, but remain firmly attached to their neighbors to create the solid nail. As the nail streams along the nail bed, new cells produced in the bed are added to it, helping compensate for surface wear.
Tough tips What purpose do toenails and fingernails serve?
They serve as mini body armor to protect the tips of our fingers and toes. Of course, fingernails also come in handy for scratching itchy spots and picking up small objects. A less obvious, but important, function of fingernails is to enhance the sensation in the fingertip.
When we use our fingertips to feel something, the nail acts as a counterforce. It increases the compression of the sensory organs between the pad of the finger and the nail, which makes us better able to distinguish fine detail on the surface we are touching.
Ashes to ashes When a person is cremated, how much do the ashes weigh? Is there anything that doesn"t burn?
The weight of the cremains (cremated remains) depends on several factors: the temperature of the cremator furnace and the duration of the cremation, and the individual"s weight, height, age, and gender. On average, the cremains of a fully developed adult weigh 5 pounds (2.3 kilograms), or approximately 3.5 percent of body weight. The range is from around 2 pounds to 8 pounds.
Cremains are not really ashes. Most of what remains after cremation is bone, often sizeable fragments. The larger and heavier a person"s skeleton is, the greater the weight of his or her cremains. As a result, the cremains of men are 2 pounds heavier on average than the cremains of women. Also, the cremains of older adults are lighter on average than those of younger adults because bone density decreases with age.
The chemical composition of cremains is mainly calcium and phosphate-the major const.i.tuents of bone. Smaller amounts of carbon, pota.s.sium, sodium, chloride, magnesium, iron, and other minerals also remain. Melted metal from dental fillings and surgical implants, such as artificial hip joints, is usually removed, and the cremains are pulverized to give them the consistency of coa.r.s.e sand.
Holey lids When I was inserting my contact lenses, I noticed a tiny hole on the inside corner of my bottom eyelids. What are these holes?
They are called puncta and are the openings to the tiny ca.n.a.ls through which tears drain. Tears flow from these ca.n.a.ls into a tear sac and then down the tear duct into the nose. In fact, you can taste eyedrops after they flow from the puncta into your nose and drip onto the back of your tongue.
Producing peepers When a human is developing, how are the eyes formed?
We start out as a ball of indistinguishable, genetically identical cells. A cell in what becomes the eye is different from a muscle cell or skin cell because it makes different proteins-the cell"s workhorses. For example, proteins called crystallins pack the lens of the eye and help focus light onto the retina.
During development, chemical signals released by other cells and physical contacts with other cells can tell a cell to switch on certain genes, thereby making it produce certain proteins. A gene called PAX6 is a master gene that initiates eye development. The same gene initiates eye development in fruit flies, and if researchers activate PAX6 randomly, flies end up with eyes in unusual places.
Eye formation begins in a 22-day-old human embryo. At this stage, the brain and head are tube-shaped and consist of sheets of cells. Outward bulges form in an inner sheet of cells. When these bulges-called optic vesicles-contact the outer sheet of cells, the formation of the eye"s lens is initiated. As the optic vesicles grow outward, their base narrows to form a stalk. This stalk eventually forms the optic nerve.
The side of the optic vesicle opposite the stalk pushes inward to become bowl-shaped, with the developing lens at its center. After many rounds of cell division, cell migration, and cell death, the layers of tissue making up the bowl behind the lens become the retina, with its ordered arrangement of light-sensitive rod and cone cells, supporting cells, and nerve cells that send electrical impulses to the brain.
A small opening, the pupil of the eye, remains in the bowl of the optic vesicle. The iris-the colored part of the eye, which is a muscle that expands and contracts the pupil-develops from the tissue in the optic vesicle that surrounds the future pupil.
The cornea, eyelids, and other parts of the eye develop in similar ways, with signals from other cells being paramount in switching on the appropriate genes for cells to take on the correct ident.i.ty.
Both optic vesicles begin from a single patch of cells. Activation of a gene called sonic hedgehog (scientists have a lot of fun naming genes) is necessary for the splitting of this patch of cells so that two optic vesicles form. A mutation in the sonic hedgehog gene can result in cyclopia, a single eye in the center of the face. Infants born with cyclopia do not survive past birth because the condition is accompanied by brain defects.