FoxI1e activates ectoderm formation and controls cell position in the Xenopus blastula

The segregation of the vertebrate embryo into three primary germ layers is one of the earliest developmental decisions. In Xenopus, where the process is best understood, the endoderm is specified by a vegetally localized transcription factor, VegT, which releases nodal signals that specify the adjacent marginal zone of the blastula to become mesoderm. However, little is known about how the ectoderm becomes specified. In this paper, we show that the forkhead protein FoxI1e (also known as Xema) is required at the blastula stage for normal formation of both the central nervous system and epidermis, the two early derivatives of the ectoderm. In addition, FoxI1e is required to maintain the regional identity of the animal cells of the blastula, the cells that are precursors of ectodermal structures. In its absence, they lose contact with the animal cap, mix with cells of other germ layers and differentiate according to their new positions. Because FoxI1e is initially expressed in the animal region of the embryo and is rapidly downregulated in the neural plate, its role in neural and epidermal gene expression must precede the division of the ectoderm into neural and epidermal. The work also shows that FoxI1e plays a role in the embryo in the poorly understood process of differential adhesion, which limits cell mixing as primary germ layers become specified.