The skin is often viewed as a static barrier that protects the body from the outside world. Emphasis on studying the skin's architecture and biomechanics in the context of restoring skin movement and function is often ignored. It is fundamentally important that if skin is to be modelled or developed, we do not only focus on the biology of skin but also aim to understand its mechanical properties and structure in living dynamic tissue. In this review, we describe the architecture of skin and patterning seen in skin as viewed from a surgical perspective and highlight aspects of the microanatomy that have never fully been realized and provide evidence or concepts that support the importance of studying living skin's dynamic behaviour. We highlight how the structure of the skin has evolved to allow the body dynamic form and function, and how injury, disease or ageing results in a dramatic changes to the microarchitecture and changes physical characteristics of skin. Therefore, appreciating the dynamic microanatomy of skin from the deep fascia through to the skin surface is vitally important from a dermatological and surgical perspective. This focus provides an alternative perspective and approach to addressing skin pathologies and skin ageing.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/exd.12832

The dynamic anatomy and patterning of skin

UVA radiation is the most prevalent component of solar UV radiation; it deeply penetrates into the skin and induces profound alterations of the dermal connective tissue. In recent years, the detrimental effects of UVA radiation were more precisely demonstrated at cellular and molecular levels, using adequate methods to identify biological targets of UVA radiation and the resulting cascade impairment of cell functions and tissue degradation. In particular gene expression studies recently revealed that UVA radiation induces modulation of several genes confirming the high sensitivity of dermal fibroblasts to UVA radiation. The major visible damaging effects of UVA radiation only appear after years of exposure: it has been clearly evidenced that they are responsible for more or less early signs of photoageing and photocarcinogenesis. UVA radiation appears to play a key role in pigmented changes occurring with age, the major sign of skin photoaging in Asians. Skin susceptibility to photoaging alterations also depends on constitutive pigmentation. The skin sensitivity to UV light has been demonstrated to be linked to skin color type.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/exd.12388

New insights in photoaging, UVA induced damage and skin types.

The skin functions as a physical barrier preventing loss of body fluids and penetration of substances (chemicals) or infectious agents (1–3). This physical, permeability barrier resides primarily in the stratum corneum (SC) (4–6). Transepidermal water loss (TEWL) and skin hydration have been widely used as indices in evaluating skin barrier function (1, 4, 7–11). TEWL represents the diffusion of condensed water through the SC (7, 12), while skin hydration reflects the water content of the SC (4, 13). An altered skin barrier function is marked by an elevated TEWL and has been observed in a number of skin diseases (e.g. atopic dermatitis and psoriasis) and experimental perturbation studies (e.g. applications of solvents and detergents) (3, 11). Furthermore, elevated TEWL values in a disturbed skin barrier are frequently correlated with low hydration of the SC (3). Several lines of evidence suggest that a disturbed, compromised skin barrier may increase dermal absorption of chemicals and other large substances (e.g. particulate), which cannot penetrate the intact skin (14, 15).

In the workplace, damage to the skin and a compromised skin barrier due to physical and mechanical irritation and chemical insults commonly occur (14). Scrubbing, friction, or abrasion may partially or completely remove the SC and thus disrupt the skin barrier by exposing the viable and water-rich epidermis to the environment (14, 16). It is suggested that organic solvents increase skin permeability by extracting and altering the structure of intercellular lipids from the SC, while desmosomes may also be damaged (14). Surfactants (detergents) interact with skin lipids and proteins leading to disorganization of extracellular lipids, reducing corneocyte cohesion and decreasing skin hydration (14, 17). Occlusion created by prolonged wearing of protective clothing, most notably protective gloves, prevents evaporation of water leading to accumulation of water in intercellular spaces across the SC and swelling of corneocytes (1).

A limited number of studies have been published in which changes in skin barrier function in workplaces have been investigated. Coenraads et al. (18) measured skin vapor loss (SVL) from the skin of metal industry workers during exposure to water-soluble oils and mineral oil. Over the 12-week study, four workers exposed to mineral oil developed contact dermatitis, with a marked increase in SVL. The SVL of other 50 workers remained normal, although SVL levels in workers exposed to water-soluble oils were slightly but not significantly higher than that of the control group. Goh and Gan (19) measured changes in TEWL in newly employed machinists exposed to cutting oils. Smit et al. (20) measured TEWL in apprentice hairdressers and nurses and results implied that hairdressers had an increased risk of hand dermatitis at TEWL >15 g/m2/h in the hand, but the increased risk was not statistically significant. No relationship was observed for nurses. In a study conducted by Berndt et al. (21), the validity of skin bioengineering methods (including skin hydration and TEWL) as predictive measures for the development of hand eczema, was investigated in a follow-up study of metalworker trainees. They concluded that no single biophysical method can be considered as a valid screening test. However, a combination of short irritation tests and skin hydration permits identification of individuals at high risk for hand dermatitis with a high sensitivity, but with low specificity. Chou et al. (22) measured TEWL in rayon manufacturing workers exposed predominantly to carbon disulfide and sulfuric acid. Significant differences in basal TEWL levels between exposed Chinese workers and a control group were reported. Chou et al. (23) measured TEWL and skin hydration changes among manufacturing workers employed in ultra-low humidity. Both TEWL and skin hydration decreased within 2 weeks of exposure to ultra-low humidity. The maximum change in TEWL occurred after 0.5–1 month and for skin hydration after 2 weeks. In another study by Chou et al. (24), TEWL was measured in cement workers exposed to chromium. They reported a significant increase in TEWL for workers exposed to high levels of chromium. More recently, du Plessis et al. (25) measured TEWL and skin hydration in base metal refinery workers. Results indicated a significant decrease in skin hydration during an 8 h work shift, which recovered to baseline (before shift) levels at the end of the shift. A significant increase was reported between TEWL measured prior to and after the work shift. Changes in skin barrier function were most probably due to exposure to sulfuric acid used in the electrowinning (refining) of nickel. Furthermore, Kutting et al. (26) measured TEWL in metal-workers and reported a slightly but significantly lower TEWL in a group of workers using barrier cream after a 1 year follow-up. However, no other significant difference in TEWL were reported for other sub-groups investigating skin care, skin protection, or both. Most evident from these studies are: (i) the variation in ambient measurement conditions [e.g. temperature and relative humidity (RH)] under which skin barrier function was measured; (ii) the range of different anatomical positions (locations) that were measured; (iii) the frequent lack of information regarding the exact methodology of measurements (e.g. acclimatization time, calibration of instruments and number of measurements for each anatomical position); and (iv) the manner in which collected data were represented and interpreted.

Guidelines exist for the measurement of TEWL and skin hydration in highly controlled clinical settings (7, 27, 28); however, measurement conditions in non-clinical settings, such as workplaces, may be more variable as a result of numerous factors that are often beyond the control of investigators. Workplaces represent ‘worst-case’ environments for in vivo measurement of skin properties. Hence, there is a need to develop guidelines on non-clinical (e.g. workplace) measurement of TEWL and skin hydration to establish best practices for data collection and reporting. In response to this need, an expert workshop was convened as part of the 5th International Conference on Occupational and Environmental Exposure of the Skin to Chemicals (OEESC) held in Toronto, Canada in June 2011. This paper presents a consensus summary of workshop participants to develop guidelines and best practices for measuring TEWL and skin hydration in non-clinical settings such as the workplace.

/pdf/international-guidelines-for-the-in-vivo-assessment-of-skin-36zpdacnox.pdf

International guidelines for the in vivo assessment of skin properties in non-clinical settings: Part 2. transepidermal water loss and skin hydration

Background
Skin is a multilayer interface between the body and the environment, responsible for many important functions, such as temperature regulation, water transport, sensation, and protection from external triggers.

Objectives
This paper provides an overview of principal factors that influence human skin and describes the diversity of skin characteristics, its causes and possible consequences. It also discusses limitations in the barrier function of the skin, describing mechanisms of absorption.

Methods
There are a number of in vivo investigations focusing on the diversity of human skin characteristics with reference to barrier properties and body-dependent factors.

Results
Skin properties vary among individuals of different age, gender, ethnicity, and skin types. In addition, skin characteristics differ depending on the body site and can be influenced by the body-mass index and lifestyle. Although one of the main functions of the skin is to act as a barrier, absorption of some substances remains possible.

Conclusions
Various factors can alter human skin properties, which can be reflected in skin function and the quality of everyday life. Skin properties and function are strongly interlinked.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/srt.12424

The relationship between skin function, barrier properties, and body-dependent factors.

Skin color diversity is the most variable and noticeable phenotypic trait in humans resulting from constitutive pigmentation variability. This paper will review the characterization of skin pigmentation diversity with a focus on the most recent data on the genetic basis of skin pigmentation, and the various methodologies for skin color assessment. Then, melanocyte activity and amount, type and distribution of melanins, which are the main drivers for skin pigmentation, are described. Paracrine regulators of melanocyte microenvironment are also discussed. Skin response to sun exposure is also highly dependent on color diversity. Thus, sensitivity to solar wavelengths is examined in terms of acute effects such as sunburn/erythema or induced-pigmentation but also long-term consequences such as skin cancers, photoageing and pigmentary disorders. More pronounced sun-sensitivity in lighter or darker skin types depending on the detrimental effects and involved wavelengths is reviewed.

https://www.mdpi.com/1422-0067/19/9/2668/pdf

Clinical and Biological Characterization of Skin Pigmentation Diversity and Its Consequences on UV Impact

People of skin of colour comprise the majority of the world's population and Asian subjects comprise more than half of the total population of the earth. Even so, the literature on the characteristics of the subjects with skin of colour is limited. Several groups over the past decades have attempted to decipher the underlying differences in skin structure and function in different ethnic skin types. However, most of these studies have been of small scale and in some studies interindividual differences in skin quality overwhelm any racial differences. There has been a recent call for more studies to address genetic together with phenotypic differences among different racial groups and in this respect several large-scale studies have been conducted recently. The most obvious ethnic skin difference relates to skin colour which is dominated by the presence of melanin. The photoprotection derived from this polymer influences the rate of the skin aging changes between the different racial groups. However, all racial groups are eventually subjected to the photoaging process. Generally Caucasians have an earlier onset and greater skin wrinkling and sagging signs than other skin types and in general increased pigmentary problems are seen in skin of colour although one large study reported that East Asians living in the U.S.A. had the least pigment spots. Induction of a hyperpigmentary response is thought to be through signaling by the protease-activated receptor-2 which together with its activating protease is increased in the epidermis of subjects with skin of colour. Changes in skin biophysical properties with age demonstrate that the more darkly pigmented subjects retaining younger skin properties compared with the more lightly pigmented groups. However, despite having a more compact stratum corneum (SC) there are conflicting reports on barrier function in these subjects. Nevertheless, upon a chemical or mechanical challenge the SC barrier function is reported to be stronger in subjects with darker skin despite having the reported lowest ceramide levels. One has to remember that barrier function relates to the total architecture of the SC and not just its lipid levels. Asian skin is reported to possess a similar basal transepidermal water loss (TEWL) to Caucasian skin and similar ceramide levels but upon mechanical challenge it has the weakest barrier function. Differences in intercellular cohesion are obviously apparent. In contrast reduced SC natural moisturizing factor levels have been reported compared with Caucasian and African American skin. These differences will contribute to differences in desquamation but few data are available. One recent study has shown reduced epidermal Cathepsin L2 levels in darker skin types which if also occurs in the SC could contribute to the known skin ashing problems these subjects experience. In very general terms as the desquamatory enzymes are extruded with the lamellar granules subjects with lowered SC lipid levels are expected to have lowered desquamatory enzyme levels. Increased pores size, sebum secretion and skin surface microflora occur in Negroid subjects. Equally increased mast cell granule size occurs in these subjects. The frequency of skin sensitivity is quite similar across different racial groups but the stimuli for its induction shows subtle differences. Nevertheless, several studies indicate that Asian skin maybe more sensitive to exogenous chemicals probably due to a thinner SC and higher eccrine gland density. In conclusion, we know more of the biophysical and somatosensory characteristics of ethnic skin types but clearly, there is still more to learn and especially about the inherent underlying biological differences in ethnic skin types.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1467-2494.2006.00302.x

A. V. Rawlings

Papers

Ethnic skin types: are there differences in skin structure and function?