SEGMENTATION OF THE VERTEBRATE HINDBRAIN - A TIME-LAPSE ANALYSIS

The chick hindbrain starts from a simple and relatively uniform axis a nd becomes segmented into repeating units, called rhombomeres. The rho mbomeres become sites of cell differentiation into specific neurons an d the location from which neural crest cells emerge from the neural tu be to form the peripheral nervous system, which has only been analyzed at distinct time points due to the lack of a method to watch the neur al tube as it is shaped into segments. We have developed a whole-embry o explant culture system in order to study cell and tissue movements w ith time-lapse video microscopy. Quantitative analyses of the neural t ube during its segmentation show that not all rhombomeres are shaped b y the same mechanism. In the rostral hindbrain, or first three segment s, rhombomeres are shaped by an expansion in the lateral width of the mid-rhombomere; either a smaller expansion or a constriction takes pla ce at the rhombomere boundaries. In the caudal hindbrain, the rhombome re boundaries constrict more than the mid-rhombomere lateral widths in crease or decrease, leading to the shaping of the segments. Throughout the segmentation process the rostrocaudal lengths of all rhombomeres remain nearly constant indicating that shape changes are influenced by lateral expansions and constrictions of the neural tube.