Comparative Morphology of the Lungs and Skin of two Anura, Pelophylax nigromaculatus and Bufo gargarizans

Gan Guangming,Zhang Chenchen,Zhuang Mei,Zhang Dongyu,Yang Zhe,Ding Jiawei

doi:10.1038/s41598-020-65746-y

Abstract

The lungs and skin are important respiratory organs in Anura, but the pulmonary structure of amphibians remains unclear due to the lack of a suitable procedure. This study improved the procedure used for fixing lungs tissues and used light microscopy, transmission electron microscopy and scanning electron microscopy to reveal the differences in the lung and skin morphologies between Pelophylax nigromaculatus (P. nigromaculatus) and Bufo gargarizans (B. gargarizans). In P. nigromaculatus and B. gargarizans, the cystic lungs comprise a continuous outer pulmonary wall on which primary, secondary, and tertiary septa attach, and a number of regular lattices form from raised capillaries and the pulmonary epithelium on the surfaces of the pulmonary wall and septa. Each lattice in P. nigromaculatus consists of several elliptical sheets and flat bottom, and the septa are distributed with denser sheets and have a larger stretching range than the pulmonary wall. The lattice in B. gargarizans consists of thick folds and an uneven bottom with several thin folds, and the septa have more developed thick and thin folds than the pulmonary wall. However, the density of the pulmonary microvilli, the area of a single capillary, the thicknesses of the blood-air barrier, pulmonary wall and septum, and the lung/body weight percentage obtained for B. gargarizans were higher than those found for P. nigromaculatus. In P. nigromaculatus, the dorsal skin has dense capillaries and a ring surface structure with mucus layer on the stratum corneum, and the ventral skin is slightly keratinized. In B. gargarizans, the stratum corneum in both the dorsal and ventral skins is completely keratinized. A fine ultrastructure analysis of P. nigromaculatus and B. gargarizans revealed that the pulmonary septa are more developed than the pulmonary walls, which means that the septa have a stronger respiratory function. The more developed lungs are helpful for the adaptation of B. gargarizans to drought environments, whereas P. nigromaculatus has to rely on more vigorous skin respiration to adapt to a humid environment.

Highlights

Amphibians have different compatible respiratory organs to adapt to diverse environments, and the structure of the respiratory organs is highly variable and prominent in the structure of the lungs and skin
We improved the procedure for the fixing of moderately inflated lungs and utilized light microscopy (LM), TEM and SEM to investigate the differences in the lung and skin morphologies between P. nigromaculatus[27] and B. gargarizans[28] adults to reveal why P. nigromaculatus and B. gargarizans live in humid and drought environments, respectively
The analysis of the shape of the lung of B. gargarizans (Fig. 1I) revealed that the inflated pulmonary wall was divided into many quadrilateral spaces by the primary septa inside the lung (Fig. 1I-I’)

Summary

Introduction

Amphibians have different compatible respiratory organs to adapt to diverse environments, and the structure of the respiratory organs is highly variable and prominent in the structure of the lungs and skin. Morphological studies have shown that amphibian lungs mainly cover with a single type of pneumocytes, and they combines features of flat type I and cubic type II alveolar cells of the mammalian lung[16,17,18,19]. Another important respiratory organ in amphibians is the skin[23]. P. nigromaculatus[28] belongs to the Pelophylax genus, Ranidae family and Anura order Adults of this species tend to live near ponds and grasses, and it is widely distributed in mainland China and is a typical amphibious amphibian in China. We improved the procedure for the fixing of moderately inflated lungs and utilized light microscopy (LM), TEM and SEM to investigate the differences in the lung and skin morphologies between P. nigromaculatus[27] and B. gargarizans[28] adults to reveal why P. nigromaculatus and B. gargarizans live in humid and drought environments, respectively

Methods

Results

Conclusion