Summary

A collision tumour in ovary comprising a small ovoid embryoid mature teratoma in the surface wall of large mucinous cyst adenoma is reported in a 36-year-old female and its pathogenesis is discussed. The oval teratoid mass externally to the mucinous cyst lining did not show any columnar epithelial or mucin-secreting cyst. Findings suggested that the mature embryoid teratoma originated from ovarian germ cell rest in the surface wall of the cyst adenomatous neoplasm. Another possibility could be that the mature embryoid teratoma resulted from parthenogenesis of ovum together with the mucinous adenoma cysts by epithelial metaplasia of the follicular lining.