The nasopalatine ducts and associated structures in the rhesus monkey (Macaca mulatta): topography, prenatal development, function, and phylogeny

Morphological and developmental characteristics of the rhesus monkey nasopalatine duct system and associated primary palatal structures are described along with functional and phylogenetic considerations. Examination of five adult palates and coronal sections of 13 fetal palates together with dissections of a sixth adult specimen and of a 119-day-old fetal palate reveal that the lateral lobes of the tripartate incisive papilla cover clefts leading into the ducts. The ducts pierce the bony palate to enter the nasal fossae in proximity to the incisive suture. The ontogenetic stability of the duct path reflects the retention of ancient duct and primitive choanae relationships and functionally maintains an optimal oral odorant-to-receptor channel. Sixteen timed pregnancy specimens (35-100 days) provided histological material for documenting rostral nasopalatal development. Duct primordia, identified at 35 days, had by 40 days formed the medial duct walls (conjoined septum-papilla from the primary medial palatal component), the lateral duct walls (maxillary processes), and the rostral walls (fused maxillary-intermaxillary components). The caudal walls derive from the fusion of palatal shelves with the papilla (45 days), thus distinguishing primary and secondary fusion modes. Duct epithelial maturation occurs between 70 and 100 days. The absence of a vomeronasal system is attributed to reduction of olfaction in reproductive behavior, while the presence of the coevolved nasopalatine ducts is linked to the persistence of epiglottal-velar valving. The ducts serve as oral food-odor conduits in otherwise functionally separated respiratory and digestive tracts.