| Summary: | Morphogenetic events consist of complex interactions of cells and extracellular materials resulting in the movement and rearrangement of groups of cells and their subsequent differentiation to form organs or organ systems. Although we can predict these movements for any given event, we have little understanding of how morphogenesis is controlled. In the starfish Pisaster ochraceus. assembly of mesenchyme cells on one particular region of the larval gut, the oesophagus, and their subsequent differentiation into muscle is an example of a simple morphogenetic event which is readily accessible for study. In their migration to the gut, the mesenchyme cells travel through a rich substrate of ECM. Upon their arrival at the presumptive esophagus, they come to settle on the BL underlying the endodermal epithelium. It is quite possible that interactions between the mesenchymal cells and 'the ECM/basal lamina are important in directing and regulating their differention into muscle.
The basal laminae and ECM of vertebrates and invertebrates is rich in glycoconjugates, including glycoproteins, proteoglycans and glycosaminoglycans. Ultrastructural studies of embryos of the asteroid Pisaster ochraceus have demonstrated that at the late gastrula stage, the endodermal basal lamina is thinner and less alcianophilic in the esophageal region. FITC and colloidal gold labelled lectins, which act as specific probes for carbohydrate moities, usually those at the terminal end of oligosaccharide chains, have been used to localize these sugars at the light and electron microscope levels. These studies show that a heterogeneity exists with respect to terminal sugars in the basal lamina, i.e. lectin binding of the basal lamina is not uniform in all regions of the embyro. Specifically, a statistical analysis of lectin binding determined that labelling with the two lectins, Au₂₅-Con A and Au₂₅-LFA was significantly reduced in the esophageal region as compared with the other regions of the embyro, while labelling of the BL with AU₂₅-DBA showed a similar intensity in all areas of the embryo. These results confirmed the alcian blue results described above and suggest that there are some sugar containing molecules, perhaps specific glycoproteins, GAGs and/or proteoglycans, which are present in reduced quantities in this region. In addition, these studies show a distinct labelling pattern of the ECM through which the mesenchymal cells migrate on route to the esophagus. Different lectins label different regions of the ECM, however it can not yet been said whether there is a regionally distinct pattern in the area of the migratory path of mesenchymal cells to the esophagus.
Proteoglycans and GAGs are involved in cell movement in vertebrates and sulfated glycoconjugates have been shown to be necessary for mesenchyme cell movement in echinoids. A decrease in proteoglycans and GAGs in the esophageal BL could therefore help to direct movement of the presumptive muscle cells to the esophagus by providing a "stop" signal. === Medicine, Faculty of === Graduate
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