Axis development in avian embryos: the ability of Hensen's node to self-differentiate, as analyzed with heterochronic grafting experiments.

A series of experiments consisting of transplantation of Hensen's nodes has been conducted to examine axis development in avian embryos. In the first group of experiments, Hensen's nodes from quail embryos were transplanted homotopically and either isochronically or heterochronically to chick embryos, and the structures derived form the grafted nodes were assessed. The grafted Hensen's nodes typically self-differentiated structures appropriate for their stages, and the host embryos developed normally; the structures formed from grafted tissue usually merged caudally with the comparable host structures. Thus, even when the stages of the donor and host tissues were significantly mismatched (e.g., stage 3 donors and stage 9 hosts or vice versa), the graft was unable to repattern the host's neuraxis, and the host was unable to respecify the types of structures derived from the graft. In the second group of experiments, Hensen's nodes from quail embryos were transplanted to sites located just lateral to Hensen's nodes of host chick embryos, thereby providing the potential for development of additional axes. A single axis always resulted in each case in which further development occurred, with the graft self-differentiating its typical stage-specific structures, all of which merged caudally with comparable host structures. A final group of experiments served principally as a control and tested the ability of a part of Hensen's node, when it was transplanted to the extraembryonic germ cell crescent, to organize an ectopic embryo. In these experiments, the entire thickness and length of each Hensen's node, but only the central one-third to one-half of its width, was transplanted to host blastoderms, yet ectopic embryos, complete with induced neuraxes, were formed. Therefore, a part of Hensen's node has the ability to function fully as an organizer when placed in a conducive environment. Collectively, these results provide further documentation of the strong ability of Hensen's node to self-differentiate, and they suggest that once morphogenetic movements are under way, neuraxial structures can form, and characteristic rostrocaudal patterning of the neuraxis can occur, without sustained influence from Hensen's node.