Cutaneous mastocytomas in the neotenic caudate amphibians Ambystoma mexicanum (axolotl) and Ambystoma tigrinum (tiger salamander)

Spontaneous mastocytomas studied in 18 axolotls (Ambystoma mexicanum) and s ix tiger salamanders (Ambystoma tigrinum) were gray-white, uni- to multilob ular cutaneous protrusions from 2 mm to 2 cm in diameter. Tumors were moder ately cellular unencapsulated masses that usually infiltrated the dermis an d hypodermis with the destruction of intervening tissues. Some tumors were invading superficial bundles of the underlying skeletal muscle. Tumors cons isted of mitotically active cells derived from a single lineage but showing a range of differentiation. Immature cells had nearly smooth to lightly cl eft or folded basophilic nuclei bordered by a band of cytoplasm with few cy toplasmic processes and containing a few small uniform eccentric granules. Mature cells had basophilic nuclei with deep clefts or folds and abundant e osinophilic cytoplasm with multiple long intertwining cytoplasmic extension s packed with metachromatic granules. The axolotls were old individuals fro m an inbred laboratory colony. The tiger salamanders were wild animals from a single polluted pond. They could have been old and inbred. Both groups w ere neotenic. These are the first mastocytomas discovered in cold-blooded a nimals.