Primary intraocular melanomas originate from melanocytes in the uveal tract.[1] Four distinct cellular types are recognized in intraocular melanoma (revised Callendar classification):[2]

1. Spindle A cells (spindle-shaped cells with slender nuclei and lacking visible nucleoli).
2. Spindle B cells (spindle-shaped cells with larger nuclei and distinct nucleoli).
3. Epithelioid cells (larger polygonal cells with one or more prominent nucleoli).
4. Intermediate cells (similar to but smaller than epithelioid cells).

Most primary intraocular melanomas contain variable numbers of epithelioid, spindle A, and spindle B cells (mixed-cell melanomas). Pure epithelioid-cell primary melanomas are infrequent (approximately 3% of cases).[1] In the Collaborative Ocular Melanoma Study, mixed-cell type melanomas predominated (86% of cases).[3]

Several microscopic features can affect the prognosis of intraocular melanoma, including cell type, mitotic activity, lymphocytic infiltration, and, possibly, fibrovascular loops.[1] Cell type remains the most often used predictor of outcome following enucleation, with spindle A cell melanomas carrying the best prognosis and epithelioid cell melanomas carrying the worst.[1,4]

References

1. Klintworth GK, Scroggs MW: The eye and ocular adnexa. In: Sternberg SS, ed.: Diagnostic Surgical Pathology. Philadelphia, Pa: Lippincott Williams & Wilkins, 1999, pp 994-6. 
   
   
2. Grossniklaus HE, Green WR: Uveal tumors. In: Garner A, Klintworth GK, eds.: Pathobiology of Occular Disease: A Dynamic Approach. 2nd ed. New York, NY: M. Dekker, 1994, pp 1423-77. 
   
   
3. Histopathologic characteristics of uveal melanomas in eyes enucleated from the Collaborative Ocular Melanoma Study. COMS report no. 6. Am J Ophthalmol 125 (6): 745-66, 1998.  [PUBMED Abstract]
   
   
4. McLean IW: Prognostic features of uveal melanoma. Ophthalmol Clin North Am 8 (1): 143-53, 1995.