Abstract
As to the fine structure of the pancreatic islets, numerous papers have been published. However, there are still some unsolved problems concerning the production and extrusion mechanisms of the secretory granules of islet cells.
In this sudy, the fine structural changes of islet cells especially insulinsecreting B-cells were observed under some experimental conditions such as the artificial hibernation of bats, Myotis lucifugus lucifgus, and administrations of alloxan, pilocarpine and dehydroascorbic acid (DHA) to dogs. Some other animals including the monkey, guinea pig, dove and snake were also used to study nervous elements in the pancreatic islets.
The islets of the bat contain A, B, C. D, and W-cells. The glucagon-secreting A-cells are a few in number and characterized by containing phloxine-staind red granules by light microscopy. These granules are electron dense spherical ones enveloped with a smooth membrane as observed by electron microscopy. The B-cells secreting insulin, which are numerous in number, are characterized by thioninestained blue secretory granules. These granules are amorphous masses or crystalloids enveloped by a smooth membrane, when observed with the electron microscope. The C-cells are rare and possess the clear cytoplasm without any granules, while D-cells which are also few in number, contain some ghostlike granules. Scarcely occurring W-cells are characterized by electron dense granules, which are similar in morphology to the A-cell granules, and contain a filamentous whorl.
The pancreas of bast and monkeys contained ganglion cells in the interlobular connective tissue or between acinar cells. The nerve endings within the pancreatic islets could be divided into two types : 1) Type 1 contained agranular synaptic vesicles (500 Å in diameter) along with a few large cored vesicles (1,000 Å), and were thought to be the cholinergic (parasympathetic). 2) Type 2 was characterized by containing vesicles of the same size as those of agranular synaptic vesicles, but a majority of these vesicles contained electron dense cores. These endings also contained a few large cored vesicles, and might be the adrenergic (sympathetic). In the pancreatic islet of the monkey, bat, guinea pig and dove, the nerve endings frequently occurred and loosely contacted with the islet cells, but in the case of the snake pancreatic islets, there were no such nerve endings.
By the artificial hibernation the pancreatic islets of bats morphologically changed as follows : 1) Islet cells diminished their volumes and contained the dark cytoplasm and pyknotic niclei. 2) Most of the secretory granules in the B-cell changed into bar-shaped granules which might contain more concentrated insulin, as compared with those of amorphous granules. These alterations of islet cells gradually recovered to normal condition by 24 hours by the refeeding.
The dog pancreas usually has A, B and D-cells. The B-cell contained bar-shaped secretory granules, while the A-cell contained electron dense round granules.
After the administration of alloxan, the B-cells changed remarkably as follows : Most bar-shaped granules became low in density or empty 4.5-6 hours after the injection of alloxan. This might suggest the increased release of insulin via diacrine secretion mechanism. The B-cell contained enlarged endoplasmic reticulum and sometimes possessed an area of the hydropic degeneration which was characterized by containing some amorphous masses. In some cases, the B-cells were severely destructed, containing some autophagic vacuoles with a large number of secretory granules. Some B-cells contained a well developed Golgi apparatus containing some immature granules. These cells also contained parallel-arranged endoplasmic reticulum.
Most B-cells were destructed 24 hours after alloxan treatment and the islets were composed of a large number of A-cells.
After the administration of pilocarpine or DHA,
