Morphology, glicohistochemistry and immunolocalization of Aquaporin 5 in the mandibular gland of the giant anteater (Myrmecophaga tridactyla)

Background: The giant anteater (Myrmecophaga tridactyla) is a toothless mammal that feeds mainly on termites and ants. Therefore, like other toothless mammals, this species has morphological and physiological adaptations of the salivary glands related to eating habits. Saliva is essential for the health of the oral cavity, chewing and lubrication of the mouth and it is constituted by an aqueous fluid containing electrolytes, enzymes, and glycoproteins which play an important role in modulating the oral microbiota. The present study investigated the morphology, glycan pattern and the aquaporin 5 (AQP5) expression in the giant anteater mandibular gland.

Methods: The morphology of the mandibular gland was studied by gross and microscopic anatomy; the glycopattern was characterized using PAS, Alcian blue, and lectin histochemistry; AQP5 was localized immunohistochemically.

Results: The mandibular gland had big dimensions, irregular shape and showed the classic morphology of the salivary glands. The gland parenchyma consisted of secretory acini, containing acinar cells and demilune cells, and intercalated, striated and interlobular ducts. Secretory acini secrete both neutral and acidic glycans (PAS and AB 2.5 positivity) and expressed a glycopattern characterized by O-linked glycans terminating with Galβl,3GalNAc (PNA affinity) and GalNAc (DBA and SBA positivity) and few N-linked glycans (Con A staining) containing also terminal lactosamine (RCA₁₂₀ reactivity). Demilune cells contained Con A and RCA120 binding sites as well as α2,6-sialoglycans (SNA binders). The epithelium lining of the striated and interlobular ducts displayed α1,6/ α1,4-fucosylated and α2,3sialoglycoproteins (AAL and MAL II affinity) in addition to the aforementioned glycans. The lumen of the secretory acini contained high-mannosylated and lactosamine-terminating glycans, fucosylated and α2,6-sialoglycans. The presence of α2,6-sialoglycans indicates the role of demilune cells in the production and acidification of saliva. Along the ductal system the saliva enriched with O-linked glycans terminating with Galβl,3GalNAc, GalNAc, and α2,3-linked sialic. As for AQP5, it was detected in the acinar and demilune cells but not in the ductal epithelium.

Conclusions: These results demonstrate that the saliva secreted by mandibular gland of the giant anteater contains a complex pattern of glycoproteins whose glycosylation also occurs in the ductal system. Lastly, AQP5 of the secretory acini is involved in the production and regulation of saliva viscosity in this toothless and myrmecophagous mammal. Overall, these findings may represent a benchmark for future studies of mandibular glands of other toothless animals.