Light and electron microscopic investigation of the endrine pancreas of the grass-snake [Natrix n. natrix (L.)]

Architecture of the Pancreatic Islets and Endocrine Cell Arrangement in the Embryonic Pancreas of the Grass Snake (Natrix natrix L.). Immunocytochemical Studies and 3D Reconstructions

During the early developmental stages of grass snakes, within the differentiating pancreas, cords of endocrine cells are formed. They differentiate into agglomerates of large islets flanked throughout subsequent developmental stages by small groups of endocrine cells forming islets. The islets are located within the cephalic part of the dorsal pancreas. At the end of the embryonic period, the pancreatic islet agglomerates branch off, and as a result of their remodeling, surround the splenic "bulb". The stage of pancreatic endocrine ring formation is the first step in formation of intrasplenic islets characteristics for the adult specimens of the grass snake. The arrangement of endocrine cells within islets changes during pancreas differentiation. Initially, the core of islets formed from B and D cells is surrounded by a cluster of A cells. Subsequently, A, B, and D endocrine cells are mixed throughout the islets. Before grass snake hatching, A and B endocrine cells are intermingled within the islets, but D cells are arranged centrally. Moreover, the pancreatic polypeptide (PP) cells are not found within the embryonic pancreas of the grass snake. Variation in the proportions of different cell types, depending on the part of the pancreas, may affect the islet function-a higher proportion of glucagon cells is beneficial for insulin secretion.

Ultrastructure of endocrine pancreatic granules during pancreatic differentiation in the grass snake, Natrix natrix L. (Lepidosauria, Serpentes)

We used transmission electron microscopy to study the pancreatic main endocrine cell types in the embryos of the grass snake Natrix natrix L. with focus on the morphology of their secretory granules. The embryonic endocrine part of the pancreas in the grass snake contains four main types of cells (A, B, D, and PP), which is similar to other vertebrates. The B granules contained a moderately electron-dense crystalline-like core that was polygonal in shape and an electron-dense outer zone. The A granules had a spherical electron-dense eccentrically located core and a moderately electron-dense outer zone. The D granules were filled with a moderately electron-dense non-homogeneous content. The PP granules had a spherical electron-dense core with an electron translucent outer zone. Within the main types of granules (A, B, D, PP), different morphological subtypes were recognized that indicated their maturity, which may be related to the different content of these granules during the process of maturation. The sequence of pancreatic endocrine cell differentiation in grass snake embryos differs from that in many vertebrates. In the grass snake embryos, the B and D cells differentiated earlier than A and PP cells. The different sequence of endocrine cell differentiation in snakes and other vertebrates has been related to phylogenetic position and nutrition during early developmental stages.

Distribution and ultrastructure of somatostatin-immunoreactive cells in the pancreas of Rana esculenta

Apart from a description of the general organization of the endocrine pancreas, the present study is focussed on the distribution and ultrastructural morphology of somatostatin-immunoreactive cells in the pancreas of the frog Rana esculenta. For light-microscopic histochemistry, the peroxidase-antiperoxidase technique was used. For the ultrastructural investigation, we employed the immunogold method. The endocrine pancreas of R. esculenta is composed of numerous islet-like structures, which contain several small somatostatin-immunoreactive cells arranged in the form of clusters. Often, however, single somatostatin cells are randomly distributed among the acinar tissue of the pancreas. These individually arranged elements possess long processes which terminate on exocrine pancreatic cells. The ultrastructural features of somatostatin-immunoreactive cells speak in favor of their endocrine and paracrine functions.

Ultrastructure of the exocrine pancreas in the snake Waglerophis merremii (Wagler)

Three types of serozymogenic cells were found in the secretory compartment of the snake exocrine pancreas. Type I cell was the most common and presented a well-developed granular endoplasmic reticulum arranged in cisternal and vesicular forms. The cisternal form was located predominantly in the basal regions of the cell and the vesicular form was found in the supranuclear regions of the cell next to a prominent Golgi complex. Mature secretory granules were seen at the cell apex. The cytoplasmic matrix of the Type II cells was electron dense but had only poorly-developed organelles. Secretory granules were rare. The cytoplasmic matrix of the Type III cells was electron lucent and the granular endoplasmic reticulum in the cisternal form was located predominantly in the supranuclear region, whereas the vesicular form was randomly distributed throughout the cytoplasm. The nucleus appeared pale due to the fine dispersion of the chromatin; the nucleolus was prominent. Centroacinar and intermediate cells were also examined.

Anatomy of the pancreas and Langerhans islets in snakes and lizards

The pancreas of snakes (18 species) was comparatively examined and classified into five major types, based on structure of the lobes and ducts, spatial relationships with the spleen and the gall bladder, and the disposition of islet cells. These types trend toward fusion of the pancreatic lobes and compaction of the pancreas--a progression that coincides with the phylogeny of the snakes. The more primitive pancreas of lizards (17 species) also was surveyed; that of Varanus is of special interest because its structure is intermediate between the extended, tri-lobate pancreas of lizards and the compact pancreas of snakes and may represent a transitional link in the evolution of this organ. Islet tissue is always confined to the dorsal lobe and is concentrated in its distal region adjacent to the spleen. In primitive snakes and in Varanus, a large islet mass is sequestered within a distinct juxtasplenic "islet body" distanced from the dorsal lobe and connected to it by a slender stalk. In some of the most advanced snake species, numerous islets of endocrine cells are found within the spleen. The occurrence and formation of these intrasplenic islets is described in detail. The anatomic "affinity" between spleen and the islet region of the pancreas is discussed. A hypothesis for the development of the pancreas from embryonal placodes on the mid-gut is presented; it proposes that the exocrine and the endocrine components derive from different progenitor cells, and that the endocrine progenitors are located in the center of the dorsal placode. The hypothesis combines embryological and evolutionary views about the origin of the pancreas, and offers a rationale for differences in its structure and in the disposition of the islets.

Immunocytochemical localization of four hormones in the pancreas of the garter snake, Thamnophis sirtalis

Insulin, glucagon, somatostatin, and pancreatic polypeptide (PP) were localized in the pancreas of the common garter snake, Thamnophis sirtalis, by light and transmission electron microscopic (TEM) immunocytochemistry. Colloidal gold-protein A was used for TEM localization and the peroxidase--antiperoxidase complex technique was used for light microscopy. The glucagon-containing A cells and the insulin-positive B cells were the most numerous cell types. The somatostatin-containing D cells made up about 15% of the endocrine cells. PP-positive F cells were a minor cell type. The only topographic arrangement of the cells within the endocrine-rich areas that was apparent was the peripheral localization of the D and F cells. Cells of a specific cell type were sometimes grouped together. At the electron microscopic (EM) level, the gold particles (indicating the presence of hormone) were localized nearly exclusively over the secretory granules of the reactive cells. The alpha-granules were the largest found and were predominantly electron dense with a moderately electron-dense periphery. PP-containing granules were the smallest. The somatostatin-reactive delta-granules were round and moderately electron opaque. The beta-granules were heterogeneous in appearance. The morphognomy of the secretory granules of the major endocrine cell types is qualitatively similar to that of mammals. Whether or not the quantitative and/or associative differences contribute to the marked metabolic differences between reptiles and mammals, remains to be determined.