Defining Issues
Changing the Tone of Skin Science
In the age-old quest to retain youthful beauty, reducing wrinkles and fine lines has often overshadowed maintaining even-toned skin. But a glance at art across cultures and centuries shows that luminous, even skin tone is considered ideal. From the palest white powders used by Japanese courtesans to the bronzers applied by women of today looking for even, sun-kissed color without the risk of tanning - changing skin tone to enhance beauty has been an ongoing interest of women for hundreds of years. In recent years, medical science has offered pharmacological tools to help with pigmentation changes due to natural aging, photodamage or skin conditions, and many of these tools and understandings are now being reapplied to the world of cosmetics. Still, the connection between skin tone, skin health and perception of age is only beginning to be illuminated.
Luminous skins has been prized through the centuries as a beautiful trait, from 15th century Venetian painters who used crushed glass in portraits to make subjects' skin glow to 18th century Japanese geishas to 20th century skin bronzers.
Scientists are gaining new insights into why skin's luminosity and uniformity of tone tends to degenerate with age. Using sophisticated medical imaging tools and computational models, scientists can now look beneath the surface of the skin and precisely characterize age-related changes in sub-surface structure, pigmentation and texture. As a result of these new understandings and better research about the underlying biology of pigment changes, the tone of skin science is changing. Scientists and dermatologists are beginning to recognize that repairing tone problems is as important as repairing texture problems, and are increasingly able to offer women better and more accessible solutions to achieve luminous and more even skin tone.
Advances in Science
Defining Tone
The wide range of skin colors - from the deepest chocolate brown of Africa's Ivory Coast to the warm olives of the Mediterranean to the pale fairness of Scandinavia - is paralleled by a similarly wide array of cultural beliefs that define "ideal skin tone" in various geographic regions. Western women may desire the healthy glow of sun-kissed skin, while Asian cultures value paleness, sometimes going to great lengths to lighten skin. Despite these seemingly contradictory goals, there is one common quality that seems to define beautiful, desirable skin the world over: evenness. Regardless of where on the color spectrum any one individual is, uniform, even-toned skin has a luminous "lit from within" quality that is emerging as one of the most critical characteristics of healthy, youthful attractiveness. Skin texture is also an important piece of the tone puzzle, because smooth, fine-textured skin reflects light more evenly.
Cultural beauty
In the case of both color and texture, contrast seems to be the key - or more precisely, the lack of contrast. That's because the human eye is drawn to edges created by contrast. Skin aging affects texture and pigmentation, creating contrasts on the face that may be a result of shadows caused by wrinkles or color changes caused by age spots. Skin tone, then, can be defined as an interplay between even color and fine texture, both of which act to reduce contrast and increase luminosity.
Skin aging affects texture and pigmentation, creating contrasts on the face that may be a result of shadows caused by wrinkles or color changes caused by age spots.
Shedding Light on "Skin Optics"
It is said that beauty is in the eye of the beholder and scientific research has shed new light on precisely how the human eye processes "beauty" and the roles played by skin tone and evenness. A rich history of research on "skin optics" has examined how the reflection and refraction of light beneath the skin's surface contributes to its outward appearance. Human skin is not a solid, opaque surface. Rather, skin has depth and layers, and its appearance is determined by how much light is ultimately reflected back to the eyes. The amount and color of reflected light is, in turn, determined by the quality and distribution of lightabsorbing molecules (chromophores) beneath the stratum corneum - particularly melanin, hemoglobin and collagen. (See diagram below.) Facial skin normally allows more than 90 percent of light to penetrate its surface.1-5 White light passing through skin's transparent surface is scattered back toward the surface by dermal collagen, which acts essentially as a mirror within the skin. The color of this light is modified by the chromophores, melanin and hemoglobin, both during its journey into the skin and as it is reflected back toward the surface. Colored light is then diffused softly at the surface by the stratum corneum, generating a luminous glow.
If, however, the delicate, even balance of chromophore distribution within the skin is disrupted - as can happen in photoaging or exposure to environmental irritants - shadowing caused by texture, along with local changes in pigment concentrations, can dull skin's luminosity by creating contrast.
Biological Facts
What Lies Beneath: The Biology of Tone
The incredible kaleidoscope of human skin color is due to each individual's unique dermal concentration and distribution of skin chromophores, molecules that absorb or reflect light. While a number of chromophores are present in human skin, various models of skin optics1- indicate that three such molecules - melanin, hemoglobin and collagen - overwhelmingly drive skin coloration differences both between different people and within the same individual.
Melanin is expressed in discrete organelles (melanosomes) that are assembled within epidermal melanocytes and are transported, when mature, to neighboring keratinocytes. Melanin is perceived as a brown-colored pigment, responsible for both "constitutive" racial pigmentation and "inducible" pigmentation (the skin's tanning response).
Hemoglobin in skin is found within proteins in red blood corpuscles comprising the rich network of blood vessels that supply the skin with oxygen. When carrying oxygen in the blood, hemoglobin takes on a red hue and, as such, gives young, healthy skin a characteristic, highly diffuse "bloom" of pink coloration. Deoxygenation of hemoglobin can produce blue-toned skin coloration.
Collagen is a fibrillar protein that represents some 75 percent of the dry weight of the dermis and provides both tensile strength and elasticity.6 While it plays an important structural role, it also contributes significantly to skin optics. Collagen acts like a mirror, scattering light back toward the skin surface. As such, it is responsible for the "brightness" of skin color.
Not So Mellow Yellow
- Glycation of collagen causes skin yellowing via a spontaneous reaction among protein and sugar in the presence of reactive oxygen - specifically, the attachment of an aldehyde group of glucose to an amino group of a collagen molecule, which produces advanced glycosylation end products (AGEs). These AGEs are yellowish in color and contribute to the yellow or sallow appearance of skin. Generally, this reaction increases with age and certain disease states, including diabetes.7-10
- Skin can also appear yellowish due to the presence of carotene, a reddish-orange chromophore. Oriental skin, for example, has a greater amount of carotene in the stratum corneum, which produces a yellowish tinge. Carotene is not synthesized in the body, but is taken in through the diet.
Changing with Age
Skin aging research is increasingly revealing how agerelated changes within the dermal layers impact skin optics and the appearance of skin tone. Cumulative changes in the three key skin color chromophores are among the primary culprits. In young skin, melanin is evenly distributed, and melanocyte activity is low, restricted to the production of constitutive pigmentation only. UV radiation in sunlight transiently activates melanocytes to produce melanin that is evenly distributed, as in a tan. In aging skin, some melanocytes may be damaged by cumulative UV exposure, causing them to be permanently "switched on" and overproduce melanin. This overzealous melanogenesis production can eventually create permanent local discoloration with sufficient size and contrast to appear as age spots (lentigines) or as diffuse hyperpigmentation. As skin turnover decreases with age, microscopic bits of melanin ("melanin dust") can become trapped in the epidermis and stratum corneum, contributing to a duller appearance.
Uneven distribution of melanin manifests itself as age spots and hyperpigmentation.
In young skin, blood vessels in the papillary dermis are normally delicate structures that are barely visible. Exercise or hot weather can cause them to dilate, producing an even, transient increase in red coloration (i.e., "flushing"). In aging skin, blood vessel walls can be damaged by repeated exposure to UV sunlight and become permanently dilated or leaky. This can eventually lead to discolorations with sufficient contrast to be seen as "broken veins" and red blotches. 15-20 P&G Beauty scientists have found that the hemoglobin content of skin increases with age (as measured by an increase in the number of permanently dilated blood vessels) and the distribution of hemoglobin in the skin becomes more uneven with age (as measured by an increase in the number of collections of dilated blood vessels).15-20 Contrast created by hemoglobin manifests as "broken" or "spider" veins ("telangiectasia") and visible, diffuse red blotchiness. In young skin, when collagen is in good condition, it is a highly efficient scatterer, reflecting light back from deep within the skin to produce a natural glow. As skin ages, solar UV damages collagen, rendering it unable to reflect light optimally, not unlike an antique mirror whose glass is cracked and clouded. Many researchers have found that the concentration and uniformity of dermal collagen decreases with age.21-24 P&G Beauty scientists conducted a study with similar findings by measuring an increase in the number of areas containing less collagen than normal.25,26 The effect of these changes is a dull, less luminous complexion.
Targeting Melanocytes
Recent research advances have helped unravel the molecular mechanisms that disrupt normal melanin processing in melanocytes and lead to hyperpigmentation. Damage to melanocytes can be initiated by UV sunlight, environmental irritants such as pollution, endogenous hormones, or free radicals (i.e., as a result of UV- or pollution-induced oxidative stress or released in inflammatory processes), among other things. All of these triggers signal the melanocyte to pump out melanin and ship it off to neighboring keratinocytes.
Red X's indicate whether the cycle may be interrupted and at which point activities intervene.
Name That Tone: Common Hyperpigmentation Disorders
Solar lentigines (also called lentigo senilis, age spots or liver spots): characterized by local communities of "cloned" melanocytes that exhibit abnormally increased rates of melanogenesis
Ephilides (freckles): small, uniformly pigmented, sharply defined macules, typically 1-5 mm in diameter
Seborrheic keratoses: common melanotic lesions found to an increasing extent in normal aging skin, presenting as sharply defined, light brown lesions, often slightly raised with a finely verrucous surface
Post-inflammatory hyperpigmentation: dyspigmentation commonly present after an inflammatory process has taken place such as acne, injury, lupus or invasive cosmetic procedures
Melasma: an acquired skin condition marked by irregular patches of light to dark brown pigmentation; it is often attributed to sun exposure, genetics or hormones and usually develops during pregnancy
The schematic (see previous page) shows the known steps involved in the production of melanin (follow the arrows), from the transcription of the gene for tyrosinase, to the activation of tyrosinase by glycosylation, to the transport of melanosomes out of the cell. Red X's indicate steps in the process where melanin production might be interrupted, along with the known therapeutic strategies that act mechanistically at each of these "intervention" points. Combining ingredients that target more than one mechanism in the process of melanogenesis may provide greater efficacy in shutting down the melanin factory and preventing the outward signs of melanin overproduction.
Lab Notes
Getting Under the Skin
Originally developed as a non-invasive tool to help detect skin cancers, the SIAscope has been repurposed to give scientists images of chromophore concentration in skin. Photo courtesy: Paul Matts.
The SIAscope is a new state-of-the-art instrument that uses a unique combination of macro-digital photography, contact-remittance spectrophotometry and hyper-spectral imaging to map the concentration and distribution of chromophores up to 2 mm beneath the surface of human skin. It has been used for many years by dermatologists and plastic surgeons to assist them in identifying and treating a wide range of skin conditions. P&G Beauty has been working with skinimaging experts at Astron Clinica to further develop and refine the SIAscope device to be small, user-friendly and more broadly available to consumers.
The SIAscope captures individual images showing how the distribution of chromophores explains skin appearance in a technique called chromophore mapping.
The SIAscope reveals the way light interacts with skin, specifically, how it is either absorbed by chromophores or scattered by internal structures. By modeling these interactions and probing skin with visible and infrared light, the SIAscope is able to determine the location and concentrations of light-absorbing and light-scattering molecules and structures within skin. In particular, this hand-held instrument is able to rapidly and non-invasively map melanin, hemoglobin and collagen, the chromophores that drive human skin coloration and play a central role in our perception of age, health and beauty. Examples of chromophore maps of normal human skin are shown above.
Emotional Attitudes & Behaviors
The Psychology of Beauty: Skin Tone Matters!28
Two leading evolutionary biologists, Professor Karl Grammer and Dr. Bernhard Fink, conducted a unique study to determine the impact of facial skin tone on the perception of a woman's age, health and attractiveness, independent of facial form and skin texture. To accomplish this, the scientists created skin color maps from digital images of women aged 10-70. Facial lines and wrinkles were removed from each image, leaving skin color distribution as the only variable.
Evolutionary biologists took photographs of women to help determine the impact of tone on the perception of age, health and attractiveness.
Using 3-D imaging technology, the scientists applied the skin color maps to one universal facial structure (see image). The resulting model faces had identically shaped features but retained the original women's skin color compositions, which were then rated by participants. The digital images were subsequently analyzed with the SIAscope technology to determine the distribution of melanin and hemoglobin chromophores.
The study found:
- Based on age estimates by raters, chromophore concentration and distribution alone may account for up to 10-12 years of age perception, independent of form and skin surface topography.
- There were close correlations between estimated age and perceived healthiness, and between estimated age and skin-specific attributes, including smoothness and firmness.
- There was a strong correlation between age and melanin homogeneity, suggesting that melanin distribution drives the majority of tone dependent age perception.
- Authors concluded that chromophore concentration and distribution have a major influence on the perception of female facial age and on judgments of attractiveness, health and youth.
Researchers used digital morphing software to "drape" study participants' skin over a universal face in order to isolate the impact of tone on the perception of age.