Mesectodermal capabilities of the trunk neural crest of birds.

Orthotopic transplantation experiments have shown that in birds, under normal conditions, mesectodermal capabilities seem restricted to the cephalic neural crest down to the level of the 5th somite. In the present study the mesectodermal capabilities of trunk and lumbar neural crest were investigated at early stages of development by heterotopic, heterospecific transplantation of the neural primordium. The quail-chick nuclear marker system was used to identify the grafted cells. Mesectodermal cells did not arise from the trunk neural crest when this was implanted orthotopically, even though the neural primordium was taken early in development at the level of unsegmented plate mesoderm just anterior to Hensen's node. Mesectodermal derivatives (connective tissues, dermis and muscle but no cartilage or bone) developed from the same trunk neural crest fragments when they were heterotopically grafted at the cephalic level and mixed with host cephalic neural crest cells. These host cephalic neural crest cells emigrated from the contralateral neural primordium when the graft was unilateral or from the fringe area of the operation in cases of bilateral transplantations. As a control, unsegmented paraxial mesoderm was inserted alongside the cephalic neural tube; its cell did not migrate ventrally in the neural crest-derived area and they differentiated in the dorsal region of the host. These results indicate that mesectodermal capabilities, thought reduced, exist in the trunk neural crest at early stages of development but the differentiation of these mesectodermal derivatives is largely dependent upon environmental influences which may be found in early cephalic levels.