The gastro-entero-pancreatic system of the turtle, Chrysemys picta

Development of the embryonic liver and pancreas of the Chinese softshell turtle Trionyx sinensis

The research on hatching ecology of the Chinese softshell turtle Trionyx sinensis has essential guiding roles to clarify the physiological and ecological mechanism of reptile evolution. The aim of this study is to describe the histological changes, differentiation, and maturation of some functional cells during the genesis and development of the liver and pancreas of the Chinese softshell turtle T. sinensis. Softshell turtle eggs were incubated under artificial conditions and hatched within 41-45 days. Hematoxylin and eosin-stained embryonic pancreas and liver were examined at various time points from 2 to 31 days and compared with that of other reptiles, amphibians, fishes, and birds in the literature. Immunohistochemical assay for glucagon and insulin was performed on paraformaldehyde-fixed embryos to identify functional cells in the pancreas. Pancreatic endocrine cells of T. sinensis have secretory ability at day 26 of embryonic development, and the dispersed pancreatic endocrine cells may be the result of the incomplete pancreatic development.

Incubation temperature and satiety influence general locomotor and exploratory behaviors in the common snapping turtle (Chelydra serpentina)

Temperature during embryogenesis determines sex and has been shown to influence other physiological traits in reptiles. The common snapping turtle (Chelydra serpentina) is an ideal model for testing how temperature impacts behavior in species that display temperature-dependent sex determination. Behavioral assays are crucial to understanding how a changing climate may affect whole organism function in the snapping turtle. Currently, there are few behavioral assays for semi-aquatic vertebrates like turtles. In this study, we used digital cameras to record behavior of fed and fasted hatchling turtles from different incubation temperatures in an open field setting for 20 min in 2018 and repeated the experiment in 2019. Open fields were circular tanks filled with water to a depth of 3.5 cm. Each field was split into four quadrants and two zones (inner and outer). The amount of time turtles spent actively moving, total distance travelled, and several other measures were collected and summarized automatically from videos with open source image analysis software (ImageJ). Satiety and incubation temperature had significant effects on total distance moved, time spent moving, and time moving in the outer zone. These findings indicate that temperature during embryogenesis has a long-lasting effect on neural mechanisms underlying exploratory or general locomotor behavior in turtles.

The comparative anatomy of islets

In the past 20 years, numerous publications on a variety of mammalian and non-mammalian species have appeared in the literature to supplement the excellent comparative work performed in the 70s and 80s by the Falkmer, Epple, and Youson groups. What emerges is that islets are much more complex than once thought and show a lot of similarities in rodents and higher primates. The diversity of lifestyles, metabolic demands, and diets has most likely influenced the great diversity in both structure and cell-type content of islets in lower vertebrate species. In this chapter, I try to provide an overview of the evolution from endocrine cell types in invertebrates to the higher mammals and focus on what has been reported in the literature and some of our own experiences and also include a description of other hormones reported to be found in islets.

Identification of the pancreatic endocrine cells of Pseudemys scripta elegans by immunogold labeling

The endocrine pancreatic cells of Pseudemys scripta elegans were investigated immunocytochemically by light and electron microscopy. Insulin-, somatostatin (SST)-1, SST-28 (1-12)-, salmon (s)SST-25-, glucagon-, pancreatic polypeptide (PP)-, peptide tyrosine tyrosine (PYY)-, and neuropeptide tyrosine (NPY)-like immunoreactivities were observed. Insulin cells were immunogold labeled with bonito insulin antiserum and secretory granules were characterized by a wide halo and a dense core of varying shape. Consecutive PAP-immunostained sections showed that SST-28 (1-12), SST-14, and sSST-25 immunoreactivities occurred in the same cells. However, preabsorption tests demonstrated that anti-sSST-25 serum detected the invariant SST-14 molecule. The SST-28 (1-12)/SST-14-immunogold-labeled cells mainly had round or ovoid medium electron-dense granules. Glucagon-IR cells were characterized by round secretory granules with an electron-dense core, with or without a narrow clear halo. There were PP, PYY, and NPY (NPY-like) immunoreactivities in a population of glucagon-IR cells in the pancreatic duodenal region (glucagon/NPY cells). Most of the secretory granules of these glucagon/NPY-like cells had an electron-dense content and were round, although there were also pyriform or ovoid secretory granules which were smaller than those of glucagon-IR cells. Preabsorption tests proved that the NPY-like peptides detected in the endocrine pancreas of P. scripta elegans were more similar to NPY or PYY than to PP.

Neuroendocrine peptides (insulin, pancreatic polypeptide, neuropeptide Y, galanin, somatostatin, substance P, and neuropeptide gamma) from the desert tortoise, Gopherus agassizii

The traditional view that Testudines (tortoises and turtles) should be regarded as the surviving clade of the anapsid reptiles rather than classified with the diapsid reptiles (snakes, lizards, and crocodiles) has recently been challenged. Neuropeptide Y, neuropeptide gamma, and somatostatin-14 were isolated from an extract of the brain, substance P and galanin from an extract of the intestine, and insulin and pancreatic polypeptide from an extract of the pancreas of the desert tortoise, Gopherus agassizii. Despite that crocodilians did not appear until the late Triassic, the amino acid sequences of the tortoise peptides resemble those of the American alligator quite closely. The primary structures of neuropeptide Y, somatostatin-14, and neuropeptide gamma are the same in tortoise and alligator. The primary structures of substance P, insulin, galanin, and pancreatic polypeptide in the two species differ by 1, 3, 5, and 8 amino acid residues, respectively. Although fewer neurohormonal peptides from squamates (lizards and snakes) have been characterized, the primary structures of neuropeptide gamma, insulin, and pancreatic polypeptide from the Burmese python and the desert tortoise differ by 3, 8, and 18 residues, respectively. The data suggest, therefore, a closer phylogenetic relationship between Testudines and Crocodilians than that derived from 'classical' analyses based on morphological criteria and the fossil record.

Purification and characterization of islet hormones (insulin, glucagon, pancreatic, polypeptide and somatostatin) from the Burmese python, Python molurus

Insulin was purified from an extract of the pancreas of the Burmese python, Python molurus (Squamata:Serpentes) and its primary structure established as: A Chain: Gly-Ile-Val-Glu-Gln-Cys-Cys-Glu-Asn-Thr10-Cys-Ser-Leu-Tyr-Glu-Leu- Glu-Asn-Tyr-Cys20-Asn. B-Chain: Ala-Pro-Asn-Gln-His-Leu-Cys-Gly-Ser-His10-Leu-Val-Glu-Ala-Leu-Tyr- Leu-Val-Cys-Gly20-Asp-Arg-Gly-Phe-Tyr-Tyr-Ser-Pro-Arg-Ser30. With the exception of the conservative substitution Phe --> Tyr at position B25, those residues in human insulin that comprise the receptor-binding and those residues involved in dimer and hexamer formation are fully conserved in python insulin. Python insulin was slightly more potent (1.8-fold) than human insulin in inhibiting the binding of [125I-Tyr-A14] insulin to the soluble full-length recombinant human insulin receptor but was slightly less potent (1.5-fold) than human insulin for inhibiting binding to the secreted extracellular domain of the receptor. The primary structure of python glucagon contains only one amino acid substitution (Ser28 --> Asn) compared with turtle/duck glucagon and python somatostatin is identical to that of mammalian somatostatin-14. In contrast, python pancreatic polypeptide (Arg-Ile-Ala-Pro-Val-Phe-Pro-Gly-Lys-Asp10-Glu-Leu-Ala-Lys-Phe- Tyr20-Thr-Glu-Leu-Gln-Gln-Tyr-Leu-Asn-Ser-Ile30-Asn-Arg-Pro-Arg -Phe.NH2) contains only 35 instead of the customary 36 residues and the amino acid sequence of this peptide has been poorly conserved between reptiles and birds (18 substitutions compared with alligator and 20 substitutions compared with chicken).

Tachykinins (substance P, neurokinin A and neuropeptide gamma) and neurotensin from the intestine of the Burmese python, Python molurus

Peptides with substance P-like immunoreactivity, neurokinin A-like immunoreactivity and neurotensin-like immunoreactivity were isolated in pure form from an extract of the intestine of the Burmese python (Python molurus). The primary structure of python substance P (Arg-Pro-Arg-Pro-Gln-Gln-Phe-Tyr-Gly-Leu- Met-NH2) shows one amino acid substitution (Phe8-->Tyr) compared with chicken/alligator substance P and an additional substitution (Lys3-->Arg) as compared with mammalian substance P. The neurokinin A-like immunoreactivity was separated into two components. Python neuropeptide gamma (Asp-Ala-Gly-Tyr- Ser-Pro-Leu-Ser-His-Lys-Arg-His-Lys-Thr-Asp-Ser-Phe-Val-Gly-Leu-Met-NH2 shows three substitutions (Gly5-->Ser, Gln6-->Pro and Ile7-->Leu) compared with alligator neuropeptide gamma and an additional substitution (His4-->Tyr) compared with mammalian neuropeptide gamma. Python neurokinin A (His-Lys-Thr-Asp-Ser-Phe-Val-Gly- Leu-Met.NH2) is identical to human/chicken/alligator neurokinin A. Python neurotensin (pGlu-Leu-Val-His-Asn-Lys-Ala-Arg-Pro-Tyr-Ile-Leu) is identical to chicken/alligator neurotensin. The data are indicative of differential evolutionary pressure to conserve the amino acid sequences of reptilian gastrointestinal peptides.

Purification and primary structure of alligator neurotensin

The gastrointestinal neurohormones of reptiles have been poorly characterized structurally. Neurotensin has been purified to apparent homogeneity from an extract of the small intestine of the alligator, Alligator mississipiensis. The primary structure of the peptide (pGlu-Leu-His-Val-Asn-Lys-Ala-Arg-Arg-Pro-Tyr-Ile-Leu) is identical to that of chicken neurotensin. The data provide further evidence for a close phylogenetic relationship between crocodilians and birds.

Neuroendocrine peptides (NPY, GRP, VIP, somatostatin) from the brain and stomach of the alligator

Despite the important position of the reptiles in phylogeny, relatively few regulatory peptides from reptilian species have been characterized structurally. Neuropeptide Y was isolated from the brain of the alligator, Alligator mississippiensis, and vasoactive intestinal polypeptide (VIP), gastrin-releasing peptide (GRP), its COOH-terminal decapeptide (GRP-10), and somatostatin-14 were isolated from the alligator stomach. The primary structures of NPY and somatostatin-14 are the same as the corresponding peptides from the human, whereas alligator VIP is identical to chicken VIP. The amino acid sequence of GRP (Ala-Pro-Ala-Pro-Ser-Gly-Gly-Gly-Ser-Ala10-Pro-Leu-Ala-Lys-Ile-Tyr -Pro-Arg-Gly-Ser20-His-Trp-Ala-Val-Gly-His-Leu-Met-NH2) contains an additional residue and six substitutions compared with chicken GRP, but alligator GRP-10 is the same as chicken GRP-10. Bombesin was not detected in the stomach extract. The data confirm that evolutionary pressure to conserve the amino acid sequence of NPY and somatostatin-14 has been very strong but demonstrate that pressure to conserve the complete primary structure of GRP has been less than that for other neuroendocrine peptides. The identity of chicken and alligator VIP is consistent with the known close phylogenetic relationship between crocodilians and birds.

Evidence for the colocalization of gastrin/CCK- and PYY/PP-immunoreactive substances in the small intestine of the lizard Podarcis hispanica: immunocytochemical and ultrastructural study

Peptide tyrosine tyrosine/pancreatic polypeptide (PYY/PP)- and C-terminal gastrin/cholecystokinin (G/CCK)-immunoreactive cells were investigated in the intestine of the lizard Podarcis hispanica, using immunocytochemistry with light and electron microscopy. Immunolabeling of consecutive semithin sections revealed coexistence of PYY/PP- and C-terminal G/CCK-like substances in some cells, while in others only PYY/PP or G/CCK immunoreactivity was found. Appropriate absorption controls excluded cross-reactivity between the antisera used. Ultrastructurally G/CCK+, PYY/PP+ cells were similar to G/CCK+, PYY/PP- cells but different from PYY/PP+, G/CCK- cells. Although virtually nothing is known concerning the physiological effects of these peptides in reptiles, their colocalization in the same cells in the intestine of Podarcis hispanica suggests a close relationship between them in the regulation of the digestive process.

Isolation and structural characterization of insulin, glucagon and somatostatin from the turtle, Pseudemys scripta

The chelonians occupy an important position in phylogeny representing a very early branching from the ancestral reptile stock. Hormonal polypeptides in an extract of the pancreas of the red-eared turtle were purified to homogeneity by reversed phase HPLC and their primary structures were determined. Turtle insulin is identical to chicken insulin. Turtle glucagon differs from chicken glucagon by the substitution of a serine by a threonine residue at position 16 and from mammalian glucagon by an additional substitution of an asparagine by a serine residue at position 28. Turtle pancreatic somatostatin is identical to mammalian somatostatin-14. The crocodilians are phylogenetically much closer to the birds than are the chelonians. Alligator insulin, however, contains three amino acid substitutions relative to chicken insulin. Thus, caution is required when inferring phylogenetic relationships based upon a comparison of amino acid sequences of homologous peptides.

Localization of insulin to gastroenteropancreatic cells in the turtle gastrointestinal tract

Insulin has been localized immunocytochemically to open-type gastroenteropancreatic endocrine cells in sections of Bouin's-fixed upper, middle, and lower intestine from Chrysemys picta, Pseudemys scripta scripta, P. scripta elegans, P. floridana, Sternotherus odoratus, and Trionyx spinifer asper. Radioimmunoassay of extracts of mucosal scrapings from Chrysemys intestine indicates differential amounts of insulin-like immunoreactivity within the intestine (higher amounts in the lower intestine) and in concentrations approximately one-tenth to one-fifth that found in extracts of Chrysemys pancreas. The presence of insulin in the gastrointestinal tract of chelonians represents a major departure from the typical vertebrate condition which is characterized by an absence of insulin from the spectrum of regulatory peptides in the gut.

Evidence that neuromedin U may regulate gut motility in reptiles but not in mammals

Neuromedin U-8 induced a monophasic and concentration-dependent contraction of intact small intestine from the turtle, Pseudemys scripta, whereas the peptide had no effect upon the motility of rat and guinea pig gut. The maximum response produced by neuromedin U-8 was 56% of that produced by acetylcholine and 62% of that produced by potassium chloride. The potency and maximum response to neuromedin U-8 were unaffected by tetrodotoxin and atropine. The data suggest that neuromedin U may play a role in regulation of gut motility in lower vertebrates but not in mammals.

Comparative immunohistochemical study of the gastroenteropancreatic endocrine system of three reptiles

The gastroenteropancreatic (GEP) endocrine system of three reptiles, Testudo graeca, Mauremys caspica, and Lacerta lepida, was investigated by means of immunocytochemistry. Single and double immunostaining methods have demonstrated immunoreactivity for insulin, glucagon, pancreatic polypeptide (PP), somatostatin, serotonin, and peptide tyrosine tyrosine (PYY) in endocrine cells of the pancreas of the reptiles studied. Islet-like structures with insulin-immunoreactive (IR) cells surrounded by glucagon-IR cells were observed only in the splenic portion of the pancreas of M. caspica. Occasionally, somatostatin- and PP-IR cells were associated with glucagon-containing cells. Endocrine cells were also observed in the excretory ducts of the exocrine glands. Serotonin, bombesin, neurotensin, gastrin, glucagon, somatostatin, PYY, and insulin were demonstrated immunocytochemically in open-type GEP cells of the digestive tract of the animals studied. Serotonin, somatostatin, and glucagon-immunoreactive cells were the most abundant endocrine cell types. In L. lepida, PP- and peptide tyrosine tyrosine-immunoreactive cells were also frequently observed. Cells containing cholecystokinin, gastric inhibitory peptide, met- and leu-enkephalin, motilin, secretin, and vasoactive intestinal peptide could not be detected. The present work demonstrates that the reptilian GEP endocrine system is a complex structure containing most of the regulatory peptides similar in structure to those found in higher vertebrates.

Comparative study on the endocrine cells in the pancreas of Mauremys caspica (chelonia) in summer and winter

Four endocrine cell types were identified using peroxidase-antiperoxidase (PAP) technique and ultrastructurally characterized in the pancreas of Mauremys caspica in both winter and summer. In winter, insulin-immunoreactive cells were more abundant and the cell groups larger in the splenic than in the duodenal region, whereas in summer, medium or small cell groups were evenly distributed. Glucagon- and somatostatin-immunoreactive cells were found throughout the gland; they were more numerous in the splenic than in the duodenal region. Polypeptide pancreatic (PP)-immunoreactive cells were found only in the duodenal region. Somatostatin-immunoreactive cells were mainly isolated in winter and grouped in summer. Glucagon- and PP-immunoreactive cells had a similar arrangement in both seasons. Somatostatin- and PP-containing cells showed cytoplasmic processes and could be found next to the pancreatic ducts; the latter were also observed near insulin-immunoreactive cells. Some large secretory granules and numerous, isolated and long rough endoplasmic reticulum (RER) cisternae were seen in winter B cells; in summer B cells numerous lysosomes and few, dilated RER cisternae were found. Summer A cells showed well-developed, dilated RER cisternae and numerous vacuoles; secretory granules were more numerous in winter A cells. In winter B cells and summer A cells some nuclear filamentous inclusions were observed. Few RER cisternae were observed in winter D cells and many in summer D cells; secretory granules were found, the shape and electron density of which differed with the season. PP cells were characterized by their small secretory granules, which were less numerous in winter than in summer, being clustered at the cell pole or dispersed in the cytoplasm, respectively; in winter, the well-developed RER cisternae were dilated and irregularly distributed.

Ultrastructural study of the endocrine cells of the gut of Testudo graeca (Chelonia)

The digestive tract of Testudo graeca (Chelonia) was investigated by means of electron microscopy using both conventional and immunocytochemical techniques. EC-, L-, D-, G-, B-, N- and EC-L-cells were detected. These cells share several common ultrastructural characteristics with the endocrine cells of mammals (i.e. clear cytoplasm, prominent Golgi apparatus, secretory granules etc.). EC and D1 cells have so far not been described in the esophagus of any animal species; in the present study these cells have been observed in the esophagus of T. graeca. Of special interest was the presence of B-cells in the intestine, suggesting that the migration of B-cells from the gut to the pancreas to constitute pancreatic islets is not concluded in T. graeca. The present study demonstrates that the gut endocrine system of T. graeca is a complex structure containing a large variety of endocrine cell types similar in morphology to those found in higher vertebrates.

Endocrine pancreas of Testudo graeca L. (Chelonia) in summer and winter: an immunocytochemical and ultrastructural study

Insulin-, glucagon-, somatostatin-, and PP-immunoreactive cells were identified immunocytochemically using antisera raised against mammalian hormones in the pancreas of Testudo graeca in both winter and summer. The endocrine cells were present throughout the gland, forming scarce islets except in the splenic region. The insulin cell islets were larger and more numerous in the splenic region than in the duodenal one. Winter glucagon-immunoreactive cells were found mainly in isolation while the summer ones occurred in groups which showed no immunoreactive central area; in both seasons these cells were more numerous in the splenic region than in the duodenal one. Somatostatin-immunoreactive cells were found isolated or grouped together more frequently in the splenic region in the summer specimens. No PP-immunoreactive cells were found in the splenic region, although they were numerous and isolated in the duodenal zone. Four cell types (B, A, D, and PP cells) were ultrastructurally characterized by the shape, size, and electron density of their respective secretory granules. Certain ultrastructural differences were detected in the summer and winter endocrine pancreatic cells. In summer specimens a fifth cellular type was observed. The presence of B, D, and PP cells among the epithelial pancreatic duct cells may confirm the comparatively primitive organization of the T. graeca endocrine pancreas.