Melanin and cellular reactions to ultraviolet radiation

Capsaicin Inhibits the Expression of Melanogenic Proteins in Melanocyte via Activation of TRPV1 Channel: Identifying an Inhibitor of Skin Melanogenesis

Chili pepper belongs to the genus Capsicum of Solanaceae family. Capsaicin is the primary capsaicinoid in placenta and flesh of chili pepper fruit, which has been shown to have various pharmacological functions, including gastric protection, anti-inflammation, and obesity treatment. Here, we revealed that capsaicin as well as chilli extract was able to inhibit synthesis of melanin in melanocytes. In cultured melanocytes, the melanin content was reduced to 54 ± 6.55% and 42 ± 7.41% with p < 0.001 under treatment of 50 μM capsaicin for 24 and 72 h, respectively. In parallel, the protein levels of tyrosinase and tyrosinase-related protein-1 were reduced to 62 ± 8.35% and 48 ± 8.92% with p < 0.001. Such an inhibitory effect of capsaicin was mediated by activation of transient receptor potential vanilloid 1-induced phosphorylation of extracellular signal-regulated kinase. This resulted in a degradation of microphthalmia-associated transcription factor, leading to reduction of melanogenic enzymes and melanin. These results revealed that capsaicin could be an effective inhibitor for skin melanogenesis. Hence, chili pepper, as our daily food, has potential in dermatological application, and capsaicin should be considered as a safe agent in treating hyperpigmentation problems.

2-(3, 4-dihydroxybenzylidene)malononitrile as a novel anti-melanogenic compound

Tyrosinase is a key player in ultraviolet-induced melanogenesis. Because excessive melanin accumulation in the skin can induce hyperpigmentation, the development of tyrosinase inhibitors has attracted attention in cosmetic-related fields. However, side effects including toxicity and low selectivity have limited the use of many tyrosinase inhibitors in cosmetics. We synthesized 12 novel 2-(substituted benzylidene)malononitrile derivatives and investigated their anti-melanogenic activities. Of these 12 compounds, 2-(3, 4-dihydroxy benzylidene)malononitrile (BMN11) exhibited the strongest inhibitory activity against tyrosinase (IC₅₀ = 17.05 μM). In parallel with this, BMN11 treatment notably decreased alpha-melanocyte-stimulating hormone-induced melanin accumulation in B16F10, cells without toxicity and also decreased melanin accumulation in a human skin model. As a mechanism underlying the BMN11-mediated anti-melanogenic effect, docking simulation showed that BMN11 can directly bind to tyrosinase by forming two hydrogen bonds with GLY281 and ASN260 residues, and via three hydrophobic interactions with VAL283, PHE264, and ALA286 residues in the tyrosinase binding pocket, and this likely contributes to its inhibitory effect on tyrosinase. Consistently, Lineweaver-Burk and Cornish-Bowden plots showed that BMN11 is a competitive inhibitor of tyrosinase. We concluded that BMN11 may be a novel tyrosinase inhibitor that could be used in cosmetics.

Identification of tyrosinase inhibitors from traditional Chinese medicines for the management of hyperpigmentation

The inhibition of tyrosinase is the most effective method to decrease melanin synthesis during the process of pigmentation. We aimed to find compounds from traditional Chinese medicines (TCM) that are more effective than the most commonly used tyrosinase inhibitor, arbutin. First, we employed homology modeling to construct a tyrosinase-modeled structure, and structure-based virtual screening to screen from 61,000 TCM compounds. We also adopted the following quantitative structure-activity relationship (QSAR) models for ligand-based validation: support vector machine, multiple linear regression, and Bayesian network. Molecular dynamics (MD) simulation was used to confirm the stability of ligand binding. We found that merresectine C might more effectively bind and inhibit the activity of tyrosinase than arbutin. This study provides useful evidence for the potential development of a novel non-toxic bleaching or whitening ingredient.

Miniature swine model of phototoxicity testing

BACKGROUND/PURPOSE

This study determined the threshold doses for 'solar erythema' and for phototoxic responses to 8-methoxypsoralen (8-MOP) in white skin Hanford and grey skin Yucatan miniature swine.

METHODS

For threshold erythema determinations, the UVR exposures included both UVA (315-400 nm) and UVB (290-315 nm) radiation by positioning one fluorescent 'sunlamp' among 10 'PUVA' lamps. With this configuration the UVR exposures ranged from 0.5-2.8 times the 'instrumental MED' (MEDi) for Hanford and from 1.0-5.6 times the MEDi for Yucatan. For phototoxicity determinations (i.e., with and without topically-applied graduated concentrations of 8-MOP), the UVB component was minimized by extinguishing the sunlamp, thus permitting higher UVA exposures.

RESULTS

The Hanford had the lower UV erythema dose threshold (1.0-1.4 times the MEDi) and the erythema that developed was readily observable. The exposure doses for the phototoxicity test were 5 J/cm(2) of UVA in 35 minutes or 10 J/cm(2) in 70 minutes. The phototoxic (vascular) response to 8-MOP was observed in the two highest concentrations (0.01% and 0.1%) in Hanford pigs, in a dose-related manner. Microscopic evidence of a dose-related response was also observed as the concentration of 8-MOP increased.

CONCLUSION

This verified that the Hanford miniature swine is the preferable strain for phototoxic effects. In contrast, UVR exposure of the Yucatan pig skin produced tanning rather than erythema, confirming that the Yucatan is the more appropriate strain for studying the melanization response. Thus, Hanford and Yucatan miniature swine have cutaneous photobiological responses that reflect their respective strain differences.

The deceptive nature of UVA tanning versus the modest protective effects of UVB tanning on human skin

The relationship between human skin pigmentation and protection from ultraviolet (UV) radiation is an important element underlying differences in skin carcinogenesis rates. The association between UV damage and the risk of skin cancer is clear, yet a strategic balance in exposure to UV needs to be met. Dark skin is protected from UV-induced DNA damage significantly more than light skin owing to the constitutively higher pigmentation, but an as yet unresolved and important question is what photoprotective benefit, if any, is afforded by facultative pigmentation (i.e. a tan induced by UV exposure). To address that and to compare the effects of various wavelengths of UV, we repetitively exposed human skin to suberythemal doses of UVA and/or UVB over 2 weeks after which a challenge dose of UVA and UVB was given. Although visual skin pigmentation (tanning) elicited by different UV exposure protocols was similar, the melanin content and UV-protective effects against DNA damage in UVB-tanned skin (but not in UVA-tanned skin) were significantly higher. UVA-induced tans seem to result from the photooxidation of existing melanin and its precursors with some redistribution of pigment granules, while UVB stimulates melanocytes to up-regulate melanin synthesis and increases pigmentation coverage, effects that are synergistically stimulated in UVA and UVB-exposed skin. Thus, UVA tanning contributes essentially no photoprotection, although all types of UV-induced tanning result in DNA and cellular damage, which can eventually lead to photocarcinogenesis.

Melanin mediated apoptosis of epidermal cells damaged by ultraviolet radiation: factors influencing the incidence of skin cancer

Ultraviolet (UV)-induced skin cancers, including melanomas and basal/squamous cell carcinomas, occur more frequently in individuals with fair skin than in those with dark skin. Melanin plays an important role in protecting the skin against UV radiation and levels of melanin correlate inversely with amounts of DNA damage induced by UV in human skin of different racial/ethnic groups. The objectives of this study are to review recent progress in our understanding of mechanisms underlying differences in cancer incidence in skins of different colors, particularly between Black and White skin. More specifically, we review DNA damage and apoptosis in various types of skin before and after exposure to UV in our human study protocols using a single UV dose, either one minimal erythema dose (MED) or a similar low dose of 180-200 J/m2. Our data and other published reports indicate that several major mechanisms underlie the increased rates of photocarcinogenesis in fair/light skin. First, UV-induced DNA damage in the lower epidermis (including keratinocyte stem cells and melanocytes) is more effectively prevented in darker skin. Second, rates of repair of DNA damage can differ significantly in individuals. Third, UV-induced apoptosis to remove potentially precancerous cells is significantly greater in darker skin. These results suggest that pigmented epidermis is an efficient UV filter and that UV damaged cells are removed more efficiently in darker skin. The combination of decreased DNA damage and more efficient removal of UV-damaged cells may play a critical role in the decreased photocarcinogenesis seen in individuals with darker skin.

Human skin responses to UV radiation: pigment in the upper epidermis protects against DNA damage in the lower epidermis and facilitates apoptosis

Melanin plays an important role in protecting the skin against UV radiation, and melanomas and basal/squamous cell carcinomas occur more frequently in individuals with fair/light skin. We previously reported that levels of melanin correlate inversely with amounts of DNA damage induced by UV in normal human skin of different racial/ethnic groups. We have now separately examined DNA damage in the upper and lower epidermal layers in various types of skin before and after exposure to UV and have measured subsequent apoptosis and phosphorylation of p53. The results show that two major mechanisms underlie the increased photocarcinogenesis in fair/light skin. First, UV-induced DNA damage in the lower epidermis (including keratinocyte stem cells and melanocytes) is more effectively prevented in darker skin, suggesting that the pigmented epidermis is an efficient UV filter. Second, UV-induced apoptosis is significantly greater in darker skin, which suggests that UV-damaged cells may be removed more efficiently in pigmented epidermis. The combination of decreased DNA damage and more efficient removal of UV-damaged cells may play a critical role in the decreased photocarcinogenesis seen in individuals with darker skin.

Increased epidermal functioning wild-type p53 expression in vitiligo

Despite the lack of protective melanin and increased oxidative stress due to mM concentrations of epidermal H2O2 in vitiligo, there is no significantly increased risk for chronic actinic damage and non-melanoma skin cancer. Therefore the question arises, which protective mechanisms could be involved in the skin of these patients preventing the initiation of these cancers. Recently an overexpression of p53 has been shown in vitiligo. Unfortunately there was no further characterization of this elevated p53. Employing a functional colour yeast assay, the study presented herein demonstrates for the first time the overexpression of a functioning wild-type p53 protein in both depigmented and 'normal' pigmented epidermis of patients with vitiligo compared with healthy controls. Surprisingly long-term narrowband UVB (311 nm) treatment does not alter this expression. Moreover, MDM-2, PCNA and p21 protein expression remain unchanged compared with healthy controls. This increased epidermal p53 in vitiligo coincides with decreased thioredoxin reductase (TR) protein levels in both depigmented and pigmented skin whereas mRNA expression is unaffected. Because TR is one transcriptional target of p53, these results support a wild-type functionality, which was further supported by the specific p53 FASAY yeast test. To our knowledge this is the first example of persistent elevated functioning wild-type p53 in humans. Based on our results we hypothesize that the low incidence for actinic damage, basal cell and squamous cell carcinoma as documented in vitiligo could well reside in a protective function of up-regulated wild-type p53.

DNA damage, histological changes and DNA repair in larval Japanese medaka (Oryzias latipes) exposed to ultraviolet-B radiation

Cyclobutane dimer formation, photorepair capability and histological damage were compared among four differently pigmented strains of larval Japanese medaka (Oryzias latipes) to determine whether pigmentation modifies the level of UV-B radiation (290-320 nm) inducible damage in these fish. One-day post-hatch medaka were exposed to one of several UV-B fluence rates with or without photoreactivating light for 5 days for 7 h per day. Their DNA was extracted for analysis by ELISA for cyclobutane pyrimidine dimers or the larvae were processed for histological examination. At the higher UV-B fluence rates tested, wild-type melanophore-containing medaka formed significantly more dimers than at least one of the other strains tested. Wild-type medaka also showed significantly less photorepair capability than the white melanophore-lacking medaka. The wild-type larvae had significantly more necrosis than the orange-red melanophore-lacking larvae at the lower UV-B fluence rate tested and at the higher fluence rate used, the wild-type medaka also exhibited significantly more necrosis than the white melanophore-lacking larvae. Of the 19 medaka observed with cellular hyperplasia, six were wild-type. These six individual larvae showed the greatest degree of cellular hyperplasia. Cellular hyperplasia appeared to be greatest at the lowest UV-B fluence rate used. The presence of melanophores in the wild-type medaka may have contributed to an increased level of tissue damage in this strain when compared to the other strains.

The sunburn cell revisited: an update on mechanistic aspects

The sunburn cell (SBC), with its pyknotic nucleus and eosinophilic cytoplasm, is characteristic of mammalian epidermis after exposure to UVC and UVB radiation or UVA radiation in the presence of psoralens. SBC may be regarded as an example of apoptosis: controlled individual cell death. Since the discovery of apoptosis over thirty years ago, there has been a considerable increase in the knowledge of mechanisms involved in this process. DNA damage has been shown to be a major determinant of SBC production both in a p53-dependent and -independent manner. Extranuclear events such as activation of membrane bound death receptors also contribute to SBC formation. The development of new technologies and techniques has resulted in a better understanding of the mechanisms and machinery involved in apoptosis, triggered by various stimuli and in different cell types. Of particular importance has been the elucidation of regulatory molecules such as caspases, inhibitor of apoptosis proteins (IAP) and the role of mitochondria as key to the process of apoptosis and consequent production of SBC. This review attempts to give an update on those mechanisms involved and the occurrence and relevance of SBC in mammalian skin are discussed.

Flow cytometric quantification of UV-induced cell death in a human squamous cell carcinoma-derived cell line: dose and kinetic studies

Exposure to ultraviolet (UV) radiation and photochemotherapy induces apoptotic cell death in epidermal cells. In this study annexin V binding and propidium iodide (PI) uptake have been measured by flow cytometry to evaluate UV-induced cell death in the human squamous cell carcinoma-derived cell line A 431. Physiological and therapeutical relevant doses of UVA, UVA1, UVB, narrow-band UVB (311 nm) and photochemotherapy using 100 ng/ml of 8-methoxypsoralen (8-MOP) with UVA or UVA1 (PUVA or PUVA1) have been applied. Doses ranged from 8 to 96 J/cm2 for UVA1 and UVA, from 8 to 128 mJ/cm2 for UVB, from 256 to 4096 mJ/cm2 for narrow-band UVB (311 nm) and from 1 to 16J/cm2 for photochemotherapy. Results show that the amount of annexin V binding, a measure of early apoptosis, as well as PI uptake, a parameter of ultimate cell death, are strictly correlated with the applied UV dose. Peak values of annexin V-positive cells are noted 12 h after UV exposure in all protocols and are followed by an increase of PI-uptaking cells with peak values at 24 h after UVA and UVA1, and 48 h after PUVA, PUVA1, UVB and narrow-band UVB. To compare the effect of different wavelengths and light sources, dose equivalents are calculated based on the induction of 50% cell death (as determined by PI uptake). The equivalents are 96 J/cm2 for UVA and UVA1, 16 J/cm2 for PUVA and PUVA1, 256 mJ/cm2 for UVB and 2048 mJ/cm2 for narrow-band UVB. Our results establish annexin V/PI double staining as an appropriate method for the quantification of UV-induced cell death. Moreover, they provide a basis for further investigations concerning mechanisms and modifications of UV-induced apoptosis.

Melanized poly(HEMA) hydrogels: basic research and potential use

Synthetic melanogenesis, using epinephrine and other melanin precursors, within the matrix of hydrophilic polymers and copolymers of 2-hydroxyethyl methacrylate resulted in hydrogels able to absorb ultraviolet and visible radiation. This significantly enhances their value as materials for extraocular (contact lenses) or intraocular (artificial crystalline lenses) devices that should protect the retina of aphakic patients from potential damage induced by light. The two-phase morphology of melanized hydrogels, as investigated by TEM, revealed a fine structure that is possibly indicative of a true sequential interpenetrating polymer network. Their biocompatibility was evaluated by a set of different assays involving human choroidal fibroblasts. No cytotoxicity was found in the aqueous extracts of materials. By using an assay with cells and polymers embedded in a collagen gel, a short-range toxic effect was detected, presumably caused by melanin itself. However, in vivo experiments in animal eyes with melanized hydrogel intraocular lenses did not reveal any toxic reaction.

In vitro cytotoxicity of melanized poly(2-hydroxyethyl methacrylate) hydrogels, a novel class of ocular biomaterials

Due to their ability to absorb ultraviolet and visible radiation, we have proposed the melanized poly(2-hydroxyethyl methacrylate) hydrogels as biomaterials suitable for the manufacture of soft artificial intraocular lenses. Their biocompatibility has not been evaluated so far. In this study, poly(2-hydroxyethyl methacrylate) containing various amounts of adrenochrome-melanin were synthesized and the cytotoxicity of their aqueous extracts was assessed by using four in vitro testing techniques (trypan blue dye exclusion, inhibition of DNA synthesis, lactate dehydrogenase release, and inhibition of cell growth). Assays were based on incubation with human choroidal fibroblasts. By the first three techniques, no cytotoxicity was found in the extracts. The inhibition of cell growth test showed a slight cellular protein loss, however only in the extracts of polymers with high melanin content. The results suggest that the release of potentially toxic agents from melanized hydrogels into an aqueous medium is not significant. However, when an assay in collagen gel was carried out in the presence of specimens of melanized hydrogels, a toxic reaction was clearly revealed. This can be caused by a delayed release of toxic molecules from melanin, or by some other mechanism. The use of melanin-containing polymers as implant materials becomes questionable and further research is necessary.

The photoprotective potential of the new superpotent sunscreens

A sun protection factor (SPF)-15 and an SPF-30 sunscreen were compared with regard to their ability to prevent sunburn cell formation after the exposure of human skin to a standardized dose of solar-simulated radiation. The SPF-30 sunscreen provided a significantly superior degree of photoprotection and almost prevented sunburn cell induction. Because sunburn cells may be markers of ultraviolet radiation-induced damage to DNA, the new superpotent sunscreens should offer an advantage in the prevention of skin cancer and long-term actinic damage to skin.

Skin type, hair color, and freckles are predictors of decreased minimal erythema ultraviolet radiation dose

In a group of 190 white healthy subjects the skin type classification method was found valuable for differentiating subgroups with various degrees of sun sensitivity (except for 33% with borderline or unclassifiable skin type). Sun-sensitive skin types I and II were significantly more common among persons with light hair color or freckles, or both (p less than 0.001). In each skin type category the proportion of subjects with a minimal erythema dose (MED) lower than the median MED of the entire group (%LMED) decreased significantly with increasing skin type number, and distinguished between skin types I through III better than did their mean MED values. Independent predictors of %LMED were skin type and hair color. The contribution of freckles to %LMED was skin type dependent. Age, sex, or eye color had no independent effect on %LMED. The association of skin types I and II, red or blond hair, and freckles with decreased MED may reflect genetically controlled predominance of pheomelanin (a photosensitizing molecule) in the skin of subjects with these phenotypes.

Ultrasonic comparison of two morphologically distinct melanosomes in malignant melanomas

Ultrasonic measurement (0.333-200 MHz) of melanosomes isolated from B16 and Harding Passey (HP) mouse melanomas indicates that the partial wave resonance and principal relaxation of the 2 kinds of melanosomes are similar, but that their stochastic resonance is markedly different. The structure of the melanosomes appears basically amorphous, but linearly ordered and copolymeric in the molecular dimension of a segment composed of 5-6 zigzag units, which are packed closely in B16 and more openly in HP.

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.

Do freckles protect the skin from actinic damage?

Epidermal sunburn (SB) cells were counted 24 h after exposure of the skin to suprathreshold doses of solar simulated radiation. The sites compared were evenly pigmented black skin and freckled white skin. SB cell numbers were dose related in all cases. There were fewer SB cells in black than white skin following equivalent erythemogenic doses of UVR. Equal doses of UVR produced fewer SB cells in freckled than non-freckled white skin. Freckles provide a significant degree of protection against acute sunburn.

Sunscreens prevent sunburn cell formation in human skin

Sunscreens with different sun protection factors were examined for their ability to prevent sunburn cell formation in human skin exposed to ultraviolet radiation from a xenon solar simulator. There was either complete or partial protection against the cytotoxic effects of ultraviolet radiation. Protection was related to the sun protection factor of the sunscreen and to the dose of radiation administered.

The influence of longwave ultraviolet radiation on sunburn cell production by UVB

The potentiation of sunburn by longwave ultraviolet radiation (photoaugmentation) has been examined with regard to the influence of these waves on sunburn cell production in human skin. It was found that UVA did not enhance sunburn cell production. Photoaugmentation is limited to the erythemal component of the sunburn reaction.

Histological changes produced in skin by equally erythemogenic doses of UV-A, UV-B, UV-C and UV-A with psoralens

The sequential light microscopic histological changes produced in human skin by a single exposure of UV-A, UV-B, UV-C and oral 8-methoxypsoralen plus UV-A (PUVA) causing approximately equal degress of delayed erythema response, have been evaluated. UV-C and UV-B affect the epidermis to a greater degree than UV-A, while UV-A affects the dermis to a greater degree than UV-B and UV-C. PUVA has prominent effects on both epidermis and dermis, differing in degree from those changes induced by UV-A, UV-B, and UV-C and are longer lasting. The sequence of histological changes following UV exposure is completed more rapidly after exposure to shorter UV wavelengths.

Decreased DNA repair activity in sunburn cells. A possible pathogenetic factor of the epidermal sunburn reaction

The pathogenesis of the formation of sunburn cells is unknown. Based on autoradiographic methods the unscheduled DNA repair synthesis of UV-induced thymin dimers was investigated in vivo in sunburn cells and in irradiated but histologically normal stratum spinosum cells. The results show a significant lower number of sparsly labeled cells in the sunburn cell-population (13.2 +/- 2.5; mean) when compared to the population of normal stratum spinosum cells (57.8 +/- 7.5; mean). These data indicate that the population of those epidermal cells, which become manifest as sunburn cells 24 h after UV exposure exhibit a reduced DNA repair of UV induced thymine dimers immediately after UV irradiation. Nuclear factors thus seem to play at least some role in the origin of sunburn cells.

Photoprotective effect of a psoralen-UVA-induced tan

To determine whether a tan produced by 8-MOP and UVA protects from subsequent solar light irradiation, volunteers were irradiated with unfiltered Xenon arc light before and 10 days after a 1 week's course of four 8-MOP-UVA treatments. Evaluation of the minimal erythema doses and of histological changes before and after 8-MOP-UVA treatment revealed that the 8-MOP-UVA induced tan protected against the erythemogenic and cell damaging effects of Xenon arc light. Unscheduled repair DNA synthesis, used as a measure for UVB-induced DNA damage and repair, was also investigated in skin irradiated with the Xenon arc before and after 8-MOP-UVA induced tanning. Both the number of grains per sparse labeled cell and the number of sparse labeled cells per 1000 cells, were found to be significantly lower in tanned skin; taking decreased unschedules repair DNA synthesis as a measure for decreased DNA-damage, these findings also demonstrate a photoprotective effect of the 8--MOP-UVA induced tan.

UV-induced unscheduled DNA synthesis in guinea pig skin melanocytes isolated in culture

Pigmented melanocytes isolated in culture from the epidermis of guinea pig ears were used to study the unscheduled DNA synthesis (UDS) induced by ultraviolet (UV) irradiation at 254 nm. After irradiation, the cells were labeled for 6 hr with 3H-thymidine (3HTdR), fixed, dehydrated, and flat embedded in Epon. Radioautographs were made on serial 0.5-1 micrometer thick sections of the cultures in order to distinguish silver grains from pigment granules. Cells irradiated with 5, 10, or 50 joules (J) m-2 showed a 3HTdR uptake due to UDS, whereas their proliferative ability, measured by the 3HTdR uptake due to scheduled DNA synthesis, was very low. For the same UV doses, UDS was lower in melanocytes than in guinea pig fibroblasts and keratinocytes.

Biologic changes due to long-wave ultraviolet irradiation on human skin: ultrastructural study

Alteration of the skin induced by single and repeated long-wave ultraviolet (UVA) exposures was studied. Following a single exposure to relatively large doses of UVA, pronounced dermal damage was observed. In the papillary dermis, superficial dermal vessels showed widely open endothelial gaps and extravasation of blood cells. Marked changes of fibroblasts were also seen in the superficial dermis. In the reticular dermis, extravascular fibrin deposition was seen. After repeated exposures to UVA the formation of cross-banded filamentous aggregations ("Zebra bodies") was observed in the superficial and reticular dermis. These were often found in amorphous masses surrounding the blood vessels. These striking dermal alterations were absent in skin irradiated by solar stimulating radiation and in control skin. Dyskeratotic "sunburn cells" were occasionally seen in the epidermis after single as well as repeated exposures to UVA. The number of these cells was less than that seen after a single exposure to solar simulating radiation.

The sunburn cell in mouse skin: preliminary quantitative studies on its production

Quantitative studies on sunburn cell (SBC) production in mouse epidermis are reported. Ultraviolet radiation (UVR), 260-300 nm, produced a dose related effect with respect to SBC formation. Maximal spectral reactivity was at wavelengths shorter than 300 nm. In 8-methoxypsoralen (8-MOP) photosensitization, SBCs were also produced and this response is dose related with respect to long wave UVR dose. In fluorescein photosensitization, provoked by 487 nm light, SBCs were not produced. The response in 8-MOP photosensitization and 254 nm UVR (given alone) differed in respect of time course, but it is considered that both may perhaps to initiated by a DNA lesion.

The induction of melanogenesis by ultraviolet light in the pigmentary system of Rhesus monkeys

Except for the face, eyelids, friction surfaces, and lips, the epidermis of the rhesus monkey contains no discernible melanocytes. After ultraviolet irradiation, however, dopa-positive dendritic cells appeared. With daily sequential irradiation, the number of histochemically demonstrable dopa-positive dendritic cells peaked after 30 exposures, then declined to a basal level which was maintained for the duration of the experiment (216 exposures or 43 weeks). Pigment cells can be restimulated by shading part of the irradiated area and then reirradiating after 3 months. While shaded, dopa-positive cells disappeared; but when reirradiated, they reappeared, increased, then declined again to a basal level. These melanocytes, unlike those in other primates, require high threshold levels of irradiation to produce a response, have a definite period during which they are active, and transfer very little melanin to the surrounding keratinocytes. Long-term ultraviolet irradiation has no discernible effect on dermal pigment-containing cells.

Epidermal apoptosis: cell deletion by phagocytosis

Apoptosis is a process found in many tissues by which devitalized cells are eliminated in an orderly manner. In skin, apoptosis occurs following sunburn, in Bowen's disease and in basal cell carcinoma. Scattered cells are observed in which the cytoplasm has become condensed and dyskeratotic. These dyskeratotic cells are phagocytized, either in toto or in fragments by surrounding keratinocytes. This process involves the condensation, fragmentation, phagocytosis and digestion of individually degenerated cells. The mass of a tissue is thus related to the balance between cell formation (mitosis) and cell destruction (apoptosis). In epidermal and other surface tissues, the rate of exfoliation must also be considered.