CELL AND MATRIX SPECIALIZATIONS OF RHOMBOMERE BOUNDARIES

Hindbrain segments, rhombomeres, define distinct cellular and molecula r domains which furnish the ground plan for important aspects of neura l and cranial development. In this study, further evidence is presente d that the interfaces between rhombomeres, rhombomere boundaries, cont ain both cells and extracellular matrix with specialised characteristi cs. Cells at rhombomere boundaries show temporally and spatially disti nct expression patterns of developmentally important genes. Towards th e end of the developmental period when rhombomeres are present, a fan- shaped array of cells at rhombomere boundaries, that constitute the ve ntricular ridge, shows decreased expression of two genes (Hoxb-1 and K rox-20), which earlier in development were expressed in all cells of s pecific rhombomeres. In contrast, these boundary cells show increased expression of another gene, Pax-6, which earlier in development has a rhombomere-specific expression pattern. A specialised identity for bou ndary cells is further suggested by increased labelling with an anti-v imentin antibody at rhombomere boundaries, indicating that at least so me boundary cells are radial glia or glial precursors. In addition to distinct cellular properties, the extracellular domain at rhombomere b oundaries is also specialised. Chondroitin sulphate proteoglycan (CSPG ) immunoreactivity is increased and, as revealed by immuno-electron mi croscopy, localised to extracellular spaces. CSPG is also enriched in boundaries regenerated after ablation, or boundaries generated ectopic ally by rhombomere transplantation. We propose that rhombomere boundar ies form their characteristic morphology at the interface between grou ps of cells with differing molecular characteristics, representing dif ferent cell states. A specialised band of cells then develops at the i nterface. Both the boundary cells and extracellular matrix have charac teristics which could be important in later events of neural developme nt such as axon guidance and cell migration. (C) 1995 Wiley-Liss, Inc.