Localization of fibronectin in the frog skin

Frog Skin Innate Immune Defences: Sensing and Surviving Pathogens

Amphibian skin is a mucosal surface in direct and continuous contact with a microbially diverse and laden aquatic and/or terrestrial environment. As such, frog skin is an important innate immune organ and first line of defence against pathogens in the environment. Critical to the innate immune functions of frog skin are the maintenance of physical, chemical, cellular, and microbiological barriers and the complex network of interactions that occur across all the barriers. Despite the global decline in amphibian populations, largely as a result of emerging infectious diseases, we understand little regarding the cellular and molecular mechanisms that underlie the innate immune function of amphibian skin and defence against pathogens. In this review, we discuss the structure, cell composition and cellular junctions that contribute to the skin physical barrier, the antimicrobial peptide arsenal that, in part, comprises the chemical barrier, the pattern recognition receptors involved in recognizing pathogens and initiating innate immune responses in the skin, and the contribution of commensal microbes on the skin to pathogen defence. We briefly discuss the influence of environmental abiotic factors (natural and anthropogenic) and pathogens on the immunocompetency of frog skin defences. Although some aspects of frog innate immunity, such as antimicrobial peptides are well-studied; other components and how they contribute to the skin innate immune barrier, are lacking. Elucidating the complex network of interactions occurring at the interface of the frog's external and internal environments will yield insight into the crucial role amphibian skin plays in host defence and the environmental factors leading to compromised barrier integrity, disease, and host mortality.

Dermal collagen organization in Bufo ictericus and in Rana catesbeiana integument (Anuran, Amphibian) under the evaluation of laser confocal microscopy

Collagen structural organization plays an important role in the mechanical property of the vertebrate integument. Bufo ictericus and Rana catesbeiana integument was investigated by light microscopy and laser confocal microscopy. Collagenous elements of the dermis were statistical analyzed. The integument is formed by the keratinized squamous stratified epidermis supported by the dermis that is subdivided into the spongious layer with a loose arrangement, and the compact layer formed by collagenous fibers arranged compactly in a criss-crossed manner. Thick collagenous columns have a perpendicular trajectory, and are formed by the assembling of alternating collagenous lamellae in both animals. Short intercolumns of collagenous fibrils connecting collagenous lamellae obliquely or transversally are observed in R. catesbeiana dorsal integument. The present study provides evidences that B. ictericus and R. catesbeiana integument has well-organized compact dermis, constituted by collagenous lamellae in a plywood manner. The integument organization is in contrast to the literature in some aspects. This dermal arrangement is important to the biomechanical property of both anuran integuments.

Structural and mechanical aspects of the skin of Bufo marinus (Anura, Amphibia)

Specific biomechanical characters and some structures possibly related to them were investigated in the skin of the toad Bufo marinus using tensile testing techniques (at constant strain till rupture) as well as morphological methods (histological, immunohistochemical and electronmicroscopical). Mechanical parameters of the native skin varied considerably according to sex, individual variability and/or site of specimen collection. In skin strips of males and females excised from different parts of the body thickness ranged from 0.45 to 0.87 mm, strain (epsilonf) from 96.52 to 211.03, tensile strength (sigmam) from 5.72 to 9.38 MPa, and stiffness (E-modulus) from 5.76 to 6.73. The dermis of B. marinus is provided with a collagenous stratum compactum of considerable thickness, a stratum spongiosum with loosely arranged fibres and a marked calcified layer (substantia amorpha). Collagen appears to be the main determinant of skin mechanics. However, the slope of the J-shaped static stress-strain curves indicates elastin to be responsible for the high values of strain. Contrary to van Gieson and orcein staining, immunostaining with a monoclonal antibody against elastin revealed very few elastic fibers between collagen bundles and in the vertical fiber tracts (perforating bundles), but a considerable amount in the tela subcutanea. This was partly confirmed at the ultrastructural level by tannic acid staining.

Mechanical properties of the skin of Xenopus laevis (Anura, Amphibia)

The skin of the aquatic pipid frog, Xenopus laevis, was examined for specific biomechanical features: 1) thickness, 2) maximal strain at break (epsilon f), 3) tensile strength (sigma m), 4) modulus of elasticity (E, stiffness), and 5) the area under the stress-strain curve (W) (breaking energy, toughness). Skin freshly removed from dorsal, ventral, and lateral areas of the body was subjected to uniaxial tension. In both sexes, the dorsal skin is thicker than the ventral. The skin of male frogs was consistently thinner in all body regions than that of females. Most biomechanical parameters showed a considerable range of values in both males (epsilon f = 59-63%, sigma m = 15-16.5 MPa, E = 33.5-38.4 MPa, W = 3.8-4.5 MJ/m3) and females (epsilon f = 102-126%, sigma m = 11.5 MPa, E = 10.4-12 MPa, W = 5.2-6.7 MJ/m3). The disparate epsilon f values in males (low) and females (high) might reflect sexual dimorphism. Static stress-strain curves were typically J-shaped; with the exception of a "toe," the curves rose approximately linearly with increasing strain. The skin of X.laevis, although heterogeneous in structure, possesses features similar to those found in tissues with aligned collagen fibers such as tendons or fish skin. However, in anurans, the skin seems to play a more passive mechanical role during locomotion than in fish.