Comparative Skin Histology Of Frogs Reveals High-elevation Adaptation Of The Tibetan Nanorana Parkeri

Adaptations to extreme environmental conditions are intriguing.An intriguing topic in evolutionary biology involves how organisms adapt to their environment.Animal skin is the first protective barrier to external stimuli,which directly interacts with surrounding environment,plays diverse and important roles in adaptive evolution.Unlike most other vertebrates whose skin is covered by either scales or hair,amphibians are thin-and bare-skinned,and their skin plays an important role in respiration.This causes amphibians to be more sensitive to the external environment than most other vertebrates.Thus,it facilitates investigations into adaptations for living in extreme environments.Especially,the QTP-endemic frogs provides an excellent model to study skin adaptations to extreme environmental conditions.Herein,we compare the structures of skin in four anuran species which belong to the same family living at elevations ranging from 100 m to 4500 m to assess phenotypic innovations in the skin of Nanorana parkeri,which lives at extremely high elevations.To distinguish between species differences/intraspecific polymorphism,the analyses include N.parkeri(two groups collected at-4500 m and?2900 m),N.medogensis(?2800 m),N.phrynoides(?1800 m),and Quasipaa spinosa(?170 m).The histological structures of the dorsal-ventral skin in four species were observed using paraffin section and hematoxylin-eosin staining.A paired t-test was used to analyse differences in thickness of skin,relative number of epidermal capillaries,glands and skin pigment content in each species.And discussed with the results of previous studies,the adaptation of Tibetan frogs to the extreme environment was discussed on the skin phenotype level.Analyses reveal the 4 species had similar basic skin structures composed of epidermis,dermnis and subcutaneous layer.Generally,the epidermis is keratinized stratified squamous epithelium,which mainly comprised of stratum comeum,stratum granusum,stratum spinpsum and stratum germinativum.The dermis is consisted of the stratum spongiosum and stratum compactum.The stratum spongiosum is mostly composed of loose connective tissue.There are two types of cutaneous glands and mucous glands in the stratum spongiosum.The stratum compactum is made up of dense connective tissue.The collagen fibers in the stratum compactum lie in bundles parallel to the body surface.The subcutaneous layer is under the stratum compactum,there are many loose connective tissues and blood capillaries.But N.parkeri differs from the other species by lacking a calcified layer at the junction between the stratum spongiosum and stratum compactum,having thinner epidermal,more epidermal capillaries and granular glands,and possessing more skin pigments,which correlate highly with responses to coldness,hypoxia and/or ultraviolet(UV)radiation.These results provide a morphological basis for understanding further the molecular adaptations of these frogs.Furthermore,it provides a theoretical basis for the medical repair of biological injury caused by hypothermia,hypoxia and ultraviolet radiation.