The extra-embryonic area opaca plays a role in positioning the primitive streak of the early chick embryo

Classical studies have established that the marginal zone, a ring of extra-embryonic epiblast immediately surrounding the embryonic epiblast (area pellucida) of the chick embryo, is important in setting embryonic polarity by positioning the primitive streak, the site of gastrulation. The more external extra-embryonic region (area opaca) was thought to have only nutritive and support functions. Using experimental embryology approaches, this study reveals three separable functions for this outer region. First, juxtaposition of the area opaca directly onto the area pellucida induces a new marginal zone from the latter; this induced domain is entirely posterior in character. Second, ablation and grafting experiments using an isolated anterior half of the blastoderm and pieces of area opaca suggest that the area opaca can influence the polarity of the adjacent marginal zone. Finally, we show that the loss of the ability of such isolated anterior half-embryos to regulate (re-establish polarity spontaneously) at the early primitive streak stage can be rescued by replacing the area opaca by one from a younger stage. These results uncover new roles of chick extra-embryonic tissues in early development.

Automated summary

Classical studies have shown that the extra-embryonic tissues in chicks, specifically the marginal zone, play a crucial role in establishing embryonic polarity. The outer region, previously thought to have only nutritive and support functions, was found to have three distinct functions: inducing a posterior marginal zone, influencing the polarity of the adjacent marginal zone, and rescuing the ability to regulate polarity in early embryos.