Ultrastructural characterization of sulfur mustard-induced vesication in isolated perfused porcine skin

Therapeutic potential of a non-steroidal bifunctional anti-inflammatory and anti-cholinergic agent against skin injury induced by sulfur mustard

Sulfur mustard (bis(2-chloroethyl) sulfide, SM) is a highly reactive bifunctional alkylating agent inducing edema, inflammation, and the formation of fluid-filled blisters in the skin. Medical countermeasures against SM-induced cutaneous injury have yet to be established. In the present studies, we tested a novel, bifunctional anti-inflammatory prodrug (NDH 4338) designed to target cyclooxygenase 2 (COX2), an enzyme that generates inflammatory eicosanoids, and acetylcholinesterase, an enzyme mediating activation of cholinergic inflammatory pathways in a model of SM-induced skin injury. Adult SKH-1 hairless male mice were exposed to SM using a dorsal skin vapor cup model. NDH 4338 was applied topically to the skin 24, 48, and 72 h post-SM exposure. After 96 h, SM was found to induce skin injury characterized by edema, epidermal hyperplasia, loss of the differentiation marker, keratin 10 (K10), upregulation of the skin wound marker keratin 6 (K6), disruption of the basement membrane anchoring protein laminin 322, and increased expression of epidermal COX2. NDH 4338 post-treatment reduced SM-induced dermal edema and enhanced skin re-epithelialization. This was associated with a reduction in COX2 expression, increased K10 expression in the suprabasal epidermis, and reduced expression of K6. NDH 4338 also restored basement membrane integrity, as evidenced by continuous expression of laminin 332 at the dermal-epidermal junction. Taken together, these data indicate that a bifunctional anti-inflammatory prodrug stimulates repair of SM induced skin injury and may be useful as a medical countermeasure.

Solute-vehicle-skin interactions in percutaneous absorption: the principles and the people

An appreciation of solute-vehicle-skin interactions underpins our current understanding of the processes of percutaneous absorption as well as in the prediction of the extent of absorption. This understanding has been reached through principles developed and validated over the last century through the work of a number of authors, including Dale Wurster, Takeru Higuchi, Irvin Blank, Robert Scheuplein, Gordon Flynn, Boyd Poulsen and Tom Franz, as well as by many scientists from my and younger generations. Their work has led to an appreciation of the rate-limiting steps in percutaneous penetration, the role played by the physicochemical properties of the solute, vehicle and skin and the variability that may arise from using various experimental/mathematical/pharmacokinetic models to quantify absorption as well as enabling the prediction of local and systemic efficacy and toxicity. In addition, unexpected behaviour may result from non-ideality in solute-vehicle-skin effects, including dehydration, chemical enhancement, supersaturation, metabolism, sequestration and vascular effects, including those of nanosystems on the local vasculature. In general, in vitro skin penetration profiles are predictive of in vivo profiles but a number of exceptions also exist.

Mechanisms of sulfur mustard analog 2-chloroethyl ethyl sulfide-induced DNA damage in skin epidermal cells and fibroblasts

Employing mouse skin epidermal JB6 cells and dermal fibroblasts, here we examined the mechanisms of DNA damage by 2-chloroethyl ethyl sulfide (CEES), a monofunctional analog of sulfur mustard (SM). CEES exposure caused H2A.X and p53 phosphorylation as well as p53 accumulation in both cell types, starting at 1h, that was sustained for 24h, indicating a DNA-damaging effect of CEES, which was also confirmed and quantified by alkaline comet assay. CEES exposure also induced oxidative stress and oxidative DNA damage in both cell types, measured by an increase in mitochondrial and cellular reactive oxygen species and 8-hydroxydeoxyguanosine levels, respectively. In the studies distinguishing between oxidative and direct DNA damage, 1h pretreatment with glutathione (GSH) or the antioxidant Trolox showed a decrease in CEES-induced oxidative stress and oxidative DNA damage. However, only GSH pretreatment decreased CEES-induced total DNA damage measured by comet assay, H2A.X and p53 phosphorylation, and total p53 levels. This was possibly due to the formation of GSH-CEES conjugates detected by LC-MS analysis. Together, our results show that CEES causes both direct and oxidative DNA damage, suggesting that to rescue SM-caused skin injuries, pleiotropic agents (or cocktails) are needed that could target multiple pathways of mustard skin toxicities.

Expression of proliferative and inflammatory markers in a full-thickness human skin equivalent following exposure to the model sulfur mustard vesicant, 2-chloroethyl ethyl sulfide

Sulfur mustard is a potent vesicant that induces inflammation, edema and blistering following dermal exposure. To assess molecular mechanisms mediating these responses, we analyzed the effects of the model sulfur mustard vesicant, 2-chloroethyl ethyl sulfide, on EpiDerm-FT™, a commercially available full-thickness human skin equivalent. CEES (100-1000 μM) caused a concentration-dependent increase in pyknotic nuclei and vacuolization in basal keratinocytes; at high concentrations (300-1000 μM), CEES also disrupted keratin filament architecture in the stratum corneum. This was associated with time-dependent increases in expression of proliferating cell nuclear antigen, a marker of cell proliferation, and poly(ADP-ribose) polymerase (PARP) and phosphorylated histone H2AX, markers of DNA damage. Concentration- and time-dependent increases in mRNA and protein expression of eicosanoid biosynthetic enzymes including COX-2, 5-lipoxygenase, microsomal PGE₂ synthases, leukotriene (LT) A₄ hydrolase and LTC₄ synthase were observed in CEES-treated skin equivalents, as well as in antioxidant enzymes, glutathione S-transferases A1-2 (GSTA1-2), GSTA3 and GSTA4. These data demonstrate that CEES induces rapid cellular damage, cytotoxicity and inflammation in full-thickness skin equivalents. These effects are similar to human responses to vesicants in vivo and suggest that the full thickness skin equivalent is a useful in vitro model to characterize the biological effects of mustards and to develop potential therapeutics.

Mechanisms mediating the vesicant actions of sulfur mustard after cutaneous exposure

Sulfur mustard (SM), a chemical weapon first employed during World War I, targets the skin, eyes, and lung. It remains a significant military and civilian threat. The characteristic response of human skin to SM involves erythema of delayed onset, followed by edema with inflammatory cell infiltration, the appearance of large blisters in the affected area, and a prolonged healing period. Several in vivo and in vitro models have been established to understand the pathology and investigate the mechanism of action of this vesicating agent in the skin. SM is a bifunctional alkylating agent which reacts with many targets including lipids, proteins, and DNA, forming both intra- and intermolecular cross-links. Despite the relatively nonselective chemical reactivity of this agent, basal keratinocytes are more sensitive, and blistering involves detachment of these cells from their basement membrane adherence zones. The sequence and manner in which these cells die and detach is still unresolved. Much has been discovered over the past two decades with respect to the mechanisms of SM-induced cytotoxicity and the intracellular and extracellular targets of this vesicant. In this review, the effects of SM exposure on the skin are described, as well as potential mechanisms mediating its actions. Successful therapy for SM poisoning will depend on following new mechanistic leads to develop drugs that target one or more of its sites of action.

Involvement of caspases and transmembrane metalloproteases in sulphur mustard-induced microvesication in adult human skin in organ culture: directions for therapy

While skin is a major target for sulphur mustard (HD), a therapy to limit HD-induced vesication is currently not available. Since it is supposed that apoptotic cell death and proteolytic digestion of extracellular matrix proteins by metalloproteases are initiating factors for blister formation, we have explored whether inhibition of these processes could prevent HD-induced epidermal-dermal separation using adult human skin in organ culture. Involvement of the caspase and the metalloprotease families was confirmed by the observation that their respective broad spectrum inhibitors, Z-VAD-fmk and GM6001, each suppressed HD-induced microvesication. The lowest effective concentrations were 10 and 100microM, respectively. Using specific inhibitors for caspase-8 (> or =10microM) and caspase-9 (> or =10microM) we learned that HD-induced apoptosis is initiated by the death receptor pathway as well as by the mitochondrial pathway. Remarkably, blocking caspase-8 activity resulted in morphologically better conserved cells than blocking caspase-9 activity. We zoomed in on the role of metalloproteases in HD-induced microvesication by testing the effects of two inhibitors: dec-RVKR-cmk and TAPI-2. Dec-RVKR-cmk is an inhibitor of furin, which activates transmembrane enzymes of the 'a disintegrin and metalloproteinase' (ADAM)-family as well as the membrane-type metalloproteases (MTx-MMP). TAPI-2 specifically inhibits TNFalpha-converting enzyme (TACE/ADAM17), which is involved in pericellular proteolysis. Both inhibitors prevented microvesication at concentrations of > or =500 and > or =20microM, respectively. This confirms that ADAMs and MT-MMPs play a role in HD-induced epidermal-dermal separation, with a particular role for TACE/ADAM17. Since TACE is involved not only in degradation of cell-matrix adhesion structures, but also in ectodomain shedding of ligands for epidermal growth factor receptor (EGFR) and in release of TNFalpha, these results imply TACE-mediated pathways as a new concept in HD toxicity. In conclusion, transmembrane metalloproteases probably form a main target for treatment of blisters in HD casualties. The observation that microvesication in the ex vivo human skin model still could be prevented when the metalloprotease inhibitor GM6001 was applied up to 8h after exposure to HD opens perspectives for non-urgent cure of HD casualties.

Treatment of keratin intermediate filaments with sulfur mustard analogs

Sulfur mustard (SM) is an alkylating agent with a history of use as a chemical weapon. The chemical reactivity of sulfur mustard toward both proteins and nucleic acids coupled with the hours long delay between exposure and appearance of blisters has prevented the determination of the mechanism of blister formation. We have treated assembled keratin intermediate filaments with analogs of sulfur mustard to simulate exposure to SM. We find that treatment of intact filaments with chloroethyl ethyl sulfide (CEES) or mechlorethamine (MEC) produces aggregates of keratin filaments with little native appearing structure. Treatment of a mix of epidermal keratins 1/10 (keratin pair 1 and 10) and keratins 5/14 with a sulfhydryl-specific modification reagent also results in filament abnormalities. Our results are consistent with the hypothesis that modification of keratins by SM would result in keratin filament destruction, leading to lysis of epidermal basal cells and skin blistering.

Skin toxicity of jet fuels: ultrastructural studies and the effects of substance P

Topical exposure to jet fuel is a significant occupational hazard. Recent studies have focused on dermal absorption of fuel and its components, or alternatively, on the biochemical or immunotoxicological sequelae to exposure. Surprisingly, morphological and ultrastructural analyses have not been systematically conducted. Similarly, few studies have compared responses in skin to that of the primary target organ, the lung. The focus of the present investigation was 2-fold: first, to characterize the ultrastructural changes seen after topical exposure to moderate doses (335 or 67 microl/cm2) of jet fuels [Jet A, Jet Propellant (JP)-8, JP-8+100] for up to 4 days in pigs, and secondly, to determine if co-administration of substance P (SP) with JP-8 jet fuel in human epidermal keratinocyte cell cultures modulates toxicity as it does to pulmonary toxicity in laboratory animal studies. The primary change seen after exposure to all fuels was low-level inflammation accompanied by formation of lipid droplets in various skin layers, mitochondrial and nucleolar changes, cleft formation in the intercellular lipid lamellar bilayers, as well as disorganization in the stratum granulosum-stratum corneum interface. An increased number of Langerhans cells were also noted in jet fuel-treated skin. These changes suggest that the primary effect of jet fuel exposure is damage to the stratum corneum barrier. SP administration decreased the release of interleukin (IL)-8 normally seen in keratinocytes after JP-8 exposure, a response similar to that reported for SP's effect on JP-8 pulmonary toxicity. These studies provide a base upon which biochemical and immunological data collected in other model systems can be compared.

Percutaneous absorption of topical N,N-diethyl-m-toluamide (DEET): effects of exposure variables and coadministered toxicants

Exposure to N,N-diethyl-m-toluamide (DEET) commonly occurs in the general population and has been implicated as a contributory factor to the Gulf War Illness. The focus of the present studies was to determine the effect of coexposure factors, potentially encountered in a military environment, that could modulate transdermal flux of topically applied DEET. Factors investigated were vehicle, dose, coexposure to permethrin, low-level sulfur mustard, occlusion, and simultaneous systemic exposure to pyridostigmine bromide and the nerve agent stimulant diisopropylfluorophosphate (DFP). Studies were conducted using the isolated perfused porcine skin flap (IPPSF), with a few mechanistically oriented studies conducted using in vitro porcine skin and silastic membrane diffusion cells. DEET was quantitated using high-performance liquid chromatography. The vehicle-control transdermal DEET flux in the IPPSF was approximately 2 micrograms/cm2/h for both 7.5 and 75% DEET concentrations, a value similar to that reported in humans. DEET absorption was enhanced by coinfusion of pyridostigmine bromide and DFP, by the presence of sulfur mustard, or by dosing under complete occlusion. The greatest increase in baseline flux was fivefold. In vitro diffusion cell studies indicated that silastic membranes had two orders of magnitude greater permeability than porcine skin, and showed vehicle effects on flux that were not detected in the IPPSF. These results suggest that coexposure to a number of chemicals that potentially could be encountered in a military environment may modulate the percutaneous absorption of topically applied DEET beyond that seen for normal vehicles at typically applied concentrations.

Gulf War related exposure factors influencing topical absorption of 14C-permethrin

Topical exposure to permethrin has often been implicated as a mitigating factor in the illnesses reported in Gulf War veterans. These studies were designed to assess the effect of co-exposure to low level sulfur mustard, JP-8 jet fuel, N,N-diethyl-m-toluamide (DEET) and fabric occlusion on the percutaneous absorption and skin disposition of topically applied 14C-permethrin (40 microg/cm(2)) in the isolated perfused porcine skin flap (IPPSF) model. Extent of dermal absorption in vehicle controls in the IPPSF was comparable to literature values for humans. These studies demonstrated a two-fold increased 14C-permethrin percutaneous absorption and almost three-fold increased penetration when JP-8 was present, compared to a one-third decreased permethrin flux in the presence of sulfur mustard. Complete occlusion slightly increased 14C-permethrin absorption, while occlusion with fabric showed no significant effect. A previously noted effect of DEET to inhibit permethrin absorption was still seen in the presence of sulfur mustard exposure. These studies suggest that co-exposure to JP-8 or sulfur mustard may modulate transdermal flux of 14C-permethrin. However, the JP-8 increase in absorption and penetration was less than the five-fold increase previously seen with arterial infusion of pyridostigmine bromide and diisopropylfluorophosphate in the IPPSF. The toxicologic significance of this moderate increase in permethrin absorption remains unclear.

Potential anti-inflammatory treatments against cutaneous sulfur mustard injury using the mouse ear vesicant model

In spite of several decades of research, no effective treatment to skin injuries following exposure to sulfur mustard (HD) has yet been found. In the present study, the mouse ear vesicant model was applied to awake mice in order to evaluate the efficiency of potential anti-inflammatory treatments in preventing HD-induced skin damages. Clinical follow-up and histological evaluation were used to characterize the injuries to the skin and to evaluate the efficiency of the drugs that were applied. Thus, the extent of mouse ear oedema and the histopathological changes following a single application of 0.2 or 1 microL of neat HD for 10 min (representing moderate and severe lesions, respectively), were monitored. Typical HD skin lesions were observed including epithelial and dermal damage. The development of the injury in mouse ears was found to be very similar to that reported in human skin. Screening of post-exposure topical steroids and non-steroidal antiinflammatory drugs (NSAIDs) proved that HD-induced inflammation could be diminished significantly as long as the treatment was applied during the early stages following exposure. A combined application of these drugs approved to be particularly effective in reducing inflammation.

Ultrastructural and histological effects of exposure to CEES or heat in a human epidermal model

Ultrastructural and terminal deoxynucleotidyl transferase nick end labeling (TUNEL) studies were conducted to compare mechanisms of 2-chloroethyl ethyl sulfide (CEES) and heat-induced injury to EpiDerm. Twenty-two hours after 2-h exposure to the monofunctional alkylating agent CEES, budding of cytoplasm, clumping of nuclear chromatin, disintegration of nuclear membranes and cytoplasmic structures, and cytoplasmic vacuolization were detected, especially in the basal cells near the pseudobasement membrane. TUNEL techniques revealed DNA fragmentation distinct from that normally associated with terminal keratinocyte differentiation. Similar evaluations 22.5 h after 90 min exposure of EpiDerm to elevated temperature (45 degrees C) produced a different pattern of cell damage. Swelling of intercellular spaces, extensive cytoplasmic vacuolization, disruption of normal nuclear shape, reduced cell membrane integrity, and release of cellular material in the basal region characterized heat injury. Heat did not alter the DNA fragmentation normally associated with keratinocyte maturation. These data suggest that CEES elicited an apoptotic mechanism of cell death with features of terminal differentiation such as nuclear membrane disintegration and loss of cytoplasmic structures. Heat, alternatively, produced changes more typical of oncotic necrosis.

Use of methyl salicylate as a simulant to predict the percutaneous absorption of sulfur mustard

Exposure to chemical vesicants such as sulfur mustard (HD) continues to be a threat to military forces requiring protectant strategies to exposure to be evaluated. Methyl salicylate (MS) has historically been the simulant of choice to assess HD exposure. The purpose of this study was to compare the percutaneous absorption and skin deposition of MS to HD in the isolated perfused porcine skin flap (IPPSF). The HD data were obtained from a previously published study in this model wherein 400 microg cm(-2) of ](14)C[-MS or ](14)C[-HD in ethanol were topically applied to 16 IPPSFs and experiments were terminated at 2, 4 or 8 h. Perfusate was collected at increasing time intervals throughout perfusion. Radioactivity was determined in perfusate and skin samples. Perfusate flux profiles were fitted to a bi-exponential model Y(t) = A(e(-bt) - e(-dt)) and the area under the curve (AUC), peak flux and time to peak flux were determined. Sulfur mustard had more pronounced and rapid initial flux parameters (P < 0.05). The AUCs determined from observed and model-predicted parameters were not statistically different, although the mean HD AUC was 40--50% greater than MS. The HD skin and fat levels were up to twice those seen with MS, but had lower stratum corneum and residual skin surface concentrations (P < 0.05). Compared with other chemicals studied in this model, HD and MS cutaneous disposition were very similar, supporting the use of MS as a dermal simulant for HD exposure.

Acute ocular effects of mustard gas: ultrastructural pathology and immunohistopathology of exposed rabbit cornea

Whole-body exposure to sulfur mustard (HD) produces cutaneous, respiratory and ocular impairment. Of these, ocular damage causes the most immediate incapacitation. Heretofore, characterization of HD ocular toxicity has been largely limited to gross and histological observations. In the present study we explore histological, ultrastructural and immunopathological acute effects of HD ocular exposure and establish correlations with HD toxicity data already documented for dermal exposure. Anesthetized rabbits were exposed to 0.4 microl of liquid HD placed directly on the cornea. Animals were euthanized at 6, 9 and 24 h post-exposure and the eyes were enucleated and processed for histopathology, ultrastructural and immunoperoxidase study. At 6 and 9 h, the most prominent histological feature was nuclear pyknosis, necrosis and loss of polarity of corneal epithelial basal cells to the exclusion of other epithelial cells. At 24 h, all corneal epithelial cells presented degenerative changes, with the epithelium eventually detaching from the underlying basement membrane at the level of the lamina lucida. Microblisters, a characteristic HD-induced skin pathology of the basement membrane zone of animals, were absent in this corneal study. Edema, degenerating fibroblasts and inflammatory cellular infiltrates were persistent stromal responses. Immunopathological effects included changes in antigenicity of bullous pemphigoid protein, laminin, desmosonal protein, Ki67 and p53. These morphological and immunopathological effects of corneal exposure to HD appear to be largely consistent with that previously reported for dermal exposures, perhaps providing shared anatomical considerations for the development of specific HD prophylaxis and therapy.

Effects of CEES on inflammatory mediators, heat shock protein 70A, histology and ultrastructure in two skin models

Chemical warfare threats require the development of diverse models for the assessment of countermeasures. Human skin products, Skin2 (differentiating keratinocytes on a fibroblast-collagen matrix) and EpiDerm (differentiating keratinocytes) were exposed (2 h) to the sulfur mustard 2-chloroethyl ethyl sulfide (CEES, 1-2 mg l(-1) min(-1)) in humidified air or to humidified air alone. Tissues were evaluated histologically, ultrastructurally and for viability 22 h later; media and tissues were also analyzed for inflammatory mediators. Histology showed that CEES induced the separation of dermal and epidermal regions in Skin2 with severe damage to basal keratinocytes. Histology and electron microscopy of both products revealed condensation of nuclear chromatin, retraction of spinous processes, collapse of the tonofibrillar network and cytoplasmic vacuolization and blebbing in those cells with loss of pseudobasement membrane integrity. Exposure of Skin2 to CEES increased extracellular interleukin-1alpha (IL-1alpha), prostaglandin-E2 (PGE2) and especially IL-1 receptor antagonist (IL-1Ra) release (56,334 vs 84,614 pg ml(-1)), but decreased interleukin-6 (IL-6, 4,755 vs 351 pg ml(-1)). Exposure of EpiDerm to CEES led to unaffected extracellular and reduced intracelluar IL-1alpha (371 vs 92 pg ml(-1)). Extracellular IL-1Ra greatly increased (2,375 vs 24,875 pg ml(-1)), whereas cellular levels decreased (16,5425 vs 96,625 pg ml(-1)). Extracellular (224 vs 68 pg ml(-1)) and intracellular (485 vs 233 pg ml(-1)) soluble interleukin-1 receptor H (sIL-1RII) decreased. Prostanglandin E2 increased (1,835 vs 2,582 pg ml(-1)), whereas heat shock protein 70A (Hsp70A) remained statistically unchanged (57,000 vs 96,000 pg ml(-1)). Failure to obtain a heat shock response to CEES may contribute to the susceptibility of tissue to the alkylating agent. Consistent and marked responses of cellular and extracellular IL-1Ra to CEES suggest a potential for use as a tissue status marker and primary antiinflammatory regulator in skin.

Systemic administration of candidate antivesicants to protect against topically applied sulfur mustard in the mouse ear vesicant model (MEVM)

The mouse ear vesicant model (MEVM) provides a quantitative edema response as well as histopathological and biochemical endpoints as measurements of inflammation and tissue damage following exposure to the chemical warfare agent sulfur mustard (HD). In the MEVM, several topically applied anti-inflammatory agents provided a significant degree of protection against HD-induced edema and dermal-epidermal separation. This study evaluated the protective effects of three of these pharmacological compounds when administered systemically in the MEVM. Alzet osmotic pumps were used to deliver a subcutaneous dose of the appropriate anti-inflammatory agent, starting 24 h before exposure to sulfur mustard and continuing until 24 h post-exposure to HD. Twenty-four hours after pump implantation, 5 microl of a 195 mM (0.16 mg) solution of sulfur mustard (density = 1.27 g ml(-1); MW = 159; purity = 97.5%) in methylene chloride was applied to the inner surface of the right ear of each mouse. Sulfur mustard injury in the mouse ear was measured by both edema response (fluid accumulation) and histopathological damage (necrosis, epidermal-dermal separation). The systemic administration of hydrocortisone, indomethacin and olvanil provided a significant reduction in edema (24%, 26% and 22%, respectively) from the positive control. Compared to HD-positive controls, hydrocortisone, indomethacin and olvanil caused a significant reduction in subepidermal blisters (71%, 52% and 57%, respectively) whereas only hydrocortisone produced a significant reduction in contralateral epidermal necrosis (41%). We show here that these anti-inflammatory drugs are effective when administered systemically in the MEVM.

Immunohistochemical characterization of the basement membrane epitopes in bis(2-chloroethyl) sulfide-induced toxicity in mouse ear skin

Sulfur mustard (bis(2-chloroethyl) sulfide (HD)), a potent cutaneous vesicant and bifunctional alkylating agent, produces significant time-dependent histopathological changes in the skin of the mouse. The right ears of male CD1 mice were exposed topically to 5.0 microl of 195 mM (0.16 mg) HD in dichloromethane and harvested at 6, 12, 18 and 24 h. The left ear control was dosed with 5.0 microl of dichloromethane. In all controls and HD-treated mouse ear, moderate immunofluorescence staining was seen at the epidermal-dermal junction with bullous pemphigoid (BP), epidermolysis bullosa acquisita (EBA) and laminin (Lam), and light staining was observed with bullous pemphigoid 180 (BP180), fibronectin (Fn) and type IV collagen (Coll IV). Mouse anti-human monoclonal antibodies for GB3, L3d and 19-DEJ-1 (Uncein) did not cross-react. In microvesicles, BP, BP180 and Fn showed areas of light focal epidermal staining and homogeneous dermal staining, and EBA, Lam and Coll IV showed moderate dermal staining. Both BP and Fn exhibited weak, inconsistent staining with time. Immunoelectron microscopy (IEM) revealed similar results, with an increase in cell damage from 6 to 24 h, which corresponded to a decrease in staining intensity. Cell proliferation, expressed as the growth fraction of proliferating cell nuclear antigen (PCNA), showed an increase in cell damage. The growth fraction was lower in the inner ear and showed time-dependent differences. The immunofluorescence and IEM results indicate that HD causes an undulating inconsistent separation in the uppermost lamina lucida with focal cleavage into the lower portion of the basal keratinocytes just above the plasma membrane. Although this pattern of separation differs from other in vivo models in which the split occurs exclusively within the lamina lucida, this should not preclude its role as a screening model to study the effects and development of specific prophylactic and therapeutic strategies.