Impaired development of the thymic primordium after neural crest ablation.

Impaired thymic development as a result of ablation of neural crest has been observed in embryos late in development. The present study was initiated to determine what changes are effected early in thymic development by neural crest ablation. The epithelial primordia of the thymus were studied in chick embryos on the sixth day of incubation. Embryos with neural crest ablations were compared with sham-operated and untreated controls. Neural crest ablation inhibited formation of epithelial thymic primordia. Primordia in experimental embryos were fewer in number and were smaller than in shams and untreated controls. When primordia from shams and controls were transplanted to the chorioallantoic membrane of chick hosts, they were able to develop into organs with the typical features of embryonic thymus. Similar transplantation from neural crest-ablated animals, on the other hand, led to small, predominantly epithelial structures with meager lymphoid development. These findings are consistent with the hypothesis that mesenchyme derived from cranial neural crest is critical in initiating and sustaining the development from pharyngeal pouches of epithelial structures competent to attract and support the proliferation and differentiation of lymphoid stem cells.