Investigations of endocrine cells in the gastrointestinal tract and pancreas during the metamorphosis of an anuran (Alytes obstetricans L.): histochemical detection of APUD cells

Immunocytochemical and ultrastructural characterization of endocrine cells in the larval stomach of the frog Rana temporaria tadpoles: a comparison with adult specimens

According to immunostaining and ultrastructural patterns, Rana temporaria tadpole stomach displays a well-differentiated endocrine population comprising, at least, six cellular types: ECL, EC [serotonin], D [somatostatin] - all three of them abundant -, P [bombesin] - less numerous -, CCK-8 [cholecystokinin/gastrin] and A [glucagon/glicentin] - both very scarce. Larval endocrine cells are mainly located in the surface epithelium and show open or closed morphologies. Cellular diversity is similar in tadpoles and frogs, with the exception of immunoreactivity for gastrin-17, found in adults in numerous cells. Larval cells display mature ultrastructural traits, although with smaller secretory granules. The different distribution of endocrine cells, which in adults are preferentially located in the glands, probably refers to different functional requirements. However, the rich vascular plexus present in larval mucosa may be an efficient transport medium of surface hormones to-gastric targets. The enhancement in adults of endocrine population and correlative increase in hormonal secretion indicates a more active functional role, probably related to the shift from herbivorous to carnivorous habits. In summary, the tadpole gastric endocrine population, although not as numerous as that of adult frogs, displays histological traits that indicate a relevant (immunoreactive and ultrastructural properties, cellular diversity) and specific (surface location, relative abundance of open-type cells) role of local regulatory factors in amphibian larval gastric function.

Ontogeny of neurohormonal peptides, serotonin, and nitric oxide synthase in the gastrointestinal neuroendocrine system of the axolotl (Ambystoma mexicanum): an immunohistochemical analysis

The ontogeny of the neurohormonal peptides vasoactive intestinal polypeptide (VIP), neurotensin (NT), substance P (SP), calcitonin gene-related peptide (CGRP), gastrin/cholecystokinin (GAS/CCK), and somatostatin (SOM) as well as serotonin (SER) and nitric oxide synthase (NOS) was investigated in the gastrointestinal tract of the urodele Ambystoma mexicanum, the axolotl, using immunohistochemical techniques. The first regulatory substances to appear were SP, SOM, and SER that could be immunohistochemically detected up from stage 1. At early stage 2, VIP immunoreactivity was observed infrequently in enteric nerve fibers. With the onset of external feeding at late stage 2, SP-immunoreactive (IR) and SER-IR endocrine cells and VIP-IR nerve fibers were present throughout the gastrointestinal tract. Furthermore, in the small intestine NT-IR and GAS/CCK-IR endocrine cells appeared. At stage 3, SER immunoreactivity was observed not only in endocrine cells but also in nerve fibers. CGRP-IR and SP-IR nerve fibers were detectable at stage 4 and stage 5, respectively. From stage 5 on, a minority of the CGRP immunoreactivity occurred in SP-IR nerve fibers. NOS immunoreactivity did not appear before stage 6 when it was found infrequently in nerve fibers. Thus, several phases of development can be distinguished: (1) at the yolk sac stages only few regulatory substances are present. (2) At the onset of external feeding, all endocrine cell types investigated were readily detectable. Thus, the onset of external feeding seems to trigger the development of the gastrointestinal endocrine system. (3) The endocrine cells are first found in the proximal part of the gastrointestinal tract and later in higher numbers in the distal parts. (4) The dually distributed neurohormonal peptides and SER first appear in endocrine cells and later additionally in nerve fibers. Thus, the nerve fibers likely set up the fine regulation of gastrointestinal blood flow and motility.

Histology and lectin-binding patterns in the digestive tract of the carnivorous larvae of the anuran, Ceratophrys ornata

Larvae of Ceratophrys ornata are carnivorous, have relatively short digestive tracts and continue to feed during metamorphic climax, in contrast to those of more typical herbivorous anuran larvae. The present study describes both histological and histochemical changes in the stomach, small intestine, and large intestine of C. ornata prior to and during metamorphic climax. Modifications in these organs were found to be similar to but less dramatic than those in herbivorous larvae. Luminal epithelial cells in the three regions develop vacuoles, suggesting degeneration, but sloughing of this epithelium, as occurs in herbivorous larvae, was not observed in C. ornata. Multicellular tubular glands develop gradually in the gastric area during the larval stages, gastric pits appear during metamorphic climax, and mucous neck cells are first visible in the juvenile. Goblet cells in the small and large intestine increase in number during larval life, as do the number of folds in the intestinal wall. Increase in diameter and thickness of the wall occurs in the stomach as well as in the small and large intestine. Such changes result in an adult-type digestive tract characteristic of frogs in general. Staining with two horseradish peroxidase conjugated lectins, soybean agglutinin (SBA) and Ulex europaeus agglutinin I (UEA I), demonstrated specific sites along the digestive tract of glycoconjugates with terminal sugars N-acetylgalactosamine and alpha-fucose, respectively. As metamorphic climax approaches, staining intensities decrease--thus providing evidence for metamorphic changes in the sugar moieties of glycoconjugates present in the digestive tract of carnivorous larvae.

Biphasic intestinal development in amphibians: embryogenesis and remodeling during metamorphosis

Intestinal development in anurans is a biphasic process. The embryogenesis of intestine resembles that in higher vertebrates. The subsequent remodeling process during metamorphosis to produce an adult organ is controlled by TH. Recent progress in studying TH action and its application to amphibian metamorphosis has provided considerable insights into the remodeling process. One possible model for the TH-induced gene regulation cascade of intestinal remodeling is presented in Fig. 9. It is assumed that TRs function as heterodimers with RXRs. In the absence of TH, the TR-RXR heterodimers can bind to TH response elements present in the target genes and repress the basal transcription of these genes (Fondell et al., 1993; Damm et al., 1989; Sap et al., 1989; Baniahmad et al., 1992; Ranjan et al., 1994). The binding of TH leads to conformational changes in the receptor complexes that in turn activate gene transcription. The products of these early response genes then participate in the activation of the remaining gene regulation cascade. Exactly how this occurs remains unknown. Interestingly, the early response genes include not only transcription factors but also other proteins such as metalloproteinases. The transcription factors could activate or repress downstream TH response genes directly. Other proteins are likely to assert their effect indirectly. For example, they could modify the ECM or cell surface. In addition, they could regulate and/or participate in signal transduction by growth factors. The cooperation between these complex intra- and extracellular processes eventually results in the degeneration of the larval organ and formation of the adult tissue. This simplified scheme immediately raises many questions. Although the mRNAs for TRs and RXRs are present in the intestine and the other tissues during metamorphosis (Yaoita and Brown, 1990; Kawahara et al., 1991; Y.-B. Shi, unpublished observations), it is unknown whether the mRNA levels reflect the protein levels. It also remains to be tested whether TR-RXR is indeed the functional complex in vivo and whether the heterodimer is responsible for the activation of the early response genes isolated to date. The majority of the early response genes are ubiquitous. Of the few intestine-specific genes, none of them have yet been identified by sequence analysis. It is of great interest to understand how the same genes expressed in tissues undergoing drastically different changes can exert their biological effects. It is likely that together with existing proteins in the intestine, these early genes regulate tissue-specific downstream genes, which in turn determine the tissue-specific transformation. An important issue is to establish the identity of these downstream genes.

Histology of the esophagus of the adult frog Rana perezi (Anura: Ranidae)

Study of the esophageal microscopic morphology of adult Rana perezi by light and electron microscopy discloses some large folds throughout the esophagus that are in themselves ringed. Glandular ostia open in the furrows of the luminal surface. The esophageal wall is made up of a connective adventitia rich in melanocytes, a muscular tunica, a connective and glandular subepithelial layer, and a pseudostratified ciliated epithelium. This epithelium basically consists of ciliated, goblet, basal, microvillous-apex, and migratory cells. Two types of goblet cells are distinguished with regard to the granular ultrastructure. The microvillous-apex cell has not been found in other amphibians. It shows a very differentiated morphology with a high number of mitochondria. The basal cells give the epithelium a pseudostratified morphology, and they have a proliferative function. Glands are branched and drain through an excretory duct that has a monolayered mucosecreting epithelium. The glandular units are formed by two principal types of cells: mucosecretory and serous.