Cellular morphology and extracellular space at rhombomere boundaries in the chick embryo hindbrain

The chick embryo hindbrain is a segmented region of the CNS characterised by repeated blocks of neuroepithelial cells, known as rhombomeres. Individual rhombomeres are polyclonal compartments, defined both by cell lineage restriction and by the restricted expression of development control genes, that later acquire specific patterns of neuronal differentiation and axon outgrowth. The interfaces between adjacent rhombomeres are defined by boundaries across which cells do not move; the boundaries contain specialised cells and are preferentially colonised at early stages of development by extending axons. In this study, routine electron microscopy and high-pressure cryopreservation, a technique that avoids artifacts of chemical fixation, have been used to examine the morphology of rhombomere boundaries through a staged series of chick embryos. We find that the boundary regions contain enlarged extracellular spaces and that these form conduits for axons subsequently extending in the circumferential plane of the hindbrain. Labeling the ventricular surface of the neuroepithelium with DiI crystals in aqueous suspension revealed the morphology of individual cells in the intact neural tube, and demonstrated unusual fan-shaped arrays of cells at the boundaries. These findings contribute further to the evidence that cells at rhombomere boundaries differ from those in rhombomere centres, and leads to hypotheses about both the mechanism of development of the boundaries, and the role they may play in hindbrain patterning.