BACKGROUND: Tonsils are secondary lymphoid organs located in the naso- and oropharynx of most mammalian
species. Most tonsils are characterised by crypts surrounded by dense lymphoid tissue. However, tonsils without
crypts have also been recognised. Gut-associated lymphoid tissue (GALT), although not well-organised and lacking
tonsillar crypts, is abundant in the avian oropharynx and has been referred to as the “pharyngeal tonsil”. In this
context the pharyngeal folds present in the oropharynx of ratites have erroneously been named the pharyngeal
tonsils. This study distinguishes between the different types and arrangements of lymphoid tissue in the pharyngeal
region of D. novaehollandiae and S. camelus and demonstrates that both species possess a true pharyngeal tonsil
which fits the classical definition of tonsils in mammals.
RESULTS: The pharyngeal tonsil (Tonsilla pharyngea) of D. novaehollandiae was located on the dorsal free surface of
the pharyngeal folds and covered by a small caudo-lateral extension of the folds whereas in S. camelus the tonsil
was similarly located on the dorsal surface of the pharyngeal folds but was positioned retropharyngeally and
encapsulated by loose connective tissue. The pharyngeal tonsil in both species was composed of lymph nodules,
inter-nodular lymphoid tissue, mucus glands, crypts and intervening connective tissue septa. In S. camelus a shallow
tonsillar sinus was present. Aggregated lymph nodules and inter-nodular lymphoid tissue was associated with the
mucus glands on the ventral surface of the pharyngeal folds in both species and represented the Lymphonoduli
pharyngeales. Similar lymphoid tissue, but more densely packed and situated directly below the epithelium, was
present on the dorsal, free surface of the pharyngeal folds and represented a small, non-follicular tonsil.
CONCLUSIONS: The follicular pharyngeal tonsils in D. novaehollandiae and S. camelus are distinct from the pharyngeal
folds in these species and perfectly fit the classical mammalian definition of pharyngeal tonsils. The presence of a
true pharyngeal tonsil differentiates these two ratite species from other known avian species where similar
structures have not been described. The pharyngeal tonsils in these ratites may pose a suitable and easily accessible
site for immune response surveillance as indicated by swelling and inflammation of the tonsillar tissue and
pharyngeal folds. This would be facilitated by the fact that the heads of these commercially slaughtered ratites are
discarded, thus sampling at these sites would not result in financial losses.