Roles of Pax-2 in initiation of the chick tectal development

Transplantation experiments have shown that the mes-metencephalic boundary (isthmus) acts as an organizer for the development of the optic tectum. We have cloned Pax-2 which is expressed in the isthmus. Previously it was show n that Pax-5, a member of the same Pax subfamily as Pax-2, transformed the diencephalon into a tectum-like structure and induced isthmus- and tectum-r elated genes both in the mesencephalon and in the diencephalon. In order to define the distinct roles between Pax-2 and Pax-5 in development of the te ctum, we expressed Pax-2 ectopically in the mesencephalon and the diencepha lon of E2 chick embryos by in ovo electroporation. Histological observation demonstrated that Pax-2 transformed the diencephalon into a tectum-like st ructure. In Pax-2, transfected embryos the expression of isthmus- and tectu m-related genes such as Fgf8 and En-2 was induced in the diencephalon. Howe ver, neither Fgf8 nor En-2 expression was induced in the mesencephalon, mak ing a striking contrast with the result of Pax-5 misexpression. In E2 chick embryos, the mesencephalon is committed of its fate to differentiate into the tectum, but the diencephalon has plasticity on its fate. Moreover, Pax- 2 expression in the isthmus precedes Pax-5 expression. Taking these results into consideration, it is suggested that Pax-2 plays a crucial role in ini tiation of the tectal development, and that Pax-5 functions to maintain the state of tectal differentiation. (C) 1999 Elsevier Science B.V. All rights reserved.