Histology and lectin-binding patterns in the skin of the terrestrial horned frog Ceratophrys ornata

The Complex Bridge between Aquatic and Terrestrial Life: Skin Changes during Development of Amphibians

Amphibian skin is a particularly complex organ that is primarily responsible for respiration, osmoregulation, thermoregulation, defense, water absorption, and communication. The skin, as well as many other organs in the amphibian body, has undergone the most extensive rearrangement in the adaptation from water to land. Structural and physiological features of skin in amphibians are presented within this review. We aim to procure extensive and updated information on the evolutionary history of amphibians and their transition from water to land-that is, the changes seen in their skin from the larval stages to adulthood from the points of morphology, physiology, and immunology.

Life history linked to immune investment in developing amphibians

The broad diversity of amphibian developmental strategies has been shaped, in part, by pathogen pressure, yet trade-offs between the rate of larval development and immune investment remain poorly understood. The expression of antimicrobial peptides (AMPs) in skin secretions is a crucial defense against emerging amphibian pathogens and can also indirectly affect host defense by influencing the composition of skin microbiota. We examined the constitutive or induced expression of AMPs in 17 species at multiple life-history stages. We found that AMP defenses in tadpoles of species with short larval periods (fast pace of life) were reduced in comparison with species that overwinter as tadpoles and grow to a large size. A complete set of defensive peptides emerged soon after metamorphosis. These findings support the hypothesis that species with a slow pace of life invest energy in AMP production to resist potential pathogens encountered during the long larval period, whereas species with a fast pace of life trade this investment in defense for more rapid growth and development.

The amphibian skin-associated microbiome across species, space and life history stages

Skin-associated bacteria of amphibians are increasingly recognized for their role in defence against pathogens, yet we have little understanding of their basic ecology. Here, we use high-throughput 16S rRNA gene sequencing to examine the host and environmental influences on the skin microbiota of the cohabiting amphibian species Anaxyrus boreas, Pseudacris regilla, Taricha torosa and Lithobates catesbeianus from the Central Valley in California. We also studied populations of Rana cascadae over a large geographic range in the Klamath Mountain range of Northern California, and across developmental stages within a single site. Dominant bacterial phylotypes on amphibian skin included taxa from Bacteroidetes, Gammaproteobacteria, Alphaproteobacteria, Firmicutes, Sphingobacteria and Actinobacteria. Amphibian species identity was the strongest predictor of microbial community composition. Secondarily, within a given amphibian species, wetland site explained significant variation. Amphibian-associated microbiota differed systematically from microbial assemblages in their environments. Rana cascadae tadpoles have skin bacterial communities distinct from postmetamorphic conspecifics, indicating a strong developmental shift in the skin microbes following metamorphosis. Establishing patterns observed in the skin microbiota of wild amphibians and environmental factors that underlie them is necessary to understand skin symbiont community assembly, and ultimately, the role skin microbiota play in the extended host phenotype including disease resistance.

Apoptosis in larval and frog skin of Rana pipiens, R. catesbeiana, and Ceratophrys ornata

Apoptosis (programmed cell death) occurs during normal development of anurans in organs such as gills, gut, and tail. For example, apoptotic cells have been reported in the luminal epithelium along the length of the digestive tract of both larvae and frogs; however, timing of the peak number of such cells varies in different species. The purpose of the present study was to ascertain whether apoptosis also varies by species during metamorphic restructuring of the skin (as larval epithelium is replaced by adult epidermis). To determine this, cross-sections of dorsal skin from representative larval stages and frogs of Rana pipiens, R. catesbeiana, and Ceratophrys ornata were incubated with monoclonal antibody against active caspase-3, one of the main enzymes in the apoptotic cascade. We observed apoptotic cells in the epidermis of the skin of the three species and found that such cells were more numerous in larval stages than in frogs and more abundant in the two ranid species than in C. ornata. These results contribute to our understanding of metamorphic changes in anuran skin.

Lectin binding properties of liver, small intestine and tail of metamorphosing marsh frog (Pelophylax ridibundus Pallas 1771)

In this present study, localization and variations of specific sugar moieties in the terminal carbohydrate chains of glycoconjugates in the small intestine, liver and tail have been investigated during the metamorphosis of Pelophylax ridibundus larvae. For this purpose, four lectins were used: wheat germ agglutinin (WGA), Ulex europaeus agglutinin (UEA-I), Dolichos biflorus agglutinin (DBA) and peanut agglutinin (PNA), in different larval stages of the frog. Some cells stained specifically in the intestinal mucosa and in tail epidermal cells with the lectins and their affinity changed during metamorphic transformation. For the most part, they decreased in the climax and postmetamorphic periods. It was also found that WGA, DBA and UEA-I lectins exhibited strong affinity to white blood cells in the liver and their binding affinities were the highest in prometamorphosis and they gradually decreased until the end of metamorphosis. These results suggest that the changes of lectin binding in metamorphosis may be an indication of some cellular events occurring in larval metamorphosis such as cell differentiation and damage of cell adhesion between death and differentiating cells. They also can be useful markers for detection of white blood cells in amphibian hematopoietic organs.

Expression pattern of glycoconjugates in the integument of Bufo ictericus (Anuran, Bufonidae): Biochemical and histochemical (lectin) profiles

Mucous consists of glycoproteins and proteoglycans produced by specific secretory cells (mucocytes). In anurans the cutaneous mucous is produced by intradermal glands and displays both mechanical and chemical protection functions. Indeed, mucous maintains the integument moist and facilitates gas exchange (cutaneous respiration). In this work, the carbohydrate moiety distribution was investigated in the integument of Bufo ictericus using conventional and lectin histochemistry to describe the pattern of cutaneous glycoconjugate expression, including both secretory and structural proteoglycans. As a preliminary step, the descendent chromatography in Whatmann 1MM paper was undertaken to prepare the histochemical trials involving the lectins. In B. ictericus, the integument exhibits the basic morphological structure found in lower terrestrial vertebrates: the epidermis is a keratinized squamous stratified epithelium supported by spongious and compact layers. The spongy dermis contain secretory portion of both mucous and serous (or poison) glands. The paper chromatography identified galactose, fucose and mannose as characteristic sugar residues. The secretory cells of the mucous gland in the dermis, as well as the interstice between the stratum corneum and the subjacent stratum spinosum in the epidermis exhibit alpha-l-fucose and alpha-galactose residues. The serous glands give no reaction. The alpha-mannose residue was detected in the extracellular matrix of spongious dermis, but not in the dermal glands. The different glycoconjugate location reflects in two glycoconjugates categories: the secretory which participate in the water flow regulation, and the structural which is involved in the dermal maintenance.

Serous cutaneous glands in the South American horned frogCeratophrys ornata (Leptodactylformes, Chthonobatrachia, Ceratophrydae): Ultrastructural expression of poison biosynthesis and maturation

Serous cutaneous glands are described in newly metamorphosed and juvenile specimens of the horned frog Ceratophrys ornata using light and transmission electron microscopy. We report patterns of biosynthesis and maturation of the specific product of the gland secretory unit. The syncytial, secretory compartment possesses a complex of endoplasmic reticulum (predominantly smooth endoplasmic reticulum after metamorphosis) and Golgi stacks. The serous product is weak in density and is contained in vesicles involved in repeating merging processes. During this maturation activity, secondary lysosomes are observed, which derive from autophagic processes (crinophagy) involving the secretory materials. Ceratophrys ornata, a species representative of the type genus of the family Ceratophrydae, belongs to the heterogeneous group of anurans that, possibly as the result of convergence, all produce cutaneous poisons consisting of vesicles or faint density granules.

Structural and Immunocytochemical Characterization of Keratinization in Vertebrate Epidermis and Epidermal Derivatives

This review presents comparative aspects of epidermal keratinization in vertebrates, with emphasis on the evolution of the stratum corneum in land vertebrates. The epidermis of fish does not contain proteins connected with interkeratin matrix and corneous cell envelope formation. Mucus-like material glues loose keratin filaments. In amphibians a cell corneous envelope forms but matrix proteins, aside from mucus/glycoproteins, are scarce or absent. In reptiles, birds, and mammals specific proteins associated with keratin become relevant for the production of a resistant corneous layer. In reptiles some matrix, histidine-rich and sulfur-rich corneous cell envelope proteins are produced in the soft epidermis. In avian soft epidermis low levels of matrix and cornified proteins are present while lipids become abundant. In mammalian keratinocytes, interkeratin proteins, cornified cell envelope proteins, and transglutaminase are present. Topographically localized areas of dermal-epidermal interactions in amniote skin determine the formation of skin derivatives such as scales, feathers, and hairs. New types of keratin and associated proteins are produced in these derivatives. In reptiles and birds beta-keratins form the hard corneous material of scales, claws, beaks, and feathers. In mammals, small sulfur-rich and glycine-tyrosine-rich proteins form the corneous material of hairs, horns, hooves, and claws. Molecular studies on reptilian beta-keratins show they are glycine-rich proteins. They have C- and N-terminal amino acid regions homologous to those of mammalian proteins and a central core with homology to avian scale/feather keratins. These findings suggest that ancient reptiles already possessed some common genes that later diversified to produce some keratin-associated protein in extant reptiles and birds, and others in mammals. The evolution of these small proteins represents the more recent variation of the process of cornification in vertebrates.

Histology and Ultrastructure of the Equine Lingual Tonsil. I. Crypt Epithelium and Associated Structures

The microstructural and ultrastructural features of the equine lingual tonsil were studied in five young horses. Located at the root of the tongue it presented an irregular surface with rounded elevations, numerous folds and crypts. Stratified squamous non-keratinized epithelium lining its outer surface was modified by heavy infiltration of lymphoid cells to form reticular epithelium within the crypt. The latter implies a role in initiating and maintaining immune responses to incoming infectious agents and antigens. Lamellated structures resembling Hassall's corpuscle were observed towards the outer surface epithelium. Microplicae were visible by scanning electron microscopy on the surface of both the outer and reticular epithelia. No microvillus cells resembling M cells were observed. The stratum superficiale of the reticular epithelium showed strong affinity for Soybean (SBA), Phosphocarpus tetragonolobus 1 (WBA 1), Ulex europaeus (UEA) and Griffonia simplicifolia 1 isolectin-B4 (GS1-B4). The characteristic lectin binding patterns may be useful for embryological and microbiological investigations. Vimentin filaments were not detected consistent with absence of M cells. Mucus glandular acini in the deeper lamina propria mucosae contained glycogen, acidic, neutral and weakly sulphated mucopolysaccharides. Transmission electron microscopy revealed that the layers of the outer surface and reticular epithelia shared characteristic features except the stratum superficiale, which had nuclei of varying shapes and an abundance of cell organelles. A few mast cells with electron lucent granules and myelinated nerve fibres were localized in the deeper portion.

Glycoconjugate localization in larval and adult skin of the bullfrog,Rana catesbeiana: A lectin histochemical study

This study investigates whether or not the distribution of specific glycoconjugates within the skin is related to the regulation of water balance in the aquatic larvae and semiaquatic adults of the bullfrog, Rana catesbeiana. A lectin histochemical study was carried out on paraffin sections of dorsal and ventral skin from tadpoles in representative stages as well as from adult frogs. Sections were stained with the following horseradish peroxidase (HRP)-conjugated lectins, which bind to specific terminal sugar residues of glycoconjugates: UEA 1 for alpha-L-fucose, SBA for N-acetyl-D-galactosamine, WGA for N-acetyl-B-D-glucosamine, and PNA for beta-galactose. Results indicate that lectins serve as markers for specific skin components (e.g., a second ground substance layer within the dermis was revealed by positive UEA 1 staining). Moreover, each lectin has a specific binding pattern that is similar in dorsal and ventral skin; the larval patterns change as the skin undergoes extensive histological and physiological remodeling during metamorphic climax. These findings enhance our understanding of glycoconjugates and their relationship to skin structure and function-in particular, to the regulation of water balance in R. catesbeiana.