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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.
FIG. 1. Fate map of the late blastula Xenopus laevis embryo. Schematic diagram of a Xenopus laevis late blastula (based on the work of
Keller, 1975, 1976) prior to formation of the bottle cells and the dorsal lip. Fate-mapping analysis has shown that the three germ layers show
patterning along the anterioposterior and dorsoventral axis. The domains fated to give rise to ventral and posterior cell fates are highlighted.
The ectoderm originates mostly from the animal pole (upper panel). The non-neural, epidermal ectoderm (blue) originates from the ventral
two-thirds of the animal pole. The neural ectoderm (striped) originates from the ectoderm overlaying the dorsal lip. Notice that the anterior
domains of both neural and non-neural ectoderm form adjacent to each other at the neural/epidermal boundary, where neural crest cells
develop. The origins of this boundary and the presence of patterning signals in this boundary are unknown. The mesoderm originates from
the marginal zone (MZ; middle panel). The dorsal end is determined as the place where the dorsal lip will form. In the dorsal marginal zone
(DMZ), the notochord (NC) and head mesoderm (HM) form most dorsally (purple). Along the MZ, tissues derived in a dorsoventral gradient
are: paraxial mesoderm (muscle), heartmesoderm, lateral plate mesoderm (pronephros), and blood (ventral-most mesoderm; yellow). Notice
that the ventral marginal zone (VMZ) encompasses 300° in the radial embryo, whereas the DMZ only 60°. Fate maps have revealed that,
along the anteroposterior axis, the somites, a dorsal mesodermal derivative, also originate from the ventral marginal zone. A minor
percentage of the blood islands, a ventral-most derivative, originate from the DMZ, close to the organizer (see text for details). The
endoderm originates exclusively from the vegetal pole (bottom panel). Image shows the fate map of the superficialendoderm. No fate maps
are available from the deependoderm. Notice how the posterior archenteron develops from mostly ventral endoderm. The anterior and
middle archenteron develops from the dorsal endoderm (yellow and striped pattern), reinforcing the notion that there is a linkage of the
ventroposterior and dorsoanterior axis in all three germ layers.
FIG. 2. Experimentally perturbed Xenopus embryos and the dorsoanterior index (DAI). Varying amounts of exposure to UV irradiation
during the first cell cycle yields embryos that are increasingly ventroposteriorized (DAI of 0, most ventroposteriorized, to DAI of 5,
untreated embryo). Exposure to LiCl for varying times yields embryos progressively more dorsoanteriorized (DAI of 10, most dorsoanteriorized).
At the extremes of the DAI index, an embryo with a DAI of 0 lacks the head and axial structures entirely. An embryo with a DAI
of 10 contains only radially symmetrical head structures. These experimentally disturbed embryos, and the DAI index, illustrate that the
ventral and posterior axis, on the one hand, and the dorsal and anterior axis, on the other, are linked in vertebrates.
FIG. 3. Signaling pathways implicated in ventral/posterior fate specification in Xenopus laevis. The four major signaling pathways
implicated in ventral and posterior fate specification are shown. (A) BMP/GDF pathway; (B) TGF-b/Activin/Nodal pathway; (C) FGF
pathway; (D) Wnt pathway. In the case of the Activin/Nodal pathway, a graded response to these molecules acts to pattern the mesoderm
along the dorsoventral and anteroposterior axis. Highest dose of these molecules results in more dorsoanterior fates, and therefore the figure
indicates their effect in the dorsal pathway. For a discussion on the role of the TGF-b, nodal and activin pathways in ventral and posterior
fates, see the text. Please see the text for a discussion on the specific involvement of each signaling pathway in the specification of ventral
and posterior fates in each of the three germ layers. Green arrows indicate stimulatory regulators, red arrows inhibitory regulators of the
signaling pathways. Question marks indicate our lack of knowledge of the molecules implicated in each step.
