Early posterior/ventral fate specification in the vertebrate embryo.

One of the central questions in developmental biology is that of how one cell can give rise to all specialized cell types and organs in the organism. Within the embryo, all tissues are composed of cells derived from one or more of the three germ layers, the ectoderm, the mesoderm, and the endoderm. Understanding the molecular events that underlie both the specification and patterning of the germ layers has been a long-standing interest for developmental biologists. Recent years have seen a rapid advancement in the elucidation of the molecular players implicated in patterning the vertebrate embryo. In this review, we will focus solely on the ventral and posterior fate acquisition in the ventral-lateral domains of the pregastrula embryo. We will address the embryonic origins of various tissues and will present embryological and experimental evidence to illustrate how "classically defined" ventral and posterior structures develop in all three germ layers. We will discuss the status of our current knowledge by focusing on the African frog Xenopus laevis, although we will also gather evidence from other vertebrates, where available. In particular, genetic studies in the zebrafish and mouse have been very informative in addressing the requirement for individual genes in these processes. The amphibian system has enjoyed great interest since the early days of experimental embryology, and constitutes the best understood system in terms of early patterning signals and axis specification. We want to draw interest to the embryological origins of cells that will develop into what we have collectively termed "posterior" and "ventral" cells/tissues, and we will address the involvement of the major signaling pathways implicated in posterior/ventral fate specification. Particular emphasis is given as to how these signaling pathways are integrated during early development for the specification of posterior and ventral fates. Copyright 2001 Academic Press.