LIGHT AND ELECTRON MICROSCOPIC STUDY ON THE ISLETS OF LANGERHANS IN THE DORPHIN, STENELLA COERULEOALBUS MAYEN

Abstract

1. The islets of Langerhans of the dorphin, Stenella coeruleoalbus MAYEN have been observed by light and electron microscopy. This may be the first morphological study on the islets of whale species.2. In the tail of the pancreas of Stenella coeruleoalbus there appear many islets that can be classified into large, middle and small islets. Besides these islets a number of solitary islet cells and small islet cell groups are found within the exocrine acini. In the pancreatic islets of the dorphin continuities between islet and exocrine acini are usually found at several portions, that is the reason why in the dorphin boundaries frequently are not clear.3. In the pancreatic islets of the dorphin, we can distinguish by light and electron microscopy three islet cell types : A, B and D cells. A cells are in general distributed at the periphery of the islet and along the blood capillaries that penetrate into the islet tissue, and show polarity to the connective tissue or blood capillaries. This property is also found in some of B and D cells, and therefore the affinity to blood capillaries is common to the three islet cell types. In the pancreatic islets of the dorphin there are often observed the overlappings of two islet cells and in such cases an A cell is always embraced completely by another islet cell, which is mostly a D cell, but very rarely a B cell.4. As revealed by electron microscopic observation, the α-granule of the dorphin A cell, like that of other animal species, consists of a smooth round membranous sac about 360mμ in diameter and a round core of homogeneously high electron density about 300mμ in diameter closely drawn to the sac, thus, leaving a narrow halo between the two constituents. The β-granule consists of a smooth membranous sac about 310mμ in diameter, which is spherical, oval or polymorphous in shape, and mostly a round, homogeneous core (about 180mμ in diameter) loosely drawn to the sac, so that the halo between the limiting membrane and the core is very wide. Among these β-granules, however, there occur a small number containing within the membranous sac one or two bar-shaped, a crescent-shaped or a rhombic core. Features suggesting the intracytoplasmic dissolution of β-granules are frequently found. The α-granule is spherical, largest (about 460mμ in diameter) and least dense, consisting of a round electron lucent core of coarse texture and a smooth membranous sac very closely attached to the core, thus leaving no empty halo between the core and the limiting membrane. Wide discontinuity of the latter is not rare. A pycnotic islet cell (X-cell) which can not be identified with any islet cell types is rarely observed.5. The argyrophil islet cells (A cells) are also demonstrated in the pancreatic islet of the dorphin by means of the silver impregnation method of Hellman and Hellerström (1960). Shape, distribution and number of these argyrophil cells almost coincide with those of the D cell, and furthermore, argyrophil cells, like D cells in the stained preparations, frequently embrace a non-argyrophil islet cell. These morphological evidences obtained in the present study indirectly support the view that the argyrophil islet cells should be identical with D cells.6. In the present study it has been revealed that in the pancreatic islet of the dorphin the number of A cells, compared with that in other mammalian species, is strikingly larger and the number of D cells is also considerably greater, so that an average A : B : D ratio is given at 47% : 43% : 10%. The A : B ratio is near to that found in the lower vertebrates which continue their body growth throughout their lives. Whales represent a particular mammalian species having characteristics that they continue, like the lower vertebrates, the body growth after their sexual maturation.