Independent assignment of antero-posterior and dorso-ventral positional values in the developing chick hindbrain

Cell patterning in the developing central nervous system seem to involve a coordinate system of positional information, in which specific fates are assigned to multipotent precursor cells by positional signals acting on the antero-posterior and dorso-ventral axes of the neural tube. Before neurons differentiate in the hindbrain, it becomes subdivided antero-posteriorly into a series of developmental compartments, the rhombomeres. When the rhombomeres are delineated from each other by interfaces at which cell mixing is transiently restricted, they are determined for expression of specific selector Hox genes that may encode aspects of their individual identity. To assess whether the phenotypic identities of the rhombomeres are also determined at this stage, we have analyzed the capacity of individual rhombomeres to realize specific neuronal fates when grafted heterotopically along both antero-posterior and dorso-ventral axes.

When rhombomere 4 (r4) is grafted unilaterally to the r2 position, both facial motor neurons and contralateral vestibulo-acoustic efferent neurons differentiate, as normal, in the ventral region of the graft. These aspects of phenotypic identity therefore appear to have been determined at or before the time of grafting. When r4 is grafted to the r2 position with its dorso-ventral polarity inverted, both types of neuron again develop, but in the ventral region of the graft, in a position appropriate to the dorso-ventral pattern of the host, rather than their original dorso-ventral position. The change in fate of these cells is restricted, however, to the repertoire characteristic of the antero-posterior position of origin, in this case r4.

Cells seem to 'know' details of their presumptive fate before more general features. At this stage of development, precursor cells in r4 seem to have been assigned an 'r4 fate', but remain multipotent in their choice of r4-specific cell type. Precursor cells seem to be committed to their fates according to position on an orthogonal grid, the coordinates of which are set (or read) independently and sequentially. Thus, at the 7-10 somite stage, dorso-ventral positional values are still labile, whereas antero-posterior values are already fixed.