Structural and histochemical aspects of epidermis development of fetal porcine skin

Commercial Silver-Based Dressings: In Vitro and Clinical Studies in Treatment of Chronic and Burn Wounds

Chronic wounds are a major health problem because of delayed healing, causing hardships for the patient. The infection present in these wounds plays a role in delayed wound healing. Silver wound dressings have been used for decades, beginning in the 1960s with silver sulfadiazine for infection prevention for burn wounds. Since that time, there has been a large number of commercial silver dressings that have obtained FDA clearance. In this review, we examine the literature involving in vitro and in vivo (both animal and human clinical) studies with commercial silver dressings and attempt to glean the important characteristics of these dressings in treating infected wounds. The primary presentation of the literature is in the form of detailed tables. The narrative part of the review focuses on the different types of silver dressings, including the supporting matrix, the release characteristics of the silver into the surroundings, and their toxicity. Though there are many clinical studies of chronic and burn wounds using silver dressings that we discuss, it is difficult to compare the performances of the dressings directly because of the differences in the study protocols. We conclude that silver dressings can assist in wound healing, although it is difficult to provide general treatment guidelines. From a wound dressing point of view, future studies will need to focus on new delivery systems for silver, as well as the type of matrix in which the silver is deposited. Clearly, adding other actives to enhance the antimicrobial activity, including the disruption of mature biofilms is of interest. From a clinical point of view, the focus needs to be on the wound healing characteristics, and thus randomized control trials will provide more confidence in the results. The application of different wound dressings for specific wounds needs to be clarified, along with the application protocols. It is most likely that no single silver-based dressing can be used for all wounds.

Characterization of Growth Secondary Hair in Min Pig Activated by Follicle Stem Cell Stimulated by Wnt and BMP Signaling Pathway

In China, the national-level protected pig, the Min pig, is characterized by the development of secondary hairs and hair follicles in winter. Factors that dominate the genotype in the growth of secondary hairs are not clear through the concrete cell signaling pathways. This study compared hair phenotypes based on morphological structure, transcriptomics, and potential targeting molecules in the breeds of Min, Berkshire, and Yorkshire pigs. The results indicated that Min pigs have specific characteristics for the growth of secondary hairs compared with the Berkshire and Yorkshire pigs. The transcriptome analyses and quantitative reverse transcription-polymerase chain reaction results revealed that secondary hair growth was activated by follicle stem cells. The specific inhibitors of Wnt and BMP were studied using respective signals. The density of follicles, activity of follicle stem cells, and relative gene expression results have shown that Wnt and BMP stimulate the activity of follicle stem cells, and the Wnt signaling molecule has a significantly better effect than the BMP signaling molecule on stem cells. Wnt and BMP can promote the growth of local secondary hair and gene expression. Therefore, this study was conducted to verify the development mechanisms of secondary hairs, which have potential applications in laboratory animals and comparative medicine.

Varying phenotypes in swine versus murine transgenic models constitutively expressing the same human Sonic hedgehog transcriptional activator, K5-HGLI2 Delta N

This study was undertaken to characterize the effects of constitutive expression of the hedgehog transcriptional activator, Gli2, in porcine skin. The keratinocyte-specific human transgene, K5-hGli2 Delta N, was used to produce transgenic porcine lines via somatic cell nuclear transfer techniques. In mice, K5-hGli2 Delta N induces epithelial downgrowths resembling basal cell carcinomas. Our porcine model also developed these basal cell carcinoma-like lesions, however gross tumor development was not appreciated. In contrast to the murine model, diffuse epidermal changes as well as susceptibility to cutaneous infections were seen in the swine model. Histologic analysis of transgenic piglets revealed generalized epidermal changes including: epidermal hyperplasia (acanthosis), elongated rete ridges, parakeratotic hyperkeratosis, epidermal neutrophilic infiltration, capillary loop dilation and hypogranulosis. By 2 weeks of age, the transgenic piglets developed erythematic and edematous lesions at high contact epidermal areas and extensor surfaces of distal limb joints. Despite antibiotic treatment, these lesions progressed to a deep bacterial pyoderma and pigs died or were euthanized within weeks of birth. Non-transgenic littermates were phenotypically normal by gross and histological analysis. In summary, constitutive expression of the human hGli2 Delta N in keratinocytes, results in cutaneous changes that have not been reported in the K5-hGli2 Delta N murine model. These findings indicate a need for a multiple species animal model approach in order to better understand the role of Gli2 in mammalian skin.

Dermatological effects of chronic exposure to 7,12-dimethylbenz[A]anthracene (DMBA) or N-methyl-N-nitrosoguanidine (MNNG) in swine

PURPOSE

To determine whether chronic exposure to DMBA or MNNG in combination with or without UVB exposure would induce skin carcinomas in swine.

METHODS

Eight gilts were exposed to 100 mJ of UVB in their left side, allowed to recuperate, and divided into two groups. Each gilt received identical high doses (DMBA 50 microM; MNNG 250 mM), low doses (DMBA 500 nM; MNNG 2.5 mM), carrier (DMSO), or nothing added treatments in the UVB and non-UVB sides. Animals were exposed weekly for 30 weeks and skin samples collected at 10, 20, and 30 weeks from initiation of exposure. An additional sample was collected 16 weeks following cessation of exposure. All samples were scored for dermal morphology, including intracellular epidermal edema, intercellular epidermal edema, papillary dermal edema, perivascular infiltrates, pyknotic stratum basale cells, collagen necrosis, and epidermal-dermal separation, and the data were analyzed by ANOVA. MNNG and UVB light had a significant effect on epidermal thickness and the number of cell layers. The greatest increase in epidermal thickness occurred from 20 weeks to 30 weeks in the UVB plus MNNG treatment. Treatment with MNNG resulted in intracellular and intercellular epidermal edema, dermal edema, and dermal inflammation at both the low and high doses of MNNG. In contrast, all the morphological evaluations of the DMBA treatments were less severe than the MNNG.

CONCLUSION

Our findings show that although chronic exposure to MNNG and DMBA, with or without UVB exposure, caused severe to mild dermatopathological changes, neither resulted in the development of skin carcinomas. These results indicate that at least with respect to responses to DMBA and MNNG, the swine model mimics more closely the responses seen in human skin.

Site-specific percutaneous absorption of methyl salicylate and VX in domestic swine

The site specificity of the percutaneous absorption of methyl salicylate (MeS) and the organophosphate nerve agent VX (O-ethyl S-(2-diisopropylaminoethyl) methylphosphonothioate) was examined in anaesthetized domestic swine that were fully instrumented for physiological endpoints. Four different anatomical sites (ear, perineum, inguinal crease and epigastrium) were exposed to the MeS and the serum levels were measured over a 6-h time period. The dose absorbed at the ear region was 11 microg cm(-2) with an initial flux of 0.063 microg cm(-2)min(-1), whereas at the epigastrium region the dose absorbed was 3 microg cm(-2) with an initial flux of 0.025 microg cm(-2)min(-1). For this reason further studies were carried out with VX on the ear and the epigastrium only. In animals treated with agent on the epigastrium, blood cholinesterase (ChE) activity began to drop 90 min after application and continued to decline at a constant rate for the remainder of the experiment to ca. 25% of awake control activity. At this time there were negligible signs of poisoning and the medical prognosis was judged to be good. In contrast, the ChE activity in animals receiving VX on the ear decreased to 25% of awake control values within 45 min and levelled out at 5-6% by 120 min. Clinical signs of VX poisoning paralleled the ChE inhibition, progressing in severity over the duration of the exposure. It was judged that these animals would not survive. The dramatic site dependence of agent absorption leading to vastly different toxicological endpoints demonstrated in this model system has important ramifications for chemical protective suit development, threat assessment, medical countermeasures and contamination control protocols.

Cytokine induction as a measure of cutaneous toxicity in primary and immortalized porcine keratinocytes exposed to jet fuels, and their relationship to normal human epidermal keratinocytes

The purpose of this study was to identify biomarkers of toxicity in primary porcine keratinocytes (PKC) and an immortalized porcine keratinocyte cell line (MSK3877) exposed to jet fuels Jet A, JP-8, and JP-8+100. Cells were exposed to 0.1% jet fuels and assayed for interleukin-8 (IL-8) and tumor necrosis factor alpha (TNF-alpha) mRNA using the TaqMan real time quantitative reverse transcriptase PCR assay. IL-8 and TNF-alpha protein release was measured using an ELISA. PKC exposed to jet fuels caused a slight upregulation of TNF-alpha mRNA at early time points, but no significant differences in TNF-alpha protein production were detected. IL-8 mRNA was increased at 4 h following exposure, and IL-8 protein was increased at 8 h. In MSK 3877 cells, jet fuels were shown to increase the production and expression of TNF-alpha mRNA and protein at 30 min and 1 h following exposure, respectively. IL-8 mRNA was only slightly induced compared to control. IL-8 protein release was suppressed by jet fuel exposure. These results were compared with those of a previous study in our laboratory to evaluate the utility of using porcine cells in lieu of normal human epidermal keratinocytes (NHEK). Similarities exist between PKC and NHEK with respect to both TNF-alpha and IL-8 production. The expression profile of TNF-alpha in MSK3877 cells mimics that of NHEK. In contrast, the profile of IL-8 expression opposes that of PKC and NHEK. These results suggest that porcine keratinocytes are susceptible to jet fuel toxicity. However, the responses of immortalized cells may vary from those of PKC and NHEK necessitating cautious interpretation of such data.

Sensitivity of cross-reacting antihuman antibodies in formalin-fixed porcine skin: including antibodies to proliferation antigens and cytokeratins with specificity in the skin

Although no animal is a perfect skin model for the study of toxicological and therapeutic agents, structurally the pig may be superior to even non-human primates. Because our work involves effects of toxicological and therapeutic agents on the skin, we wanted to identify stains which may prove useful as well as determine cross-reactivity of some newer antihuman antibodies. We performed a battery of formalin-fixed skin from weanling pigs and minipigs. The battery of antibodies included LCA, CD3, OPD-4, CD34, UCHL-1, L-26, KP-1, MAC-387, Factor XIIIa, Leu-7, S-100 protein, HMB-45, GFAP, synaptophysin, neurofilament protein, ubiquitin, vimentin, type IV collagen, laminin, fibronectin, Factor VIII related antigen, Desmin-M, smooth muscle actin, cytokeratin 7, cytokeratin 20, AEI/AE3, CAM 5.2, EMA, GCDFP, Ki-67, and PCNA. Immunohistochemical stains for CD3, Leu-7, S-100 protein, type IV collagen, laminin, Factor VIII related antigen, GFAP, synaptophysin, neurofilament protein, ubiquitin, smooth muscle actin, vimentin, Desmin-M, cytokeratin 7, cytokeratin 20, AE1/AE3, CAM 5.2, Ki-67 and PCNA showed consistent cross-reactivity. In formalin-fixed tissue, only antibodies to lymphoreticular cells showed poor cross-reactivity. A high percentage of the remaining antibodies did show good cross-reactivity but with some interesting similarities and differences in specificity.

Depth of morphologic skin damage and viability after one, two, and three passes of a high-energy, short-pulse CO2 laser (Tru-Pulse) in pig skin

BACKGROUND

CO2 laser energy is absorbed by water, which is present in all tissue. The depth of penetration of CO2 lasers is narrow with minimal reflection, scatter, or transmission. However, thermal damage has limited the usefulness of conventional, continuous-wave CO2 lasers for debridement as demonstrated by wound healing studies. The development of high-energy CO2 lasers, with pulse durations that are less than the thermal relaxation time of tissue, have made vaporization of skin for resurfacing and wound debridement possible because of the decreased risk of thermal damage.

OBJECTIVE

This study was performed to evaluate thermal damage produced by a CO2 laser.

METHODS

Routine histopathologic examination and nitroblue-tetrazolium chloride (NBTC) staining were used to evaluate the depth of tissue damage and viability in weanling pig skin after one, two, and three passes of the laser.

RESULTS

At a pulse energy of 300 mJ, with a pulse duration of 60 microseconds, one pass of the laser produced vaporization of the epidermis with minimal thermal damage. Two passes produced areas of denatured collagen with loss of viable cells in the superficial papillary dermis. Three passes extended the damage into the papillary dermis.

CONCLUSION

Hyalinization of collagen appears to correspond well with the level of thermal damage as measured by NBTC staining. Our findings suggest that the energy necessary to vaporize the dermis may be greater than that needed to vaporize epidermis.

Histopathologic features seen with different animal models following cutaneous sulfur mustard exposure

In an effort to understand the pathophysiology of sulfur mustard (2,2' dichlorodiethyl sulfide, HD)-induced cutaneous lesions, a number of animal models have been used. Animal models have been and will continue to be used in the development of therapeutic strategies to protect against and/or moderate lesions, and to potentiate wound healing after HD exposure. Upon reviewing the histopathologic features seen after HD-exposure, we propose roles for different animal models in HD-research. Hematoxylin and eosin slides from protocols done originally as dose response studies for either liquid or vapor HD-exposures were examined. The animal models reported include the hairless guinea pig (HGP), weanling pig (WP), mouse ear (ME) and hairless mouse (HM). In all these animal models. HD induces subepidermal blister formation as well as epidermal cell death. The HGP appears to be the most sensitive model for epidermal necrosis. The HGP and, to a lesser degree, the HM react with a marked neutrophilic infiltrate. The ME provides a quantitative measure for HD effects and a mild inflammatory infiltrate similar to what is seen in human skin. Doses necessary to produce microblister formation in the WP are usually associated with more significant stromal and vascular changes than in other animal models. In addition to a quantitative measure of the HD effect and a mild inflammatory response, the cost, as well as the availability of specific antibodies, and DNA and RNA probes and primers gives the ME advantages for both drug screening and for the study of the pathophysiology of HD-induced cutaneous lesions. The sensitivity of the HGP and the abundant experience with vapor exposures establishes a place for this animal model in barrier cream and drug screening. The similarity of WP skin to human skin is important in the study of wound healing after HD exposure, as well as in the study of the pathophysiology of the cutaneous lesion and in more definitive therapeutic studies.

Cutaneous histopathologic features in weanling pigs after exposure to three different doses of liquid sulfur mustard

Sulfur mustard (2,2' dichlorodiethyl sulfide, HD) is a chemical warfare agent that is easily produced, and may be used against civilian populations as well as against military troops. However, good therapeutic and prophylactic measures await a better understanding of the pathophysiology of lesions produced by this agent. Because the skin remains is one of the principal routes for HD exposure and damage, the study of HD-induced skin lesions is of major interest. Blister formation is a characteristic of HD-induced skin lesions in humans. Attempts have been made to find an animal model that produces cutaneous microblisters after exposure to the naturally occurring liquid as well as vaporized HD. Weanling pigs were exposed to three different doses of liquid HD. Histopathologic findings showed microblister formation as well as variable apoptosis and/or necrosis of epidermal keratinocytes and vascular endothelium. Pig skin is morphologically similar to human skin. In the pig, the epidermal lipids, the density of hair follicles, the presence of sweat glands, the proliferation kinetics, and the antigenicity are all closer to human skin than are rodent models. All these features may be important in lesions induced by HD, and may mean that the pig is a superior model for studying the pathophysiology of HD-induced cutaneous lesions.

Immunohistochemical demonstration of non-muscle myosin in the developing porcine epidermis

Using an immunohistochemical technique, the study demonstrates for the first time non-muscle myosin stress fibres in fetal mammalian epidermis, with special regard to cellular development during integumentary ontogenesis in the sparsely haired domesticated pig. The results obtained are discussed in view of possible cytoskeletal functions of the cytofilaments.

Indirect immunohistochemistry and immunoelectron microscopy distribution of eight epidermal-dermal junction epitopes in the pig and in isolated perfused skin treated with bis (2-chloroethyl) sulfide

Sulfur mustard (bis [2-chloroethyl] sulfide, HD) is a potent cutaneous vesicant that causes gross blisters by separation of the epidermal-dermal junction (EDJ). The EDJ of the skin is a highly specialized and complex structure composed of several components and plays a major role in the integrity of the skin. The isolated perfused porcine skin flap (IPPSF) was dosed with 0.2 mg/ml (n = 4), 5.0 mg/ml (n = 4), and 10.0 mg/ml (n = 5) HD or ethanol (n = 4) for 8 hr (dose-response study) and 10.0 mg/ml HD or ethanol for 1, 3, 5, and 8 hr (n = 4/treatment) (time-response study). Successful EDJ mapping was carried out in normal pig skin (NPS), ethanol-treated IPPSFs, and HD-treated IPPSFs using the following antibodies: laminin, type IV collagen, fibronectin, GB3 (Nicein), bullous pemphigoid (BP), and epidermolysis bullosa acquisita (EBA). Two mouse anti-human monoclonal antibodies, L3d and 19-DEJ-1 (Uncein), did not cross-react with the EDJ of the pig. Antibody staining in NPS, ranging from very intense for laminin and type IV collagen to weak for fibronectin, was generally more discrete than in the IPPSF. No differences in staining were noted between the ethanol and nonblistered areas of the HD-treated IPPSFs. In HD-blistered areas, BP stained only the epidermal hemidesmosomes, and laminin, fibronectin, and GB3 stained primarily the dermis with fragments attached to the basal pole of the stratum base cells, while type IV collagen and EBA stained only the dermis. Mapping of these epitopes determined that the precise plane of EDJ separation in the HD-treated skin occurred beneath the hemidesmosomes within the upper portion of the lamina lucida. The conservation of human epitopes in the EDJ of the pig further emphasizes the similarities between human skin and pig skin. Therefore, pig skin and the IPPSF may be used to study HD-induced vesication and blistering diseases.

Prenatal development of the integument in Delphinidae (Cetacea: Odontoceti)

The prenatal development of epidermis, dermis, and hypodermis was studied in embryos of different age of two delphinid species (Stenella attenuata, Delphinus delphis), using light and transmission electron microscopical methods. The delphinid embryo is covered by a multilayered tissue formed by four different epidermal generations (periderm, stratum intermedium-I, str. intermedium-II, str. spinosum) produced by the str. basale. The first layer appears at about 40-50 mm of body length, the second type (s.i.-I) about 60-160 mm, and the third type (s.i.-II) is present at 160-500 mm. The first spinosal cells are produced at 225-260 mm body length; thenceforth, the epidermis increases continuously in thickness. Epidermal ridge formation begins about 400-mm body length. The development of the dermis is characterized by the early production of thin connective tissue fibers (40-70-mm body length) and simultaneously the cutaneous muscle matures in structure. Vascular development intensifies between embryos of 150-225 mm, and collagen production increases markedly in fetuses of 225-260-mm length. These events are paralleled by an increase in dermal thickness. The first elastic fibers can be recognized in the skin from the abdomen at about 600-mm body length. The development of the hypodermis is marked by very rapid and constantly progressing growth, beginning about 60-mm body length. The first typical fat cells appear in animals of 360-400 mm. Regional differences are obvious for all skin layers with regard to the flippers, where structural maturation proceeds more rapidly than in dorsal or abdominal regions.

[Differentiation of the ruminal epithelium of cattle during intrauterine development]

The prenatal development of the bovine ruminal epithelium was studied with light- and electronmicroscopical techniques. During the period of the nonstratified epithelium a pseudostratified epithelium is found in the dorso-cranial part, whereas the other areas possess a one-layered epithelium, which is, like the pseudostratified epithelium, transformed to a multilayered epithelium from the 7th week onwards. From the 9th week the period of the stratified epithelium starts with the formation of the stratum profundum and stratum superficiale. First signs of keratinization are seen in the superficial cells from 2.3 months onwards. With 4 months fetal cornified cells can be identified, with 5.5 months a single-layered stratum basale is seen on the differentiating papillar connective tissue, and the superficial cells are transformed to balloon-cells. In suprapapillar areas, a stratum spinosum is formed at the prenatal age of 7.5 months. During epitheliogenesis a horizontal and vertical differentiation of the cells can be observed. The first one includes the differentiation of undifferentiated, embryonal cells to the basal cells of the stratum profundum, the latter the development of the basal cells to spinous cells and then to fetal cornified and balloon-cells. The ultrastructural changes during the process of keratinization were especially considered.

Tissue expansion in pig skin--a histochemical approach

In the present study, a porcine model for controlled skin expansion was investigated to improve our understanding of epidermal and vascular responses following stretching. The model is of outstanding importance not only for the clinical use of tissue expansion but provides interesting data for skin physiology and oncology, too. Thirteen out of 15 animals, who underwent silicone tissue expander implantation showed good clinical results. In all of them, skin biopsies were taken at the end of a controlled tissue expansion procedure (final expander volumes 350 or 500 ccm): one tissue specimen was obtained from the centre of the expanded skin area and a second from the neighbouring but nonexpanded skin. The tissue specimens were immediately frozen in liquid nitrogen and processed to 4 microns thick acetone-fixed frozen sections. Lectin histochemistry and immunohistology were performed using the following techniques: direct and indirect immunofluorescence technique (DIFT, IIFT), immunoperoxidase technique (POX) with either 3,3'-diamino-benzidine (DAB) or 3-amino-9-ethyl-carbazole (AEC). The histochemical findings were supplemented by measurements of the number of vital epidermal cell layers, the epidermal thickness (microns), and the papillary vascular count per visual field. There was a significant diminuation of the vascular count (mean +/- S.D. = 55.0% +/- 12.5%; U-test: p less than 5%). By immunohistochemistry, a loss of the basal cell reactivity for the following antibodies was noted: ACAM (against calmodulin), K 8.12 (against keratins 13 +/- 16) and A51-B/H4 (against keratins 8, 14, 18). There was a remarkable increase of filaggrin expression in the uppermost spinal cell layers in expanded skin, which was most pronounced in those specimens with the shortest interval to the last fluid injection into the expander. We gained no evidence for alterations of the expression of suprabasal epidermal keratins, lectin binding sites (UEA I, PNA, ConA, WGA), and vascular lectin- and immunoreactivity due to tissue expansion. The subdermal capsule, which had formed around the silicone expander, was strongly vimentin-reactive. In conclusion, controlled tissue expansion is capable to change the basal cell phenotype--a feature which is shared with a number of conditions with increased proliferative activity and with the epidermis covering different skin tumours. The regular expression of suprabasal keratins and epidermal lectin binding sites provides evidence for a normal epidermal cell differentiation. Furthermore, the porcine skin is a reliable model for studying physiology and pathophysiology of human skin.

Histochemistry of the porcine pilosebaceous unit

The present study describes lectin and immunoreactivity in the pilosebaceous unit of porcine skin. Complex carbohydrates of mucin and biantennary Man/Gluc types were distributed among hair follicle epithelia (hair root sheaths, cuticula, shaft, and shaft matrix). Sebaceous glands expressed biantennary Man/Gluc carbohydrates and GalNAc residues. The expression of simple-type and epidermis-like keratins was confirmed by immunohistochemistry with monoclonal antibodies. Filaggrin-positive cells were found in the keratinizing zone of Henle's layer in anagen follicles. The innermost layer of the outer hair root sheath was stained with antibodies against the epidermal growth factor-receptor, keratin 10 and Ki67 antigen. The differences to humans were remarkably small.

[The deciduous hoof capsule (Capsula ungulae decidua) of the equine fetus and newborn foal]

The term Eponychium is used to describe the deciduous hoof capsule in veterinary-medical and embryological literature. In other aspects of veterinary medicine, the term is generally reserved for the perioplic corium of the permanent hoof. In order to clarify this double usage, the structure and origin of the hoof epidermis from 10 equine fetus at different stages of development and 4 newborn foals were investigated and described using light microscopical techniques. Epidermal tubules and lamellae are already present in the non-cornified fetal hoof epidermis. These structures, along with the formation of a white line, allow this epidermis to be divided into the same segments as are commonly used when referring to the permanent hoof. The greatest part of the deciduous hoof epidermis consists of the sole and frog, with significant portions forming in the coronary corium and that of the hoof wall as well. The perioplic corium only makes up insignificant portions of the fetal hoof capsule, however. Between the second half of the gestation period and birth, this deciduous (primary) hoof capsule is continually being replaced from below by newly-forming, cornified permanent hoof capsule. On the basis of its structure and origin, we therefore suggest that the term "deciduous hoof capsule (Capsula ungulae decidua)" be used as a replacement for the word "Eponychium" when referring to the primary hoof epidermis.

Immunohistochemistry of porcine skin

The present paper reports immunohistological findings in porcine skin, which were obtained by use of mono- and polyclonal antihuman antibodies and either alkaline phosphatase anti-alkaline phosphatase (APAAP) or peroxidase (POX) technique. Epidermal staining was observed with antibodies to keratins (K 8.12, RSKE 60), filaggrin, and calmodulin (ACAM). Staining of connective tissue and vessels was achieved using antibodies to vimentin (V9(1)), collagen type IV, and fibronectin. In general, these antibodies gave a staining pattern similar to that of normal human skin. The similarities of immunoreactivity to poly- and monoclonal antihuman antibodies in porcine and human skin render porcine skin a reliable model in biomedical research.