JPC SYSTEMIC PATHOLOGY

INTEGUMENTARY SYSTEM

September 2025

I-N30

Signalment (JPC #2681629): Adult boa constrictor (Boa constrictor constrictor)

HISTORY: A 1 cm diameter skin mass that recurred 1.5 years after surgical excision

HISTOPATHOLOGIC DESCRIPTION: Scaled skin: Expanding the dermis and elevating the ulcerated epidermis is a 2 cm x 1.5 cm, unencapsulated, infiltrative, multilobular, moderately cellular, pigmented neoplasm composed of round to spindle cells arranged in packets as well as short streams and whorls on a scant to moderate fibrovascular stroma. Neoplastic cells have indistinct cell borders; a moderate amount of eosinophilic granular cytoplasm that contains variable amounts of gold-brown to green, granular, highly birefringent pigment; and an oval to elongate nucleus with finely stippled chromatin and one variably prominent nucleolus. Anisocytosis and anisokaryosis are marked. There are 3 mitotic figures per 2.37 mm². Vessels within the neoplasm are occasionally surrounded by variable numbers of lymphocytes, plasma cells, histiocytes, and granulocytes. There is a focally extensive area of ulceration with subjacent granulation tissue formation and adjacent epidermal hyperplasia with hyperkeratosis.

MORPHOLOGIC DIAGNOSIS: Skin: Iridophoroma, Boa constrictor (Boa constrictor constrictor), reptile.

GENERAL DISCUSSION:

• Rare neoplasm of reptiles; most common in snakes; composed of chromatophores (contractile pigment-bearing and light-bearing cells of neuroectodermal origin) found in the epidermis or dermis of reptiles, fish, and amphibians
• Snake skin contains three classes of pigment producing cells; tumors are classified according to the type of chromatophores they contain:
  • Melanophores- dark brown to black melanin-producing cells (melanophoroma or melanoma); only pigmented cell found both in epidermis and dermis; can be benign or malignant with metastasis (intravascular and visceral)
  • Xanthophores- reddish carotenoid and pteridine producing cells (xanthophoroma)
  • Iridophores - crystalline purine-producing cells containing refractile (birefringent) granules of guanine, adenine, hypoxanthine, or uric acid (iridophoromas)
• Neoplasms from all three classes have been reported in snakes, including mixed chromatophoromas, melanophoromas most common
• Iridophoromas are generally benign; rare malignant forms are reported
  • In Siamese fighting fish the iridophoromas were invasive (Diniz, 2025)
• In snakes, common species to be affected are colubrids, vipers, and boids (Taggard, J Comp Path, 2021)

TYPICAL CLINICAL FINDINGS:

• Presents as ulcers or blisters on the skin
• Underlying mass is often firmly attached

TYPICAL GROSS FINDINGS:

• Iridophoroma are typically single, white to light grey, or black dermal mass or plaque; typically with overlying ulceration
• Occasionally overlying integument is hyperkeratotic

TYPICAL LIGHT MICROSCOPIC FINDINGS:

• Iridophoromas: Pleomorphic population of cells with abundant golden brown to yellow-green pigment that is birefringent with polarized light
• Melanophoromas: Spindle cells with variably prominent black cytoplasmic granules
• Indicators for malignancy of chromatophoromas is nuclear atypia, mitotic count, lymphatic invasion, infiltrative growth, and metastasis

ADDITIONAL DIAGNOSTIC TESTS:

• Microscopic examination is typically sufficient for diagnosis
• Fontana Masson
  • Highlights cytoplasmic pigment-containing pterinosomes of xanthophores
  • Highlights intracytoplasmic black pigment in melanophores
• Immunohistochemistry:
  • S-100 for all chromatophoromas, weak immunolabeling in iridophomoras; PNL2 and MelanA for melanophoromas
• Low specificity for differentiating snake chromatophore tumor types (Taggard, J Comp Path, 2021)
• Electron microscopy
  • Iridophores contain cytoplasmic bundles of reflecting parallel plates oriented to the basal lamina or haphazardly
• PAS for mucin rich melanophoromas

DIFFERENTIAL DIAGNOSIS:

• Gross:
  • Granulomas due to bacteria, fungi, trauma, or septicemia
  • Parasitic masses due to plerocercoids, larval dracunculids, acanthocephalans, or spirurids
  • Steatitis
  • Cryptosporidium spp.
  • Other neoplasms: Fibrosarcoma, squamous cell carcinoma, other chromatophoromas
  • Melanophomoras are gray or black
  • Xanthophoromas are difficult to distinguish on light microscopy; may share overlapping features with other chromatophoromas
• Microscopic (other chromatophoromas):
• Chromatophore hyperplasia: Occur at sites of skin injury
• Melanophoroma/melanoma:
• Amorphous granular melanin granules (melanosome);
  • These are the most common of the chromatophoromas in snakes
  • Can be invasive with intravascular and distant metastases;
  • Immunohistochemistry: Sensitivity is poor, but Melan-A and S-100 immunostaining are reported in snake melanophoromas
  • Histochemistry: melanin granules stain with Fontana-Masson
• Mixed chromatophoroma: Combined non refractile and refractile pigment granules
• Xanthophoroma: Concentric lamellar pteridines (pterinosome); pterinosomes may stain with Fontana-Masson, making differentiation from poorly pigmented melanophoroma difficult

COMPARATIVE PATHOLOGY:

• Chromatophoromas have been reported in a variety of snakes and amphibians
  • Melanophoroma and iridophoromas have been reported in amphibians
  • Melanophoromas have a higher malignancy probability in reptiles
• Reported in several fish species
• Melanophomora reported in several lizard species: Bearded dragons, veiled chameleons, leopard gecko, savannah monitor, green iguana
• Iridophomora reported in several lizard species: bearded dragon (Pogona vitticeps), veiled chameleon, savanna monitor, dwarf bearded dragon (Pogona henrylawsoni)

REFERENCES:

1. Crevoiserat L, Katzenstein N, LaDouceur EEB. Uveal iridophoroma in a betta fish (Betta splendens). J Comp Pathol. 2024;215:71-75.
2. Diniz SLP, Oliveira Filho HS, Santos KMOL, et al. Gross, histologic, and ultrastructural features of iridophoromas in Siamese fighting fish (Betta splendens). Vet Pathol. 2025;62(2):237-247
3. Monahan CF, Meyer A, Garner MM, Kiupel M. Gross, histologic, and immunohistochemical characteristics of cutaneous chromatophoromas in captive bearded dragons. J Vet Diagn Invest. 2021;33(5):932-938.
4. Ossiboff RJ. Serpentes. In: Terio KA, McAloose D, St. Leger J, eds. Pathology of Wildlife and Zoo Animals. Cambridge, MA: Elsevier; 2018: 907-908.
5. Origgi FC. Lacertilia. In: Terio KA, McAloose D, St. Leger J, eds. Pathology of Wildlife and Zoo Animals. London, UK: Academic Press; 2018: 889.e6.
6. Pessier AP. Amphibia. In: Terio KA, McAloose D, St. Leger J, eds. Pathology of Wildlife and Zoo Animals. Cambridge, MA: Elsevier; 2018: 930, 932
7. Taggart PL, Woolford L, Dunstan N, et al. Cutaneous chromatophoromas in four species of Australian elapid snake. J Comp Path. 2021;183:33-38.

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