Chapter 11, Problem 1DQ

Summary Introduction

To review:

The nature of processing of chordin and BMP pathways in flies and vertebrates, and the mechanism by which chordin-BMP axis allow the regulation of the same pattern of embryonic axis in both smaller and larger embryo.

Introduction:

The dorso-ventral patterning of the bilaterally symmetrical organism is controlled by the chordin-BMP pathway. This pathway forms the signaling centers at antipodal positions and despite of being conserved during evolution, the component perform different functions. The pattern forming reactions are responsible for the generation of signaling centers which are self regulated and acts as a organizer to form the primary embryonic axes. Basically, two organizer acts at each terminal position, one acts as primary system and this system helps in establishing the secondary system at a distance. A number of related models helps in describing the dorso-ventral pattern formation in different phyla, chordin and BMP pathway is one of the important model for this signalling pattern.

Explanation of Solution

Bone morpogenetic proteins (BMPs) are considered as a long-ranging inhibitory component which restrict the size of the organizer, wheras the chordin acts as an activator and both are expressed in partially overlapping region. The ectoderm is induced to be the epidermis in the presence of BMPs, while the inhibition of BMPs leads to the development of the nervous tisue from the ectoderm. To fullfill this requirement organizer produces the BMP inhibiting molecules. The three primary BMP inhibitors are Chordin, Noggin and Follistatin. The gene

expressing these proteins are considered as one of the most important genes activated by the Smad2 and Siamois/Twin. Norrin is the fourth inhibitor helps in blockage of BMP in dorsal ectoderm. Chordin mRNA has been localized in dorsal blastopore lip and in notocord at the later stage, it helps in inducing secondary nervous system. In different organism BMP-chordin pathway bears its homologue like Decapentaplegic (Dpp) is a BMP homologue and BMP antagonist short gastrulation (Sog) is a homologue of chordin. In Drosophila embryo chordin produces ventral nervous system and in Xenopus embryo it is localized at dorsal site. In both of these cases sog/chordin helps in the specification of neural tissue by blocking the adverse effects Dpp/BMP4.

Conclusion

Thus it is concluded that in both flies and vertebrates chordin and BMP pathway follows a closely related pattern involving BMP inhibition by chordin and few other protein molecules to induce the nervous system from the ectoderm and also regulate the embryonic axis pattern formation.