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Axis formationComments

Axis formation

Germ layers and body plan axes

In spite of the fact that the blastula stage embryos for various organisms look very different, and hence the movements of gastrulation leading to the organization of the body plan are different, the molecular control of DV axis formation is conserved across a wide phylogenetic distance.

Not only is BMP-4 a homolog of Drosophila Dpp and Chordin a homolog of Drosophila Sog, but Chordin can actually substitute for the function of Sog in the Drosophila embryo. Note that Dpp promotes formation of the most dorsal position in the Drosophila embryo, while BMP-4 promotes formation of the most ventral position in the Xenopus embryo. These observations have led to the proposal that the mechanism for establishing the dorsal-ventral axis has been conserved over 600 million years (the time between the divergence of the line leading to arthropods from that leading to chordates). The fact that Dpp promotes development of the dorsal side (where the heart is located in insects) and BMP-4 promotes development of the ventral side (where the heart is located in vertebrates) indicates that the dorsal-ventral axis became inverted in one of the ancestors. This is a beautiful example of how studying genes that control development can give us profound insights into evolutionary history.

We do not have any idea how the DV axis is established in mammals. However, we do know that BMPs and their antagonists are important for specifying D vs V mesoderm in mammals. For example, if mesoderm is explanted from a mouse embryo, exposure to BMP4 tends to lead to the development of ventral cell types (blood, heart). It is thought that a gradient of BMPactivity (high ventrally and low dorsally, due to the presence of BMP inhibitors) patterns the mesoderm.

HOW DID THE GERM LAYERS AND THE EMBRYONIC AXES EVOLVE?

These are among the most fundamental unanswered questions in biology. Despite the great variation in body form among metazoans (multicellular animals), the body plans of most species are organized along a longitudinal axis (A/P) and a perpendicular dorsal-ventral (D/V) axis. This defines a plane of bilateral symmetry, and such organisms are called bilaterian (bi = two). An exception is the sea urchin, which has a D/V axis, but is radially symmetric.

It is generally thought that bilaterian animals evolved from hollow, ciliated radially symmetric organisms. It is not clear which axis evolved first. The hollow ciliated muticellular organisms in turn are thought to have evolved from flagellated protozoan (unicellular) organisms.

Gastrulation is probably the most important event in metazoan evolution. It is the reason that animals are not hollow balls of cells, and permits the interaction of different tissue layers that build complex tissues and organs. The site of gastrulation is associated with the embryonic axes in almost all animals. We have no idea how gastrulation first evolved. Most theories hypothesize that a hollow animal underwent an invagination of ingression to create a second internal layer, which would have been the origin of the endomesoderm. Modern day organisms such as cnidarians (sea corals, sea anenomes) have only two germ layers (ectoderm and endomesoderm).

Although gastrulating embryos vary greatly in their morphology, some common patterns of gene expression can be found. For example, nuclear-beta-catenin location corresponds with the site of formation of endomesoderm across the animal kingdom. It seems that the mechanisms by which beta-catenin becomes localized are divergent. Another common feature of development is that the presence of an organizer region appears to be associated with the development of a gut tube that contains both a mouth and anus. Also, nodal expression is associated with the appearance of the mesodermal germ layer (as opposed to an endomesodermal layer). Finally, it appears that the Hox gene family, which we address in a separate lecture, is responsible for patterning along the A/P axis in all multicellular organisms.