How is pigmented skin more acidic?

 How is Pigmented Skin More Acidic?


As we discussed in an earlier post, darkly pigmented skin is superior to lightly pigmented skin. It functions better in holding in body water. It does a better job of preventing skin infections. And it is better at resisting mechanical trauma.  We then discussed work from the Elias laboratory demonstrating that this superiority is due to the lower pH of the skin surface.  In other words, darkly pigmented skin is more acidic. Yet darkly pigmented skin has the same number of pigment-forming cells (‘melanocytes’) as does lightly colored skin.  So, what then is the difference in the melanocytes of darkly- vs. lightly-pigmented skin? Well, one big difference is in the the clusters of pigment (or ‘melanin granules’) made by melanocytes in darkly pigmented skin. They are larger and more numerous than the sparser and smaller melanin granules made by pale skin. Melanocytes transfer these granules to the cytoplasm of neighboring epidermal cells (‘keratinocytes’) using their long arms (or ‘dendrites’).  This transfer process happens in both light and dark skin, but here’s where things proceed differently.


Comparison of melanin granules in darkly pigmented (left side) vs. lightly pigmented (right side) epidermis. Both skin types have the same endowment of pigment cells or melanocytes. But, the pigment granules produced by melanocytes in dark skin are larger and more numerous than those in light skin. When transferred to cells of the epidermis (keratinocytes), pigment granules in dark skin (left side) persist as the cells move towards the skin surface and some of the granules “escape” into the spaces outside the cells. In contrast in lighter skin (right side) the small granules tend to cluster over the top of the nucleus (‘nuclear cap’) where they can protect the DNA from sun damage. Very few make it into the outer skin cell layers and few of those are found outside the cells there. Drawing by Jessica Kraft.

The small, crumbly (read ‘crummy’, if you wish) pigment granules of very lightly pigmented skin are degraded within the keratinocytes well before they can reach the outermost layers of the epidermis. In contrast, the larger, more robust melanin granules of darkly-pigmented skin persist within cells as they mature and move outward towards the skin surface. [Read more…]

Our Sour Skin Surface

Did you know that the surface of our skin is acidic? That we have a sour skin surface, like vinegar or lemon juice?  Scientists have long known that we are covered by an acid mantle. But how the epidermis – the outer layers of skin – achieves this feat, conferring a pH of 5.0 or less to our skin surface, when the cells of our body and the blood and fluids bathing those cells have a more neutral pH of ~7.4, has until recently been something of a mystery. lemons crop2


To our surprise, as we were studying the characteristics of epidermis in relation to skin pigmentation, we noted that the pH of darkly pigmented stratum corneum is  lower – that is, it is more acidic – than the surface of lightly pigmented skin. We saw that the pH of darkly pigmented skin was lower by about a half a unit – from ~5.0-5.5 in lighter skin to ~5.0-4.5 in darker skin . While that may seem like a small difference, it is actually rather huge. Differences in skin pH are a bit like those in earthquakes measured on the Richter scale, because both are logarithmic scales. Thus, a change of half a pH unit denotes a 50-fold more acidic environment! [Read more…]

Black Skin Is Better

This proposition – that black skin is better skin – is not a political statement. Rather, it is based upon scientific observations. Darkly pigmented skin is functionally superior to lighter colored skin in several ways. Our purpose here is give our readers a scientific perspective to a socially fraught question. And to acquaint our readers with some of the most important functions of skin that are influenced by skin color.

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Image by Mary L. Williams, M.D.

Some of the benefits of skin color are obvious and widely recognized. Darkly pigmented skin is less susceptible to skin cancer. And dark skin is less prone to develop wrinkles over time.

Poetically speaking, age has a lighter stroke on the canvas of black skin.

But there are other, less well-known, but very important ways in which darkly pigmented skin  functions better than pale skin. We are not talking about “racial” differences, because race is not a valid scientific construct. The differences we speak of are attributable to skin pigmentation per se in whomever and wherever it is found.  Whether in an African-American, Filipino, or Sri Lankan, it is dark pigmentation itself that confers multiple advantages to the part of the skin called ‘epidermis’, which comprises the outermost and protective layers of the skin. [Read more…]

Highlights of the 2013 IID. Part 6: Does barrier repair therapy improve skin defense against infection?

Prior work from our group has shown that the permeability and antimicrobial barriers in normal skin share many common features, and are co-regulated, such that perturbations in one function inevitably impact the other. We also showed that a variety of conditions that compromise the permeability barrier, including neonatal and aging skin, are also accompanied by a reduction in the production of the cathelicidin antimicrobial peptide, LL-37, a key defender against S. Aureus and Streptococcal infections.

In this study, we asked whether strategies that improve permeability barrier function also enhance production of LL-37. [Read more…]

HIghlights of the IID 2013. Part 4: The Good Side of Psychological Stress

It is widely held that the stress of our modern lives is bad for our health. And it is certainly true that stress (or, more precisely, ‘psychological stress’) can be harmful to the function of our skin. It delays wound healing. It can reduce our immune defenses against infection.  And, as we demonstrated several years ago, it can compromise the skin’s permeability barrier. It is also accepted by many that stress exacerbates chronic inflammatory diseases, like rheumatic disorders and inflammatory bowel diseases, such as ulcerative colitis and Crohn’s disease, as well as skin conditions, like psoriasis and eczema (atopic dermatitis).

It makes sense, of course, that the stress response to acute threats – like a tiger attack – was beneficial to early man.  This is the famous “Fight or Flight” response system. But why has the stress response to chronic challenges, like illness, persisted? If the effects of psychological stress during illness are all bad – then would not mutations that weakened this deleterious stress response have been favored by natural selection long ago? The answer, as we reported here, may be that the stress response to illness is, in fact,  not always harmful. [Read more…]

HIghlights of the IID. Part 3: How Melanin Is Good For The Skin Barrier

In previous work, we demonstrated that the skin of darkly-pigmented people possesses a tighter, more competent barrier to water leakage than does the skin of lightly-pigmented individuals.  Although we could attribute this to the much more acidic (lower pH) skin surface in individuals with dark pigmentation, just how either melanin or its parent cell, the melanocyte, confers these benefits was unknown.

For the studies reported here, we took advantage of two closely related strains of hairless mice – one that is darkly pigmented (hrs/J), and a a closely-related strain that is pink or albino (hr/hr). As expected, the barrier was better in the pigmented mice, and these darker mice, like their more heavily pigmented human counterparts, also exhibited a more acidic stratum corneum. In the pigmented mice (just as in darkly pigmented human skin), melanin granules persisted out into the stratum corneum, and eventually some of these granules are released into the spaces between the cells of the stratum corneum (corneocytes).  [Read more…]