Palatine tonsils -: are they branchiogenic organs?

The palatine tonsil is not a real branchiogenic organ, even though it develops together with the thymus, parathyroids and ultimobranchial body in the pharyngeal region, in which in Anamniots, the gills are situated. It evolved much later as a phyletic successor of the thymus, but a large time difference exists between both organs also in ontogeny. The tonsil develops in the region of the previous second branchial cleft which is the only one whose branchial membrane breaks open and allows a mixing of cleft ectoderm with the pouch entoderm. The authors suppose that there is only a slight participation of ectoderm in the early development of crypt epithelium, which can explain the formation of tonsillar corpuscles. The connective tissue structures develop from the ectomesenchyme of epibranchial placode and neural crest origin. Whereas the thymus represents an archaic organ, common in all vertebrates, the tonsil belongs to the very modem mammalian organs, which is not yet fully anchored among most modem immune structures, and which evolves in coevolution with the changing antigenic situation. The full development of tonsils also depend in ontogeny on their antigenic stimulation, as was proved in the experiments on germ-free piglets. (C) 2003 Elsevier B.V. All rights reserved.