Purpose: The pluripotent embryonic stem cells can differentiate into various kinds offormal tissues. There is no previous report on the differentiation of embryonic stem cellin the intraocular environment. In this paper, the authors tried to investigate theintraocular growth character of mice embryonic stem cells in nude mice.Methods: Murine embryonic stem cells were cultured and maintained in anundifferentiated state in vitro. They were transplanted into the right eyes of 20 nude miceby microinjection under operating microscope. Animal eye observation, light microscopeand immunohistochemical examinations were implemented.Results: Two to three days after transplantation, small pieces of gray-white materialcould be viewed in the vitreous cavity. Between the 15th and 20th day, the gray-whitemass grew into the anterior chamber in 4 nude mice eyes. Then, the mass at the anteriorchamber extended extraocularly. On the 30th day, a remarkable proptosis was observedin two of the four nude mice. In 6 to 45 days, the mice were executed for morphologicalexamination which showed the following typical structures: (1) Undifferentiated cellswith prominent nucleolius. (2) Flexner-Wintersteiner-like rosettes. (3) Medulloepithe-lioma-like structure: the cells were arranged in sheets, cords, tubes, and cysts. (4) Large,spindle-or astrocyte-like cells. (5) Cartilage-like structure. Immunohistochemically, mostof the cells were highly positive in NSE staining and a few cells were moderately positivein GFAP staining.Conclusions: Both animal eye findings and morphologic examinations certificated thatthe transplanted embryonic stem cells could grow in the eyes of nude mice anddifferentiate into intraocular medulloepithelioma.