JPC SYSTEMIC PATHOLOGY

INTEGUMENTARY SYSTEM

October 2022

I-N32 (NP)

 

Signalment (JPC# 3054993):  9-year-old male Dutch shepherd

 

HISTORY:  Swollen right rear fourth digit

 

HISTOPATHOLOGIC DESCRIPTION:   Haired skin, digit: Expanding the deep dermis, compressing adnexa, and elevating the overlying moderately hyperplastic epidermis is a 5 X 2 mm cyst lined by a thick wall of squamous epithelium that undergoes gradual keratinization characterized by the presence of deeply basophilic keratohyaline granules within the stratum granulosum; there is cytoplasmic melanin pigment throughout all cell layers. The cyst lumen is filled with laminations of anucleate (orthokeratotic) keratin admixed with hemorrhage, scant fibrin and edema, and cellular debris. The dermis adjacent to the cyst is thickened with increased fibrous tissue (fibrosis) and is infiltrated by numerous macrophages and neutrophils, few lymphocytes and plasma cells, free melanin pigment, and clusters of melanin laden macrophages (pigmentary incontinence), and hemorrhage, fibrin, and edema. Adjacent to the third phalanx (P3) are multifocal to coalescing foci of inflammatory infiltrates as previously described admixed with eosinophilic cellular and karyorrhectic debris (necrosis) and abundant hemorrhage, fibrin, and edema; these foci are surrounded by dense fibrosis. The margin of P3 is rarely scalloped with increased numbers of osteoclasts within Howship’s lacunae (bone resorption) and with foci of increased woven bone (bony remodeling).  The adjacent epidermis is moderately hyperplastic with prominent anastomosing rete ridges and parakeratotic hyperkeratosis. Multifocally, hair follicles contain lamellations and fragments of keratin. Aprocrine and eccrine glands are mildly ectatic.

 

MORPHOLOGIC DIAGNOSIS:  Haired skin, digit: Nail bed epidermal inclusion cyst with pyogranulomatous dermatitis, hemorrhage, and necrosis, Dutch shepherd, canine.

 

GENERAL DISCUSSION:  

• Subungual epidermal inclusion cysts are infundibular cysts within the bone of the third phalanx of dogs
• Often grouped with other infundibular cysts such as epidermoid cysts, epidermal cysts, and epidermal inclusion cysts that can be found on other areas of the body
• Non-neoplastic; rare reported transformation to neoplasm (one case)
• Can cause pyogranulomatous inflammation if ruptured (foreign body response to free keratin)

 

PATHOGENESIS: 

• Proliferation of epidermal cells within a circumscribed space of the dermis
• Most likely caused by trauma or surgical implantation of epidermal tissue into subjacent connective tissue
  • Previous onychectomy sites and dewclaws are most often affected 

 

TYPICAL CLINICAL FINDINGS: 

• Single swollen digit with or without a cracked or irregular nail
• Radiographically, there can be partial to total bone loss of the third phalanx with little periosteal response

 

TYPICAL CYTOLOGICAL FINDINGS:

• FNA reveals a dense collection of keratinized, sometimes pigmented, squamous epithelial cells
• Rupture of the cyst wall can induce a localized pyogranulomatous cellulitis:

• Neutrophils and macrophages may be frequent

 

TYPICAL GROSS FINDINGS:  

• Nodule size difficult to assess until sectioned, but generally 0.5 - 2 cm 
• Cyst contents may be semi-fluid, caseous, or composed of dry granular material that can be pigmented

 

TYPICAL LIGHT MICROSCOPIC FINDINGS: 

• Similar histologic appearance to follicular cysts of infundibular origin:  Cyst lined by stratified squamous epithelium that undergoes gradual keratinization with an obvious granular layer and filled with lamellations of keratin
• Pressure lysis of third phalanx may be present depending on size
• Cyst rupture leads to pyogranulamatous inflammation (may include periosteal fibrosis or periosteal bone proliferation if severe)

 

DIFFERENTIAL DIAGNOSIS:

 

COMPARATIVE PATHOLOGY:

• Epidermal cysts can occur in a variety of places in multiple species 
• Horses: Epidermal inclusion cysts reported in the nasal diverticulum
• Postoperative conjunctival inclusion cysts result from intraorbital migration of bulbar conjunctival epithelium due to improper surgical enucleation

 

REFERENCES:

1. Caswell JL, Williams KJ. Respiratory system. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 2.  6th ed. Philadelphia, PA: Elsevier Saunders; 2016: 478.
2. Hargis AM, Myers S. The integument. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 6th ed. St. Louis, MO: Elsevier; 2017: 1119.

3. Mauldin 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 Saunders; 2016: 704.
4. Raskin RE, Conrado FO. Integumentary system. In: Raskin RE, Meyer DJ, eds. Canine and Feline Cytopathology: A Color Atlas and Interpretation Guide. 4th ed. St. Louis, MO: Elsevier; 2023: 61-62.
5. Welle MM, Linder KE. The integument. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 6th ed. St. Louis, MO: Elsevier; 2022: 1219-1220.

Click the slide to view.

---