The Formation of the Ectoderm in Vertebrate Embryos
All tissues of the vertebrate body arise from three primary germ layers. The outer layer, the ectoderm, forms the central nervous system and epidermis. The middle layer, or mesoderm,forms all connective tissues of the body, as well as the muscles, immune, urino-genital, and cardiovascular organ systems. The inner layer, or endoderm, forms the lining of the gastro intestinal tract, and the glands associated with it (including the liver and pancreas).
How are the primary germ layers formed in the early embryo? Our lab studies this problem, using the Xenopus embryo as a model. In this project, we are studying the formation of the ectoderm, the most poorly understood. We started this work with an Affymetrix screen to identify genes whose expression was most up-regulated in the cells which form ectoderm. This identified a Forkhead class gene, Foxi1e, which is expressed after zygotic transcription has started, and in a highly patterned manner in the embryo. First, it is expressed only in the animal half of the embryo. Second it is expressed in a mosaic pattern; expressing cells are surrounded by non-expressing cells. Third it is largely excluded from the outermost layer of cells of the animal hemisphere. Both gain and loss of function experiments showed that Foxi1e is essential for normal differentiation of the ectodermal tissues, and this raised some very interesting and novel questions about ectoderm formation. First, how is Foxi1e switched on in the embryo? Second, how does it become patterned in its expression? Third, not all cells in the ectoderm-forming region express Foxi1e, but they all become ectoderm. How does Foxi1e control ectoderm formation in cells that do not express it?
These are the questions we are addressing now in this NIH-funded project.
