High Doses Of UV-B Research Articles

Specific induction of gadd45 in human melanocytes and melanoma cells after UVB irradiation.

The purpose of our study was to analyze the p53-mediated response of human melanocytes and human melanoma cells to UVB (natural environmental carcinogen) or UVC irradiation (experimental carcinogen). A semi-quantitative RT-PCR method was developed to allow the analysis of the expression of 5 p53 effector genes (p21(WAF1), mdm2, cyclin G1, gadd45, bax) at the same time with a small amount of RNA (1 microg). In human melanocytic cells, the p53 downstream genes were found to be differentially activated after UVB and UVC irradiation. After UVB irradiation, p53 protein accumulation was sustained up to 48 hr that was not the case after UVC irradiation. Among the p53 effector genes tested, gadd45 was the only 1 to show a strong and specific induction after UVB irradiation. With high UVB doses, gadd45 was also the only gene to be transcribed. By contrast, after UVC irradiation, all the p53 effector genes tested were transcriptionally induced. Experiments conducted with fibroblasts and keratinocytes didn't show such a striking activation of gadd45 after UVB irradiation. These results point out the potential role of gadd45 in response to UVB irradiation in human melanocytes and the different p53-mediated responses to different carcinogens.

Photoinduced toxicity of retene to Daphnia magna under enhanced UV-B radiation

The effects of UV radiation on the acute toxicity of retene (7-isopropyl-1-methylphenanthrene) to Daphnia magna Straus were studied. Dehydroabietic acid (DHAA) from which retene is formed in the vicinity of pulp and paper industry was also studied. Pyrene, anthracene, and phenanthrene were used as model PAH compounds. The time taken for immobilization (ET 50) was monitored under biologically effective UV-B dose rates of 240, 365, 565, and 650 mW m −2 (UV-A and visible light also present). Median effective concentrations (EC 50) were determined after a 15-min UV exposure (565 mW m −2) followed by 24 h in the dark. Retene ( 10–320 μg l −1 ) was not acutely toxic in the dark. The induction of phototoxicity was in agreement with the absorption properties of the compounds (absorption peak of retene at around 300 nm). Photoinduced toxicity followed an order pyrene>anthracene>retene. Phenanthrene and DHAA were not acutely phototoxic. Accumulation of the compound in Daphnia before UV exposure was essential. Some changes in the absorption spectra of the compounds were seen after a 5-h UV irradiation (565 mW m −2) , but none of the irradiated compounds were acutely toxic without further UV exposure. Therefore, the enhanced acute toxicity was primarily due to internal photosensitization reactions rather than photomodification. The dissolved fraction of 25% pulp and paper mill effluent reduced phototoxicity by attenuating UV radiation. The phototoxicity of retene was a function of both the exposure concentration and the UV-B dose rate, but relatively high UV-B dose rates and concentrations were needed for the acute photoinduced toxicity.

The consequence of p53 overexpression for liver tumor development and the response of transformed murine hepatocytes to genotoxic agents.

To analyse the effect of p53 on liver tumor development, we generated transgenic mice overexpressing wild-type p53 in the liver and crossed them with transgenic mice in which the expression of the SV40 large T antigen (TAg) induces hepatic tumors. Remarkably, whereas preneoplastic TAg liver exhibited anisocaryosis and anisocytosis, TAg/p53 liver never presented any dysplastic cells. Moreover, whereas expression of p53 did not affect hepatic development, its constitutive expression in tumorigenic livers resulted in a significantly enhanced apoptosis once nodules had appeared. In contrast, p53 overexpression did not modify the elevated proliferation of TAg-transformed hepatocytes and had no effect on hepatocarcinoma progression. In vitro analysis of primary hepatocytes exposed to various genotoxic agents showed that p53 failed to sensitize normal or TAg-transformed hepatocytes to apoptosis, except when high doses of doxorubicin, UV-B and UV-C radiation were used. Our results confirmed that the hepatocyte cell type is very resistant to genotoxic agents and showed that constitutive expression of p53 failed to improve their responsiveness. In addition, our results showed that suppression of dysplastic cells, probably by restoring normal cytokinesis and karyokinesis, and enhancement of apoptosis by means of p53 overexpression were insufficient to counteract or delay the TAg-induced liver tumoral progression. Oncogene (2000) 19, 3498 - 3507

Timecourse study of UVB-induced cytokine induction in whole mouse skin.

Ever since the skin was recognized as a site of immunologic activity, a number of laboratories have studied the production of cytokines by skin cells and the effects of chemicals, allergens, contact irritants and UVB radiation on their production. Most research to date has been carried out using either purified populations of primary cells, or established cell lines. Cytokines, however, do not function in isolation but they appear in human tissues within the context of other cytokines that can, in turn, strongly influence the final biological outcome. Therefore, in vivo studies using whole skin are more physiologically relevant since all cell types are present and interactions among them are allowed to proceed. We report here the results of a detailed timecourse study using whole mouse skin, consisting of both dermis and epidermis, irradiated with either a low or high dose of UVB and analyzed using a Multi-probe RNase protection assay system. The results show that in whole skin the kinetics of cytokine induction are different than what was previously observed in tissue culture. In addition to already known skin-associated cytokines, we report here the presence and UVB induction of cytokines not previously reported.

Ultraviolet-B radiation and nitrogen effects on growth and yield of maize under Mediterranean field conditions

The effects of an increase in UV-B radiation on growth and yield of maize ( Zea mays L.) were investigated at four levels of applied nitrogen (0, 100, 200 and 300 kg ha −1 of N) under Mediterranean field conditions. The experiment simulated a 20% stratospheric ozone depletion over Portugal. Enhanced UV-B and N deficiency decreased yield and total biomass production by 22–49%. High UV-B dose reduced yield, total biomass and growth of N-fertilized maize plants but did not affect N-stressed plants to the same extent. The response of grain yield to N was smaller with enhanced UV-B radiation. The underlying mechanisms for these results are discussed.

Successful separation of apoptosis and necrosis pathways in HaCaT keratinocyte cells induced by UVB irradiation.

UVB irradiation can induce apoptotic, necrotic, and differentiation pathways in normal human keratinocytes. The present study was undertaken to determine at what dose of UVB each of these pathways is induced and whether these pathways are distinct or overlapping. We have observed that UVB induces fragmentation of DNA in human HaCaT keratinocytes, in a bimodal manner. Low doses of UVB, 5-20 mJ/cm2, increase the levels of apoptosis as shown by increased levels of fragmented DNA, Fas, PARP, and FasL protein, and the number of apoptotic cells as assessed by FACS analysis. At higher doses of UVB (20 and 30 mJ/cm2) the number of apoptotic cells becomes reduced, as does the amount of Fas, PARP, and FasL protein. At these higher doses, cell viability is decreased as measured by DNA synthesis (BrdU labeling) neutral red uptake, which represents an increasing necrotic phenotype. Expression of markers of keratinocyte differentiation, involucrin, keratin K1, and keratin K10, are also observed to decrease with increasing UVB dose. These changes are accompanied by a further increase in DNA fragmentation. We conclude that low doses of UVB (5-20 mJ/cm2) induced an apoptotic pathway, whereas increasing doses (greater than 20 mJ/cm2) of UVB produce a direct necrotic effect and inhibit terminal differentiation.

DNA damage formation, DNA repair, and survival after exposure of DNA repair-proficient and nucleotide excision repair-deficient human lymphoblasts to UVA1 and UVB

Purpose : The comet assay has been used to visualize DNA damage in single cells after exposure to UV light. These comets are commonly thought to reflect transient, repair-induced DNA breaks. The goal of the work presented here was to further characterize the nature of UV-induced comets and to further elucidate DNA damage formation by different wavelengths of ultraviolet light. Materials and methods : Detailed dose–response and time-course experiments with comet formation were carried out with normal and nucleotide excision repair (NER)-deficient xeroderma pigmentosum (XP) lymphoblasts. Irradiation was carried out with low, intermediate, or high doses of UVA1 or UVB, comet formation was observed, cell survival and viability were determined, and UV-induced apoptosis was measured. Results : All responses were dose-dependent. With the intermediate dose of UVA1, a pronounced comet formation was observed without subsequent growth inhibition. Raising levels of porphyrins, which act as photosensitizers, by preincubation with 5-amino-levulinic acid increased comet formation with UVA1, but not with UVB. UVA1-sensitivity and comet formation in XP cells was not significantly different from the normal cells. With UVB no comet formation was seen without subsequent apoptotic cell death. XP cells exhibited the known UVB-hypersensitivity, but their comet formation was not significantly different from that of normal cells. Conclusions : The findings are compatible with the hypothesis that UV-induced comets represent transient repair-induced DNA breaks. Both, the NER of dimers and the base excision repair of oxidative DNA modifications are thought to contribute to comet formation.

Ultraviolet radiation-induced tolerance.

Ultraviolet (UV) radiation is one of the most important environmental factors. Besides its well-known advantages and its indispensable effects on human life, UV light, particularly the middle wavelength range (290-320 nm [UVB]), can harm human health by inducing skin cancer, premature skin aging, inflammation, and cell death (1-4). Over the last 25 years, it has become apparent that exposure to UVB radiation may also suppress immune reactions. Review of the numerous studies on the immunosuppressive properties of UV radiation led to two quite surprising observations: 1) that UV exposure can have systemic immunologic consequences 2) that UV radiation can induce antigen-specific tolerance. This paper briefly reviews the phenomenon of UV-induced tolerance.

Light and death: photons and apoptosis.

Phototherapies like photodynamic therapy (PDT), UVA1, UVB, and PUVA treat skin diseases. These phototherapies work because they alter cytokine profiles, change immune cytotoxicity in the skin, and directly kill diseased cells by apoptosis. Apoptosis is a term that only describes the morphologic changes a cell undergoes during this mode of cell death. The terms "immediate", "intermediate", and "delayed" apoptosis segregate the different apoptotic mechanisms into three kinetic categories, whereas the terms preprogrammed cell death (pre-PCD) and programmed cell death (PCD) describe the underlying mechanisms. Immediate apoptosis (T< or =0.5 h post-exposure) is triggered by singlet-oxygen damage that opens the mitochondrial megachannel, which can be mediated by PDT or UVA1 radiation. It is a pre-PCD mechanism of apoptosis, i.e., protein synthesis is not required post-insult, because all the necessary components are constitutively synthesized and only need to be activated. Intermediate apoptosis (T< or =4 h>0.5 h) is initiated by receptor cross-linking on the plasma membrane, which can be achieved using high doses of UVB or UVC radiation. It is also a pre-PCD mechanism. Delayed apoptosis (T>4 h) is induced by DNA damage that can be caused by X-rays, PUVA, UVC, UVB, UVA, and PDT. It is a PCD mechanism of apoptosis, i.e., protein synthesis is required post-insult. These three apoptotic mechanisms each access one of two "points-of-no-return" located on the mitochondrial membrane, which activate different, but not mutually exclusive, final pathways of apoptosis. This review discusses the latest findings on these apoptotic mechanisms and their implications in phototherapies.

Vitality of plaice embryos ( Pleuronectes platessa) at moderate UV-B exposure

The effects of moderate UV-B radiation (280–320 nm) on mortality and on physiologically vital parameters (heart rate, quality of respiratory control) were studied in plaice embryos ( Pleuronectes platessa L.). If UV-B exposure started at the early embryonic stage Ib (with a daily dose similar to present maximum outdoor levels in mid-April: 4.86 kJ m −2), 100% of embryos were dead after 5 days (control: 61% mortality). If light exposure started later (after gastrulation at stage II), however, mortality in test (UV-B irradiated) and control embryos (not UV-B irradiated) was not significantly different. Instantaneous or short-term effects of UV-B exposure on heart rate were absent even at high intensity (0.5 W m −2). With exposure to a daily dose of 4.86 kJ m −2, long-term accumulated effects of UV-B on heart rate were not detectable: heart rate in test and control embryos increased similarly during development from stage IIIγ to hatching. Exposure for a few days at a daily dose of 8.64 kJ m −2 (similar to present maximum outdoor levels in mid-May), however, resulted in significantly lower heart rates in test embryos than in control embryos at stage IVβ. Even 45 days after exposure, sublethal effects of UV-B on dry weight of the larvae, heart rate and quality of respiratory control were detectable in test animals. It is concluded that the vitality of plaice embryos can be impaired, either by relatively low UV-B doses during early development or by higher UV-B doses during late development.

Subphysiological concentrations of extracellular calcium sensitize normal human keratinocytes to UVB-induced apoptosis

Normal human keratinocytes are stimulated to proliferate in serum-free medium containing subphysiological concentrations of calcium (0.09 mM, low calcium). In this study, we examined the effect of increased levels of extracellular calcium (2.0 mM, normal calcium) on UVB-induced apoptosis. Apoptosis was assessed by changes in cellular morphology, annexind V-FITC flow cytometry, and the formation of internucleosomal DNA ladders. High doses of UVB induced keratinocytes grown in low calcium medium to undergo apoptosis. In contrast, keratinocytes grown for 72 h in normal calcium medium were completely resistant to UVB-induced apoptosis. No apoptosis was observed even at UVB doses as high as 1200 J/m2. However, despite the lack of UVB-induced cell death, keratinocytes grown in normal calcium medium lost the ability to proliferate following high levels of UVB irradiation. High doses of UVB also increased the expression of the differentiation-specific proteins involucrin and cytokeratin 10 in a dose-dependent manner. In addition, growth in normal calcium medium lowered the UVB-induced stimulation of the p53 protein and altered the normal subcellular localization pattern of p53. UVB irradiation of human keratinocytes grown in normal calcium medium may be inducing further cell differentiation in the absence of overt cell death.

The Martian and extraterrestrial UV radiation environment—1. Biological and closed-loop ecosystem considerations

The Martian surface is exposed to both UVC radiation (<280 nm) and higher doses of UVB (280–315 nm) compared to the surface of the Earth. Terrestrial organisms have not evolved to cope with such high levels of UVC and UVB and thus any attempts to introduce organisms to Mars, particularly in closed-loop life support systems that use ambient sunlight, must address this problem. Here we examine the UV radiation environment of Mars with respect to biological systems. Action spectra and UV surface fluxes are used to estimate the UV stress that both DNA and chloroplasts would experience. From this vantage point it is possible to consider appropriate measures to address the problem of the Martian UV environment for future long term human exploration and settlement strategies. Some prospects for improving the UV tolerance of organisms are also discussed. Existing artificial ecosystems such as Biosphere 2 can provide some insights into design strategies pertient to high UV environments. Some prospects for improving the UV tolerance of organisms are also discussed. The data also have implications for the establishment of closed-loop ecosystems using natural sunlight on the lunar surface and elsewhere in the Solar System.

TNF modulates susceptibility to UVB-induced systemic immunomodulation in mice by effects on dermal mast cell prevalence.

The mechanisms by which UV radiation is immunosuppressive are controversial, but there is growing evidence that processes of UVB-induced suppression of the immune response towards a sensitizing antigen are different if this antigen is applied to irradiated compared with non-irradiated sites. Consistent with this is our recent observation (Hart et al., J. Exp. Med. 1998. 187: 2045-2053) that the prevalence of dermal mast cells determines the extent of susceptibility of different mouse strains to UVB-induced systemic, but not local, immunosuppression. Using C57BL/6 and BALB/c mice exposed to low and high doses of UVB, respectively, in the presence of a polyclonal anti-TNF antibody, we found that TNF is directly involved as a mediator in the suppression by UVB of local immune responses. To determine whether TNF indirectly regulates UVB-induced systemic immunomodulation by altering the prevalence of dermal mast cells, dermal mast cell numbers in gene-targeted mice deficient in TNF or TNF receptors (p55/p75-/- mice) were quantified by video image analysis. A reduced dermal mast cell prevalence in these mice correlated with decreased susceptibility for systemic immunosuppression caused by UVB. We hypothesize that TNF is one molecule that controls dermal mast cell prevalence by as yet unknown mechanisms. However, it is the mediators released from mast cells upon UVB-induced degranulation, which do not include TNF, that directly signal suppressive events relevant to systemic immunosuppression.

Biological Effects of High Ultraviolet Radiation on Early Earth—a Theoretical Evaluation

The surface of early Earth was exposed to both UVC radiation (<280nm) and higher doses of UVB (280–315 nm) compared with the surface of present day Earth. The degree to which this radiation environment acted as a selection pressure on organisms and biological systems has rarely been theoretically examined with respect to the biologically effective irradiances that ancient organisms would receive. Here action spectra for DNA inactivation and isolated chloroplast inhibition are used to estimate biologically effective irradiances on archean Earth. Comparisons are made with present day Earth. The theoretical estimations on the UV radiation screening required to protect DNA on archean Earth compare well with field and laboratory observations on protective strategies found in present day microbial communities. They suggest that many physical and biological methods may have been effective and would have allowed for the radiation of life even under the high UV radiation regimes of archean Earth. Such strategies would also have provided effective reduction of photoinhibition by UV radiation. The data also suggest that the UV regime on the surface of Mars is not a life limiting factorper se, although other environmental factors such as desiccation and low temperatures may contribute towards the apparent lack of the surface biota.

Natural ultraviolet radiation and photosynthetically active radiation induce formation of mycosporine-like amino acids in the marine macroalga Chondrus crispus (Rhodophyta)

The UV-absorbing mycosporine-like amino acids (MAAs) are hypothesized to protect organisms against harmful UV radiation (UVR). Since the physiology and metabolism of these compounds are unknown, the induction and kinetics of MAA biosynthesis by various natural radiation conditions were investigated in the marine red alga Chondrus crispus collected from Helgoland, Germany. Three photosynthetically active radiation (PAR, 400–700 nm) treatments without UVR and three UV-A/B (290–400 nm) treatments without PAR were given. Chondrus crispus collected from 4–6 m depth contained only traces of the MAA palythine. After 24 h exposure to 100% ambient PAR, traces of three additional MAAs, shinorine, palythinol and palythene, were detected, and their concentrations increased strongly during a one-week exposure to all PAR treatments. The concentration of all MAAs varied directly with PAR dose, with palythine and shinorine being four- to sevenfold higher than palythinol and palythene. Likewise, naturally high doses of both UV-A and UV-B resulted in a strong accumulation of all MAAs, in particular shinorine. While shinorine accumulation was much more stimulated by UVR, the content of all other MAAs was more affected by high PAR, indicating an MAA-specific induction triggered by UVR or PAR.

UV-B Induced Alterations in Composition of Thylakoid Membrane and Amino Acids in Leaves of Rhizophora Apiculata Blume

Seedlings of Rhizophora apiculata were exposed to UV-B radiation at four doses equivalent to 10, 20, 30, and 40 % ozone depletion. The seedlings irradiated with high doses of UV-B had characteristic decline in contents of specific proteins with molecular masses of 33, 23, and 17 kDa. On the contrary, proteins of 55, 33, 25, 23, and 17 kDa were accumulated in the seedlings exposed to low doses of UV-B. The UV-B, in general, enhanced formation of saturated fatty acids and reduced unsaturated fatty acids, to a maximum extent of 88 and 26 %, respectively. The low dose of UV-B increased content of oleic acid by 9 %, and the high dose reduced it by 34 %. The high dose of UV-B enhanced the lipid peroxidation by 48 %, whereas the low dose of UV-B did not show any significant effect. The contents of amino acids such as aspartate, glutamate, asparagine, serine, glutamine, threonine, and histidine were increased in low UV-B doses by 53, 86, 142, 72, 3, 119, and 32 %, respectively; while in high doses they were reduced significantly.

Effect of ultraviolet radiation on pigmentation and nitrogen metabolism of antarctic phytoplankton and ice algae

Summary Antarctic phytoplankton and ice algae were exposed to UV-A or UV-B radiation under laboratory conditions during the «Polarstern» cruise (ANT XII/2) in Antarctica (23,11,1994 to 5,1,1995). The pattern of pigments varied in dependence on the dose and the UV source used as well as on the nitrogen compound applied. UV-B radiation in conjunction with addition of 15 NH 4 + caused a reduction of the main pigment contents of phytoplankton from the water column: the damaging effect was more pronounced at a relatively high UV-B dose (1.2 or 1.6 Wm −2 ) whereas UV-A radiation led to an increase in the main pigments. A UV-B irradiance of 5 h caused a strong reduction of all the pigment contents from ice algae after addition of 15 N-nitrate (60μmol/L) compared with the effects after 15 N-ammonium (60μmol/L). Uptake rates of 15 N-ammonium and 15 N -nitrate of phytoplankton and ice algae were more affected by UV-B than by UV-A radiation. Nitrate uptake was more reduced by UV than that of ammonium. Patterns of pools and 15 N labelled free amino acids of phytoplankton and ice algae varied in dependence of the UV source applied and inorganic nitrogen compound. 15 N incorporation into all amino acids of phytoplankton was markedly reduced after addition of 15 N-nitrate and 3 h UV radiation. UV-B radiation and addition of 15 N-ammonium led also to a strong reduction of 15 N labelling of all amino acids except for glutamine. However, UV-A irradiance had no significant effect. Patterns of free amino acid pools of phytoplankton samples varied in dependence of the UV source: no effect or an increase in the pools after UV-A exposure. Patterns of free amino acids of ice algae were less affected by UV radiation than those of phytoplankton: pools of main amino acids were enhanced after 3 or 5 h UV radiation. Results are discussed with reference to the UV source applied and the species-dependent response of the Antarctic microalgae under field conditions. The influence of UV radiation on the key enzymes of nitrogen metabolism in conjunction to the pigments was another topic of the discussion.

Skin tumor development and keratin expression in differentexperimental models. Relation to inducing agent and target tissue structure

The applicability of the experimental skin carcinogenesis model for studies of tumor development was examined by exposing the skin of various mouse strains to different chemical carcinogens and UV radiation regimens, in order to analyze the development and progression of the neoplastic process and the role of differentiation markers such as keratins. In tumor-sensitive hairy NMRI mouse skin, the chemical carcinogen, 7,12-dimethylbenz(a)-anthracene (DMBA) induced an abnormal epidermal cell differentiation and structural irregularities associated with an altered keratin expression, as well as numerous papillomas and squamous cell carcinomas. A suboptimal dose of UVB irradiation increased the number of DMBA-induced benign squamous neoplasms. Low doses of benzo(a)pyrene resulted in mild epidermal alterations, but only in one tumor. High doses of UVB induced a large number of undifferentiated spindle cell tumors with few keratinpositive cells in NMRI mice, similar though fewer tumors in hairy, heavily pigmented C57BL/6 mice, numerous papillomas and squamous cell carcinomas in hairless hr/hr mice but only two papillomas in hairy, moderately pigmented DBA/2 mice while UVA exposure produced only two papillomas in hairless SKH-1 mice. In conclusion, the extent and type of skin tumor development depended upon the induction regimen: physical, chemical, dose and duration, as well as on the skin structure: pigmentation and adnexal development, all of which have to be taken into account when relating experimental results to human conditions.

Aqueous humour and crystalline lens changes associated with ultraviolet radiation or mechanical damage to corneal epithelium in freshwater rainbow trout eyes

In vivo irradiation of the eye of the rainbow trout with moderately high doses of UV-B (0.75 to 1.5 J cm 2) can produce corneal damage and persistent cataractous changes in the anterior part of the crystalline lens within hours. Irradiation with UV-C (0.25 J cm 2) also produces corneal damage but only temporary lens changes after 2–3 days. Cataractous changes can also occur following mechanical damage to the cornea ex vivo, and develop concurrently with decreases in the osmolarity of the aqueous humour of at least 30 mOsm l −1; no change in aqueous humour osmolality was detected at the onset of cataracts associated with UVB irradiation in vivo. The UVB-associated anterior cataracts are thus not obviously caused by aqueous humour changes although, when there is concurrent mechanical damage with the ultraviolet radiation, hypo-osmotic changes may contribute to the cataract formation.

Structural and biochemical basis for the UVB-induced alterations in epidermal barrier function.

Ultraviolet light (UVR) induces a myriad of cutaneous changes, including delayed disruption of the permeability barrier with higher doses. To investigate the basis for the UVB-induced barrier alteration, we assessed the epidermal lamellar body secretory system at various time points before and after barrier disruption with a single high dose of UVB (7.5 MED) to murine epidermis. Morphological data were correlated with changes in epidermal proliferation and lipid synthesis, indicative of lamellar body generation. Twenty-four hours following UVB, the stratum corneum (SC) is normal, but a layer of abnormal, vacuolated, and lamellar body (LB)-deficient cells is present, immediately beneath the stratum granulosum (SG)/SC interface. Immediately subjacent to this band of damaged cells, normal keratinocytes that contain intact LBs are present. By 72 h, concomitant with the appearance of a barrier abnormality, extensively damaged cells persist at the SC/SG interface, and abnormal lamellar membrane structures appear in the lower SC. Upper stratum spinosum (SS) and lower SG cells appear normal, with increased numbers of LBs. A barrier abnormality is still present at 96 h, in association with membrane abnormalities in the lower SC interstices, but up to four normal appearing, subjacent SG cell layers are present. By 120 h, accelerated LB formation and precocious LB extrusion occur throughout the thickened SG; normal lamellar membranes are present in the lower SC; and barrier recovery is almost complete. Whereas, epidermal synthesis of the major barrier lipid species (i.e., cholesterol, fatty acids, and ceramides, including acylceramides) is reduced or unchanged at 24 and 48 h, it increases significantly 72 h after exposure to UVB. Therefore, the delayed disruption of the permeability barrier following acute UVB exposure results from the arrival of a band of lamellar body-incompetent (i.e., damaged) cells at the SG/SC interface. The subsequent, rapid recovery of the barrier, in turn, results from compensatory hyperplasia of subjacent, undamaged SS/SG cells, generating increased numbers and contents of LB. These results underscore the critical role of the stratum compactum in mediating barrier function, and suggest that beneficial therapeutic effects of UV exposure may be due to enhanced lipid production and barrier regeneration.