Evolutionary relationships between the amphibian, avian, and mammalian stomachs

Histological and gene-expression analyses of pyloric sphincter formation during stomach metamorphosis in Xenopus laevis

During anuran metamorphosis from herbivorous tadpoles to carnivorous frogs, the gastrointestinal (GI) tract undergoes drastic remodeling, such as the formation of the stomach-intestine boundary and the development of the pyloric sphincter at the posterior end of the stomach. However, the morphogenetic process and molecular mechanisms of how the pyloric sphincter is formed during metamorphosis, instead of during embryogenesis as in amniotes, are largely uninvestigated. Using the African clawed frog Xenopus laevis, we histologically examined the development of the pylorus region from embryonic to froglet stages and performed spatiotemporal gene expression analyses. We found that the pyloric sphincter is formed at a flexure within the pyloric region during metamorphic climax, and that the pyloric and duodenal epithelia, which are morphologically indistinguishable before sphincter formation, become clearly demarcated by the sphincter at the end of metamorphosis. Consistent with these morphological changes, expression domains of a stomach marker barx1 and an intestine marker cdx2 overlapped until late metamorphic climax, but became separated after metamorphosis. Despite the absence of the sphincter before metamorphosis, various genes crucial for sphincter formation in amniotes were already expressed in the pylorus region of Xenopus embryos. RNA-sequencing analysis at pre-metamorphic and metamorphic-climax stages suggest unappreciated roles of genes, such as those for retinoic acid signaling and various transcription factors, in suppressing or promoting sphincter formation. These data provide histological and molecular insights into the heterochrony of the pyloric sphincter formation in amniotes and anurans.

Morphofunctional analysis of the gastrointestinal tract of an annual killifish: In search of structural traits related to the annual life history

Annual killifishes have active and voracious rates in acquisition of food resources with display of behaviors that allow them to maintain high metabolic rates to deal with the harsh and unpredictable conditions prevalent in temporary pools where they inhabit. The objective of this research was to describe histologically and ultrastructurally, the digestive system of the annual killifish Millerichthys robustus to identify morphological traits related to its annual life history and digestive physiology. Also, we quantify food items along the intestine as a proxy for rates of digestion. Millerichthys has a short digestive system, associated with a carnivorous diet, with no evidence of a stomach. Instead, the presence of pharyngeal jaws with caniform teeth was documented, related to the breakdown of invertebrate exoskeletons, allowing prey fluids to be tasted by taste corpuscles related to selection of food items, and that digestive enzymes penetrate once in the intestine. The histological morphology of the intestine showed four different regions, associated with its digestive rates: (i) reception of food from the esophagus with intact pray; (ii) digestion with enzymes from the pancreas and liver/gallbladder of simple exoskeleton prey (Entognatha), and beginning of absorption; (iii) absorption of nutrients, and digestion of large-complex exoskeleton prey (Hexanauplia, Brachiopoda, and Ostracoda); and (iv) probable absorption of intact macromolecules. The second region of the intestine presented two anatomical loops and the highest thickness that may be related to reducing the speed of food transit, allowing for more efficient digestion given the large amount of food ingested by this species.

Pseudogenization of NK3 homeobox 2 (Nkx3.2) in monotremes provides insight into unique gastric anatomy and physiology

The enzymatic breakdown and regulation of food passage through the vertebrate antral stomach and pyloric sphincter (antropyloric region) is a trait conserved over 450 million years. Development of the structures involved is underpinned by a highly conserved signalling pathway involving the hedgehog, bone morphogenetic protein and Wingless/Int-1 (Wnt) protein families. Monotremes are one of the few vertebrate lineages where acid-based digestion has been lost, and this is consistent with the lack of genes for hydrochloric acid secretion and gastric enzymes in the genomes of the platypus (Ornithorhynchus anatinus) and short-beaked echidna (Tachyglossus aculeatus) . Furthermore, these species feature unique gastric phenotypes, both with truncated and aglandular antral stomachs and the platypus with no pylorus. Here, we explore the genetic underpinning of monotreme gastric phenotypes, investigating genes important in antropyloric development using the newest monotreme genomes (mOrnAna1.pri.v4 and mTacAcu1) together with RNA-seq data. We found that the pathway constituents are generally conserved, but surprisingly, NK3 homeobox 2 (Nkx3.2) was pseudogenized in both platypus and echidna. We speculate that the unique sequence evolution of Grem1 and Bmp4 sequences in the echidna lineage may correlate with their pyloric-like restriction and that the convergent loss of gastric acid and stomach size genotypes and phenotypes in teleost and monotreme lineages may be a result of eco-evolutionary dynamics. These findings reflect the effects of gene loss on phenotypic evolution and further elucidate the genetic control of monotreme stomach anatomy and physiology.

Binding and sequestration of poison frog alkaloids by a plasma globulin

Alkaloids are important bioactive molecules throughout the natural world, and in many animals they serve as a source of chemical defense against predation. Dendrobatid poison frogs bioaccumulate alkaloids from their diet to make themselves toxic or unpalatable to predators. Despite the proposed roles of plasma proteins as mediators of alkaloid trafficking and bioavailability, the responsible proteins have not been identified. We use chemical approaches to show that a ~50 kDa plasma protein is the principal alkaloid-binding molecule in blood of poison frogs. Proteomic and biochemical studies establish this plasma protein to be a liver-derived alkaloid-binding globulin (ABG) that is a member of the serine-protease inhibitor (serpin) family. In addition to alkaloid-binding activity, ABG sequesters and regulates the bioavailability of 'free' plasma alkaloids in vitro. Unexpectedly, ABG is not related to saxiphilin, albumin, or other known vitamin carriers, but instead exhibits sequence and structural homology to mammalian hormone carriers and amphibian biliverdin-binding proteins. ABG represents a new small molecule binding functionality in serpin proteins, a novel mechanism of plasma alkaloid transport in poison frogs, and more broadly points toward serpins acting as tunable scaffolds for small molecule binding and transport across different organisms.

Morpho-Histological Studies of the Gastrointestinal Tract of the Orange-Rumped Agouti (Dasyprocta leporina Linnaeus, 1758), with Special Reference to Morphometry and Histometry

Simple Summary

The agouti (Dasyprocta leporina) is a neo-tropical rodent that has the potential to be domesticated. These animals are considered to be omnivores from studies conducted in the wild and in captivity. However, an in-depth morphometric and histometric analysis of the gastrointestinal tract of this animal was never conducted. Thus, the objective of this paper was to give a quantitative analysis of the gastrointestinal tract of the agouti and relate it to the feeding habits of the animals. This study showed that the agouti had an esophagus which was lined with keratinized epithelium and esophageal glands. The stomach was simple, with well-developed gastric glands, and the small intestines accounted for the majority of the animal’s digestive tract in length. The long cecum and well-developed colon indicated that this animal had the ability to digest fibrous material in the diet. Based on the analysis of the gastrointestinal tract, these animals can be fed a dry omnivorous diet, with protein sources from either vegetation or animal matter.

Abstract

The morphology of the gastrointestinal tract (GI) is a strong indicator of a species’ dietary habits. The objective of this study was to describe and quantitatively analyze the gross and microanatomy of the digestive tract of the orange/red-rumped agouti (Dasyprocta leporina) and relate it to the feeding habits of this animal. The digestive tracts of six adult males were used for this study. The results showed that the esophagus was thick (mean thickness of 1023.78 ± 28.97 μm) and lined by keratinized epithelium with scant esophageal glands. Mucosa-associated lymphocytic infiltration was robust throughout the GI tract. These findings suggest that the esophagus was well adapted to a coarse diet. The simple stomach with well-developed gastric glands in the fundus region (mean thickness of 605.39 ± 28.68 μm) was indicative of an adaptation to a carnivorous diet. The small intestine constituted approximately 80% of the length of the GI tract. The remarkable development of the jejunum with a greater villus length (mean thickness of 182.50 ± 27.38 μm) indicated a greater absorptive capacity in frugivorous and carnivorous diets. The long cecum and well-developed colon clearly indicated that the GI tract was well adapted to frugivorous and herbivorous diets. Overall, D. leporina showed that it is well adapted to an omnivorous diet. These results suggest that D. leporina can be fed a balanced omnivorous dry diet with a high protein content of plant or animal origin that is well suited to the architecture of the GI tract in captivity.

High-resolution ultrasound and speckle tracking: a non-invasive approach to assess in vivo gastrointestinal motility during development

Gastrointestinal motor activity has been extensively studied in adults; however, only few studies have investigated fetal motor skills. It is unknown when the gastrointestinal tract starts to contract during the embryonic period and how this function evolves during development. Here, we adapted a non-invasive high-resolution echography technique combined with speckle tracking analysis to examine the gastrointestinal tract motor activity dynamics during chick embryo development. We provided the first recordings of fetal gastrointestinal motility in living embryos without anesthesia. We found that, although gastrointestinal contractions appear very early during development, they become synchronized only at the end of the fetal period. To validate this approach, we used various pharmacological inhibitors and BAPX1 gene overexpression in vivo. We found that the enteric nervous system determines the onset of the synchronized contractions in the stomach. Moreover, alteration of smooth muscle fiber organization led to an impairment of this functional activity. Altogether, our findings show that non-invasive high-resolution echography and speckle tracking analysis allows visualization and quantification of gastrointestinal motility during development and highlight the progressive acquisition of functional and coordinated gastrointestinal motility before birth.

HCN2 Channel-induced Rescue of Brain, Eye, Heart, and Gut Teratogenesis Caused by Nicotine, Ethanol, and Aberrant Notch Signaling

Organogenesis is a complex process that can be disrupted by embryonic exposure to teratogens or mutation-induced alterations in signaling pathways, both of which result in organ mispatterning. Building on prior work in Xenopus laevis that showed that increased HCN2 ion channel activity rescues nicotine-induced brain & eye morphogenesis, we demonstrate much broader HCN2-based rescue of organ patterning defects. Induced HCN2 expression in both local or distant tissues can rescue CNS (brain & eye) as well as non-CNS (heart, & gut) organ defects induced by three different teratogenic conditions: nicotine exposure, ethanol exposure, or aberrant Notch protein. Rescue can also be induced by small-molecule HCN2 channel activators, even with delayed treatment initiation. Our results suggest that HCN2 (likely mediated by bioelectric signals) can be an effective regulator of organogenesis from all three germ layers (ectoderm, mesoderm, and endoderm) and reveal non-cell-autonomous influences on organ formation that work at considerable distance during embryonic development. These results suggest molecular bioelectric strategies for repair that could be explored in the future for regenerative medicine. This article is protected by copyright. All rights reserved.

The expression of the gastrin/cholecystokinin (GAST/CCK) family and their receptors (CCKAR/CCKBR) in the chicken changes in response to quantitative restriction and reveals a functional role of CCK in the crop

Gastrin and cholecystokinin peptides bind a common G-protein coupled receptor, cholecystokinin receptor B (CCKBR) whilst cholecystokinin receptor A (CCKAR) is preferentially bound by CCK. Gastrin and cholecystokinin mediate signalling from the gastrointestinal tract to regulate appetite and digestive function. In this study, expression of the cholecystokinin/gastrin family and distribution of their receptors expression was measured to understand the target organs for the peptides and how expression responds to changes in food intake. We confirmed the restricted expression of gastrin in the antrum and the abundant expression of cholecystokinin in the hypothalamus. The expression of gastrin in the antrum was significantly elevated in broiler breeders when released from feed restriction. CCKBR was most abundant in the hypothalamus and proventriculus. CCKAR was most abundant in the pancreas and crop, more than tenfold greater than the gastrointestinal tract. Cholecystokinin expression in the pancreas increased after removal of food restriction. CCKAR in the gastrointestinal tract peaks around the distal ileum, distal to the peak of cholecystokinin expression. There was virtually no cholecystokinin expression in the caecum but CCKAR expression was high. The CCKAR expression in the crop was unexpected, supporting a role of cholecystokinin in mediating crop emptying which was supported by the observation of in-vitro contraction after cholecystokinin administration. The response to changes in food intake and the expression pattern of the cholecystokinin/gastrin family and their receptors will stimulate and inform new hypotheses on their role in growth in poultry.

Developmental stages of the ballan wrasse from first feeding through metamorphosis: Cranial ossification and the digestive system

We have described six developmental stages for the ballan wrasse, from the first feeding until the juvenile stage, supported by specific descriptions of cranial ossification, maturation of the digestive tract, and growth-correlated stages. The initial formation and development of bones are closely linked to the functional anatomical structures required for the mechanics of its feeding behavior and ingestion, particularly the jaws and branchial regions involved in opening the mouth and capturing food particles. The overall ontogeny of the cranial structure compares to that of other teleosts. The cranial ossification of the ballan wrasse skull and the development of its dentary apparatus-first pharyngal teeth and later oral teeth-is linked to the development of the digestive system and to their feeding habits, from preying on zooplankton to feeding on crustaceans and invertebrates on rocks and other substrates. As ballan wrasse is a nibbler, eating small meals, the digestive tract is short compared to the length of the fish; there is no stomach or peptic digestion and also no distinctive bulbus and pyloric ceca. The liver and exocrine pancreas and their outlets terminating in the lumen of the most anterior part of the intestine are important in the digestive process and develop with a larger volume than that in gastric teleosts, relative to the digestive system.

Chick fetal organ spheroids as a model to study development and disease

BACKGROUND

Organ culture models have been used over the past few decades to study development and disease. The in vitro three-dimensional (3D) culture system of organoids is well known, however, these 3D systems are both costly and difficult to culture and maintain. As such, less expensive, faster and less complex methods to maintain 3D cell culture models would complement the use of organoids. Chick embryos have been used as a model to study human biology for centuries, with many fundamental discoveries as a result. These include cell type induction, cell competence, plasticity and contact inhibition, which indicates the relevance of using chick embryos when studying developmental biology and disease mechanisms.

RESULTS

Here, we present an updated protocol that enables time efficient, cost effective and long-term expansion of fetal organ spheroids (FOSs) from chick embryos. Utilizing this protocol, we generated FOSs in an anchorage-independent growth pattern from seven different organs, including brain, lung, heart, liver, stomach, intestine and epidermis. These three-dimensional (3D) structures recapitulate many cellular and structural aspects of their in vivo counterpart organs and serve as a useful developmental model. In addition, we show a functional application of FOSs to analyze cell-cell interaction and cell invasion patterns as observed in cancer.

CONCLUSION

The establishment of a broad ranging and highly effective method to generate FOSs from different organs was successful in terms of the formation of healthy, proliferating 3D organ spheroids that exhibited organ-like characteristics. Potential applications of chick FOSs are their use in studies of cell-to-cell contact, cell fusion and tumor invasion under defined conditions. Future studies will reveal whether chick FOSs also can be applicable in scientific areas such as viral infections, drug screening, cancer diagnostics and/or tissue engineering.

Macro- and microstructures of the digestive tract in the Eurasian collared dove, Streptopelia decaocto (Frivaldszky 1838): Adaptive interplay between structure and dietary niche

We describe the functional morphology of the digestive tract of the Eurasian collared dove, Streptopelia decaocto using anatomical, morphometric, histological, histochemical, and ultrastructure techniques, and relate our findings to the species' dietary niche. Our results revealed that the esophagus is displaced on both sides of the neck and has highly folded tunica mucosa, which confer greater elasticity for efficient swallowing and passage of food to the crop. The proventriculus is delicate and its mucosal layer contains polymorphic glands with dense profound and superficial secretory units that open to the luminal surface by gastric pores. The ventriculus is biconvex and lined with a keratinized koilin membrane. The tubular glands within the mucosal lining include the isthmus, the neck, and the basal segment that comprise chief and basal cells with prominent nuclei. At the cuticle-mucosal interface, pyramidal vertical rodlets of the cuticle are secreted and superficially covered by a thin film of a horizontal matrix. The mucosa of the ileum form pyramidal villi that are oriented perpendicularly to the central lumen. Enterocytes infiltrated with goblet cells make up the epithelial lining of the villi. There are subtle differences in the thicknesses of corresponding tunics together with histochemical reactions of alcian blue (AB) and Masson-Goldner trichrome (MT) for their microstructures. Overall, our findings reveal remarkable convergence of both macro-and microstructures in S. decaocto to other granivorous species, and offer further evidence of the close association between functional morphology and feeding style relative to food swallowing, digestion, and absorption.

Infantile hypertrophic pyloric stenosis in patients with esophageal atresia

BACKGROUND

Patients born with esophageal atresia (EA) have a higher incidence of infantile hypertrophic pyloric stenosis (IHPS), suggestive of a relationship. A shared etiology makes sense from a developmental perspective as both affected structures are foregut derived. A genetic component has been described for both conditions as single entities and EA and IHPS are variable components in several monogenetic syndromes. We hypothesized that defects disturbing foregut morphogenesis are responsible for this combination of malformations.

METHODS

We investigated the genetic variation of 15 patients with both EA and IHPS with unaffected parents using exome sequencing and SNP array-based genotyping, and compared the results to mouse transcriptome data of the developing foregut.

RESULTS

We did not identify putatively deleterious de novo mutations or recessive variants. However, we detected rare inherited variants in EA or IHPS disease genes or in genes important in foregut morphogenesis, expressed at the proper developmental time-points. Two pathways were significantly enriched (p < 1 × 10^-5 ): proliferation and differentiation of smooth muscle cells and self-renewal of satellite cells.

CONCLUSIONS

None of our findings could fully explain the combination of abnormalities on its own, which makes complex inheritance the most plausible genetic explanation, most likely in combination with mechanical and/or environmental factors. As we did not find one defining monogenetic cause for the EA/IHPS phenotype, the impact of the corrective surgery could should be further investigated.

Understanding the gastrointestinal physiology and responses to feeding in air-breathing Anabantiform fishes

The Mekong Delta is host to a large number of freshwater species, including a unique group of facultative air-breathing Anabantiforms. Of these, the striped snakehead (Channa striata), the climbing perch (Anabas testudineus), the giant gourami (Osphronemus goramy) and the snakeskin gourami (Trichogaster pectoralis) are major contributors to aquaculture production in Vietnam. The gastrointestinal responses to feeding in these four species are detailed here. Relative intestinal length was lowest in the snakehead, indicating carnivory, and 5.5-fold greater in the snakeskin, indicating herbivory; climbing perch and giant gourami were intermediate, indicating omnivory. N-waste excretion (ammonia-N + urea-N) was greatest in the carnivorous snakehead and least in the herbivorous snakeskin, whereas the opposite trend was observed for net K⁺ excretion. Similarly, the more carnivorous species had a greater stomach acidity than the more herbivorous species. Measurements of acid-base flux to water indicated that the greatest postprandial alkaline tide occurred in the snakehead and a potential acidic tide in the snakeskin. Additional findings of interest were high levels of both PCO₂ (up to 40 mmHg) and HCO₃ ^- (up to 33 mM) in the intestinal chyme of all four of these air-breathing species. Using in vitro gut sac preparations of the climbing perch, it was shown that the intestinal net absorption of fluid, Na⁺ and HCO₃ ^- was upregulated by feeding but not net Cl^- uptake, glucose uptake or K⁺ secretion. Upregulated net absorption of HCO₃ ^- suggests that the high chyme (HCO₃ ^- ) does not result from secretion by the intestinal epithelium. The possibility of ventilatory control of PCO₂ to regulate postprandial acid-base balance in these air-breathing fish is discussed.

The morphological development of the proventriculus of Dandarawi chick: Light and electron microscopical studies

This study was carried out on 40 chick embryos collected from incubated eggs of Dandarawi chicken (Gallus gallus domesticus) on the 5th to 19th incubation day (27 to 45 Hamburger and Hamilton, H&H stages). In addition, 15 chicks were collected on the day of hatching (stage 46 H&H), one week and two weeks post-hatching to demonstrate the histological, histochemical, and electron microscopic developmental changes of the proventriculus (of the digestive tract). Histologically, the proventriculus was observed as a narrow tube at 27 H&H stage. It was lined by pseudostratified columnar epithelium through 27-39 H&H stages and from the stage 43 till post-hatching, it was lined by simple columnar epithelium. The Lamina muscularis mucosa could be identified at stage 43. The proventricular glands were detected firstly at stage 31 and branching at stage 35. Histochemically, the surface epithelium and proventricular glands reacted positively to PAS, alcian blue and bromophenol blue from stage 31 till maturity. The glands displayed an apocrine mode of secretion at stage 39 and their cytoplasm contained abundant mitochondria, RER, secretory granules, and lipid droplets. Enteroendocrine cells could be observed among the glandular and surface epithelium at stage 45 H&H. The interstitial tissue contained fibroblasts and telocytes. The telocytes were firstly detected at stage 35 H&H and composed of a cell body and two long cell processes called telopodes. The tunica muscularis differentiated into three layers of smooth muscle fibers at stage 37 H&H. The cellular and stromal organizations of the proventriculus and their relations to the development and function were discussed.

BMP Signaling in Development, Stem Cells, and Diseases of the Gastrointestinal Tract

The bone morphogenetic protein (BMP) pathway is essential for the morphogenesis of multiple organs in the digestive system. Abnormal BMP signaling has also been associated with disease initiation and progression in the gastrointestinal (GI) tract and associated organs. Recent studies using animal models, tissue organoids, and human pluripotent stem cells have significantly expanded our understanding of the roles played by BMPs in the development and homeostasis of GI organs. It is clear that BMP signaling regulates GI function and disease progression that involve stem/progenitor cells and inflammation in a tissue-specific manner. In this review we discuss these new findings with a focus on the esophagus, stomach, and intestine.

Expression of the matrix metalloproteinases 2 and 9 in the rat small intestine during intrauterine and postnatal life

The expressions of matrix metalloproteinases 2 and 9 have been described during the development, as an example in heart and tooth but not in the small intestine yet. In this context, this study aimed to evaluate the expressions of MMP-2 and MMP-9 in the small intestine of Wistar rats during intrauterine (IU) and postnatal (PN) life. Expressions were determined on the 15th and 18th days of IU life and the 3rd, 10th, 17th, 25th, and 32nd days of PN life. Intestinal samples obtained from six animals were submitted to zymography, immunohistochemistry, and staining with Masson's trichrome. The results showed that MMP-2 and MMP-9 were not expressed during IU life; however, after birth, MMP-9 was immunolocalized in the goblet and mast cells. In conclusion, our results showed that MMP-2 and MMP-9 were not expressed in absorptive epithelial cells during the IU period of the small intestine but after birth, MMP-9 was expressed in the goblet cells, and mast cells present in the lamina propria.

Molecular ontogeny of the stomach in the catshark Scyliorhinus canicula

The origin of extracellular digestion in metazoans was accompanied by structural and physiological alterations of the gut. These adaptations culminated in the differentiation of a novel digestive structure in jawed vertebrates, the stomach. Specific endoderm/mesenchyme signalling is required for stomach differentiation, involving the growth and transcription factors: 1) Shh and Bmp4, required for stomach outgrowth; 2) Barx1, Sfrps and Sox2, required for gastric epithelium development and 3) Cdx1 and Cdx2, involved in intestinal versus gastric identity. Thus, modulation of endoderm/mesenchyme signalling emerges as a plausible mechanism linked to the origin of the stomach. In order to gain insight into the ancient mechanisms capable of generating this structure in jawed vertebrates, we characterised the development of the gut in the catshark Scyliorhinus canicula. As chondrichthyans, these animals retained plesiomorphic features of jawed vertebrates, including a well-differentiated stomach. We identified a clear molecular regionalization of their embryonic gut, characterised by the expression of barx1 and sox2 in the prospective stomach region and expression of cdx1 and cdx2 in the prospective intestine. Furthermore, we show that gastric gland development occurs close to hatching, accompanied by the onset of gastric proton pump activity. Our findings favour a scenario in which the developmental mechanisms involved in the origin of the stomach were present in the common ancestor of chondrichthyans and osteichthyans.

Morphogenesis and motility of the Astyanax mexicanus gastrointestinal tract

Through the course of evolution, the gastrointestinal (GI) tract has been modified to maximize nutrient absorption, forming specialized segments that are morphologically and functionally distinct. Here we show that the GI tract of the Mexican tetra, Astyanax mexicanus, has distinct regions, exhibiting differences in morphology, motility, and absorption. We found that A. mexicanus populations adapted for life in subterranean caves exhibit differences in the GI segments compared to those adapted to surface rivers. Cave-adapted fish exhibit bi-directional churning motility in the stomach region that is largely absent in river-adapted fish. We investigated how this motility pattern influences intestinal transit of powdered food and live prey. We found that powdered food is more readily emptied from the cavefish GI tract. In contrast, the transit of live rotifers from the stomach region to the midgut occurs more slowly in cavefish compared to surface fish, consistent with the presence of churning motility. Differences in intestinal motility and transit likely reflect adaptation to unique food sources available to post-larval A. mexicanus in the cave and river environments. We found that cavefish grow more quickly than surface fish when fed ad libitum, suggesting that altered GI function may aid in nutrient consumption or absorption. We did not observe differences in enteric neuron density or smooth muscle organization between cavefish and surface fish. Altered intestinal motility in cavefish could instead be due to changes in the activity or patterning of the enteric nervous system. Exploring this avenue will lead to a better understanding of how the GI tract evolves to maximize energy assimilation from novel food sources.

Structure of the alimentary tract in the Atlantic mudskipper Periophthalmus barbarus (Gobiidae: Oxudercinae): anatomical, histological and ultrastructural studies

The alimentary tract of oxudercine gobies is characterized by a lack of an anatomically distinct stomach, owing to which they are classified as stomachless. Since the environment, food requirements, and feeding habits have a significant impact on the anatomy of the alimentary tract of fish, it was assumed that predominantly carnivorous, semi-terrestrial mudskippers would have a stomach. In order to verify this hypothesis, anatomical, histological, histochemical and ultrastructural analysis of the alimentary tract of the Atlantic mudskipper Periophthalmus barbarus was performed. The results revealed that despite a lack of clear anatomical distinction within the alimentary tract, there were four well-distinguished sections visible at the histological level: oesophagus, stomach, intestine, and rectum. The division was enhanced by the presence of a pyloric sphincter and an ileorectal valve. The stomach contained tubular glands composed of oxynticopeptic cells. Gland cells had pepsinogen granules and a well-developed tubulovesicular network of smooth membranes, which indicates the secretion of gastric juice. The presence of neutral mucus in the apical region of surface epithelial cells as protection against hydrochloric acid as well as the presence of active pepsin also confirm gastric function. However, low pepsin activity seems to implies low protein digestion. The results of this study indicate that the Atlantic mudskipper P. barbarus has a functional stomach.

Gastrointestinal distribution of chicken gastrin-cholecystokinin family transcript expression and response to short-term nutritive state

The related peptide hormones cholecystokinin (CCK) and gastrin are conserved throughout vertebrate clades and implicated in energy homeostasis. CCK is generally accepted as a satiety hormone in poultry, but the role of gastrin remains poorly studied. Functional dissection of these ligands is required to characterise the molecular control of growth & satiety in the domestic chicken, for which there is an increasingly pressing mandate. There are limited descriptions of physiological distributions for the two genes in birds, and these are mostly reliant on immunohistochemistry which can prove problematic due to the shared structure of the targets. Therefore, we have defined the tissue distributions of CCK and gastrin in the chicken, focussing on the gastrointestinal tract, by using transcript-dependent techniques to improve reliability by increasing specificity. Though considerably more highly expressed in the brain, gastrointestinal CCK transcripts were dispersed throughout the small intestine and particularly around the proximal ileum. Gastrin expression was strictly limited to the gastric antrum region of the intestinal tract, albeit very highly expressed. We demonstrate that CCK mRNA expression does not respond as expected for a short-term satiety hormone, and that the short-term response of gastrin expression is paradoxical compared to its role in mammals. These results partially corroborate previous peptide distribution studies and initiate exploration of the nutrient-responsive roles of these hormones in avian energy balance.

Influence of feeding habits in the endocrine pancreas of insectivore bat Pteronotus personatus and nectarivore bat Anoura geoffroyi: A comparative stereological and immunohistochemical study

Pteronotus personatus as an insectivore bat and has a diet that consists of a high protein diet, whereas the diet of Anoura geoffroyi, a predominantly nectarivore bat, is rich in simple sugars like sucrose, glucose and fructose. Considering that diet influences the activation of different pathways, which may influence morphological adaptations in the gastrointestinal system, the aim of this study was to compare the morphology of the endocrine pancreas in P. personatus and A. geoffroyi. For this, histological, stereological and immunohistochemical methods were used. In P. personatus, the average diameter of the pancreatic islet was 40.47μm±13.94, while in A. geoffroyi was 88.16μm±36.40. The total number of pancreatic islets in P. personatus was 26150±2346 and in A. geoffroyi was 15970±1666. In P. personatus, the volume density of the pancreatic islets was 3.4%± 2.6, whereas in A. geoffroyi the volume density was 6.1%±3.7. In addition, the immunodensity of the α, β and δ cells, in P. personatus was 25.8%±11.9, 35.5%±13.5, 3.9%±0.7, respectively, and in A. geoffroyi was 33.10%±12.7, 55.08%±7.4, 6.2%±4.6, respectively. In conclusion, the results of this study indicate differences in the pancreatic weight/body, weight ratio, diameter and volume density of pancreatic islets and in immunodensity of the β and α cells between both species, which have different dietary habits.

Goblet cells and intestinal Alkaline phosphatase expression (IAP) during the development of the rat small intestine

This study aimed to evaluate the temporal and spacial distribution of the mucins produced by goblet cells and intestinal alkaline phosphatase (IAP) expression during the development of the small intestine of the rat. Intestines were removed from rats on the 15th, 17th and 18th days of intratuterine life (i.u.) and on the 3rd, 10th, 17th and 25th days after birth (a.b.). Intestines were processed for routine histological procedures and sections were submitted to histochemistry using PAS to stain neutral glycoproteins and Alcian blue for acidic glycoproteins, as well as immunohistochemistry to detect IAP. In rats, glycoprotein production was seen to begin in the intestinal epithelium cell at around the 17th day of i.u. life; however, this production was not accompanied by morphological indications of the presence of goblet cells. By the 18th i.u. day, the villus epithelium was undergoing differentiation and the first goblet cells could be identified from this time. At around the 10th day a.b., both compartments of the small intestine were detected; i.e. the villi and the crypts. At this timepoint, goblet cells were present in the villi, and also in the upper regions of the crypts. On the 3rd, 10th 17th and 25th days a.b., the presence of the goblet cells increased and presented regional differences in the sections evaluated. IAP was not detected during i.u. life, but was weakly detected in the cells of the villi from the 3rd day a.b., along the entire extension of the villi. On the 10th day, IAP was detected at the tip of the villi, while on the 25th day, it was detected along the extension of the villi, but with a weaker intensity. In conclusion, a temporal and spacial distribution of goblet cells and IAP activity occurs during the development of the small intestine, suggesting a possible regulatory control in accordance with the suckling and weaning phases of food intake in the rat's life.

Metabolomic approach for identifying and visualizing molecular tissue markers in tadpoles of Xenopus tropicalis by mass spectrometry imaging

In developmental and cell biology it is crucial to evaluate the dynamic profiles of metabolites. An emerging frog model system using Xenopus tropicalis, whose genome sequence and inbred strains are available, is now ready for metabolomics investigation in amphibians. In this study we applied matrix-assisted laser desorption/ionization (MALDI)-mass spectrometry imaging (MSI) analysis to identify and visualize metabolomic molecular markers in tadpoles of Xenopus tropicalis. We detected tissue-specific peaks and visualized their distribution in tissues, and distinguished 19 tissues and their specific peaks. We identified, for the first time, some of their molecular localizations via tandem mass spectrometric analysis: hydrocortisone in artery, L-DOPA in rhombencephalon, taurine in eye, corticosterone in gill, heme in heart, inosine monophosphate and carnosine in muscle, dopamine in nerves, and phosphatidylethanolamine (16:0/20:4) in pharynx. This is the first MALDI-MSI study of X. tropicalis tadpoles, as in small tadpoles it is hard to distinguish and dissect the various organs. Furthermore, until now there has been no data about the metabolomic profile of each organ. Our results suggest that MALDI-MSI is potentially a powerful tool for examining the dynamics of metabolomics in metamorphosis as well as conformational changes due to metabolic changes.

Summary: We applied matrix-assisted laser desorption/ionization−mass spectrometry imaging analyses to identify and visualize metabolomic molecular markers in tadpoles of Xenopus tropicalis. We found new molecular markers in various tissues and cells.

A histological atlas of the tissues and organs of neotenic and metamorphosed axolotl

Axolotl (Ambystoma Mexicanum) has been emerging as a promising model in stem cell and regeneration researches due to its exceptional regenerative capacity. Although it represents lifelong lasting neoteny, induction to metamorphosis with thyroid hormones (THs) treatment advances the utilization of Axolotl in various studies. It has been reported that amphibians undergo anatomical and histological remodeling during metamorphosis and this transformation is crucial for adaptation to terrestrial conditions. However, there is no comprehensive histological investigation regarding the morphological alterations of Axolotl organs and tissues throughout the metamorphosis. Here, we reveal the histological differences or resemblances between the neotenic and metamorphic axolotl tissues. In order to examine structural features and cellular organization of Axolotl organs, we performed Hematoxylin & Eosin, Luxol-Fast blue, Masson's trichrome, Alcian blue, Orcein and Weigart's staining. Stained samples from brain, gallbladder, heart, intestine, liver, lung, muscle, skin, spleen, stomach, tail, tongue and vessel were analyzed under the light microscope. Our findings contribute to the validation of the link between newly acquired functions and structural changes of tissues and organs as observed in tail, skin, gallbladder and spleen. We believe that this descriptive work provides new insights for a better histological understanding of both neotenic and metamorphic Axolotl tissues.

Orchestrating change: The thyroid hormones and GI-tract development in flatfish metamorphosis

Metamorphosis in flatfish (Pleuronectiformes) is a late post-embryonic developmental event that prepares the organism for the larval-to-juvenile transition. Thyroid hormones (THs) play a central role in flatfish metamorphosis and the basic elements that constitute the thyroid axis in vertebrates are all present at this stage. The advantage of using flatfish to study the larval-to-juvenile transition is the profound change in external morphology that accompanies metamorphosis making it easy to track progression to climax. This important lifecycle transition is underpinned by molecular, cellular, structural and functional modifications of organs and tissues that prepare larvae for a successful transition to the adult habitat and lifestyle. Understanding the role of THs in the maturation of organs and tissues with diverse functions during metamorphosis is a major challenge. The change in diet that accompanies the transition from a pelagic larvae to a benthic juvenile in flatfish is associated with structural and functional modifications in the gastrointestinal tract (GI-tract). The present review will focus on the maturation of the GI-tract during metamorphosis giving particular attention to organogenesis of the stomach a TH triggered event. Gene transcripts and biological processes that are associated with GI-tract maturation during Atlantic halibut metamorphosis are identified. Gene ontology analysis reveals core biological functions and putative TH-responsive genes that underpin TH-driven metamorphosis of the GI-tract in Atlantic halibut. Deciphering the specific role remains a challenge. Recent advances in characterizing the molecular, structural and functional modifications that accompany the appearance of a functional stomach in Atlantic halibut are considered and future research challenges identified.

Antigenotoxic and Antimutagenic Activities of Probiotic Lactobacillus rhamnosus Vc against N-Methyl-N'-Nitro-N-Nitrosoguanidine

The present study provides experimental evidence of in vivo reduction of genotoxic and mutagenic activities of potent carcinogen N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) by the strain Lactobacillus rhamnosus Vc. In vitro studies revealed that coincubation of MNNG with viable cells of L. rhamnosus Vc resulted in the detoxification of the parent compound accompanied with reduction in genotoxicity (69%) and mutagenicity (61%) as evaluated by SOS-Chromotest and Ames test, respectively. Oral feeding of probiotic bacteria L. rhamnosus Vc (10(9) cfu) to Gallus gallus (chicks) for 30 days provided protection against MNNG-induced damage as evidenced from the significant decrease (P = 0.009) in glutathione S-transferase activity in the L. rhamnosus Vc+MNNG-treated chicks in comparison to the MNNG-treated chicks. Histopathology of colon and liver showed intact cells and mild inflammation in the L. rhamnosus Vc+MNNG-treated chicks, whereas heavy inflammation and degenerative changes were observed in MNNG-treated chicks. The results indicate that the probiotic L. rhamnosus Vc provided in vivo protection against MNNG-induced colon damage by detoxification of MNNG to less toxic metabolites.

Mesenchymal-epithelial interactions during digestive tract development and epithelial stem cell regeneration

The gastrointestinal tract develops from a simple and uniform tube into a complex organ with specific differentiation patterns along the anterior-posterior and dorso-ventral axes of asymmetry. It is derived from all three germ layers and their cross-talk is important for the regulated development of fetal and adult gastrointestinal structures and organs. Signals from the adjacent mesoderm are essential for the morphogenesis of the overlying epithelium. These mesenchymal-epithelial interactions govern the development and regionalization of the different gastrointestinal epithelia and involve most of the key morphogens and signaling pathways, such as the Hedgehog, BMPs, Notch, WNT, HOX, SOX and FOXF cascades. Moreover, the mechanisms underlying mesenchyme differentiation into smooth muscle cells influence the regionalization of the gastrointestinal epithelium through interactions with the enteric nervous system. In the neonatal and adult gastrointestinal tract, mesenchymal-epithelial interactions are essential for the maintenance of the epithelial regionalization and digestive epithelial homeostasis. Disruption of these interactions is also associated with bowel dysfunction potentially leading to epithelial tumor development. In this review, we will discuss various aspects of the mesenchymal-epithelial interactions observed during digestive epithelium development and differentiation and also during epithelial stem cell regeneration.

Histological and histochemical analysis of the gastrointestinal tract of the common pipistrelle bat (Pipistrellus pipistrellus)

Bats have a very high mass-specific energy demand due to small size and active flight. European bat species are mostly insectivorous and the morphology of the gastrointestinal tract should be adapted accordingly. This study investigated the general anatomy by histology and the function by analysing carbohydrate distribution in particular of the mucus of the GI tract of the insectivorous bat Pipistrellus pipistrellus. The GI tracts of three individuals were dissected, fixed in formaldehyde, and embedded in paraffin wax. The tissues and cells of the GI tract of P. pipistrellus were analysed by classical (Acid Alizarin Blue, Haematoxylin-Eosin, and Masson Goldner Trichrome), histochemical (periodic acid-Schiff, Alcian blue at pH 2.5) and lectin histochemical (lectins WGA and HPA) staining procedures. The GI tract of P. pipistrellus was organised into the typical mammalian layers. The short, narrow, and thin-walled esophagus was simple with a folded stratified squamous epithelium without glands but mucous surface cells secreting neutral mucus. The stomach was globular shaped without specialisation. Mucous surface cells produced neutral mucus whereas neck and parietal cells secreted a mixture of neutral and acid mucus. Chief cell surface was positive for N-acetylglucosamine and the cytoplasm for N-acetylgalactosamine residues. The intestine lacked a caecum and appendix. The small intestine was divided into duodenum, jejunum‑ileum and ileum‑colon. The epithelium consisted of columnar enterocytes and goblet cells. The large intestine was short, only represented by the descending colon-rectum. It lacked villi and the mucosa had only crypts of Lieberkühn. Towards the colon-rectum, goblet cells produced mucus with N-acetylglucosamine residues increasing in acidity except in colon-rectum where acidity was highest in the base of crypts. Along the tube the surface of enterocytes was positive for N-acetylglucosamine and N-acetylgalactosamine. All over the mucus filling the lumen of the GI tract was positive for N-acetylglucosamine and increased in acidity in all parts except of the stomach. In conclusion, the simple GI tract showed an anatomical reduction of tissue enabling for a short retention time and a reduction of the load carried during flight: short GI tract, lack of lymphoid tissue, missing of glands in certain regions, and a distinct pattern of mucus distribution, indicating different physiological functions of these areas. The GI tract of P. pipistrellus was typical for an insectivorous species probably representing the ancestral condition.

Histochemical and biometric study of the gastrointestinal system of Hyla orientalis (Bedriaga, 1890) (Anura, Hylidae)

This study was carried out to assess the localization of hyaluronic acid (HA) and the distribution of glycoproteins in the gastrointestinal system of adult Hyla orientalis. Histochemical analysis of the gastrointestinal system in H. orientalis showed that mucous content included glycogene and/or oxidable dioles [periodic acid/Schiff (PAS)+], neutral or acid-rich (PAS/AB pH 2.5+), sialic acid residues (KOH/PAS+) and acid sulphate [Aldehyde fuchsin (AF)+] glycoproteins. However the mucus content was not the same in stomach, small and large intestine. The mucus content of stomach included only glycogene and/or oxidable dioles and sialic acid residues. Besides these histochemical methods, the localization of HA was detected using biotinylated hyaluronic acid binding protein labeled with streptavidin-fluorescein isothiocyanate (FITC). In the extracellular matrix of the submucosa, the reaction for HA was evident. Since HA was located in submucosa beneath the epithelial layer of gastrointestinal system, it has a significant role in hydric balance, and essential to provide the gastrointestinal system integrity and functionality. According to biometric results, there were statistical differences between small and large intestine in terms of the amount of material stained positive with PAS/AB, PAS, KOH/PAS and AF/AB. Additionally, number of goblet cells in the small and large intestine was significantly different.

LIM homeodomain transcription factor Isl1 directs normal pyloric development by targeting Gata3

Background

Abnormalities in pyloric development or in contractile function of the pylorus cause reflux of duodenal contents into the stomach and increase the risk of gastric metaplasia and cancer. Abnormalities of the pyloric region are also linked to congenital defects such as the relatively common neonatal hypertrophic pyloric stenosis, and primary duodenogastric reflux. Therefore, understanding pyloric development is of great clinical relevance. Here, we investigated the role of the LIM homeodomain transcription factor Isl1 in pyloric development.

Results

Examination of Isl1 expression in developing mouse stomach by immunohistochemistry, whole mount in situ hybridization and real-time quantitative PCR demonstrated that Isl1 is highly expressed in developing mouse stomach, principally in the smooth muscle layer of the pylorus. Isl1 expression was also examined by immunofluorescence in human hypertrophic pyloric stenosis where the majority of smooth muscle cells were found to express Isl1. Isl1 function in embryonic stomach development was investigated utilizing a tamoxifen-inducible Isl1 knockout mouse model. Isl1 deficiency led to nearly complete absence of the pyloric outer longitudinal muscle layer at embryonic day 18.5, which is consistent with Gata3 null mouse phenotype. Chromatin immunoprecipitation, luciferase assays, and electrophoretic mobility shift assays revealed that Isl1 ensures normal pyloric development by directly targeting Gata3.

Conclusions

This study demonstrates that the Isl1-Gata3 transcription regulatory axis is essential for normal pyloric development. These findings are highly clinically relevant and may help to better understand pathways leading to pyloric disease.

Functional modifications associated with gastrointestinal tract organogenesis during metamorphosis in Atlantic halibut (Hippoglossus hippoglossus)

BACKGROUND

Flatfish metamorphosis is a hormone regulated post-embryonic developmental event that transforms a symmetric larva into an asymmetric juvenile. In altricial-gastric teleost fish, differentiation of the stomach takes place after the onset of first feeding, and during metamorphosis dramatic molecular and morphological modifications of the gastrointestinal (GI-) tract occur. Here we present the functional ontogeny of the developing GI-tract from an integrative perspective in the pleuronectiforme Atlantic halibut, and test the hypothesis that the multiple functions of the teleost stomach develop synchronously during metamorphosis.

RESULTS

Onset of gastric function was determined with several approaches (anatomical, biochemical, molecular and in vivo observations). In vivo pH analysis in the GI-tract lumen combined with quantitative PCR (qPCR) of α and β subunits of the gastric proton pump (H+/K+-ATPase) and pepsinogen A2 indicated that gastric proteolytic capacity is established during the climax of metamorphosis. Transcript abundance of ghrelin, a putative orexigenic signalling molecule produced in the developing stomach, correlated (p < 0.05) with the emergence of gastric proteolytic activity, suggesting that the stomach's role in appetite regulation occurs simultaneously with the establishment of proteolytic function. A 3D models series of the GI-tract development indicated a functional pyloric sphincter prior to first feeding. Observations of fed larvae in vivo confirmed that stomach reservoir function was established before metamorphosis, and was thus independent of this event. Mechanical breakdown of food and transportation of chyme through the GI-tract was observed in vivo and resulted from phasic and propagating contractions established well before metamorphosis. The number of contractions in the midgut decreased at metamorphic climax synchronously with establishment of the stomach's proteolytic capacity and its increased peristaltic activity. Putative osmoregulatory competence of the GI-tract, inferred by abundance of Na+/K+-ATPase α transcripts, was already established at the onset of exogenous feeding and was unmodified by metamorphosis.

CONCLUSIONS

The functional specialization of the GI-tract was not exclusive to metamorphosis, and its osmoregulatory capacity and reservoir function were established before first feeding. Nonetheless, acid production and the proteolytic capacity of the stomach coincided with metamorphic climax, and also marked the onset of the stomach's involvement in appetite regulation via ghrelin.

Pattern and process during sea urchin gut morphogenesis: the regulatory landscape

The development of the endoderm is a multistage process. From the initial specification of the endodermal domain in the embryo to the final regionalization of the gut, there are multiple stages that require the involvement of complex gene regulatory networks. In one concrete case, the sea urchin embryo, some of these stages and their genetic control are (relatively) well understood. Several studies have underscored the relevance of individual transcription factor activities in the process, but very few have focused the attention on gene interactions within specific gene regulatory networks (GRNs). Sea urchins offer an ideal system to study the different factors involved in the morphogenesis of the gut. Here we review the knowledge gained over the last 10 years on the process and its regulation, from the early specification of endodermal lineages to the late events linked to the patterning of functional domains in the gut. A lesson of remarkable importance has been learnt from comparison of the mechanisms involved in gut formation in different bilaterian animals; some of these genetic mechanisms are particularly well conserved. Patterning the gut seems to involve common molecular players and shared interactions, whether we look at mammals or echinoderms. This astounding degree of conservation reveals some key aspects of deep homology that are most probably shared by all bilaterian guts.

Proper development of the outer longitudinal smooth muscle of the mouse pylorus requires Nkx2-5 and Gata3

BACKGROUND & AIMS

Infantile hypertrophic pyloric stenosis is a common birth anomaly characterized by obstruction of the pyloric lumen. A genome-wide association study implicated NKX2-5, which encodes a transcription factor that is expressed in embryonic heart and pylorus, in the pathogenesis of infantile hypertrophic pyloric stenosis. However, the function of the NKX2-5 in pyloric smooth muscle development has not been examined directly. We investigated the pattern of Nkx2-5 during the course of murine pyloric sphincter development and examined coexpression of Nkx2-5 with Gata3 and Sox9-other transcription factors with pyloric-specific mesenchymal expression. We also assessed pyloric sphincter development in mice with disruption of Nkx2-5 or Gata3.

METHODS

We used immunofluorescence analysis to compare levels of NKX2-5, GATA3, and SOX9 in different regions of smooth muscle cells. Pyloric development was assessed in mice with conditional or germline deletion of Nkx2-5 or Gata3, respectively.

RESULTS

Gata3, Nkx2-5, and Sox9 are coexpressed in differentiating smooth muscle cells of a distinct fascicle of the pyloric outer longitudinal muscle. Expansion of this fascicle coincides with development of the pyloric sphincter. Disruption of Nkx2-5 or Gata3 causes severe hypoplasia of this fascicle and alters pyloric muscle shape. Although expression of Sox9 requires Nkx2-5 and Gata3, there is no apparent hierarchical relationship between Nkx2-5 and Gata3 during pyloric outer longitudinal muscle development.

CONCLUSIONS

Nkx2-5 and Gata3 are independently required for the development of a pyloric outer longitudinal muscle fascicle, which is required for pyloric sphincter morphogenesis in mice. These data indicate that regulatory changes that alter Nkx2-5 or Gata3 expression could contribute to pathogenesis of infantile hypertrophic pyloric stenosis.

Analysis of the expression of p53 during the morphogenesis of the gastroesophageal mucosa of Gallus gallus domesticus (Linnaeus, 1758)

Ontogenesis comprises a series of events including cell proliferation and apoptosis and resulting in the normal development of the embryo. Protein p53 has been described as being involved in the development of several animal species. The aim of this study was to analyze the expression of protein p53 during the morphogenesis of the gastroesophageal mucosa of Gallus gallus domesticus and to correlate it with the histogenesis of structures present in this tissue. We used 24 embryos (at 12-20 days of incubation) and the thymus of two chickens. Immunohistochemical analysis was performed with the ABC indirect method. The expression of p53 in the gastroesophageal mucosa increased during the formation of the organ, mainly at the stages during which tissue remodeling and cell differentiation began. In the esophagus at stages 42 and 45, we observed immunoreactive (IR) cells in the surface epithelium and in early esophageal glands. In the proventriculus at stages 39-45, IR cells were present in the epithelial mucosa and rarely in the proventricular glands. In the gizzard after stage 42, we found IR cells mainly in the medial and basal epithelial layers of the mucosa and especially within the intercellular spaces that appeared at this phase and formed the tubular gland ducts. Thus, protein p53 occurs at key stages of development: in the esophagus during the remodeling of esophageal glands, in the proventriculus during the differentiation of the epithelium of the mucosa and in the gizzard during the formation of tubular glands.

Developmental origins of a novel gut morphology in frogs

Phenotypic variation is a prerequisite for evolution by natural selection, yet the processes that give rise to the novel morphologies upon which selection acts are poorly understood. We employed a chemical genetic screen to identify developmental changes capable of generating ecologically relevant morphological variation as observed among extant species. Specifically, we assayed for exogenously applied small molecules capable of transforming the ancestral larval foregut of the herbivorous Xenopus laevis to resemble the derived larval foregut of the carnivorous Lepidobatrachus laevis. Appropriately, the small molecules that demonstrate this capacity modulate conserved morphogenetic pathways involved in gut development, including downregulation of retinoic acid (RA) signaling. Identical manipulation of RA signaling in a species that is more closely related to Lepidobatrachus, Ceratophrys cranwelli, yielded even more similar transformations, corroborating the relevance of RA signaling variation in interspecific morphological change. Finally, we were able to recover the ancestral gut phenotype in Lepidobatrachus by performing a reverse chemical manipulation to upregulate RA signaling, providing strong evidence that modifications to this specific pathway promoted the emergence of a lineage-specific phenotypic novelty. Interestingly, our screen also revealed pathways that have not yet been implicated in early gut morphogenesis, such as thyroid hormone signaling. In general, the chemical genetic screen may be a valuable tool for identifying developmental mechanisms that underlie ecologically and evolutionarily relevant phenotypic variation.

Expression patterns of genes encoding small GTPases Ras-dva-1 and Ras-dva-2 in the Xenopus laevis tadpoles

Small GTPases of the recently discovered Ras-dva family are specific to the Vertebrate phylum. In Xenopus laevis, Ras-dva-1 is expressed during gastrulation and neurulation in the anterior ectoderm where it regulates the early development of the forebrain and cranial placodes (Tereshina et al., 2006). In the present work, we studied the expression of Ras-dva-1 at later developmental stages. As a result, the Ras-dva-1 expression was revealed in the eye retina, epiphysis (pineal gland), hypophysis (pituitary), branchial arches, pharynx, oesophagus, stomach and gall bladder of swimming tadpoles. Additionally, we investigated for the first time the expression pattern of Ras-dva-2. This gene encodes a protein belonging to a novel sub-group of Ras-dva GTPases that we identified by phylogenetic analysis within Ras-dva family. In contrast to Ras-dva-1, Ras-dva-2 is not expressed before the swimming tadpole stage. At the swimming tadpole stage, however, Ras-dva-2 transcripts can be detected in the eye retina and brain. Later in development, the expression of Ras-dva-2 can also be revealed in the mesonephros and stomach.

Morphogenesis of the primitive gut tube is generated by Rho/ROCK/myosin II-mediated endoderm rearrangements

During digestive organogenesis, the primitive gut tube (PGT) undergoes dramatic elongation and forms a lumen lined by a single-layer of epithelium. In Xenopus, endoderm cells in the core of the PGT rearrange during gut elongation, but the morphogenetic mechanisms controlling their reorganization are undetermined. Here, we define the dynamic changes in endoderm cell shape, polarity, and tissue architecture that underlie Xenopus gut morphogenesis. Gut endoderm cells intercalate radially, between their anterior and posterior neighbors, transforming the nearly solid endoderm core into a single layer of epithelium while concomitantly eliciting "radially convergent" extension within the gut walls. Inhibition of Rho/ROCK/Myosin II activity prevents endoderm rearrangements and consequently perturbs both gut elongation and digestive epithelial morphogenesis. Our results suggest that the cellular and molecular events driving tissue elongation in the PGT are mechanistically analogous to those that function during gastrulation, but occur within a novel cylindrical geometry to generate an epithelial-lined tube.

Dynamic patterning at the pylorus: formation of an epithelial intestine-stomach boundary in late fetal life

In the adult mouse, distinct morphological and transcriptional differences separate stomach from intestinal epithelium. Remarkably, the epithelial boundary between these two organs is literally one cell thick. This discrete junction is established suddenly and precisely at embryonic day (E) 16.5, by sharpening a previously diffuse intermediate zone. In the present study, we define the dynamic transcriptome of stomach, pylorus, and intestinal tissues between E14.5 and E16.5. We show that establishment of this boundary is concomitant with the induction of over a thousand genes in intestinal epithelium, and these gene products provide intestinal character. Hence, we call this process intestinalization. We identify specific transcription factors (Hnf4 gamma, Creb3l3, and Tcfec) and examine signaling pathways (Hedgehog and Wnt) that may play a role in this process. Finally, we define a unique expression domain at the pylorus itself and detect novel pylorus-specific patterns for the transcription factor Gata3 and the secreted protein nephrocan.

Dividing the tubular gut: generation of organ boundaries at the pylorus

The discrete organs that comprise the gastrointestinal tract (esophagus, stomach, small intestine, and large intestine) arise embryonically by regional differentiation of a single tube that is initially morphologically similar along its length. Regional organ differentiation programs, for example, for stomach or intestine, involve signaling cross-talk between epithelium and mesenchyme and result in the formation of precise boundaries between organs, across which dramatic differences in both morphology and gene expression are seen. The pylorus is a unique area of the gut tube because it not only marks an important organ boundary in the tubular gut (the stomach/intestinal boundary) but is also the hub for the development of multiple accessory organs (liver, pancreas, gall bladder, and spleen). This chapter examines: (a) our current understanding of the molecular and morphogenic processes that underlie the generation of the dramatic epithelial tissue boundary that compartmentalizes stomach and intestine; (b) the tissue interactions that promote development of the accessory organs in this area; and (c) the molecular interactions that specify patterning of the pyloric sphincter. Though the focus here is primarily on the mouse as a model organism, the molecular underpinnings of organ patterning near the pylorus are shared by chick and frog. Thus, further study of these conserved developmental programs could potentially shed light on the mechanisms underlying human pyloric malformations such as infantile hypertrophic pyloric stenosis.

Left-right asymmetry in gut development: what happens next?

The gastrointestinal tract is an asymmetrically patterned organ system. The signals which initiate left-right asymmetry in the developing embryo have been extensively studied, but the downstream steps required to confer asymmetric morphogenesis on the gut organ primordia are less well understood. In this paper we outline key findings on the tissue mechanics underlying gut asymmetry, across a range of species, and use these to synthesise a conserved model for asymmetric gut morphogenesis. We also discuss the importance of correct establishment of left-right asymmetry for gut development and the consequences of perturbations in this process.

Six2 activity is required for the formation of the mammalian pyloric sphincter

The functional activity of Six2, a member of the so/Six family of homeodomain-containing transcription factors, is required during mammalian kidney organogenesis. We have now determined that Six2 activity is also necessary for the formation of the pyloric sphincter, the functional gate at the stomach-duodenum junction that inhibits duodenogastric reflux. Our data reveal that several genes known to be important for pyloric sphincter formation in the chick (e.g., Bmp4, Bmpr1b, Nkx2.5, Sox9, and Gremlin) also appear to be required for the formation of this structure in mammals. Thus, we propose that Six2 activity regulates this gene network during the genesis of the pyloric sphincter in the mouse.

Cross-fostering of the tammar wallaby (Macropus eugenii) pouch young accelerates fore-stomach maturation

There are two phases of fore-stomach development during the first 200 days of pouch life in tammar wallaby. For the first 170 days, the mucosa displays an immature gastric glandular phenotype that changes to a cardia glandular phenotype, which remains for the rest of the animal's life. During this 200-day period after birth, the pouch young (PY) is dependent on maternal milk, which progressively changes in composition. We showed previously that PY cross-fostered to host mothers at a later stage of lactation accelerated development. In this study, we investigated whether cross-fostering and exposure to late lactation stage milk affected the transition to cardia glandular phenotype. In fostered PY fore-stomach, there was increased apoptosis, but no change in cell proliferation. The parietal cell population was significantly reduced, and expression of gastric glandular phenotype marker genes (ATP4A, GKN2, GHRL and NDRG2) was down-regulated, suggesting down-regulation of gastric phenotype in fostered PY fore-stomach. The expression of cardia glandular phenotype genes (MUC4, KRT20, CSTB, ITLN2 and LPLUNC1) was not changed in fostered PY. These data suggest that fore-stomach maturation proceeds via two temporally distinct processes: down-regulation of gastric glandular phenotype and initiation of cardia glandular phenotype. In fostered PY, these two processes appear uncoupled, as gastric glandular phenotype was down-regulated but cardia glandular phenotype was not initiated. We propose that milk from later stages of lactation and/or herbage consumed by the PY may play independent roles in regulating these two processes.