INDUCTION OF A 2ND NEURAL AXIS BY THE MOUSE NODE

The anterior aspect of the mouse primitive streak resembles the organi zer of Xenopus and chick in terms of its developmental fate, ability t o alter pattern in the chick limb bud and with respect to the repertoi re of genes that its constituent cells express. However, until now the re has been no direct evidence that the mouse node organizes pattern d uring gastrulation, nor that the exceptionally small mouse embryonic e gg cylinder:can be induced to form a second axis. Grafts of transgenic ally marked mid-gastrulation mouse node, or node labelled with DiI, to a posterolateral location in a host embryo of the same developmental stage results in the induction-of a second neural axis and the formati on of ectopic somites. The graft gives rise predominantly to notochord and endoderm tissue whereas the neurectoderm and somites are mainly o f host origin. The ectopic notochord formed is derived solely from the donor node which suggests that the node can serve as a 'stem cell' so urce of axial mesoderm. This is corroborated by the observation that l abelling in situ the population of cells on the outer surface of the m id-gastrulation node with DiI results in continuous labelling of the n otochord. DiI-labelled cells are present throughout the notochord from a rostral boundary in the cranial region to its most caudal extreme a nd the node itself always remains labelled.