Scanning electron microscopy of the rat exocrine pancreas.

The three-dimensional architecture of the rat exocrine pancreas was studied by scanning electron microscopy (SEM) with the HCl digestion method. The treatment with HCl effectively removed connective tissue elements enclosing the ducts and the acini. Thus their shapes and relations were clearly revealed under the SEM. Furthermore fine surface features of the acinar or ductal cells were also demonstrated. Although recent studies have suggested that the pancreas might be reticular in architecture due to anastomoses between adjacent ducts or acini, the present investigation confirmed that rat pancreas is a compound acinar (alveolar) gland. Excretory ducts extended into interlobular spaces, branching dichotomously. Intercalated ducts, which showed an almost constant thickness, issued directly from the excretory ducts, entered the pancreatic lobuli, and branched several times to connect with numerous acini of varying size and shape. Basal surfaces of the excretory ducts showed conspicuous reticular undulation, while those of the intercalated ducts were smooth except for some areas with fine unevenness near the epithelial cell boundaries. It remains to be elucidated whether these basal structures of the intercalated ducts are involved in their assumed function, i.e., the secretion of water and bicarbonate in pancreatic juice. The acinar lumen was a narrow secretory canaliculus covered with finger-like microvilli. Intercellular canaliculi branched from the central canaliculus, taking a straight course toward the acinar cell base, and ended blindly. Some canaliculi extended to the close vicinity of the basement membrane, providing a locus minor is resistantiae for the leakage of pancreatic enzymes into blood. The SEM findings on the canalicular system were confirmed by light microscopic observation of the pancreatic excretory passages stained by the rapid Golgi method, and by three-dimensional reconstruction of the pancreatic acini from serial semithin sections.