A morphological study of the oral cavity, pharyngeal cavity and oesophagus of the Nile crocodile, Crocodylus Niloticus (Laurenti, 1768)

Abstract

In view of the paucity of detailed information in the literature relevant to the upper digestive tract of the Nile crocodile, this study describes the morphological and histological features of the oral cavity (gingivae, palate and tongue), pharyngeal cavity and oesophagus of the Nile crocodile, Crocodylus niloticus (Laurenti, 1768) using light microscopy. The findings, which were supplemented by scanning electron microscopy, were compared with published information. The ciliated component of the oesophagus was also examined using transmission electron microscopy. The oral cavity had the form of a triangle and was dorso-ventrally flattened. The dorsal limit was formed by the palate and the ventral limit by the broad-based tongue. The close proximity of the tongue and palate severely limited the space within the cavity. The caudal border of the cavity was formed by the dorsal and ventral components of the gular valve. The epithelium of the palate, gingivae and tongue was stratified squamous in nature and appeared lightly keratinised. Specialised epithelial structures in the palate, gingivae and tongue, revealed by both light microscopy (LM) and scanning electron microscopy (SEM), bore characteristics resembling structures responsible for pressure and taste reception. Glandular tissue in the tongue was arranged in a triangular formation in the posterior region and displayed morphological features ascribed to salt secreting glands described in other Crocodilia. There were no palatine glands in the oral region of the palate, except that the oral surface of the dorsal gular fold contained branched tubular mucus secreting glands. The pharyngeal cavity was also dorso-ventrally flattened and was bordered rostrally by the flaccid dorsal gular fold, which displayed a median apical notch, and the ventral gular fold, which was supported internally by the broad rostral tip of the basihyal plate (hyaline cartilage). In the occluded mouth, the dorsal gular fold and the more rostrally positioned ventral component of the gular valve isolated the pharyngeal cavity. This arrangement is essential in preventing the crocodile from drowning (flooding of the pharyngeal cavity) while capturing prey. The roof of the pharyngeal cavity was characterised by the opening to the internal nares (an extension of the nasal passage from the external nares), the fibrous Eustachian plug sealing the common opening to the paired Eustachian ducts and a nodular tonsillar region, which was situated caudo-laterally to the Eustachian plug. Throughout this region, the epithelium was typically ciliated with goblet cells. However, the tonsillar nodules displayed regions of partial or no ciliation on their surface. SEM and stereomicroscopic observations showed fine longitudinal mucosal folding throughout the pharynx the distension of which, together with the large capacity for mucus production (produced by intraepithelial glands and mucus secreting glands), would facilitate the swallowing of large chunks of food in the living state. The ventrally situated laryngeal mound containing the slit-like glottis also displayed longitudinal folds and a ciliated epithelium. Anatomically, the oesophagus could be divided into two clear regions. The cranial, approximate two-thirds appeared broad and flabby. At the tracheal bifurcation, the oesophagus narrowed significantly and indicated a greater muscular content, confirmed by light microscopy. LM and SEM examination of the oesophagus, however, revealed three regional components, viz., the cranial, mid- and caudal regions. In the cranial region, the epithelium was densely ciliated with intervening goblet cells being present. In the mid-region the ciliated component decreased with a concomitant increase in the goblet cell component. In the caudal region there was a further decrease in the number of ciliated cells and a higher concentration of goblet cells. Transmission electron microscopy (TEM) of the ciliated component of the oesophagus showed typical ultrastructural features of both the ciliated and goblet cells.