Melanomas (amelanotic type) in albino rats

Albino rats lack melanin production due to a defect in the tyrosinase gene. Melanomas arising in albino rats, which originate from neural crest cells, are non-melanin-producing and are referred to as amelanotic melanomas (AM). AM in albino rats represents one of the most challenging tumors to diagnose pathologically using light microscopy, primarily because of the absence of melanin pigment. Although electron microscopy remains the gold standard for AM diagnosis, PNL2 serves as a practical alternative, particularly in toxicologic studies. To date, only eight reports have definitively diagnosed AM in albino rats by confirming the presence of premelanosomes through ultrastructural analysis. This review aims to provide toxicologic pathologists with information to aid in diagnosing melanomas by analyzing eight previous reports along with data from our own case studies.