Neural crest interactions in the development of the immune system.

The thymus is formed by contributions from different sources which must interact in a precisely timed sequence for proper development. Pharyngeal pouch epithelium interacts with embryonic connective tissue in preparation for migration of blood-borne lymphoid stem cells into the thymic primordium. The embryonic connective tissue in this region is derived from neural crest. Ablation of small portions of neural crest prevents or alters the development of the thymus. Other pharyngeal pouch derivatives such as thyroids and parathyroids are affected. Furthermore, defects in the heart and great vessels are produced. These observations establish experimentally the role of neural crest in defective immune development such as that seen in the DiGeorge syndrome. It seems likely that various combinations of defects will be rendered more understandable through recognition of the contributions made from neural crest.