TRANSCRIPTION FACTORS AND HEAD FORMATION IN VERTEBRATES

Evidence from Drosophila and also vertebrates predicts that two differ ent sets of instructions may determine the development of the rostral and caudal parts of the body. This implies different cellular and indu ctive processes during gastrulation, whose genetic requirements remain to be understood. To date, four genes encoding transcription factors expressed in the presumptive vertebrate head during gastrulation have been studied at the functional level: Lim-1, Otx-2, HNF-3 beta and goo secoid. We discuss here the potential functions of these genes in the formation of rostral head as compared to posterior head and trunk, and in the light of recent fate map and expression analyses in mouse, chi ck, Xenopus and zebrafish. These data indicate that Lim-1, Otx-2 and H NF-3 beta may be involved in the same genetic pathway controlling the formation of the prechordal mesendoderm, which is subsequently require d for rostral head development. goosecoid may act in a parallel pathwa y, possibly in conjunction with other, yet unidentified, factors.