Scanning electron microscopy and light microscopy of tongue, fingers, toes, nuptial pad, and humeral gland in Sylvirana nigrovittata (Anura: Ranidae)

Abstract

The micromorphology and histology of anuran skin, particularly in Thai species, remain relatively understudied. Anurans use their tongues for prey capture and their hands and feet for adhesion and mating, suggesting a close relationship between micro-surface features and histological structure. Information on Sylvirana nigrovittata is particularly limited. This study provides a detailed examination of the micro-surface and histological characteristics of the tongue, fingers, toes, nuptial pad, and humeral gland in S. nigrovittata. Specimens collected from Phu Luang Wildlife Sanctuary in Loei province, northeastern Thailand, were studied using scanning electron microscopy (SEM) for micro-surface features and H&E staining for histological details. SEM micrographs revealed that the tongue was cordate-shaped, with about 75% of its surface covered by filiform papillae and 25% by fungiform papillae housing taste buds. Filiform papillae were trapezoidal, averaging 23.88±5.68 µm in width and 38.83±6.57 µm in length, while fungiform papillae were disc-like, averaging 52.95±14.64 µm in width and 66.70±15.88 µm in length. Digital pads on fingers and toes presented prismatic cells, with fingertip cells averaging 7.09±0.97 µm in width and 11.81±1.24 µm in length, and toe cells averaging 8.40±1.37 µm in width and 11.28±1.63 µm in length. Subdigital surfaces were smoother, containing squamous cells with dimensions of 10.65±3.21 µm in width and 21.29±4.35 µm in length. Subarticular tubercles were composed of prismatic cells with hexagonal or pentagonal surfaces, averaging 10.02±2.85 µm in width and 13.08±1.94 µm in length. Nuptial pad displayed cylindrical structures with dome-like tips, averaging 24.01±3.96 µm in width and 21.16±4.37 µm in length, and feature mucous openings. They consisted of a thick keratinized stratified squamous epithelium and a dense dermis with specialized mucous glands. Humeral gland exhibited rough surfaces with squamous cells and microridges, showing both small and large mucous openings, with average cell dimensions of 16.23±1.70 µm in width and 19.11±2.05 µm in length. Histological analysis confirmed that these structures were predominantly composed of keratinized stratified squamous epithelium, with variations in dermal collagen density and mucous gland distribution. This study enhances our understanding of the skin and gland morphology in S. nigrovittata, highlighting on its anatomical and functional adaptations.