| In the process of evolution from aquatic to terrestrial life,amphibians are faced with the dilemma of air breathing and water conservation.As the change of the respiratory medium,terrestrial amphibian breathes with lung instead of gill,but the structure of amphibian lung is simple and the cutaneous respiration is still required as an assisted breathing pathway.The high permeability of amphibian skin is an important cause of evaporation water loss(EWL).Amphibians skin must not only complete breathing,but also play an important role in keeping water,it is also an important organ for water uptake.So,amphibian skin is an important organ both for breathing and water regulation.In addition to the skin,the kidneys are also involved in the water balance of amphibians.How amphibians adapt to the diverse terrestrial environment through morphological and structural changes is an important issue in amphibian ecology and adaptive evolution.In this study,8 species of Anuran in Yunnan Province that inhabit different environments were selected as experiment material.The sample species covered various types of life form from aquatic to terrestrial environments.Including aquatic species of Fejervarya multistriat and Limnonectes bannaensi;Terrestrial frog species of Nanorana phrynoide,Odorrana graham,Rana chaochiaoensis,Babina pleuraden,and completely terrestrial toad species of Bufo gargarizans andrewsi and Duttaphrynus melanostictus.histological examination of respiratory and water regulation related organs structure,including dorsal and ventral skin,lungs,and kidney of the eight anuran species adapted to different habitats was performed by paraffin slicing and H.E.staining methods The differences in lung,skin,and kidney structure between species of different habitat types were analyzed in order to explore the ecological adaptability of different structures of respiratory and water regulation organs to different habitats of the anurans species,and better understand the adaptive evolution of amphibians' respiratory and water regulating organs.The results showed:1.The basic structure of the skin,lungs,and kidneys of the eight species in Yunnan were similar to most of the species that have been studied.The skin consisted of the epidermis and the dermis,and the dermis included the stratum spongiosum and the stratum compactum.The lungs could be divided into unstructured and structured areas,the structured area was seperated by septum,which can be distinguished into two levels.Many septum divided the structural area of the lung into many small chambers.The basic structure of the skin,lungs,and kidneys of the eight species in Yunnan were similar to most of the species that have been studied.The skin consisted of the epidermis and the dermis,and the dermis included the stratum spongiosum and the stratum compactum.The lungs could be divided into unstructured and structured areas,the structured area was seperated by septum,which can be distinguished into two levels.Many septums divided the structural area of the lung into many small chambers.The structure of the kidney could be identified as the external zone and central zone,and the nephron unit consists of two parts: the renal corpuscles and the renal tubules.The renal corpuscles included glomeruli and renal capsular space.Neck segment,proximal convoluted tubules,intermediate segments,and distal convoluted tubules builded up the renal tubules.2.The thickness of the skin was relation to the size of the amphibian body.The skin of toad is thick,with a thickness of 400-700 ?m,and only 100-300 ?m for the frogs.Both skin stratum corneum thickness,keratinocyte morphology and degree of keratinization were different between the dorsal and ventral skin and among the different species.Both dorsal and ventral skin was rich in mucous glands.Except the Limnonectes bannaensi,all of the other seven species had granular gland.3.The comparison between the dorsal and ventral skin structure of the eight anuran species showed that the epidermis,stratum spongiosum and stratum compactum of the dorsal skin were thicker than that of the ventral skin,and the stratum corneum was also thicker,with a higher degree of keratinization and a richer mucous gland and pigment cells.The difference in cutaneous structure was related to its function.The dorsal skin had respiratory and water retention functions,while the main function of the ventral skin is water absorption.4.The structure of the respiratory organs was closely related to the habitat types.Two species of aquatic frogs,with a small relative lung volume,a wide central unstructured area,a simple lung structure,and a few septum;However,in their dorsal skin,the stratum corneum was thinner,the degree of keratinization was lower,the density of capillaries was higher,and the blood-gas barrier was relatively thinner.However,the lung of terrestrial frogs and toads was relatively bulky and more complex in structure,with a narrow unstructured area,more septa in the lung wall,a wider breathing surface,and relatively thinner blood gas barriers,but their dorsal skin had a thicker stratum corneum with a high degree of keratinization,and a thicker blood gas barrier.Structural analysis of the lung and dorsal skin revealed that the habitat type affected the distribution of respiration on the two respiratory pathways.The stronger the aquatic,the simpler the lung structure,the more the amphibians relies on skin respiration.The more the terrestrial,the more complex the lung structure was,and the stronger the respiratory function of the lungs.As a result,habitat type might influence the evolution of the respiratory organ structure.5.The structure of water regulation organ was also associated with the habitat types.Higher terrestrial toads and frogs have well-developed glomeruli and relatively large total filtration area.The renal tubule of the two aquatic frog species and the frogs Babina pleuraden are more developed than other species,and the proximal tubules are relatively The relative diameter of the distal tubules and the relative cell height of the distal tubules were greater,indicating a stronger reabsorption capacity of nutrients and ions.The two higher terrestrial toads had stronger and drier dorsal skin,thicker keratosis,higher degree of keratinization,and sparser mucous glands.There were also calcified layers in the skin of the toads,which could effectively prevent evaporation of water;Two aquatic frogs species have relatively thinner epidermis and stratum corneum,lower degree of keratinization,plentiful of mucinous glands,and easily evaporation water loss;4 terrestrial frog species had more keratinized in dorsal skin than aquatic frog species,but less than 2 terrestrial toad species,richer in mucous glands,which was in favour of keep water.The ventral skin of all the eight anuran species had a higher degree of vascularization and could effectively absorb water from the substrate.Amphibians achieved the environmental adaptation of the water regulation organ structure to the water conditions of different habitats through a combination of different water retention,water absorption,and renal reabsorption capabilities of the dorsal and ventral skin.There are differences in the structure of skin,lungs,and kidneys among the eight anuran species in Yunnan.In the process of adaptation from water to land,an increase in skin thickness,a thickening of the stratum corneum,and an increase in keratinization to adapt to the dry environment had occurred in amphibian skin.At the same time,the respiration of the skin had been gradually weakened,lung respiration had evolved,and the structure of the lungs become more and more complicated.In the kidney structure,terrestrial species have a larger glomerular filtration area,strong filtration capacity of blood,and aquatic species have developed renal tubules with strong reabsorption capacity.These changes in structure of respiration and water regulation organs are the result of long-term adaptation of amphibians to diverse aquatic and terrestrial habitats.The results of this study have a certain theoretical significance in the adaptive evolution of amphibians' tissues structure. |
PDF Full Text Request