The three-dimensional microanatomy of the pancreatic duct system in the Japanese monkey, Macaca fuscata, with special reference to fine proximal passages of a high species-specificity

Abstract

We have previously shown the duct system in the rat pancreas to consist of two parts: a fine proximal (intercalated) duct and thicker distal (intralobular and interlobular) duct, with the latter part displaying morphological signs indicative of a bicarbonate-rich fluid secretion. In this study the pancreatic duct system in the Japanese monkey Macaca fuscata was observed by scanning electron microscopy after the hydrolytic exposure of cell surfaces as well as by transmission electron microscopy of ultrathin sections. Cellular expression of the water channel aquaporin 1 (AQP1) was also examined immunohistochemically. In contrast to the segmented duct system in the rat, all the duct cells in the monkey pancreas consistently displayed rich mitochondria in the cytoplasm, elaborate interdigitations of cell processes, and an intense immunoreactivity for AQP1 on the apical and basolateral cell membrane to favor active ion transport and osmotic water movement across the epithelium. Both the existence of secretory canaliculi and basal trabeculae in the duct epithelium and randomized localization of primary cilia on the luminal cell surfaces were demonstrated for the first time in monkeys, and the physiological implications of these phenomena are discussed.The pancreatic duct system is known as the site of a secretin-stimulated secretion of a bicarbonate-rich fluid which solubilizes acinar enzymes and neutralizes acidic chyme in the duodenum (3). Micropuncture studies with intact gland preparations and isolated duct segments directly demonstrated the secretory ability of the interlobular ducts (10, 20). However, the functions of smaller ducts in the pancreatic lobules remain unclear because of their inaccessibility by fine analytical techniques. By fuchsin staining of the guinea-pig pancreas, Bensley (1) reported centroacinar and intercalated duct cells in the lobules to contain considerable amounts of mitochondria that can energize active transport. His finding was subsequently corroborated in dogs and humans by transmission electron microscopy (TEM) (9,13,15). By scanning electron microscopy (SEM), Fujita et al. (5) demonstrated dense microvilli on basal surfaces of intercalated duct cells in the dog pancreas, indicating the transcellular permeation of large amounts of substances across the duct epithelium. However, our previous TEM and SEM observations have shown the rat pancreatic duct system to consist of two parts: a fine proximal (intercalated) duct which lacks morphological specialization for active transport, and a thicker distal (intralobular and interlobular) duct characterized both by the accumulation of mitochondria and elaboration of the cell membrane (21, 25). More systematical analyses are called for concerning the duct cell morphology in different animal species.Employing physiological experiments and geneexpression analyses, Ko et al. (14) identified the