JPC SYSTEMIC PATHOLOGY

INTEGUMENTARY SYSTEM

September 2022

I-M19

 

Signalment (JPC# 2370088): Adult male Sprague-Dawley rat

 

HISTORY: This rat was in the control group of a subchronic toxicity study. Gross morphological changes were limited to bilaterally enlarged pinnae.

 

HISTOPATHOLOGIC DESCRIPTION: Pinna: Affecting approximately 50% of this section, the pinna is expanded up to 5mm by nodules of disrupted and disorganized cartilage surrounded by inflammatory infiltrates. Auricular cartilage is affected by one or more of the following changes: loss of matrix basophilia (degeneration); cartilage fragmentation with shrunken, hypereosinophilic, pyknotic chondrocytes or empty lacunae (necrosis); nodules of amphophilic cartilaginous matrix containing closely approximated chondrocyte lacunae that occasionally contain two to multiple chondrocytes (chondrones, chondromatous regeneration); or replacement by eosinophilic, homogenous osteoid that surrounds osteocytes within lacunae, multifocally contains extensive areas of basophilic, angular to granular, mineralized matrix that occasionally forms trabeculae of woven bone containing marrow elements (osseous metaplasia). Diffusely infiltrating the dermis, separating and surrounding myocytes, and multifocally infiltrating the perichondrium and disrupting auricular cartilage is a dense inflammatory infiltrate composed of many viable neutrophils, macrophages, lymphocytes, plasma cells, and few Langhans-type multinucleated giant cells admixed with fibroblasts, immature fibrous connective tissue, and small caliber blood vessels (granulation tissue) that progresses to more mature fibrous connective tissue (fibrosis). Myocytes in affected areas are affected by one or more of the following changes: swollen with vacuolation and loss of cross striations (degeneration), shrunken (atrophy), or shrunken, hypereosinophilic with a pyknotic nucleus (necrosis). Multifocally, dermal collagen fibers are loosely arranged and separated by an eosinophilic amorphous material, and lymphatics are moderately ectatic and filled with eosinophilic proteinaceous fluid (edema). Focally, the overlying epidermis is mildly hyperplastic with mild orthokeratotic hyperkeratosis, acanthosis and intracellular edema.

 

MORPHOLOGIC DIAGNOSIS: Pinna, cartilage: Degeneration, multifocal, moderate, with granulomatous chondritis and dermatitis, cartilaginous regeneration, and osseous metaplasia, Sprague-Dawley rat, rodent.

 

ETIOLOGIC DIAGNOSIS: Idiopathic auricular chondritis

 

CONDITION: Auricular chondritis (relapsing polychondritis)

 

GENERAL DISCUSSION:

• Affects multiple strains of rats and mice
• Often associated with metal ear tags

 

PATHOGENESIS:

• Spontaneous or result of trauma
• Often associated with metal ear tags in mice/rats
• Thought to be immune-mediated: 

• Inoculation with type II collagen can induce similar lesions
• In ear tagged animals contralateral (non-tagged) pinna also affected

• May stimulate formation of squamous cell carcinoma (Percy and Barthold, p 96)

 

TYPICAL CLINICAL FINDINGS:

• Pinnae are misshapen and contain firm nodules
• Usually non-painful

 

TYPICAL GROSS FINDINGS:

• Firm, multinodular to diffuse thickening of pinnae
• Bilateral lesions extend peripherally from the base of the pinnae

 

TYPICAL LIGHT MICROSCOPIC FINDINGS:

• Granulomatous inflammation with degeneration and lysis of auricular cartilage 
• Foci of chondroid hyperplasia and osseous metaplasia 

 

ADDITIONAL DIAGNOSTIC TESTS:

 

COMPARATIVE PATHOLOGY:

• Auricular chondritis rarely reported in dogs/cats; resembles human relapsing polychondritis (RP) but generally restricted to auricle; reported in young cats < 3 years of age, FeLV+ cats, single cat with lymphoma, and single dog; auricles bilaterally swollen, alopecic, erythematous, painful, curled; hyaline cartilage becomes eosinophilic and surrounded by inflammation; may also occur secondary to trauma (JKP p 697); one or both ears and often painful (PBVD Ch17)
• Humans: Relapsing polychondritis is an immune mediated response to type II collagen and matrilin 1 (MATN1); also affects cartilage in nose, larynx, trachea, bronchi (PBVD Ch 20, Muller and Kirk’s p 489)
• Horses: Auricular chondrosis lacks inflammation (chondritis), not responsive to corticosteroids; cause undetermined; nodular thickening of pinnae with degeneration and necrosis, invasion by adipocytes, and new cartilage formation (PBVD Ch 20)

 

REFERENCES:

1. Maudlin EA, Peters-Kennedy J. Integumentary System. In: Maxie MG ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 1. 6th ed. Philadelphia, PA: Elsevier Ltd. 2016: 697.
2. Miller WH, Campbell KL, Griffin CE. Autoimmune and immune-mediated dermatoses. In: Muller & Kirk’s Small Animal Dermatology. 7^th ed. Philadelphia, PA: WB Saunders Company; 2013: 489-490.
3. Njaa BL. Ear. In: Maxie MG ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 1. 6th ed. Philadelphia, PA: Elsevier Ltd. 2016: 504.
4. Njaa BL. The Ear. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 7^th ed. St. Louis, MO: Elsevier, Inc.; 2022: 1371, 1377.
5. Njaa, BL. Tumors of the Ear. In: Tumors in Domestic Animals. 5^th ed. Ames, IA: Donald Meuten, Jr. 2017: 939.
6. Percy DH, Barthold SW. Pathology of Laboratory Rodents and Rabbits. 4th ed. Ames, IA: Blackwell Publishing; 2016:96,159-160.
7. Welle MM, Linder KE. The Integument. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 7th ed. St. Louis, MO: Elsevier; 2022:1260.

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