CELLULAR MORPHOLOGY AND EXTRACELLULAR-SPACE AT RHOMBOMERE BOUNDARIES IN THE CHICK-EMBRYO HINDBRAIN

The chick embryo hindbrain is a segmented region of the CNS characteri sed by repeated blocks of neuroepithelial cells, known as rhombomeres. Individual rhombomeres are polyclonal compartments, defined both by c ell lineage restriction and by the restricted expression of developmen t control genes, that later acquire specific patterns of neuronal diff erentiation and axon outgrowth. The interfaces between adjacent rhombo meres are defined by boundaries across which cells do not move; the bo undaries contain specialised cells and are preferentially colonised at early stages of development by extending axons. In this study, routin e 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 chi ck embryos. We find that the boundary regions contain enlarged extrace llular spaces and that these form conduits for axons subsequently exte nding in the circumferential plane of the hindbrain. Labeling the vent ricular surface of the neuroepithelium with DiI crystals in aqueous su spension revealed the morphology of individual cells in the intact neu ral tube, and demonstrated unusual fan-shaped arrays of cells at the b oundaries. These findings contribute further to the evidence that cell s at rhombomere boundaries differ from those in rhombomere centres, an d leads to hypotheses about both the mechanism of development of the b oundaries, and the role they may play in hindbrain patterning. (C) 199 3 Wiley-Liss, Inc.