Guardian lashes What is the purpose of the lashes on your lower eyelid?

Their function is partly cosmetic-to frame those baby blues (or greens or browns)-but they also help protect the eye. They can deflect dust, foil insects, and shield the eye from reflected sunlight. If you gently touch the tips of your upper or lower lashes, you will see how exquisitely sensitive the nerves at the base of the lashes are to the deflection of the lashes. Because the lashes project outward, they trigger a protective blink reflex when an object comes too close to your eye.

Stopping short Why don't eyelashes grow beyond a certain length, unlike the hair on one's head?

Some people who want thicker eyelashes have hair follicles from their scalp transplanted onto their eyelids. The transplanted hairs act like head hairs. They keep growing and require tr.i.m.m.i.n.g.

Within each hair follicle (a pit containing the hair) is a biological "clock" that determines speed of hair growth and how long the hair grows before it falls out. Unfortunately for folks hoping for more hair on their heads, or less on their backs, the exact genes and molecules responsible for the hair cycle clock are still an enigma.

Grating habit What happens when you crack your knuckles/joints? Is it bad for you?

Different parts of a joint-ligaments, tendons, cartilage, and synovial fluid-can snap, crackle, and pop for different reasons.

Ligaments connect bone to bone to strengthen the joint. Tendons connect muscle to bone and move the bone by transmitting the force created by the muscle. When a joint moves, cracking noises are created by the loosening and tightening of ligaments, as well as the change in position of tendons and their snapping back into place. Such noises are normal and are especially common in the knee and ankle joints.

On the other hand, grinding cartilage is a sign of an arthritic or injured joint. Smooth cartilage coats the ends of the bones that come together to form the joint. A joint capsule containing a lubricant surrounds the cartilage surfaces.

In a normal joint, the lubricated cartilage surfaces glide past each other with less friction than a skate on ice. Unfortunately, cartilage has very little ability to repair itself. Worn cartilage can make noise as it grates together. Loose pieces of cartilage can even break off and get caught in the joint, causing it to lock.

Some people can pop their knuckles by pulling on their fingers, which increases the s.p.a.ce in the joint capsules. This reduces the pressure on the synovial fluid-the lubricant in the joints. Synovial fluid contains dissolved gases (carbon dioxide, oxygen, and nitrogen). Like the bubbles of gas that form when a bottle of sparkling water is opened, the reduced pressure on the synovial fluid can cause a bubble of gas to pop into existence. The bubble can be seen on an X-ray and takes approximately 20 minutes to redissolve in the synovial fluid.

Microphones on a knuckle detect two separate sounds when the joint is cracked. One is the sound of the gas bubble forming. The other is probably the sound of the joint capsule (which would be pulled inward slightly as the pressure in the joint decreased) snapping back into place because the formation of the gas bubble increases the pressure within the capsule.

Habitual knuckle crackers are not more likely to develop arthritis, but they are more likely to experience minor swelling and have poorer grip strength. However, the researchers who reported these findings pointed out that they do not prove knuckle cracking causes these problems. Only some people can crack their knuckles. It is possible that these people have looser ligaments to begin with, and the looser ligaments may predispose them to hand weakness and swelling.

Local harvest What type of tissue would be used if stem cells were harvested from an adult?

Many adult tissues have stem cells, including the skin, gut, respiratory tract, liver, muscle, and brain, where they play a role in tissue repair and renewal. Not all of these stem cells are amenable to being harvested and transformed into other cell types.

Many studies have employed hematopoietic stem cells, which are found in bone marrow and generate all the types of cells in the blood. They have been used to treat blood disorders for three decades, and under the right conditions they can be coaxed to give rise to many other cell types.

Recently, researchers discovered stem cells in fat and have transformed them into other tissue types. It would be ideal if stem cells from fat prove to be as versatile as those from bone marrow. Liposuction is simpler than removing bone marrow, and even slender people carry a large-enough fat supply for their own treatment.

Cell selection Embryonic stem cell research is controversial in many countries. Does adult stem cell research hold the same therapeutic promise(s)?

The jury is still out. Depending on who you ask, you will be told either that adult stem cells have demonstrated a surprising ability to transform into other cell types and repair damaged tissue, or that such transformations are relatively rare and can sometimes be accounted for by alternative explanations.

Embryonic stem cells are taken from three- to five-day-old embryos. These cells are exciting to researchers because at this stage, they have the potential to give rise to any cell type (muscle, bone, nerve, skin). On the other hand, it was initially thought that adult stem cells-which are found in many tissues (in children and adults), as well as umbilical cord blood and the placenta-could produce only progeny cells corresponding to their tissue of origin. For example, skin stem cells give rise to the various types of cells in the skin.

However, many recent studies suggest that adult stem cells can generate cell types other than those in their tissue of origin. Researchers coax stem cells into taking on a specific ident.i.ty by selectively exposing them to chemicals that cells normally use to communicate with each other. Coaxing cells to take on a specific ident.i.ty and verifying that they have indeed taken on that ident.i.ty is technically challenging, and many studies have proven difficult to replicate.

Preliminary results of efforts to develop adult stem cell treatments do provide reason for optimism. For example, a few small human trials have shown that injecting adult stem cells into the blood stream can lead to some improvement in heart function after bypa.s.s surgery. But scientists still have much to learn about both embryonic and adult stem cells before clinical therapies live up to their promise.

If adult stem cell research advances to the point where scientists can consistently generate large numbers of cells of the tissue of interest, adult stem cells have three potential advantages over embryonic stem cells. First, ethical controversy has arisen over the destruction of embryos to obtain stem cells. Second, more research is needed to overcome the risk that rapidly dividing embryonic stem cells could lead to tumors. Third, using a patient's own adult stem cells in a treatment would overcome the issue of immune rejection.

Whiter shade of pale Why don't some scars tan?

The most obvious possible explanation is that the scar tissue has fewer melanocytes-cells that produce the dark pigment melanin-than the surrounding skin. However, this does not appear to be the case.

In one study, researchers took biopsies from old, pale scars and from the adjacent normal skin of Caucasian volunteers. The researchers were surprised to discover that the number of melanocytes was about the same in scar tissue and nonscar tissue. In addition, the amount of melanin appeared to be similar in the scarred and normal skin.

The researchers proposed two hypotheses to explain why scars may appear pale even though melanocytes are present and appear to be functioning normally. First, scar tissue may have fewer blood vessels, resulting in decreased blood flow and whiter skin. Second, the structural properties of scar tissue can cause it to reflect light differently than normal skin.

In normal skin, fibers of the structural protein collagen are randomly oriented. As a result, skin scatters light in random directions. When skin is injured, the interwoven arrangement of collagen is destroyed. In an effort to repair the damage as quickly as possible, the body lays down new collagen fibers in linear strips parallel to each other. The scar reflects light mainly along a direction perpendicular to the skin.

Also, the upper layer of the skin over the scar may be thinner and may absorb less light. Thus, the scar may reflect more light toward the observer and appear whiter.

Healing potion How does vitamin E lotion help scars?

Since vitamin E was found to be a major antioxidant in skin, physicians have recommended that patients apply it to injured skin to reduce scarring. Antioxidants mop up free radicals-highly reactive molecules that are produced at the site of a wound. Free radicals can damage cells and can also interfere with the production of collagen. Therefore, vitamin E should protect skin and promote healing.

Yet, despite its popularity, there is little scientific evidence that vitamin E reduces scarring. In fact, some studies have found the opposite. In one carefully designed study, published in Dermatologic Surgery Dermatologic Surgery (April 1999), patients applied a regular ointment (Aquaphor) to one side of a surgical wound and the same ointment mixed with vitamin E to the other side of the wound. In the majority of cases (90 percent), vitamin E had no effect or actually worsened the scar's appearance. Also, about one-third of patients developed a rash on the vitamin-E-treated skin. (April 1999), patients applied a regular ointment (Aquaphor) to one side of a surgical wound and the same ointment mixed with vitamin E to the other side of the wound. In the majority of cases (90 percent), vitamin E had no effect or actually worsened the scar's appearance. Also, about one-third of patients developed a rash on the vitamin-E-treated skin.

Replacement parts How can I donate an organ to give someone else a chance at a longer life?

Lack of donor organs is a big problem in the United States, where more than 100,000 people are on waiting lists, and 19 people die each day while waiting for an organ, according to the official U.S. government website for organ and tissue donation (www.organdonor.gov). Some countries, including France, Spain, and Belgium, have solved the problem of lack of organ donors by adopting "opt out" policies. This means that everyone is considered a potential donor when they die unless they have said otherwise. Typically only about 2 percent of people choose to opt out.

In the United States, you must "opt in" to be considered as an organ donor. You can request an organ donor card at the Department of Motor Vehicles, or download one from the Donate Life website at www.donatelife.net. The argument against an opt-out system has to do with informed consent. Silence is not consent, because if people do not know about the policy, they cannot opt out. Therefore, an opt-out policy raises ethical questions but, of course, so does the current opt-in policy and the resulting chronic donor shortage.

You can also become a "living donor" and donate a kidney, partial liver, lung, or partial pancreas. Medical costs are paid through the organ recipient's insurance, but the donor is not compensated for taking time off from work. More information is available on the Donate Life website. To learn about becoming a bone marrow donor, see www.marrow.org or contact your local blood bank. or contact your local blood bank.

Not so wise What is Nature's purpose for wisdom teeth?

Wisdom teeth were the greatest thing before sliced bread. The extra surface area they provide is handy for chewing nuts, coa.r.s.e grains, and raw meat. In other words, they helped our long-lost ancestors extract more calories from tough stuff. As humans have found ways to make food more toothsome, wisdom teeth have become disadvantageous, except to surgeons who make a living extracting them.

Our jaws are considerably smaller than those of our ancient ancestors. It is often impossible to squeeze in an extra set of molars, and the resulting prevalence of malocclusions-poor alignment-of teeth have made braces a rite of pa.s.sage for many adolescents. No other mammals, even other primates, suffer from malocclusions to the extent that we do.

Part of the explanation for our shrinking jaws lies with genetic changes dating back to early human history. Those changes led to the remodeling of the skull and made room for a larger brain. Our jaws have also gotten smaller as a result of changes in our diet that have reduced the amount of muscular force we need to chew our food and the amount of time we must spend chewing it. Teeth have also gotten smaller over time, but not as rapidly as jaws, because tooth size is more strongly controlled by genetic factors and less influenced by diet.

For at least 300,000 years, humans have fragmented food with tools and used cooking to reduce its toughness. The advent of agriculture over 10,000 years ago increased humans' intake of cereals and other soft foods. More recently, improved techniques for milling grain and a host of other food-processing techniques have made it even easier to take in calories without exercising our jaws, a fact to which anyone who has gulped down a burger, fries, and shake can attest.

The result is a case of "use it or lose it." The fact that exerting muscular forces during chewing has an important effect on the jaw's development has been demonstrated experimentally. In one study, young pigs were fed a diet of soft food. After just a few months, their snouts were shorter and narrower and had thinner bones than pigs fed a diet of hard food.

"Magdalenian Girl," a 13,000- to 15,000-year-old skeleton found in southwestern France, had the earliest recorded case of impacted wisdom teeth. Anthropologists consider it evidence that dietary changes have long been a source of tooth troubles.

4. Bodily functions

Music of maturity

You can tell someone's approximate age by listening to his or her voice. I also think women's voices age more rapidly than men's, because I can more readily tell it is an older lady than an older man. What happens to the vocal cords as a person ages?

Shakespeare wrote about the aging individual, "turning again toward childish treble, pipes and whistles in his sound" (As You Like It, Act 2, Scene 7). Tests with modern acoustic equipment validate these poetic observations. Older people's voices can be distinguished by their characteristic decreases in loudness and clarity, changes in pitch, tremulousness, and breathiness.

The medical term for the normal age-related changes of the voice is presbylarynx. The prefix "presby" means elder. The larynx is the voice box. It lies in the middle of the neck and is composed of nine cartilages, held together by ligaments and controlled by muscles. Within the larynx are the vocal cords-paired ligaments covered by mucous membranes. Varying the length and tension of the ligaments produces sounds of different pitch.

All parts of the larynx have been observed to undergo age-related changes. Cartilage hardens, muscles atrophy, and nerves degenerate. The composition of the tissue in the vocal cords changes, which alters their mechanical properties. Dryness caused by diminished function of the mucus membranes in the larynx and decreased production of saliva affects the voice.

Respiratory health is also important, because air exhaled through the larynx creates the vibrations that produce sound. Therefore, the voice ages with decreases in the size and elasticity of the lungs, changes in the structure of the chest wall, and decreases in the force and rate of contraction of the muscles that control respiration.

Some physiological changes that age the voice differ by gender. In men, thinning of the outer layer of the vocal cords is common. As a result, the vocal cords may become bowed and fail to close completely, permitting air to escape through the gap and creating a wheezing sound. In women, the outer layer of the vocal cords tends to thicken, altering the vibration pattern and resulting in frequent breaks in pitch.

Changes in the thickness of the vocal cords are thought to be related to the testosterone/estrogen ratio, especially after menopause in women. Voice changes vary tremendously from person to person and appear to be more dependent on physiological age-overall health-than chronological age.

Prune people What causes skin to wrinkle like a prune when a person is in a pool or bath?

The standard "stratum corneum" explanation is that we get wrinkly fingers and toes when water soaks into the outer layer of skin, the stratum corneum stratum corneum (Latin for "h.o.r.n.y layer"). The stratum corneum is thickest on the palms and soles and consists of stacks of dead cells. When we dilly-dally in the tub, these dead cells absorb water and swell. The stratum corneum gets prune-like instead of puffy because it is firmly attached to the living skin beneath. Its surface area increases, but the surface area of the living skin stays the same. As a result, the stratum corneum buckles into a series of little ridges and valleys to accommodate its new surface area. (Latin for "h.o.r.n.y layer"). The stratum corneum is thickest on the palms and soles and consists of stacks of dead cells. When we dilly-dally in the tub, these dead cells absorb water and swell. The stratum corneum gets prune-like instead of puffy because it is firmly attached to the living skin beneath. Its surface area increases, but the surface area of the living skin stays the same. As a result, the stratum corneum buckles into a series of little ridges and valleys to accommodate its new surface area.

However, the observation that replanted fingers do not wrinkle after water immersion suggests that a different mechanism is responsible, or partly responsible, for wrinkling. A recent study found that blood flow to normal fingers decreased when people's hands were immersed in warm water, but in fingers that had been successfully reattached after accidental amputation, blood flow did not change. Wrinkling occurred and blood flow declined in the normal fingers of the same hand, and even the normal portion of the injured finger up to the reattachment point. Nerve damage in the reattached fingers may explain the blood flow response difference.

Based on these observations, the researchers suggested that constriction of blood vessels plays a key role in wrinkling. The digits contain large numbers of glomus organs-cl.u.s.ters of large, convoluted arteries that are involved in temperature regulation. The glomus organs are attached to the upper and lower layers of skin, so if they shrink, they would cause the overlying skin to be pulled inward. Uneven skin folds would then form because of the varying levels of tautness between the upper and lower layers of skin, at and amid the attachment points that anchor together the two layers.

The constriction of blood vessels in warm water is considered to be paradoxical. Usually it is a cold environment that causes a decrease in blood flow in the extremities to conserve body heat. When hands are heated with warm air, rather than warm water, blood flow increases.

The blood vessel constriction mechanism explains the difference in wrinkling between normal and replanted fingers. But it does not explain why blood vessel constriction in response to a cold environment does not lead to wrinkling. It may be that stratum corneum swelling and blood vessel constriction must occur together to cause fingers and toes to get all crinkly.

Blinky Why do some people blink more than others?

One reason is that some people have dry eyes. Tear film, consisting of a layer of mucus, a layer of salty water, and a layer of oil, protects the outer surface of the eye. When the tear film thins or breaks up, nerve endings in the eye are exposed to environmental pollutants, including smoke, smog, and vapors from paint and cleaning products. Blinking helps alleviate the irritation by sweeping debris from the surface of the eye and stimulating the meibomian glands in the eyelid to release oil into the tear film.

Certain medications, such as allergy medicines, may cause dry eyes. Contact lenses interfere with the maintenance of a uniform tear film. Women are much more likely to suffer from dry eyes than men, in part because eye cosmetics can cause the tear film to break up. In addition, tear production declines with age, especially in women. The decline is probably related to decreased levels of estrogen and testosterone, which scientists postulate may help maintain the health of the glands that produce the tear film.

Destabilization of the tear film is not the only factor that affects blink rate. Typical blink frequencies at rest are about 12 to 20 blinks per minute. Studies have found that blink rate increases during conversation and when someone is anxious, but it can be suppressed during visual tasks that require concentration, such as reading.

Blink frequency is also affected in diseases, such as Parkinson's disease and Tourette's syndrome, which involve alterations in dopamine-a chemical that nerve cells use to communicate with each other-in the brain. These diseases may affect a "blink generator" in the brain thought to control involuntary blinking.

Blepharospasm is a condition that results in the forceful closing of one or both eyes. It appears to be a blink reflex gone awry.

Twitchy Why does a human face occasionally twitch or have muscle spasms?

Twitching is a sudden, involuntary contraction and release of a muscle. Minor eyelid or facial spasms occur frequently and can be induced by stress, fatigue, eyestrain, caffeine, and certain medications. The exact mechanism that leads to these twitches is unknown, but normally muscle fibers are stimulated to contract by the release of calcium from little storage compartments within a muscle cell.

Hemifacial spasm is a more serious condition that results when an artery presses on the nerve to the facial muscles. Involuntary movements of the face can also result from disorders involving an area of the brain called the basal ganglia.

Summertime blues If the human body runs at 98.6 degrees, why do we consider it hot when it's that warm outside?

To maintain a constant temperature, heat loss and heat production must be in balance. Our bodies produce heat as a byproduct of muscular activity and the chemical reactions of metabolizing food. Body heat radiates to the environment, but at a rate that decreases dramatically as the temperature of the environment increases.

When a part of the brain called the hypothalamus receives the message that the body is heating up, it sends out signals to make the blood vessels in the skin dilate. (This makes you feel more flushed but allows more heat to be released.) It also makes the sweat glands increase sweat output. Sweating cools you down because the evaporation of water uses heat.

If you have experienced the Midwest or East Coast during the three H's (hot, hazy, humid), you will appreciate how much more effective sweating is in a dry climate like San Diego!

Staying cool Why do some people sweat more than others?

Age is one factor. The ability to sweat increases with maturation. Compared to sweat glands in adults, those in children are less sensitive to increases in body temperature and produce sweat more slowly. Sweating capacity is also lower in older adults relative to younger and middle-aged adults.

Gender plays a role. Women have a greater sweat gland density-number of sweat glands per unit area. Men produce more sweat per gland. Overall, women have a slightly lower sweat rate than men.

Heat acclimation has a large effect on the production of sweat and its composition. A person who is not acclimated to the heat usually cannot produce more than a quart (or liter) of sweat per hour. After someone has been exposed to hot weather for a few weeks, the sweat rate can double or triple. At the same time, the concentration of sodium chloride in the sweat declines to conserve body salt.

Hormones control the changes in sweating that result from heat exposure. Sweat comes from the fluid between cells, which is supplied by the blood vessels. Therefore, sweat is filtered blood plasma-the liquid (cell-free) portion of the blood. The sweating-related decrease in the water content of the blood leads to the production of antidiuretic hormone by the pituitary gland and to the production of aldosterone by the adrenal glands.

Antidiuretic hormone stimulates the kidneys to reabsorb water. Aldosterone stimulates the kidneys to reabsorb sodium. Repeated days of exercise in the heat can increase the volume of the blood plasma and the fluid between cells by 20 percent. Retention of water and salt prepares the body for subsequent sweat losses.

Aldosterone also stimulates the reabsorption of sodium and chloride by the cells that comprise the long, coiled tube of the sweat gland. However, pota.s.sium, calcium, magnesium, and other electrolytes found in sweat are not conserved, because the sweat gland does not have a mechanism to reabsorb them.

Sweating is initiated more quickly in physically fit people. More copious amounts of sweat are produced compared to less-fit people exercising at the same relative intensity (not engaging in the same task, but exerting themselves equally hard with respect to their own limitations).

Body size and composition can also play a role in sweating by limiting the body's ability to radiate heat to the environment so that more heat must be lost via evaporation. Other influences include hormonal imbalances and medications that stimulate the part of the nervous system that controls sweating.

Low thermostat I have always sweated profusely. My normal body temperature is 96.8, and this is not a transposed figure. Is it possible that with my very low body temperature I suffer more in temperatures that others find chilly?

Average normal body temperature is 98.6 degrees Fahrenheit (37 degrees C), but temperatures as low as 95.9 degrees Fahrenheit (35.5 degrees C) and as high as 101.2 degrees Fahrenheit (38.4 degrees C) have been recorded in healthy people.

Maintenance of body temperature occurs through the balance of thermal energy generation from metabolizing food and the loss of thermal energy to the environment by conduction to other objects, convection due to air currents, radiation of infrared energy, and evaporation of sweat.

At rest, conduction, convection, and especially radiation account for most of the thermal energy transferred to the environment. The hotter a body is in relation to the environment, the more effective are these ways of getting rid of excess thermal energy. So someone with a naturally low body temperature must rely more on sweating to cool down.

Temperature is tightly regulated in humans, and relatively small increases in body temperature trigger sweating. On the other hand, camels can allow their body temperature to increase more than 10 degrees Fahrenheit (5.6 degrees C), which reduces the need for evaporative cooling through sweating and conserves water.

Sweaty gourmet My daughter sweats when she eats, regardless of the temperature of the food or weather. I have never seen anyone else react the same. The sweat pours down her face.

Gustatory sweating-sweating in response to food-has various causes. Spicy food can stimulate the nerves that control the sweat glands. Also, thermal energy is generated as a byproduct of the digestion, absorption, and storage of food.

The amount of thermal energy generated in response to consuming an identical meal varies considerably among individuals. Gustatory sweating can also occur as a rare complication of diabetes.

Frey's syndrome is a special case of gustatory sweating that occurs when the nerve that controls the salivary gland is damaged by an accident or infection. The nerve's regrowth may be misdirected so that it connects with the nerve fibers that control the sweat glands. If this happens, any of the stimuli that would normally cause salivation-eating, the smell of food, or even the thought of food-can cause sweating on one or both sides of the face.

Impulsive impulses When I observe my finger touching my toe, the touch feeling in finger and toe and the visual observation all occur simultaneously. How can the three nerve impulses (6 feet, 3 feet, and 4 inches) arrive at the brain simultaneously? I understand that nerve impulse speed is about 6 feet per second. This seems awfully slow, since I seem to feel the touch instantaneously.

If all nerve impulses traveled that slowly, you would be in trouble if you were a giraffe! Some nerve impulses do travel as slowly as 3 feet per second (about 1 meter per second), but others travel at speeds of over 200 feet per second (70 meters per second). Impulses travel more slowly along axons-long processes of the nerve cell-with smaller diameters.

The speed also depends on whether an axon is surrounded by myelin. Myelin consists of layers of membrane produced by special cells that envelop the nerve cells. Myelin acts as an electrical insulator and dramatically increases the speed at which the nerve impulse can travel. In diseases such as multiple sclerosis, in which myelin is destructively removed from the nerve, nerve impulses are slowed.

Myelin is rare in invertebrate organisms but is ubiquitous among vertebrates. Not all vertebrate axons are myelinated, but sensory nerves and nerves involved in movement are myelinated. Therefore, it takes only a fraction of a second for a nerve impulse to travel from the toe to the brain. As a result, the difference in impulse arrival times from the toe, finger, and eyes is too small for us to consciously distinguish.

American Lilliputians How do our bodies know when to stop growing so that we do not become giants?

We would seem like giants to some populations of the past. A higher standard of living (better nutrition, less infectious disease) in many developed nations has led to significant increases in height with each generation. For example, in the past century, average height has increased about 4 inches in j.a.pan and many European countries.

Intriguingly, Americans, who were the tallest in the world from colonial times to after World War II, have been surpa.s.sed by the Dutch, Swedes, Norwegians, Danes, British, and Germans, according to a study in Economics and Human Biology Economics and Human Biology led by economist John Komlos. Komlos argues that universal access to health care and greater social equality in Northern Europe, relative to the United States, have led to healthier and taller populations. led by economist John Komlos. Komlos argues that universal access to health care and greater social equality in Northern Europe, relative to the United States, have led to healthier and taller populations.