Neuraxial patterning is a continuous process that extends over a protracted period of development. During gastrulation a crude anteroposterior pattern, detectable by molecular markers, is conferred on the neuroectoderm by signals from the endomesoderm that are largely inseparable from those of neural induction itself. This coarse-grained pattern is subsequently reinforced and refined by diverse, locally acting mechanisms. Segmentation and long-range signaling from organizing centers are prominent among the emerging principles governing regional pattern.

Identification of specific neuronal populations and their projections in the developing hindbrain reveals a segmental organization in which pairs of metameric epithelial units cooperate to generate the repeating sequence of cranial branchiomotor nerves. Neurogenesis also follows a two-segment repeat, suggesting parallels with insect pattern formation.

In the chick embryo hindbrain, morphological segmentation into rhombomeres is matched by metameric patterns of early neuronal differentiation and axonogenesis. Boundaries between rhombomeres coincide with boundaries of expression of murine regulatory genes. By clonal analysis using intracellular marking, we show here that the rhombomere boundaries are partitions across which cells do not move. When a parent cell is marked before the appearance of rhombomere boundaries, the resulting clone is able to spread into the neighbouring rhombomere. When marked after boundary appearance, the clone still expands freely within the rhombomere of origin, but it is now restricted at the boundaries. Rhombomeres in the chick embryo thus behave like polyclonal units, raising the possibility that they are analogous to the compartments of insects.

Fifteen years ago, cell lineage restriction boundaries were discovered in the embryonic vertebrate hindbrain, subdividing it into a series of cell-tight compartments (known as rhombomeres). Compartition, together with segmentally reiterative neuronal architecture and the nested expression of Hox genes, indicates that the hindbrain has a truly metameric organization. This finding initiated a search for compartments in other regions of the developing brain. The results of recent studies have clarified where compartment boundaries exist, have shed light on molecular mechanisms that underlie their formation and have revealed an important function of these boundaries: the positioning and stabilization of local signalling centres.