Age-related morphological and ultrastructural changes in the palate and pharyngeal masticatory apparatus of grass carp (Ctenopharyngodon idella) juveniles.

Grass carp (Ctenopharyngodon idella or C. idella) is a Cyprinid fish frequently utilized for aquaculture, medical, and research purposes. In C. idella, the palate is followed by a well-developed pharyngeal masticatory apparatus. The latter consists of an upper chewing pad superimposing a paired set of teeth. The present study investigated morphological, morphometric, histochemical, and surface ultrastructural changes involving these structures in C. idella juveniles at three different timepoints namely 35 mm total length (TL; 59 days posthatching (dph); fry-fingerling transition), 70 mm TL (90 dph; fingerling), and 210 mm TL ( 365 dph: yearling). The palatal epithelium revealed a constant number of taste buds. However, the height and width of these buds revealed an age-dependent increase. The number of palatal acidic goblet cells increased gradually with age. Enhanced keratinization of pad epithelium, and increased teeth dimensions were age-associated characteristics. Ultrastructurally, the palatal surface of C. idella was slightly papillated at 35 mm TL after which it formed brick-like structural units that tended to cluster into longitudinally paralleled rows toward the palate-pad junction. Goblet cell openings appeared oval at 35 mm TL and became club-shaped by 210 mm TL at which the epithelium appeared compact and heavily coated in mucus. Indentations of pad surface and signs of dental wear and tear were evident ultrastructurally at 70 mm TL and onwards. The current study reports for the first time age-related developmental features of the palate and pharyngeal masticatory apparatus of grass carp. Results of the present work will help to understand aging-associated factors involving the studied fish and other related aquatic species.