Proliferative and Non-Proliferative Lesions of the Rat and Mouse Integument (PDF Download Available)
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of articles inArticle · Literature Review with 90 ReadsDOI: 10.1293/tox.26.27S · Source: + 720.9825.8Charles River Laboratories International, IncShow more authorsAbstractThe INHAND (International Harmonization of Nomenclature and Diagnostic Criteria for Lesions in Rats and Mice) project is a joint initiative of the societies of toxicological pathology from Europe (ESTP), Great Britain (BSTP), Japan (JSTP) and North America (STP). Its aim is to develop an internationally-accepted nomenclature for proliferative and non-proliferative lesions in laboratory rodents. A widely accepted international harmonization of nomenclature in laboratory animals will decrease confusion among regulatory and scientific research organizations in different countries and will provide a common language to increase and enrich international exchanges of information among toxicologists and pathologists. The purpose of this publication is to provide a standardized nomenclature for classifying microscopical lesions observed in the integument of laboratory rats and mice. Example colour images are provided for most lesions. The standardized nomenclature presented in this document and additional colour images are also available electronically at http://www.goreni.org. The nomenclature presented herein is based on histopathology databases from government, academia, and industrial laboratories throughout the world, and covers lesions that develop spontaneously as well as those induced by exposure to various test materials. (DOI: 10.1293/tox.26.27S; J Toxicol Pathol S-57S).Discover the world's research14+ million members100+ million publications700k+ research projects
J Toxicol Pathol 2013; 26 (3 Suppl): 27S–57S27SReviewProliferative and Non-Proliferative Lesions of the
Rat and Mouse IntegumentLars MeckLenburg1, Donna kusewitt2, carine koLLy3, siLke treuMann4, e. terence aDaMs5, keLLy DiegeL6,
JyoJi yaMate7, woLfgang kaufMann8, susanne MüLLer9, DiMitry DaniLenko10, anD aLys braDLey111 mecklenburg-consulting, Hamburg, Germany2 UT MD Anderson Cancer Center, Smithville, Texas, USA3 Novartis Pharma AG, Basel, Switzerland4 BASF SE, Ludwigshafen, Germany5 Experimental Pathology Laboratories, Inc, Research Triangle Park, North Carolina, USA6 Hoffmann La-Roche Nonclinical Safety, Nutley, New Jersey, USA7 Osaka Prefecture University, Osaka, Japan8 Merck Serono Research & Development, Darmstadt, Germany9 Boehringer Ingelheim Pharma GmbH & Co. KG, Biberach, Germany10 Genentech, South San Francisco, California, USA11 Charles River, Tranent, Edinburgh, UKabstractThe INHAND (International Harmonization of Nomenclature and Diagnostic Criteria for Lesions in Rats and Mice) project is a joint initiative of the societies of toxicological pathology from Europe (ESTP), Great Britain (BSTP), Japan (JSTP) and North America (STP). Its aim is to develop an internationally-accepted nomenclature for proliferative and non-proliferative lesions in laboratory rodents. A widely accepted
international
harmonization
of nomenclature
in laboratory
will decrease
regulatory and scienti?c research organizations in different countries and will provide a common language to increase and enrich international exchanges of in-formation among toxicologists and pathologists. The purpose of this publication is to provide a standardized nomenclature for classifying microscopical lesions observed in the integument of laboratory rats and mice. Example colour images are provided for most lesions. The standardized nomenclature presented in this document and additional colour images are also available electronically at http://www.goreni.org. The nomenclature presented herein is based on histopathology databases from government, academia, and industrial laboratories throughout the world, and covers lesions that develop spontaneously as well as those induced by exposure to various test materials. (DOI: 10.1293/tox.26.27S; J Toxicol Pathol 2013; 26: 27S–57S)Keywords: diagnostic pat histopat nomenclatu rodent pathology; integumentary system; skin; rodent.introDuctionThis manuscript provides a standardized nomenclature for classifying microscopic lesions observed in the skin and its appendages, namely hair follicles, sebaceous glands, apocrine glands and eccrine glands. It does not cover claws. Lesions in the Zymbal’s gland, mammary gland, preputial and clitoral gland are covered in a separate manuscript (Rudmann et al., 2012). Lesions in the dermis are covered only if not discussed in the manuscript about soft tissue (Greaves et al., 2013), in the manuscript about the nervous system (Kaufmann et al., 2012) or in the manuscript about the cardiovascular system (Berridge et al., in preparation).Ontology, anatomy and function of the rodent skinThe integument derives from the embryonal ectoderm and the underlying primitive mesenchyme. Its ontogeny depends Financial disclosure: No money was paid for the preparation of this manuscript. During preparation of this manuscript, salaries of contribu-tors were paid by their respective employer. None of the content of the manuscript contains any information that could be patentable or claimed as intellectual property of the contributors or their respective companies.Address correspondence to: Lars Mecklenburg, mecklenburg-con-sulting, Saseler Chaussee 17, 22391 Hamburg, Germany. e-mail: lm@mecklenburg-consulting.de(C)2013 The Japanese Society of Toxicologic PathologyThis is an open-access article distributed under the terms of the Cre-ative Commons Attribution Non-Commercial No Derivatives (by-nc-nd) License &http://creativecommons.org/licenses/by-nc-nd/3.0/&.Abbreviations: BSTP, British Society of Toxicological Pathologis EMA/CHMP, European Medicines Agency/Committee for Medicinal Products for Human U ESTP, European Society of Toxicologic Pa-thology; FDA/CDER, Food and Drug Administration, Center for Drug Evaluation and Resear H&E, hematoxylin and eosin; INHAND, Inter-national Harmonization of Nomenclature and Diagnostic Criteria for Le-sions in Rats and Mice; JSTP, Japanese Society of Toxicologic Pathology; OECD, Organisation for Economic Co-operation and Development; STP, Society of Toxicologic Pat Tg.AC, zetaglobin promoted v-Ha-ras.
MECKLENBURG ET AL.28Son a multitude of complex ectodermal-mesenchymal interac-tions and is further in?uenced by the migration of neuroecto-derm-derived melanocytes. The integument is considered the largest organ of the body and functions as a barrier against the environment. As such it protects from external chemical, physical and microbiological agents. In addition, the skin and its appendages function as a reservoir of electrolytes, water, vi-tamins, lipids, carbohydrates, and proteins. The skin possesses immunological and metabolic competence and is essential for the production of vitamin D.The basic anatomy of the rodent skin is largely comparable to that of other mammals, but species-speci?c differences need to be taken into account when evaluating the morphology of the skin and its appendages in biomedical research. The reader is referred to standard texts on mouse and rat anatomy (Krinke, 2004; Hofstetter et al., 2006).The number of transgenic, conditional, and spontaneous mutant mice that display skin and hair abnormalities is increas-ing rapidly (Nakamura et al., 2001; Nakamura et al., 2002). The researcher analyzing such mice is referred to detailed descrip-tions of mouse skin and hair follicle anatomy (Paus et al., 1999; Yamanishi, 1998; Mueller-Roever et al., 2001; Sundberg et al., 2005) and to general reviews about the phenotyping procedure itself (Zeiss et al., 2012).Common diseases of the rodent skinBased on its direct interaction with the environment, the skin is subject to many spontaneous and housing-related dis-eases. This emphasizes the importance of husbandry, particu-larly if dealing with delicate animal models. The pathologist evaluating the integument from experimental studies in ro-dents should be aware of such conditions. Morphological le-sions should be described using the nomenclature listed herein. However, spontaneous diseases of the rodent skin should not be diagnosed on the histomorphology of the lesions alone but in combination with clinical data. In a pathology report, the disease terms listed below could be used as ‘syndromes’ to summarize and interpret morphological lesions.Alopecia: There are several strains of mice and fewer in rats that display a congenital form of hair loss, due to abnormal hair follicle formation. The hallmark morphological lesion is ‘abnormal development’ of hair follicles. A detailed descrip-tion of these forms of alopecia is beyond the scope of this manuscript, and the reader is referred to textbooks and reviews covering this issue (Sundberg, 1994; Nakamura et al., 2001). Alopecic mice frequently used in biomedical research are hair-less mice (Hr) and nude (Foxn1/nu) mice. Several mutations in the hairless (Hr) gene, encoding a transcriptional co-repressor, have
hairlessness
in homozygous animals. Outbred SKH1 mice are the most widely used hairless mice. Alopecia develops after a single cycle of relatively normal hair growth and is caused by dysplasia of hair follicles, which lose their ability to form hair shafts and transform into large intradermal cysts (Benavides et al., 2009). Mutations in the nude (Foxn1) gene, encoding a member of the winged helix/forkhead family of transcription factors, lead to macroscopic nudity and thymic dysgenesis in mice and rats. Relatively normal hair follicles develop that still produce hair shaf however, presumably because of a lack of some hair keratins, the hair shafts that are generated twist and coil in the hair follicle infundibulum, which becomes dilated. Because hair shafts fail to penetrate the epidermis, the animals appear alopecic (Mecklenburg et al., 2001).Hair loss in haired rodents can also occur due to external trauma
in?amma-tory or degenerative processes that impair formation of new hair shafts. It is not always easy to distinguish between these two processes and detailed clinical or histopathological inves-tigations may be needed (Mecklenburg, 2009). Alopecia due to barbering occurs frequently in group-housed mice. It occurs mostly in males, and less dominant animals are primarily af-fected. Genetic components may also be involved (Kalueff et al., 2006). Overcrowding needs to be considered as a contribut-ing factor in dominance bevavior (Kurien et al., 2005; Kalueff et al., 2006). Mechanical denuding of facial hair is a conse-quence of improperly constructed feeder openings or water-ing devices and is a differential diagnosis for barbering (Percy and Barthold, 2007). Hair loss due to external trauma is mor-phologically characterized by a lack of hair shafts from hair follicle infundibula outward and is frequently associated with erosion/ulcer of the epidermis and in?ammation in the dermis.C3H mice are known to develop a spontaneous follicular in?ammation
follicular
resembles alopecia areata in humans (McElwee et al., 2003).Ulcerative dermatitis: Chronic ulcerative dermatitis in mice can result from barbering in group-housed mice, but it can also be caused by self-trauma/overgrooming. In the C57BL/6 sub-strain, chronic ulcerative dermatitis is a common skin problem. Female animals are predisposed, and there is marked seasonal variation in the disease frequency with a peak in spring and fall. Furthermore, the severity and prevalence of the disease within a colony are dependent on nutritional and husbandry factors. Lesions occur along the dorsum and in the cervico-thoracic area. Microscopically, there is epidermal erosion/ulcer with crusting and dermal in?ammation. A hypersensitivity re-action has been proposed as pathogenesis (Kastenmayer et al., 2006), although recent investigations suggest follicular devel-opmental abnormalities as the primary pathogenesis with sec-ondary rupture of severely affected follicles. The dermis may contain granulomatous in?ammation (Sundberg et al., 2011). A secondary infection with Staphylococcus spp. or Streptococ-cus
associated
neutrophilic
in?ammation. Ulcerative dermatitis is less common in rats than in mice, and has mainly been described in the Sprague Dawley strain (Ash, 1971; Fox et al., 1977; Wagner et al., 1977).Ulcerative pododermatitis in rats: Epidermal erosion/ulcer at the footpads occurs frequently in rats housed in wire cages, especially in strains of rats with higher body weights, such as
Nomenclature for Rodent Integument 29Ssome diabetic phenotypes. Initial traumatic abrasions and sec-ondary infections may lead to dermal granulomatous in?am-mation of the foot pads, known as “bumblefoot” (Morrow et al., 1977).Necrotizing dermatitis of the tail: A ring-like constriction of the tail skin with dermal hemorrhage, thrombosis, edema and full-thickness necrosis is known as ‘ringtail’ in mice and rats. Epidermal hyperkeratosis and secondary infection oc-cur frequently. This condition has historically been associated with low environmental humidity (&40%) and high tempera-tures (&80°F/27°C). Poor diet, hydration status, genetic suscep-tibility, and other predisposing factors may also be involved (Lawson and Churchman, 1993; Crippa et al., 2000; Percy and Barthold, 2007).Auricular chondritis: A granulomatous in?ammation in the dermis in the ear pinnae associated with chondrolysis has been described in rats (McEwen and Barsoum, 1990). The ear pin-nae present with intradermal nodules and crusting or erosion/ulcer of the overlying epidermis. The etiology is unknown.Dermatophytosis (syn. Favus, ringworm): Dermatophytes such as Trichophyton spp. and Microsporum spp. can infect mice and rats (Balsari et al., 1981) and might be transmitted to humans. Although most infections are subclinical, alopecia, crusting, and
dermal in?ammation may occur. In?ammatory cell in?ltration of the hair follicle is present and dermatophytes are recognized as arthrospores and hyphae along the hair shafts. The organisms can be stained with periodic acid-Shiff (PAS) or GMS.Demodicosis: Mice and rats are susceptible to Demodex musculi, although infestations with clinical symptoms are very rare in immunocompetent animals (Walberg et al., 1981; Hill et al., 1999). However, clinical disease may occur in transgenic mice lacking mature T cells and NK cells (Percy and Barthold, 2007). The hallmark lesion is in?ammation of hair follicles and dermis in association with elongate mites that are easily de-tected within the follicular infundibulum.Acariasis: Fur mites include Myobia musculi (most clini-cally signi?cant), Radfordia af?nis, and Myocoptes musculinis in mice and Radfordia ensifera in rats (Csiza and McMartin, 1976; Jungmann et al., 1996; Peper, 1994). Myocoptes muscu-linis is the most common fur mite and frequently occurs as a mixed infestation with Myobia musculi (Percy and Barthold, 2007). Mite infestation leads to hypersensitivity resulting in gross clinical manifestations that vary from focal alopecia to widespread ulcerative dermatitis. Lesions are mostly located along the dorsum, including the head and are characterized by dermal
in?ammation,
hyperplasia
and hyperkeratosis. Mites are easily found within the stratum cor-neum.Hyperkeratosis in nude mice: Nude mice infected with Corynebacterium bovis may develop a diffuse orthokeratotic epidermal hyperkeratosis associated with a sparse super?cial dermal in?ammation. Small gram-positive rods
dem-onstrated in the hyperkeratotic stratum corneum (Clifford et al., 1995).Ectromelia in mice: The Ectromelia virus is the cause of mousepox, an epizootic disease with high mortality. The vi-rus is transmitted via direct animal-to-animal contact and cu-taneous trauma. While some strains such as C3H, DBA and BALB/c are highly susceptible to the virus, others such as C57BL/6 are not. Infected animals present with an acute swell-ing of the extremities caused by follicular necrosis and dermal in?ammation. The necrotic keratinocytes contain pale, slightly eosinophilic intracytoplasmic inclusion bodies (Wallace and Buller, 1985; Dick et al., 1996).Dermatotoxicology in rodentsBecause of its function as an outer barrier, the skin is ex-posed to a wide variety of toxic agents. These can harm the skin directly, causing irritation or corrosion, or can cause im-mune-mediated toxic effects. A special mechanism of toxicity is phototoxicity, which is due to the interaction of a toxic agent with ultraviolet light (Haschek et al., 2010).Direct toxicity is caused by direct cell damage of epidermal keratinocytes or by a direct activation of the effector pathway of the immune system. It is often referred to as ‘non-immuno-logical activation’, since it occurs independently of antibodies. Activation of mast cells or complement or prostaglandin syn-thesis results in reversible damage to the skin referred to as ‘irritation’. It usually occurs within 4 hours following topical application of the irritating substance. Histologically, skin irri-tation is characterized by dermal in?ammation and epidermal hyperkeratosis and hyperplasia, associated with variable other epidermal changes such as erosion/ulcer, necrosis or vesicle. If the damage to the skin is irreversible, the lesion is clinically referred to as ‘corrosion’. Corrosion is characterized by full thickness necrosis of the epidermis penetrating into the under-lying dermis.Immune-mediated cutaneous toxicity can be subdivided into anaphylactic reactions mediated by IgE antibodies (type I hypersensitivity), immune-complex reactions mediated by IgG or IgM antibodies (type III hypersensitivity), and delayed-type (type IV) hypersensitivity reactions mediated by effector T cells. While type I and type III hypersensitivity reactions are mostly caused by toxic agents that are taken up systemically, type IV hypersensitivity reactions usually occur with toxic agents in direct contact with the skin. Type IV hypersensitiv-ity reactions in the skin are frequently referred to as ‘allergic contact dermatitis’ and present clinically with pruritus, ery-thema, edema, and development of papules, vesicles or bullae. The ability of a compound to initiate type IV hypersensitivity (‘skin sensitization’) is usually investigated in the guinea pig (OECD, 1992); however, the local lymph node assay in mice is gradually replacing traditional hypersensitivity testing in
MECKLENBURG ET AL.30Sguinea pigs (OECD, 2010a, 2010b, 2010c). Skin sensitization lesions are characterized by dermal in?ammation and various epidermal changes ranging from hyperplasia to erosion/ulcer. These lesions are not reliably different from those seen with skin irritation by histomorphology alone.Additional forms of immune-mediated dermal toxicity in-clude ‘erythema multiforme’ and ‘toxic epidermal necrolysis’. These represent a continuum of reactions, presumably caused by cytotoxic T cells that induce apoptosis or necrosis of epider-mal keratinocytes (Haschek et al., 2010). The predominant his-tological ?nding is epidermal necrosis, either single cell type (erythema multiforme) or full thickness type (toxic epidermal necrolysis).Phototoxicity can occur due to direct reaction of a toxic agent with ultraviolet light or indirectly if the toxic agent alters endogenous proteins thus making them reactive with ultraviolet light. Phototoxicity can be immune-mediated (‘photoallergy’) or non-immunological (‘photoirritation’). For pharmaceutical chemicals, the assessment for any need to conduct experi-mental photosafety testing is mandatory in both the European Union (CPMP/SWP, 2002) and the United States of America (FDA/CDER, 2003). Although there are in vitro test systems to determine the general reactivity of a chemical with UV light, in vivo testing in hairless mice may still be necessary. This is particularly true when testing the potential for carcinogenesis due to a chemical in association with ultraviolet light (‘pho-tocarcinogenesis’) (Forbes, 1996). Histologically, ultraviolet light-induced skin lesions are not distinguishable from other toxic skin changes.Testing of topically applied chemicals for acute dermal ir-ritation/corrosion is typically performed in rabbits (OECD, 2002). However, as outlined in the supplement to the OECD test guideline 404 on dermal irritation/corrosion testing (OECD, 2002), consideration of existing data, structure activ-ity relationships, physiochemical properties of the test item and testing in validated in vitro and ex vivo systems are recom-mended, before an in vivo study is conducted. Several in vitro testing methods for skin corrosion have been validated such as the transcutaneous electrical resistance test, the human skin model test, and the reconstructed human epidermis test. Acute and chronic cutaneous and systemic toxicity of a topically ap-plied chemical is usually investigated in rats, rabbits or guinea pigs (OECD, 1987, 1981a, 1981b), although minipigs are now becoming more frequently used, due to the high degree of mor-phological and physiological similarity between porcine skin and human skin (Mahl et al., 2006).The carcinogenic risk of a chemical after its topical admin-istration is traditionally investigated in rats. In recent years, however, the Tg.AC (zetaglobin promoted v-Ha-ras) transgenic mouse has become a popular alternative to the rat. The skin of the Tg.AC mouse is genetically initiated, thus the induction of epidermal papillomas in response to dermal or oral exposure to a chemical agent acts as a reporter phenotype for the car-cinogenicity of the test chemical. In Tg.AC mouse bioassays, the test agent is typically applied topically for up to 26 weeks, and the number of tumors in the treated area is counted weekly (Dunson et al., 2000). If chemicals are applied topically to the skin, their effect is in?uenced not only by their primary mode of toxicity but also by the processes of absorption and metabo-lism. The outer surface of the skin has an oily coating of sebum from the sebaceous glands which forms a barrier to polar wa-ter soluble compounds. Lipophilic nonpolar compounds more readily pass through the surface epithelium and the intercel-lular spaces, and can also enter through the hair follicles and sebaceous glands. Absorption is further affected by the integ-rity and thickness of the epidermis and dermal vascularization. Microsomal enzymes in keratinocytes are able to metabolize topically applied chemicals, thus rendering them inactive or active. For instance, dimethylbenz(a)anthracene (DMBA) be-comes a potent skin carcinogen after metabolic activation by keratinocytes (Peckham and Heider, 1999). Some systemically administered chemicals, particularly some antiproliferative an-ticancer
and cytokines,
can induce skin in?ammation, atrophy and necrosis (Greaves, 2000).For neoplasms in the epidermis and cutaneous adnexa, it might be dif?cult to determine whether they are spontaneous or related to treatment. In that case, combining the incidence of different tumors with similar histogenesis might provide addi-tional value, particularly if progression from benign to malig-nant neoplasms is known such as for squamous cell papilloma/carcinoma, basal cell tumor/carcinoma or sebaceous gland ad-enoma/carcinoma (Bruner et al., 2001; Brix et al., 2010).Alopecia, i.e. the loss of hair, is a frequent ?nding in rodent toxicity studies. Although it can be caused by excessive bar-bering or other external trauma (see Common diseases of the rodent skin), alopecia can also be a sequel of systemic toxicity induced by insuf?cient supply of nutrients or due to hair cycle disturbances caused by hormonal dysregulation. While the un-derlying pathogenesis cannot always be determined, histologi-cal examination of the alopecic skin will usually differentiate among an abnormality in hair follicle cycling, atrophy of cuta-neous adnexa and in?ammation or necrosis of the hair follicle. Hypertrichosis, i.e. increased occurrence of hair (also known as ‘hirsutism’), is not covered in this manuscript, because it rarely occurs in rodent toxicity studies. However, in nude mice, visible hair growth can be stimulated by treatment with cy-closporine (Sawada et al., 1987) or recombinant keratinocyte growth factor (Danilenko et al., 1995).noMencLatureIt is common practice in diagnostic dermatopathology to approach cutaneous lesions histopathologically by a method known as `pattern analysis` (Ackerman, 1978). The main as-pect of this approach is the use of a scanning magni?cation (i.e. 1.0 – 2.5x objective) followed by a deliberate, orderly, logical description of the lesion. The latter is facilitated by an analy-sis and description of the individual anatomic compartments of the skin. Despite the difference in objectives, this approach is also extremely helpful in experimental dermatopathology. Therefore,the nomenclature that is suggested to be used for the description of lesions in the rat and mouse integument is sepa-
Nomenclature for Rodent Integument 31Srated into the compartments ‘EPIDERMIS’, ‘CUTANEOUS ADNEXA’, and ‘DERMIS AND SUBCUTIS’.Normal morphologySKIN – EPIDERMISEpidermal thickness varies greatly among species. Even within one species, the epidermal thickness will vary between body regions, and within one region will vary depending on physiological parameters such as the hair cycle stage and gen-der (Hansen et al., 1984; Azzi et al., 2005). Mouse epidermis is approximately 10 um thick, and rat epidermis is 10 to 20 um thick. Human epidermis is 50 to 120um thick. The greater epidermal thickness in man is due to the presence of more cell layers. Rete pegs, a prominent feature of human epidermis, are not observed in normal rodent skin. Rodent and human skin also differ with respect to skin adnexae. Human haired skin contains both eccrine and apocrine glands, while these struc-tures are absent from the haired skin of rats and mice.The epidermis is derived from ectoderm. The epidermis is a strati?ed epithelium characterized by a unique differentiation process known as keratinization (syn: corni?cation). Keratini-zation is due to formation of intracellular tono?laments, com-posed of cytokeratins. Cytokeratins are usually found in pairs of a type I cytokeratin and a type II cytokeratin. The composi-tion of cytokeratins changes during the differentiation process. As an example, epidermal basal cell keratinocytes express the cytokeratin pairs 5 and 14, while suprabasal epidermal kera-tinocytes lose these cytokeratins, but express cytokeratins 1 and 10 instead (Galvin et al., 1989). Cells are continuously lost from the epidermal surface by desquamation, and new cells are continuously produced in the basal epidermal layer by prolif-eration of transit amplifying cells which arise from stem cells and divide a ?nite number of times until they become differ-entiated. The basal layer of the epidermis is connected to the underlying connective tissue via a basement membrane and adhesion structures called ‘hemidesmosomes’. Differentiating cells from the stratum basale move upwards into the stratum spinosum, which represents the thickest layer of the normal human epidermis. Keratinocytes within the stratum spinosum change
polyhedral
up-ward. With further differentiation, keratinocytes develop in-tracellular keratohyalin granules. These granules contain the protein
cross-linking
cyto-keratin ?laments. Keratohyalin granules are readily visible in H&E-stained histological sections and are characteristic of the stratum granulosum. As cross-linking of cytokeratin ?laments occurs,
transformed
‘corni?ed envelope’, the nucleus vanishes, and lipid-containing lamel-lar
bodies are
?ll the intercellular spaces of the stratum corneum and surround terminally differentiated keratinocytes (corneocytes). Corneo-cytes eventually desquamate into the environment. Keratino-cytes within the stratum basale, stratum spinosum and stratum granulosum are connected to eachother via desmosomes and adherens junctions. Because the normal epidermis of rodents is only 2-4 cells thick, the stratum spinosum and stratum granu-losum are not usually visible; however, these layers become evident in hyperplastic epidermis.Interspersed among keratinocytes are antigen-presenting cells (‘Langerhans cells’), Merkel cells (neuroendocrine cells interacting with nerve endings), T cells, and melanocytes (Peckham and Heider, 1999). The epidermis has an impor-tant function in the skin immune system (Loser and Beissert, 2007). There are known species differences with regard to the distribution of immunocompetent cells within the epidermis. Rodents, for example, exhibit many gamma/delta T-cells in epithelia such as the epidermis, while in humans these cells predominate in lymph nodes, not the epithelium (Salerno and Dieli, 1998).Local epidermal thickenings in mouse skin that are asso-ciated with a tylotrich hair follicle and that are composed of many Merkel cell-neurite complexes are known as ‘Haars-cheibe’ (Smith, 2004) (Figure 1).Melanocytes in the skin of mice are located in the epidermis in some glabrous areas of the body and within hair follicles of the trunk. They derive from neural crest cells that migrate to the epidermis in early development and become interdigi-tated between keratinocytes. These cells reside only brie?y in the epidermis of the trunk (Noonan et al., 2000). Melanoblasts may also reside in the dermis where they can proliferate and cause pigmented neoplasms (Kanno, 1989).SKIN – CUTANEOUS ADNEXAThe skin would not be able to ful?ll its numerous functions without cutaneous adnexa. Cutaneous adnexa all derive from the ectodermal component of the skin and consist of hair fol-licles, sebaceous glands, apocrine glands, and eccrine glands. Claws represent further cutaneous adnexa in rodents and are the analogues of nails in humans. For reporting purposes, it is recommended to use ‘cutaneous adnexa’ as a primary locator and ‘hair follicles’, ‘sebaceous glands’, ‘apocrine glands’, ‘ec-crine glands’ and ‘claws’ as secondary locators.Mice and rats have up to 8000 hairs per cm2 of skin. Hair follicles can be divided into primary hair follicles (also referred to as ‘tylotrich’ or ‘guard’ hair follicles) and secondary hair fol-licles (also referred to as ‘non-tylotrich’ or ‘wool’ hair follicles). Primary hair follicles have large sebaceous glands, prominent innervations, a prominent blood supply, and they are associated with a focal epidermal thickening, known as the ‘Haarscheibe’. In contrast, secondary hair follicles are smaller and have small sebaceous glands. Eccrine glands in rodents are restricted to the foot pads.In the rodent, secondary hair follicles are the predominant type (about 70%) in animals that are kept indoors (Meyer, 2009). If evaluating morphological defects of hair follicles, it is important to understand the morphology of hair follicle morphogenesis and cycling, which has been described in de-tail (Paus et al., 1999; Yamanishi, 1998; Mueller-Roever et al., 2001; Sundberg et al., 2005). The hair cycle may also func-
MECKLENBURG ET AL.32Stionally in?uence the skin response towards topically applied chemicals (Argyris, 1963). Hair shafts that are produced by hair follicles in mice are divided based on their morphology into zigzags, auchenes, awls and monotrichs (Dry, 1926). In addition to the primary and secondary hairs of the trunk, ro-dents have a number of specialized hair follicle types with unique histomorphologic features. Included in these special-ized hair follicle types are vibrissae (large tactile hairs of the face/ muzzle featuring blood sinuses and trigeminal nerve in-nervation), cilia (eyelashes, hairs with specialized sebaceous glands called ‘Meibomian glands’), perinanal/ genital hairs (also larger with unique sebaceous secretions functioning in marking/ territorial scent behaviors), tail hairs, and possibly others (Sundberg, 1994).Hair follicles can be divided into three morphologically and functionally different segments: The upper segment is repre-sented by the follicular infundibulum, which is composed of a central lumen that is ?lled with hair shafts and keratin and that is lined by a strati?ed squamous epithelium which is continu-ous with the interfollicular epidermis. The middle segment is known as the ‘isthmus’. It is a very complex and still not com-pletely understood structure that plays an important role in hair follicle cycling. The isthmus is lined by an epithelial sheath with distinct tricholemmal keratinization and contains the opening of the sebaceous gland duct. The deepest segment is known as the ‘inferior portion’. It is again a very complex structure that contains the hair bulb at its lower end. The hair bulb is com-posed of epithelial matrix cells. These cells produce the hair shaft and encircle mesenchymal cells of the dermal papilla. Matrix cell differentiation and upward migration build the in-ner root sheath and the hair shaft, while the outer root sheath is supposedly formed from basal cells deriving from the isthmus region. While the upper segment remains largely unchanged throughout the hair cycle, the middle segment undergoes strik-ing morphological changes throughout the hair cycle, and the lower segment disintegrates and almost completely vanishes during the catagen and telogen phase, with complete separation of the dermal papilla from the epithelial matrix cells. During the course of the synchronized hair cycle in rodents, there is a marked change in skin thickness.Sebaceous glands derive from the follicular isthmus and are composed of a glandular and a ductal component. The duct is lined by keratinizing squamous epithelium. Peripheral glandu-lar cells are basaloid. They differentiate towards the glandular centre by accumulating lipid droplets within their cytoplasm. Cell rupture and disintegration result in ‘holocrine’ secretion of sebum.Rodents do not have thermoregulatory apocrine sweat glands. Eccrine glands in mice and rats are limited to the foot-pads (Peckham and Heider, 1999; Taylor et al. 2012).Lesions of other specialized cutaneous adnexa such as Zymbal’s gland, auditory sebaceous gland, and circumanal gland are covered in other manuscripts of the INHAND initia-tive.SKIN – DERMIS AND SUBCUTISDermis and subcutis (syn. hypodermis, subdermis) are of mesodermal origin. They provide the tensile strength to the skin,
responsible
elasticity
?ex-ibility. The extracellular matrix of the dermis mainly consists of type I,
V, and VI collagen ?bers, accompanied
by re-ticular
ground substance (composed of glycosaminoglycans, proteoglycans, hyaluronic acid, and dermatan sulphate). The extracellular ma-trix
?broblasts
and nerves. The hypodermis is characterized by densely-packed adipocytes.Morphological lesions in the dermis and subcutis are simi-lar to lesions in other soft tissues. Therefore, the reader is re-ferred to the manuscript on proliferative and non-proliferative lesions of the rat and mouse soft tissue, skeletal muscle and me-sothelium (Greaves et al., 2013). Vascular lesions in the dermis are covered by the manuscript on cardiovascular abnormalities in rats and mice (Berridge et al., in preparation).Non-proliferative lesionsSKIN – EPIDERMISAtrophy, epidermal (Figure 2)Synonyms: Epidermal thinning.Pathogenesis/cell of origino
Result of reduced basal cell turnover.Diagnostic featureso
noncorni?ed
epidermal layers.o
Reduced number and size of nucleated epidermal ke-ratinocytes.Differential diagnoseso
super?cial
cell layers (erosion) or complete loss of epidermis (ulcer-ation).o
Necrosis, epidermal: Loss of cellular detail.Comment:
laboratory
to detect, because the normal epidermis in mice and rats is only 2 to 4 cell layers. Atrophy of the epidermis can occur in association with underlying expansile masses or with chemicals such as corticosteroids that reduce the metabolic activity of epidermal keratinocytes. Par-tial ischemia and severe malnutrition have also been cited as possible causes of epidermal atrophy. Epider-mal atrophy has been described as a spontaneous ?nd-ing in B6C3F1 mice. (Hargis and Ginn, 2007; Slaga et al., 1975; Peckham and Heider, 1999)Erosion/ ulcer (Figure 3)Pathogenesis/cell of origin
Nomenclature for Rodent Integument 33So
Loss of epidermal cell layers.Diagnostic featureso
Loss of super?cial epidermal cell layers (erosion) or complete loss of epidermis (ulceration).o
In case of ulceration, the basement membrane is dis-rupted.Differential diagnoseso
Necrosis, epidermal: Loss of cellular detail.o
Atrophy, epidermal: Decreased thickness of all non-corni?ed epidermal cell layers.Comment: Erosion and ulcer are a continuum. Erosions are nearly always caused by external trauma, mostly from scratching. Ulcerations are also often caused by external trauma, but may also derive from internal sources such as necrotizing dermatitis (see ‘Necrosis, epidermal’) or vesicular dermatitis (see ‘Vesicle’) with detachment of the entire epidermis. Toxic epidermal ulceration needs to be differentiated from spontaneous ulcerative dermatitis, which is known to occur in cer-tain strains of mice and rats (see ‘Common diseases of the rodent skin’). Ulceration is nearly always associ-ated with dermal in?ammation. As external pathogens get direct access to the dermis, neutrophils are usually heavily involved in the in?ammatory reaction and ac-cumulate in the upper dermis. The exposed dermis is often covered with cellular debris and exudate (crust). (Kastenmayer et al., 2006; Wuepper et al., 1975; Peck-ham and Heider, 1999)Necrosis, epidermal (Figure 4)Modi?ers: Full thickness type, Single cell typeSynonyms: Necrolysis (full thickness type), Sunburn cells (see Comment)Pathogenesis/cell of origino
Non-speci?c cell death of epidermal keratinocytes.Diagnostic featuresFull thickness typeo
Complete loss of cellular detail from the epidermis.o
associated
separation
from dermis (necrolysis).Single cell typeo
Individual
keratinocytes
show hyaline
hypereosino-philic cytoplasm and nuclear pyknosis.o
keratinocytes
surrounded
lym-phocytes (satellitosis).Differential diagnoseso
Erosion/ ulcer: Loss of super?cial epidermal cell lay-ers (erosion) or complete loss of epidermis (ulcer).Comment: Apoptosis should no longer be used as a di-agnostic term unless molecular techniques have con-?rmed the apoptotic pathway. Single necrotic epiermal cells are therefore refered to as necrosis, single cell type rather than apoptosis. Necrosis of epidermal ke-ratinocytes occurs in a continuum of spontaneous and experimentally-induced lesions. Single cell type necro-sis of epidermal keratinocytes is typical for ‘erythema multiforme’, whereas full thickness necrosis is typical for ‘toxic epidermal necrolysis’. Single cell type necrot-ic keratinocytes in ultraviolet light-exposed epidermis are known as ‘sunburn cells’. The term ‘dyskeratosis’ is frequently misused to describe apoptotic keratino-cytes, because keratinocytes undergo a process of pro-grammed cell death during terminal keratinisation, which cannot reliably be differentiated from apoptosis. (Young, 1987; Gross et al., 2005)Edema, intracellular, epidermal (Figure 5)Synonyms: Hydropic degeneration, Vesicular degenera-tion, Vacuolar degeneration, Ballooning degeneration, Reticular degenerationPathogenesis/cell of origino
Intracellular ?uid accumulation.Diagnostic featureso
Increased cell size.o
Cytoplasmic pallor and presence of intracellular vac-uoles.o
Displacement
the periphery
the cell.Differential diagnoseso
intercellular,
epidermal:
inter-cellular spaces without disruption of epidermal archi-tecture.o
Vesicle: Fluid-?lled cavity within or beneath the epi-dermis.Comment:
Intracellular
occur-rence of intracellular vacuoles. It is mostly associated with reversible cell injury indicating alterations of the membranes, mitochondria and endoplasmic reticu-lum
subsequent
How-ever, there is a continuum to irreversible cell damage. If intracellular edema occurs in the basal layer of the epidermis it is usually named ‘hydropic degeneration’ or ‘vacuolar degeneration’. If occurring in suprabasal epidermal cell layers, it is usually named ‘ballooning degeneration’. Severe cytoplasmic swelling may result in the rupture of keratinocytes and in the formation of intraepidermal vesicles (see ‘Vesicle’). (Hargis and Ginn, 2007)Edema, intercellular, epidermal (Figure 6)Synonyms: SpongiosisPathogenesis/cell of origino
Intercellular edema within the epidermis.Diagnostic featureso
Widening of intercellular spaces.
MECKLENBURG ET AL.34So
Accentuation of intercellular desmosomes.Differential diagnoseso
intracellular,
epidermal:
Intracellular
?uid accumulation with increased cell size, cytoplasmic pallor and displacement of the nucleus to the periph-er y.o
Vesicle: Fluid-?lled cavity within or beneath the epi-dermis.Comment: Spongiosis results from widening of the inter-cellular spaces by ede however the keratinocytes remain connected to each other via desmosomal at-tachments. Severe intercellular edema of the epidermis may be associated with rupture of desmosomes and formation of intraepidermal vesicles. Intercellular ede-ma is a common feature of skin in?ammation. (Gross et al., 2005; Hargis and Ginn, 2007)Vesicle (Figure 7)Synonyms: Bulla, Cleft, Reticular degenerationPathogenesis/cell of origino
Loss of cohesion between epidermal keratinocytes or between epidermis and dermis, resulting in the ac-cumulation of ?uid within a cavity.Diagnostic featureso
Disruption of epidermal architecture with open inter-cellular spaces.o
Fluid-?lled cavities within or beneath the epidermis.o
Cavities do not contain in?ammatory cells.Differential diagnoseso
Edema, intracellular, epidermal: Intracellular edema with increased cell size, cytoplasmic pallor and dis-placement of the nucleus to the periphery.o
intercellular,
epidermal:
inter-cellular spaces without disruption of epidermal archi-tecture.o
Pustule: Intraepidermal or subepidermal cavity ?lled with in?ammatory cells.Comment: Vesicles can result from immune mediated injury (e.g. loss of desmosomal attachments known as ‘acantholysis’) or as a result of epidermal or dermal edema as a consequence of poxvirus infection, fric-tional trauma, or burns. Intraepidermal vesicles may develop from severe intercellular edema and/or severe intracellular edema with rupture of keratinocytes. These are also known as ‘reticular degeneration’. Vesi-cles have been described in association with hydrogen peroxide treatment. (Jeong et al., 2010)In?ltrate, in?ammatory cell, epidermalModi?ers:
Lymphocytic,
Mononuclear,
Neutrophilic, EosinophilicSynonyms: ExocytosisPathogenesis/cell of origino
In?ltration of leukocytes into the epidermis.Diagnostic featureso
Leukocytes are present in between epidermal kerati-nocytes.o
Usually associated with dermal in?ammation.o
Frequently associated with hyperkeratosis and inter-cellular edema.Lymphocytico
Lymphocytes predominate.Mononuclearo
Lymphocytes and macrophages predominate.Neutrophilico
Neutrophils predominate.Eosinophilico
Eosinophils predominate.Differential diagnoseso
Pustule: leukocytes accumulate in a cavity.Comment: The epidermis is non-vascularized, hence in-?ammatory cells in the epidermis all derive from the dermal
compartment.
in?ltrations
neutro-philic and /or eosinophilic in nature. In these cases, bacteria, fungi or parasites should be looked for in the stratum corneum. Monomorphic lymphocytic in-?ltrates
an epitheliotrophic lymphoma (syn. ‘mycosis fungoides’). (Veldman and Feliciani, 2008)Pustule (Figures 8A and 8B)Modi?ers:
Lymphocytic,
Mononuclear,
Neutrophilic, EosinophilicSynonyms: MicroabscessPathogenesis/cell of origino
Focal accumulation of leukocytes within the epider-mis.Diagnostic featureso
Intraepidermal or subepidermal cavity ?lled with in-?ammatory cells.o
Mostly degenerate neutrophils and/or eosinophils.o
Frequently associated with cellular
debris and inter-cellular edema.Lymphocytico
Lymphocytes predominate.Mononuclearo
Lymphocytes and macrophages predominate.Neutrophilico
Neutrophils predominate.Eosinophilico
Eosinophils predominate.Differential diagnoseso
In?ltrate, in?ammatory cell, epidermal: Leukocytes are diffusely distributed throughout the epidermis without formation of a cavity.
Nomenclature for Rodent Integument 35So
Vesicle: Fluid ?lled space within the epidermis that is not ?lled with leukocytes.Comment: Intraepidermal pustules are a frequent sequel of super?cial skin in?ammation. Pustules can be fur-ther named according to the predominant leukocyte population, i.e. neutrophilic, eosinophilic or lympho-cytic. Lymphocytic pustules occur in epitheliotrophic lymphoma (syn. ‘mycosis fungoides’). Pustules that contain isolated rounded keratinocytes with a normal nucleus are referred to as ‘acantholytic pustules’. This is a common feature in pemphigus diseases. (Veldman and Feliciani, 2008)Hyperkeratosis, epidermal (Figures 9A and 9B)Modi?ers: Orthokeratotic, Parakeratotic, CrustPathogenesis/cell of origino
Alteration in epidermal cell turnover and differentia-tion of super?cial keratinocytes.Diagnostic featureso
Increased thickness of the stratum corneum.Orthokeratotico
Normal non-nucleated corneocytes.Parakeratotico
Corneocytes are nucleated.Crusto
Desiccated
accumulation
in?ammatory
cells, erythrocytes, epithelial squames and clotted plasma proteins.Differential diagnoseso
Hyperplasia,
epidermal:
the non-keratinized layers of the epidermis.Comment: Hyperkeratosis is a common sequel of chronic epidermal disease and is caused by increased turnover of epidermal cells or decreased desquamation of cor-neocytes. Hyperkeratosis can be a sign of skin irritancy. It also occurs in association with ulcerative dermatitis of the tail, acariasis and due to Corynebacterium bovis infections in nude mice (section 1.2) (Greaves 2000)SKIN – CUTANEOUS ADNEXAAtrophy, adnexal (Figures 10A and 10B)Synonyms: Alopecia, Fading folliclesPathogenesis/cell of origino
pilosebaceous
skin glands.Diagnostic featureso
Hair follicles and sebaceous glands are markedly re-duced in size beyond a normal telogen stage.o
Small remnants of keratinocyte strands are surround-ed by a thickened connective tissue sheath.o
Most hair follicles will have lost their hair shaft, but occasional presence of hair shafts is possible.o
There may be accompanying dermal atrophy or scar-ring.Differential diagnoseso
Dysplasia, adnexal: Abnormalities in the shape of the hair follicle and/or the hair shaft with no evident re-duction in size.o
Necrosis, adnexal: Degeneration of hair follicle kera-tinocytes that may be associated with distortion of the hair follicle.Comment: Hair follicles lose cells when they undergo regression in the catagen stage of the hair cycle. There-fore, hair follicle atrophy must be distinguished from catagen and telogen stages of the physiological hair cycle. Hair follicle atrophy is a loss of cells beyond the physiological telogen stage. Hair follicle atrophy can be caused by a number of different compound classes such as anti-proliferatives and steroid hormones. It can also be linked to dermal ischemia caused by vasculopathy and some autoimmune conditions. Genetically engi-neered mice may display a loss of eccrine glands from the foot pads, resembling anhidrotic ectodermal dys-plasia in humans. (Cerundolo and Mecklenburg, 2009; Taylor et al. 2012)Dysplasia, adnexal (Figures 11A and 11B)Synonyms: Abnormal developmentPathogenesis/cell of origino
Hair follicle keratinocytes.Diagnostic Featureso
Abnormalities in the shape of the hair follicle and/or the hair shaft.Differential diagnoseso
Atrophy, adnexal: Hair follicles and sebaceous glands are markedly reduced in size beyond a normal telogen stage.o
Irreversible
degeneration
hair follicle keratinocytes that may be associated with dis-tortion of the hair follicle.Comment: Adnexal dysplasia primarily affects hair fol-licles. It is not a preneoplastic hyperplastic lesion. Many genetically modi?ed mice have been described that exhibit various forms of congenital hair follicle malformation. The loss of pigment from hair follicles, e.g. in coat color mutants, could also be classi?ed as a dysplasia. (Walsh and Gough, 1989; Sells and Gibson, 1987; Nakamura et al., 2001)In?ammation, adnexal (Figures 12A and 12B)Modi?ers:
Lymphocytic,
Neutrophilic,
Eosinophilic, GranulomatousSynonyms: Folliculitis, Perifolliculitis, Adenitis, Periad-
MECKLENBURG ET AL.36Senitis, FurunculosisPathogenesis/cell of origino
Unspeci?c in?ammatory reaction.Diagnostic featureso
In?ltration of in?ammatory cells (lymphocytes, plas-ma cells, macrophages, eosinophils, mast cells, baso-phils, granulocytes, or combinations of any of these) within or surrounding the adnexae.o
May be associated with necrosis of the adnexa.o
Edema, congestion, neovascularization or ?broplasia might be present.Lymphocytico
Lymphocytes predominate.Neutrophilico
Neutrophils predominate.Eosinophilico
Eosinophils predominate.Granulomatouso
Histiocytes with epithelioid appearance predominate, giant cells may be present.Differential diagnoseso
Dysplasia, adnexal: Abnormalities in the shape of the hair follicle and/or the hair shaft with no evident re-duction in size.o
Atrophy, adnexal: Hair follicles and sebaceous glands are markedly reduced in size beyond a normal telogen stage.o
Irreversible
degeneration
hair follicle keratinocytes that may be associated with dis-tortion of the hair follicle.Comment: As is the case for the epidermis, in?ltration of leukocytes into the cutaneous adnexa is always asso-ciated with dermal in?ammation. Hair follicle in?am-mation can
classi?ed according
pre-cise localization, namely mural (anywhere within the hair follicle epithelium), bulbar (within the hair follicle bulb), and luminal (within the hair follicle lumen). In diagnostic pathology, the term ‘interface folliculitis’ is used when perifollicular
in?ammation
are associated with distinct necrosis of follicular keratino-cytes. The term ‘furunculosis’ describes a penetrating and perforating follicular in?ammation, meaning that the
follicular
in?ammatory process. In?ammation of the cutaneous adnexa in ro-dents can occur in association with dermatophytosis (see ‘Common skin diseases of laboratory rodents’) or as a sequel of topical or systemic treatment with chemi-cals. (Mecklenburg, 2009; Brown et al., 2008)Necrosis, adnexal (Figure 13)Modi?ers: Single cell type, Diffuse typeSynonyms: Vacuolar degeneration (see Comment)Pathogenesis/cell of origino
Non-speci?c,
irreversible cell
death of the follicular or glandular epithelium.Diagnostic featureso
Cell death of hair follicle keratinocytes, either as sin-gle cells (single cell type) or as multiple cells (diffuse type)o
Necrosis of keratinocytes may be associated with un-even distribution of melanin within the hair follicle and hair shaft, perifollicular melanophages, dilation of the follicular canal, or distortion of the entire hair follicle.Single cell type:o
Individual
keratinocytes
show hyaline
hypereosino-philic cytoplasm and nuclear pyknosis.o
keratinocytes
surrounded
lym-phocytes (satellitosis).Diffuse type:o
Complete loss of cellular detail from the adnexa.Differential diagnoseso
In?ammation,
In?ltration
in?ammatory cells predominates.o
Dysplasia, adnexal: Abnormalities in the shape of the hair follicle and/or the hair shaft with no evident re-duction in size.o
Atrophy, adnexal: Hair follicles and sebaceous glands are markedly reduced in size beyond a normal telogen stage with no evidence of apoptosis, vacuolar degen-eration or necrosis.Comment: The term ‘dystrophy’ denotes a degenera-tive process that is due to ‘malnutrition’ of an organ. Morphological hallmarks of hair follicle dystrophy are uncoordinated vacuolar degeneration or apoptosis of keratinocytes. Since these are features of necrosis, hair follicle dystrophy can be grouped under ‘necrosis’. The term
ef?uvium’
medicine to emphasize that hair shafts are lost despite the fact that hair follicles are in the anagen stage of the hair cycle
ef?uvium’).
Chemothera-py-induced alopecia is a good example of hair follicle necrosis of the single cell type. (Hendrix et al., 2005; Cerundolo and Mecklenburg, 2009)Hyperkeratosis, adnexal (Figures 14A and 14B)Synonyms:
Chloracne,
Hypercorni?cation
Com-ment)Pathogenesis/cell of origin:o
keratinocytes
hair follicle infundibulum or sebaceous gland duct.Diagnostic featureso
The follicular infundibulum is dilated and ?lled with keratin that resembles epidermal differentiation.o
The hair follicle canal may be cystic.o
Nomenclature for Rodent Integument 37Splugged.o
Keratin plugging can result in retained hairs or secre-tions.Differential diagnoseso
Keratoacanthoma: Well demarcated mass with large central keratin-?lled cavity surrounded by hyperplas-tic squamous epithelium.Comment: Hyperkeratosis with dilation of the follicular infundibulum can be observed as a treatment-related lesion. The follicular hyperkeratosis manifested in dioxin toxicosis known as ‘chloracne’ is an example. (Peckham and Heider 1999)SKIN – DERMIS AND SUBCUTISNon-proliferative lesions in the dermis and subcutis such as in?ammation (syn. dermatitis or panniculitis), necrosis, ?-brosis (syn. sclerosis), metaplasia, mineralization and amyloid deposition, lesions in the hypodermal adipose tissue such as lipogranulomatous in?ammation, necrosis, atrophy and hyper-plasia, and lesions in the subcutaneous muscle such as necrosis and
in?ammation,
hypertrophy,
degeneration,
vacu-olation and mineralization are all described in the manuscript on proliferative and non-proliferative lesions of the rat and mouse soft tissue, skeletal muscle and mesothelium (Greaves et al., 2013). Amyloidosis in the dermis of mice is usually a manifestation of systemic amyloidosis which occurs frequently in some strains of mice, particularly CD-1 mice. Dermal min-eralization is usually a sequel of generalized mineralization in the body and may lead to dermal in?ammation and erosion/ul-ceration of the epidermis. In terms of dermal peripheral nerves and vasculature, lesions are described in the manuscript on the nervous system (Kaufmann et al., 2012) and the cardiovascular system (Berridge et al., in preparation).Atrophy, dermal (Figure 15)Pathogenesis/cell of origino
Reduced metabolic activity of dermal ?broblasts.Diagnostic Featureso
Loss of collagen ?bers and extracellular matrix.o
Fibroblasts are smaller and more ovoid.o
Mast cell numbers are decreased.Differential diagnoseso
Separation
a pale amorphous or slightly granular substance.o
nuclei with or without replacement by cellular debris.Comment: Atrophy of the dermis occurs typically with long-term administration of corticosteroids and is usu-ally associated with atrophy of the epidermis and cuta-neous adnexa. (Lavker et al., 1986)Edema, dermal (Figure 16)Pathogenesis/cell of origino
Accumulation of interstitial ?uid.Diagnostic Featureso
Separation of collagen ?bers by a pale amorphous or slightly granular substance.Differential diagnoseso
Atrophy, dermal: Loss of both collagen ?bers and ex-tracellular matrix.Comment: Edema frequently accompanies dermal in-?ammation.
skin when there is poor blood ?ow and prolonged immobil-ity. Edema can also occur spontaneously in the dermis or subcutis of mice. (Peckham and Heider, 1999; Chan et al., 1982; Hirouchi et al., 1994)ElastosisPathogenesis/cell of origino
Dermal ?broblasts producing elastic ?bers.Diagnostic Featureso
Accumulation
lightly basophilic,
irregular,
thick-ened elastic ?bers in the upper dermiso
oriented parallel to one anotherSpecial techniques for diagnosiso
Orcein stainDifferential diagnoseso
Fibrosis: Increase in extracellular collagen, mostly ar-ranged in long strands.Comment: Elastosis is observed after excessive exposure to ultraviolet light. (Sams et al., 1964; Nakamura and Johnson, 1968; Berger et al., 1980)Non-neoplastic proliferative lesionsSKIN - EPIDERMISHyperplasia, epidermal (Figures 17A and 17B)Modi?ers: With cellular atypiaSynonyms: Acanthosis, Squamous hyperplasia, Epider-mal hyperplasiaPathogenesis/cell of origino
Derives from epidermal keratinocytes.Diagnostic featureso
of non-keratinized
layers of the epidermis, especially the stratum spinosum and gran-ulosum.o
especially
in stratum spinosum.o
Rete ridge formation is often present.
MECKLENBURG ET AL.38So
Hyperkeratosis
(orthokeratotic
hyperkeratotic)
is frequently also present.o
It might include the epithelial lining of the hair follicle infundibulum.o
The underlying basement membrane is intact.With cellular atypia:o
Irregularly
non-keratinized layers of the epidermis, especially the stratum spino-sum and granulosum.o
Differentiation into stratum basale, stratum spinosum and stratum granulosum is lost.o
keratinocytes
hyperchromatic nuclei are found in the stratum basale and the lower stratum spinosum.Differential diagnoseso
Papilloma, squamous cell: Thickened epidermis with exophytic or papilliform grow regular squamous differentiationo
Carcinoma, squamous cell (keratinizing type): Inva-sive growth into basement membrane and surround-ing
numerous with nuclear aty epithelial cells have variable squamous differentiation and keratinization.Comment: Squamous cell hyperplasia is observed as a re-sponse to a variety of insults including spontaneous or induced in?ammation, toxic irritation, repeated
abra-sion of the super?cial stratum corneum, or prolonged exposure to ultraviolet light. Rarely, chemicals directly induce proliferation of epidermal keratinocytes. Direct induction of proliferation by chemicals such as tetra-decanoyl phorbol acetate (TPA) is used for tumor ini-tiation/promotion studies. Treatment-related epidermal hyperplasia was reported from mouse studies with xy-lene sulfate.Hyperplasia of basal keratinocytes only is not con-sidered to represent a separate morphological entity. It has been described to occur within papillomas or kera-toacanthomas. Hyperplasia of basal epidermal kerati-nocytes is not considered a precursor lesion of basal cell tumor, since the latter is supposed to be of follicu-lar origin (see ‘Basal cell tumor’).Squamous cell hyperplasia with cellular atypia is frequently found in transgenic mice which show an increase in keratinocyte proliferation or in mice that were treated with topical carcinogens. Cellular atyp-ia occurring in a squamous cell papilloma should be diagnosed as Papilloma, squamous cell, with cellular atypia (see ‘Papilloma, squamous cell’). (Evans et al., 1997; Bader et al., 1993; Greaves, 1990; Gopinath et al., 1987; Hasegawa et al., 1989; Bruner et al., 2001; Greaves, 2000; Stenb?ck et al., 1986)Cyst, squamous (Figures 18A and 18B)Synonyms: Epidermal cyst, Horn cyst, Keratin cystPathogenesis/cell of origino
Keratinocyte.Diagnostic featureso
Cyst within the upper dermis.o
Cyst wall is composed of strati?ed keratinizing epi-thelium.o
The cyst lumen contains concentrically arranged la-mellar keratin.Differential diagnoseso
Hyperplasia, epidermal: No cyst formation.o
Papilloma,
papilliform growth of the epidermis with no discernible lumen.o
Keratoacanthoma: central cavity surrounded by well differentiated, hyperplastic squamous epithelium, oc-casionally with papillary projections into the lumen; there may be intraepithelial whorls with central kera-tinisation, frequently containing cholesterol crystals intermingled with the keratin; edges of the wall may have prominent foci of basaloid cell.Comment: Squamous cysts may occur as ‘horn pearls’ within squamous cell carcinoma (see ‘Carcinoma, squamous cell’). Squamous cysts are not considered a simple hyperplastic lesion. Their cause is generally unknown. Most likely they arise from injured pilose-baceous units in which squamous epithelial cells pro-ducing keratin are trapped in the dermis. The lesion is comparable with ‘infundibular cyst’ or ‘isthmus cyst’ as described for dogs. Squamous cell cysts can sponta-neously occur in mice, particularly the B6C3F1 strain. (Peckham and Heider, 1999)SKIN – CUTANEOUS ADNEXAHyperplasia, adnexal (Figure 19)Modi?ers: Hair follicle, Sebaceous glandSynonyms: Sebaceous hyperplasiaPathogenesis/cell of origino
Derives from hair follicle or sebaceous gland epithe-lium.Diagnostic featureso
otherwise normal architectureSebaceous glando
Sebaceous glands are enlarged and show an increased number of sebaceous cells in individual acini with few immature germinative cells and many mature glandular cells arranged around prominent central ducts.Differential diagnoseso
gland architecture is distort growth pattern may be exo-phytic; large numbers of immature germinative basa-
Nomenclature for Rodent Integument 39Sloid cells are present at the periphery.Comments: Hyperplasia of cutaneous adnexa mostly af-fects sebaceous glands. Sebaceous cell hyperplasia can be found in cases of chronic in?ammatory irritation of the skin. It can also occur simultaneously with squa-mous cell hyperplasia. An increase in the overall size of hair follicles associated with an increased number of hair follicle keratinocytes can occur in genetically engineered mice. As an example, an increased size of anagen hair follicles has been described in p27Kip1 knockout mice. The increased size of hair follicles is associated with an increase in the diameter of the fol-licular dermal papilla. (Evans et al., 1997; Peckham and Heider, 1999; Bruner et al., 2001; Sharov et al. 2006)SKIN – DERMIS AND SUBCUTISHyperplasia of the subcutaneous adipose tissue is covered by the manuscript on proliferative and non-proliferative lesions of the rat and mouse soft tissue, skeletal muscle and mesothe-lium (Greaves et al., 2013).Hyperplasia, melanocyte (Figure 20)Pathogenesis/cell of origino
Melanocyte.Diagnostic featureso
Accumulation of pigmented cells within the dermis, located between hair follicles and sebaceous glands.Differential diagnoseso
Melanoma, benign: Dense nodular proliferation in the dermis with or without association towards the epi-dermis.Comment: Melanocyte hyperplasia has been observed in some initiation-promotion and skin painting studies in mice. Melanocyte hyperplasia should be differentiated from
in?ltration
macrophages
phagocytozed pigment. (Peckham and Heider, 1999)Neoplastic proliferative lesionsSKIN - EPIDERMISPapilloma, squamous cell (Figure 21)Modi?ers: Exophytic, Endophytic, With cellular atypia, Non-keratinizingPathogenesis/cell of origino
Derives from epidermal keratinocytes.Diagnostic featureso
circumscribed
papilliform
endo-phytic mass with no capsule.o
The mass is composed of keratinizing squamous cells overlying a well vascularised stroma.o
Basal cells are fusiform or columnar with distinct cell borders and little basophilic cy they show few mitotic ?gures.o
Suprabasal cells show gradual squamous differentia-tion and keratinization with a thickened granular cell layer and irregularly enlarged keratohyalin granules.o
Mitotic ?gures are common.o
Individual suprabasal cells may show premature ke-ratinization (dyskeratosis).o
There is a variable degree of parakeratotic hyperkera-tosis.o
Ulceration and in?ammation may be seen.Exophytico
A more or less distinct stalk is present at the base of the mass (also known as ‘pedunculated’ papilloma).Endophytico
There is no evidence
the mass is continuous with the adjacent hyperplastic epidermis, and a crater is produced by invaginat the tumor stroma is indistinctly demarcated from the underlying dermis.With cellular atypiao
hyperchromatic nuclei are found primarily in the basal and suprabasal layer of the epidermis. Mitotic ?gures might be found in suprabasal layers.Non-keratinizingo
The epithelium lacks typical keratinization.Differential diagnoseso
Hyperplasia,
epidermal:
with-out formation of a well circumscribed papilliform mass.o
Keratoacanthoma: A large central cavity or multiple smaller
concentrically
arranged keratin material.o
Carcinoma,
into basement membrane and surrounding tissues is pres- mitotic ?gures are numerous with nuclear aty epithelial cells have variable squamous differentia-tion and keratinization.Comment: Spontaneous squamous cell papilloma in rats and mice is not associated with papilloma virus and its occurrence is rare except for in aged animals. In Tg.AC (v-Ha-ras) transgenic mice, squamous cell pap-illomas occur at typical sites of chronic grooming (eg. ears, nose, lips, paws, ano-urogenital area) as early as 8 weeks of age (Usui et al., 2001). The overall incidence is low (0–2%) in hemizygous mice but up to 17% in homozygous animals (Tennant et al., 2001). Chemi-cal carcinogens readily induce squamous cell papil-lomas in mice. Papillomas can give rise to squamous cell carcinomas and may show prominent foci of basal cells (see ‘Hyperplasia, squamous cell’). Fibropapil-lomas, i.e. papillomas with an increased amount of ?-brous tissue at their base, as they occur in turtle and
MECKLENBURG ET AL.40Ssome domestic animal species (e.g. cattle, horse, cat), have not been described in rodents. (Bader et al., 1993; Bogovski, 1994; Deerberg et al., 1986; Elwell et al., 1990; Evans et al., 1997; Faccini et al., 1990; Frith and Ward, 1988; Greaves and Faccini, 1984; Kovatch, 1990; Maita et al., 1988; Mohr and Hunt, 1989; Peckham and Heider, 1999; Poteracki and Walsh, 1998; Thomas and Rohrbach, 1975; Zackheim et al., 1990; Zwicker et al., 1992; Hasegawa et al., 1989; Rehm et al., 1989; Fuk