THE EFFECT OF PROLONGED SOLAR RADIATION ON MELANOCYTES OF THE HUMAN EPIDERMIS

Comparison of photodamage in non-pigmented and pigmented human skin equivalents exposed to repeated ultraviolet radiation to investigate the role of melanocytes in skin photoprotection

Introduction

Daily solar ultraviolet (UV) radiation has an important impact on skin health. Understanding the initial events of the UV-induced response is critical to prevent deleterious conditions. However, studies in human volunteers have ethical, technical, and economic implications that make skin equivalents a valuable platform to investigate mechanisms related to UV exposure to the skin. In vitro human skin equivalents can recreate the structure and function of in vivo human skin and represent a valuable tool for academic and industrial applications. Previous studies have utilised non-pigmented full-thickness or pigmented epidermal skin equivalents to investigate skin responses to UV exposure. However, these do not recapitulate the dermal-epidermal crosstalk and the melanocyte role in photoprotection that occurs in vivo. In addition, the UV radiation used in these studies is generally not physiologically representative of real-world UV exposure.

Methods

Well-characterised pigmented and non-pigmented skin equivalents that contain human dermal fibroblasts, endogenous secreted extracellular matrix proteins (ECM) and a well-differentiated and stratified epidermis have been developed. These constructs were exposed to UV radiation for ×5 consecutive days with a physiologically relevant UV dose and subsequently analysed using appropriate end-points to ascertain photodamage to the skin.

Results

We have described that repeated irradiation of full-thickness human skin equivalents in a controlled laboratory environment can recreate UV-associated responses in vitro, mirroring those found in photoexposed native human skin: morphological damage, tanning, alterations in epidermal apoptosis, DNA lesions, proliferation, inflammatory response, and ECM-remodelling.

Discussion

We have found a differential response when using the same UV doses in non-pigmented and pigmented full-thickness skin equivalents, emphasising the role of melanocytes in photoprotection.

Reference values for skin microanatomy: A systematic review and meta-analysis of ex vivo studies

BACKGROUND

Few studies have characterized reference values of normal human skin microanatomy parameters.

OBJECTIVE

To quantify histologic measurements of epidermal thickness, melanocyte density, hair follicle density, and eccrine gland density as a function of age and anatomic site.

METHOD

We searched the PubMed, Embase, Web of Science, and Cochrane databases for articles published through May 25, 2017. Two reviewers independently screened 2016 articles; 327 relevant articles and 151 additional articles found via forward or reference citations underwent full-text review by 1 of 4 reviewers for relevance, data extraction, and critical appraisal. Weighted averages, meta-analysis, and meta-regression were used in statistical analysis.

RESULTS

A total of 56 articles were included; when all anatomic locations were used, the overall estimates for epidermal thickness, melanocyte density, hair follicle density, and eccrine gland density were 99.75 μm (95% confidence interval [CI], 83.25-116.25), 955.05 cells/mm² (95% CI. 880.89-1029.21), 1.40 hairs/mm² (95% CI. 0.91-1.89), and 1.28 glands/mm² (95% CI. 0.91-1.64), respectively.

LIMITATIONS

There was significant data heterogeneity across studies, possibly because of differences in histological techniques and absence of standardized microanatomy definitions.

CONCLUSIONS

We established summary estimates for normal human skin microanatomy parameters.

The melanomas: a synthesis of epidemiological, clinical, histopathological, genetic, and biological aspects, supporting distinct subtypes, causal pathways, and cells of origin

Converging lines of evidence from varied scientific disciplines suggest that cutaneous melanomas comprise biologically distinct subtypes that arise through multiple causal pathways. Understanding the respective relationships of each subtype with etiologic factors such as UV radiation and constitutional factors is the first necessary step toward developing refined prevention strategies for the specific forms of melanoma. Furthermore, classifying this disease precisely into biologically distinct subtypes is the key to developing mechanism-based treatments, as highlighted by recent discoveries. In this review, we outline the historical developments that underpin our understanding of melanoma heterogeneity, and we do this from the perspectives of clinical presentation, histopathology, epidemiology, molecular genetics, and developmental biology. We integrate the evidence from these separate trajectories to catalog the emerging major categories of melanomas and conclude with important unanswered questions relating to the development of melanoma and its cells of origin.

The Density and Distribution of Melanocytes Adjacent to Melanoma and Nonmelanoma Skin Cancers

BACKGROUND

The determination of clear margins during Mohs surgery for melanoma in situ arising on sun-damaged skin is often made difficult by the presence of background atypical melanocytic hyperplasia.

OBJECTIVE

To determine the density and distribution patterns of melanocytes adjacent to melanoma and nonmelanoma skin cancers.

METHODS

180 skin specimens obtained during the routine repair of defects resulting from the removal of melanoma and nonmelanoma skin cancers were analyzed using H and E-stained permanent sections to determine the quantity and distribution of epidermal melanocytes.

RESULTS

The mean melanocyte density was 7.97 melanocytes per 1 mm of epidermis (SD,+/-6.7). Contiguous melanocytes were found in 30 (16.7%), atypical melanocytes were observed in 8 (4.4%), and follicular extension of melanocytes was observed in 11 (6.1%) of the specimens. These features were significantly associated with higher melanocyte densities (p< .001) and were more commonly observed in specimens from patients with melanoma.

CONCLUSIONS

There is a high degree of variability in melanocyte densities seen adjacent to melanoma and nonmelanoma skin cancers. Contiguous melanocytes, atypical melanocytes, and follicular melanocytes can be seen in the sun-damaged skin surrounding both melanoma and nonmelanoma skin cancers, but especially with melanoma. Because some of the features of melanoma in situ can be seen in chronically sun-damaged skin, the Mohs surgeon should be cautious when assessing the margins for melanoma in this setting.

Melanocytic nevi are associated with neurofibromas in neurofibromatosis, type I, but not sporadic neurofibromas: a study of 226 cases

BACKGROUND

Neurofibromatosis, type 1, is associated with cutaneous melanin pigmentation, but an association with ordinary melanocytic nevi has not been described.

METHODS

This retrospective case-control study was designed to see if neurofibromas in patients with neurofibromatosis, type 1 (NF-1) differ from sporadic neurofibromas (SN) in their incidence of associated melanocytic nevi and other histologic features. Slides from 114 NF-1 were compared with 112 SN and 300 intradermal melanocytic nevi (IDN).

RESULTS

Small lentiginous melanocytic nevi were identified over 13 NF-1 (11%) but no SN (P=0.0002). Compared with other NF-1, NF-1 with nevi were more frequently associated with melanocytic hyperplasia, giant melanosomes and diffuse neurofibroma (P<0.03). Compared with SN, NF-1 were also more frequently associated with melanocytic hyperplasia, lentigo simplex-like changes, diffuse neurofibroma and plexiform neurofibroma (P<0.001). Sebaceous hyperplasia (14%), dermal elastosis (9%), lipomatous change (8%), epithelial cysts (4%) and keratin granulomas or folliculitis (3%) were not significantly different in prevalence between NF-1, SN and the control group of IDN.

CONCLUSIONS

This study suggests that there is a difference in the potential for melanocytic proliferation in NF-1 compared with SN. NF-1, SN and IDN are associated with a similar range of incidental histologic changes. Ball NJ, Kho GT. Melanocytic nevi are associated with neurofibromas in neurofibromatosis, type 1, but not sporadic neurofibromas. A study of 226 cases.

Melanoma in situ versus melanocytic hyperplasia in sun-damaged skin. Assessment of the significance of histopathologic criteria for differential diagnosis

For differentiation of melanoma in situ (MIS) from melanocytic hyperplasia (MH) in sun-damaged skin, several criteria have been proposed. To assess sensitivity and specificity of those criteria, we examined the epidermis adjacent to 50 consecutive basal cell carcinomas and 50 MISs in skin with significant solar elastosis. The most valuable criteria for the diagnosis of MIS, as opposed to MH, were presence of nests of melanocytes, irregular distribution of melanocytes, descent of melanocytes far down adnexal epithelial structures, irregular distribution of pigment, presence of melanocytes above the junction, a high number of melanocytes, pleomorphism of melanocytes, and atypical nuclei of melanocytes. Other criteria, e.g., collapse of cytoplasm around nuclei of melanocytes; flattening of rete ridges; differences in the area, shape, and contour of nuclei of melanocytes as assessed by nuclear morphometry; and presence of melanocytes stained by HMB-45 and Ki-67/MIB-1 monoclonal antibodies, were found to be of low or no value for differential diagnosis.

Re-excisions of scar in primary cutaneous melanoma: a histopathological study

In many centres the wide excision and split-skin grafting remains the standard therapy for primary cutaneous melanoma. One-hundred and thirty-seven primary cutaneous melanomas were seen in our department during a 3-year period; 25 patients were subsequently subjected to re-excision of scar following the initial excision biopsy. Lymphatic permeation, field change in the epidermal melanocytes and micrometastases were sought in the re-excision specimens in order to examine the pathological basis for this therapy. Two of the 25 primary melanomas included in this study were known to have been incompletely excised at the time of initial biopsy and both re-excision specimens included a few nests of atypical melanocytes adjacent to one edge of the biopsy wound. Examination of the re-excision specimens failed to demonstrate evidence of direct lymphatic permeation by melanoma, or of a field change in the epidermal melanocytes adjacent to the melanomas, although five specimens from sun-exposed sites showed slight melanocyte atypia. One re-excision specimen did include a single small group of melanocytes, less than 120 microns in size, in the dermis within 2 mm of the initial excision biopsy site of a melanoma 8.4 mm in thickness. These results support the view that more extensive local therapy than complete excision of primary cutaneous melanoma with a narrow margin of adjacent normal skin, is unlikely to benefit the patient.

Sun-induced freckles in children and young adults. A correlation of clinical and histopathologic features

Sun-induced freckles are a risk factor for epidermal and melanocytic neoplasia. Whereas sun-induced freckles in children and older adults may be clinically indistinguishable, and sun-induced freckles in older adults usually consist of increased numbers of intraepidermal melanocytes, the histology of sun-induced freckles in children remains unsettled. Using L-3,4 dihydroxyphenylalanine (DOPA)-paraffin sections, the authors examined six sun-induced freckles and adjacent nonpigmented skin (ANP) in as many white male subjects, ages 10 to 23 years. Melanocyte frequency was expressed as the ratio of DOPA-reactive melanocytes to total epidermal basal unit cells. For each case, melanocyte frequencies in freckles were significantly greater than in ANP. Cellular atypia of melanocytes was noticed in four of six freckles. Reactivity of melanocytes for HMB-45 was noticed in two freckles studied, compared with no reactivity in three specimens of ANP studied. The authors conclude the sun-induced freckles in the young may consist of a hyperplasia of melanocytes (i.e., similar to solar lentigines in the elderly), sometimes with cellular atypia, and that these findings may be relevant to melanocytic neoplasia.

Ultrastructure of melanocytes in chronically sun-exposed skin of elderly subjects

Chronically sun-exposed facial skin of three females aged 68, 71, and 78 years, and of a male aged 78, was examined by electron microscopy in order to study the condition of the epidermal melanocytes. Considerable heterogeneity of morphological and functional characteristics of the cells was observed. The majority of melanocytes were large, active, with occasionally lobulated or double nuclei, an appearance indicative of hyperstimulation. Some cells exhibited an appearance of having reached the end of an active life cycle and were labelled "aged." Others, in the upper end of the outer root sheath of hair follicles and adjacent interfollicular epidermis, presented a typically inactive appearance, indistinguishable from that of fetal melanocytes, or of those in unexposed skin of younger subjects. A cell with indented nucleus, fully melanised melanosomes, and hypertrophic Golgi apparatus was sporadically seen. Minute foci of dissociation of keratocytes were present, and melanocytes included in these were frequently disrupted. Swelling of mitochondria and cytoplasmic lipid droplets occurred sporadically within all the above variants of melanocytes. It proved difficult to establish criteria of specific sun damage of melanocytes. It is suggested that either the melanocytes exhibiting stimulation or the relatively inactive ones could be the precursors of the proliferating cells of lentigo maligna.

Melanocytic lesions and melanocyte populations in human epidermis

Basal melanocytes were counted and atypia assessed on an arbitrary scale in punch biopsies from the sun-exposed extensor aspect of the forearm of normal skin and from the covered skin of the buttock of patients with pigmented naevi and control subjects. The difference in melanocyte counts and in the presence of atypia between sun-exposed and covered skin was statistically highly significant. The only other difference was between melanocyte counts in covered skin from those with multiple atypical naevi and all other groups; the counts in the former were greater than those in the latter. No further difference was attributable to sun exposure, skin type or diagnostic group. Some degree of melanocyte atypia was seen in approximately half of the biopsies of sun-exposed skin, but atypia was seen in only six of 84 biopsies of covered skin. In each case atypia was present in the corresponding forearm biopsy and all six subjects had pigmented lesions (four with melanoma and two with multiple atypical naevi). It seems possible that while increased melanocyte counts in covered skin could correlate with the presence of atypical naevi, atypia of covered epidermal melanocytes could possibly relate to melanoma risk.

Epidermal melanocytes adjacent to melanoma and the field change effect

The presence of a field change, affecting epidermal melanocytes in the skin surrounding melanomas, has been cited as a justification for performing radical excision of these lesions. Twenty-five consecutive re-excisions of melanoma specimens were examined and melanocytes per 100 keratinocytes counted continuously across the width of each, in order to ascertain whether melanocyte counts decreased from the centre to the periphery, as would be expected if there were a field change effect associated with the tumour. Melanocyte counts did not appear to diminish with increasing distance from the tumour site and were within the range seen in a previous study in which we demonstrated increased numbers of basal melanocytes and melanocyte atypia in sun-exposed skin from a control population. Melanocyte atypia of a mild degree was present in five of the 25 cases studied, but in no case was this greater than that observed in sun-exposed forearm skin. We therefore suggest that the 'field change effect' in epidermal melanocytes adjacent to melanoma could be a result of chronic sun exposure alone.

Prostaglandin E2 and D2 but not MSH stimulate the proliferation of pigment cells in the pinnal epidermis of the DBA/2 mouse

Epidermal melanocytes proliferate following a variety of physical stimuli, for example, mechanical injury to the skin or exposure to UV radiation. We suggest that some transducer in the epidermis converts the physical modality into a biochemical signal which is responsible for initiation of mitosis. Melanocyte stimulating hormone, both alpha and beta variants, administered parenterally for periods up to 4 weeks do not alter the number of melanocytes per mm2 in several strains of neonatal or adult mice. Ultraviolet B and arachidonic acid both stimulate proliferation of pigment cells. Indomethacin which inhibits cyclooxygenase and the formation of prostaglandins (PGs) blocks the proliferation induced by both agents. We tested a wide variety of PGs. We observed that PGD2 applied daily to the skin of a mouse causes a small increase in melanocyte density (cells/mm2). PGE2 in similar doses applied topically caused a large increase. PGE2 caused an increase in the uptake of tritiated thymidine by dopa-positive dendritic cells. This indicates that PGE2 stimulated some melanocytes to proliferate. Histologic studies indicate that PGE2 also enhances melanogenesis. PGE2 is synthesized in the skin and affects keratinocytes and Langerhans cells as well as pigment cells. We postulate that it is one compound that can modulate the interaction of these 3 main cells of the epidermis.

Ecological factors in skin color variation among Papua New Guineans

An EEL reflectance spectrophotometer was used to measure the skin color of the inner upper arm and the forearm of 913 Karkar Islanders (Madang District) and 684 Lufa villagers (Eastern Highlands District). The samples were subdivided to study sex, age, and population variation against a background of ecological observations, including sunlight exposure, clothing, and erythemally effective wavelengths of ultraviolet light (Robertson, unpublished Ph.D. thesis, 1974). Population differences in sex and age variation in upper arm skin color may largely be attributable to the effects of culturally associated clothing differences. Not only do the Lufa villagers wear substantially less clothing than the Karkars, but also their arms are exposed more frequently to ultraviolet light during heavy manual work in unshaded gardens. For the melanin content of the forearm skin there are similar patterns of age variation in both populations; however, the populations differ in mean percentage of reflectance throughout most of the age span. These between-population differences are interpreted as a consequence of greater average daily exposure to sunlight and the higher intensity of ultraviolet light in the highland environment. On the forearm the percentage of reflectance at 685 nm decreases more rapidly with age in the prepubertal and adult age groups, a result attributed to endocrine changes superimposed on cumulative changes in the melanin pigmentary mechanism.

The epidermal melanocyte population in the skin of ultraviolet-irradiated crested newt

The response of the epidermal melanocyte population to repeated ultraviolet (UV) exposure (wavelength spectrum 275-350 nm) has been investigated in the crested newt, Triturus cristatus carnifex. The effects of different doses of UV light were studied. The animals were killed 7 months after the first UV exposure. Only a slight decrease in the number of pigment cells was found after 85 sequential irradiations with a total dose of 1.3 x 10(5) J/m2, whereas striking decreases were observed when the same total dose was fractionated into 14 exposures or when a double dose was given in 57 exposures. The relationship between the square roots of the epidermal melanocyte densities and single doses appeared to be roughly linear, at least over the range of doses administered. The main factor in melanocyte damage seemed to be the single dose of irradiation rather than the cumulative dose administered. Decreased melanin content of the keratinocytes was observed in most irradiated animals.

The proliferative and toxic effects of ultraviolet light and inflammation on epidermal pigment cells

The ear of the mouse is useful for studying the effects of ultraviolet light on epidermal pigment cells. The quantity of light penetrating into the skin causing an inflammatory response can be assessed easily by measuring with an engineering calipers the swelling of the ear. The inflammatory response of the ear exhibits a linear relationship to the dose of light delivered. We observed that doses of shortwave ultraviolet light which are noninflammatory when repeated at daily intervals induce moderate to severe inflammation. Small doses of psoralen and prolonged exposure to UVA (PUVA) were more inflammatory than larger amounts of psoralen and short exposure to light. Doses of shortwave ultraviolet light and PUVA which produce only a minimal inflammation of the skin stimulate the proliferation of epidermal melanocytes. In contrast, PUVA in doses sufficiently large to cause a marked inflammatory reaction in the skin seems injurious to pigment cells and kills them or causes only a minimal proliferative response. The inflammatory reaction itself does not seem to stimulate or inhibit the proliferation of melanocytes. Prostaglandins A, E, and F2 alpha have no effect on the proliferation of epidermal pigment cells. In contrast, dimethyl sulfoxide (DMSO) and allergic contact dermatitis increase the numerical density of pigment cells. Steroids may block the function of the enzyme tyrosinase. Our experiments indicate that pigment cells, like many other varieties of cells, are susceptible to injury and can be killed at least by large doses of PUVA.

The epidermal melanocyte system in individuals of Scandinavian origin, determined by DOPA-staining and TEM

The quantitative evaluation of DOPA-positive epidermal melanocytes in 16 patients of Scandinavian origin showed both individual and regional differences in the melanocyte count. Our data is in agreement with other published studies. The distribution in the number of melanocytes varies significantly in some specimens. This is due partly to the preparation procedure and partly to normal biological variations. We believe that we have demonstrated a cyclic function of the melanocyte in the epidermis. The varying density of cells in epidermal sheets as well as their varying morphology support the theory concerning the presence of the epidermal melanin unit.

The epidermal melanocyte system in patients with malignant melanoma. Quantitative and qualitative investigation of dopa positive melanocytes

The results from this study indicate that in 5 out of 22 patients increased activity of the melanocytes in the vicinity (a few mm to 1 cm) of the tumour is to be found. In 8 out of 22 patients suspect cells were also found a few mm to 1 cm from the tumour; a finding which accounts for the frequent recurrence following close excision of the melanoma. Furthermore, it was ascertained that the total melanocyte population is increased in 14 out of 22 patients. An increased melanocyte population can contain a greater number of instabile cell forms that can easier be malignantly transformed under the influence of external factors.

Mitotic activity of epidermal melanocytes in UV-irradiated mouse skin

The role of mitosis in the increase in the epidermal melanocyte population following repeated ultraviolet (UV) irradiation has been studied in C57B1 mice. Cumulative labeling with 3H-methyl-thymidine (3HTdr) was used in combination with autoradiography. A 4-6 fold increase in the melanocyte population was associated with an isotope labeling of 65-80% of the melanocytes. This indicates that cell division during the irradiation period can account for the total increase in the number of melanocytes. Labeled melanocytes were also found in the epidermis from unirradiated skin. Thus, the epidermal melanocytes seem to constitute a dynamic cell population under continuous renewal.

The use of ultraviolet light to induce melanogenesis in the epidermis of the rhesus monkey: an ultrastructural and biochemical study

The general body epidermis of the rhesus monkey (Macaca mulatta) contains no discernible melanocytes, but after repeated ultraviolet irradiation DOPA-positive melanocytes appear and increase numerically up to 30 exposures. With continued irradiation, however, the number again declines. Experiments to determine how melanogenic activity, assayed by the incorporation of labeled DOPA or tyrosine, is related to DOPA positivity indicated that biochemical activity corresponded to the histochemical pattern. Ultrastructural studies demonstrated that after the exposure to ultraviolet light a pool of indeterminate cells in the skin of rhesus monkeys developed into malanocytes. The melanosomes formed by these cells, however, differed from the eumelanin melanosomes described in other species; they had no internal filamentous matrix with periodicity but appeared similar to phaeomelanin melanosomes. Long term ultraviolet light irradiation may damage keratinocytes and render them incapable of phagocytizing melanosomes.