LIGHT AND ELECTRON MICROSCOPIC STUDY ON THE ISLETS OF LANGERHANS IN THE SOFT-SHELLED TURTLE, TRIONYX SINENSIS JAPONICUS

Abstract

Pancreatic islets of the soft-shelled turtle, Trionyx sinensis japonicus, have been light and electron microscopically observed.1. In the soft-shelled turtle the islet cells occur within the exocrine acini of the pancreas making islet-cell groups in their periphery or occupying the entire space of a part of them. Between these islet-cell groups and the exocrine cells, however, there occur no fibrous boundaries consisiting of connective tissue fibers. The islet-cell groups sppear also in the basal parts of the epithelium of the excretory duct ; especially in the small ducts and the intercalated portion, the islet cells occur on the outside of their epithelium making a single layer of the islet cells, which surrounds completely the duct epithelium.2. Islet cells of the soft-shelled turtle in azan preparations can be classified into three types, A, B, and D. A and B cells are common islet cells, while D cells are rare. A and B cells usually make their respective cell groups, though occasionally mixed groups are observed. By means of the silver impregnation method of Hellman and Hellerstrom (1960) argyrophil islet cells have occasionally been demonstrated within the islets, though it has been impossible to identify them as D cells.3. In the pancreatic islets of the soft-shelled turtle average A : B ratio is 55.5% : 44.5 %, thus strikingly larger than that for the mammals, which is approximately 1 : 4. This finding may support the view that the A : B ratio is generally larger in lower vertebrates which continue their body growth after the sexual maturation.4. In the pancreatic islet of the soft-shelled turtle no definite seasonal change has been observed in regard to the specific granule contents of both A and B cells. Glycogen has not been demonstrated in the islet cells by means of PAS-method in any season of the year. The most remarkable seasonal change of the islet of the soft-shelled turtle may be hydropic degeneration of B cells observed in the hibernation season. The degeneration appears already in October and December, becomes most intensive in January, February and March, but is promptly reduced in intensity in April. In accordance with study by Kano (1960) on the pancreatic islet of the Japanese hard-shelled turtle (Clemmys japonica), the present experiment has revealed that the hydropic degeneration of the B cell can appear physiologically in hibernating animals. But different from Kano, the auther failed to demonstrate any glycogen infiltration in hydropically degenerated B cells of the soft-shelled turtle. Thus, the view that the hydropic degeneration should be induced by glycogen infiltration into the B cell has not been supported in the present study.5. In regard to the ultrastructural properties of the specific granule the islet cells of the soft-shelled turtle can be classified also into A, B and D cells, and further B cells into B1 and B2 cells. B1 cells are rare, while B2 cells are numerous and common.6. Each α-granule (approximately 250-400mμ in diameter) consists of a smooth membrane sac and an electron dense spherical core, and between the two components, there exists, contrary to α-granules of other animal species, a considerably wide light space. The spherical core frequently shows an irregular outline and more or less conspicuous density decrease, probably suggesting the intracytoplasmic dissolution.7. Specific granules of B1 cells, the largest specific ones, in the islet cells of the soft shelled turtle, measuring approximately 500-890mμ in diameter, each consist of a smooth membranous sac and one or several rod-shaped crystalline cores contained loosely in it.