Olfactory And Skeletal Reconstruction. Amphibian Metamorphosis Process

Amphibian metamorphosis is a model system to study organs remodeling. Olfaction organ is one of special sense organs of vertebrates and appears earliest among special senses in phylogeny. In addition, olfaction organ plays important roles in animal behavior, including feeding, reproduction, migration, kin recognition and predator avoidance. The skeleton remains primarily cartilagenous to form the basic structural components and framework of the tadpole before metamorphosis. The chondrocranium is the primitive cartilagionous skeletal structure in the tadpole. The majority of the chondrocranium is succeeded by the bony skull after metamorphosis, and the development of the axial skeleton and the presence of limbs are also very obvious after metamorphosis. This change in the life style from aquatic to terrestrial during metamorphosis may be reflected in the structure of the olfactory organ and the skeleton system. The olfactory organ and skeleton remodelings during amphibian metamorphosis were investigated using histological method and double-stained method with alcian blue and alizarin red, in order to reveal phylogenic evolution of aquatic animal into a terrestrial animal. The main results as follows:1 It is general accepted that Bowman's glands are responsible for secreting mucus substances present on the olfactory epithelium in terrestrial vertebrates. The late-larval appearance of Bowman's glands in the cavum principale suggests that the olfactory epithelium in tadpoles begins to perceive the odorants in the air in preparation for the transition from water to land, and secretion of intermaxillary glands makes the tongue sticky and facilitates the hunting and swallowing of prey. Appearance of Bowman's glands, the intermaxillary glands and the forelimb during amphibian metamorphosis may be indispensable to the transition of frogs on land.2 The olfactory epithelium in tadpole of Bufo gargarizans and Rana chensinensis is divided into dorsal and ventral olfactory epithelium from G24-G42. Jermakowicz et al (2004) thought that the ventral olfactory epithelium exposed to the organic matter in the buccal cavity, and it has been hypothesized that the ventral olfactory epithelium provide an analogous chemosensory function in the tadpoles as tastes buds in the adult tongues. The ventral olfactory epithelium and valves at the junction between the buccal cavity and the olfactory chamber disappear from G43. It is suggested that the ventral olfactory epithelium in the tadpole be used to sense water-borne odors, and the dorsal olfactory epithelium appears to be used to sense air-borne odors. After metamorphosis, the olfactory epithelium in principal cavity may begin to have aerial olfaction for land life, and lose the waterborne olfaction.3 The results showed that the naso-lacrimal duct of B. gargarizans arises from the lateral portion of the cavum medium. The nasolacrimal duct arises from the lateral wall of the cavum principale in Rana chensinensis. It is concluded that the Harderian glands in frog secrete fluids for the olfactory organ through the nasolacrimal duct. The origin of nasolacrimal duct in metamorphosis seems to be a turning point that means the termination of aquatic life and the onset of terrestrial life.4 In B. gargarizans and R. chensinensis tadpoles, the vomeronasal epithelium begins to appear as a blind sac, which is not connected with principal chamber before metamorphosis. After metamorphosis, the cavum principale is confluent with the cavum medium. The medial part of cavum medium becomes confluent with the vomeronasal organ. Therefore, the cavum principale is connected with the cavum principale. The pheromones can enter into the vomeronasal organ through the cavum principale. On the basis of the evidence, it is agured that the vomeronasal organ be functional before metamorphosis.5 Our experience supports Eisthens' attitude that the most recent common ancestor to the tetrapods was aquatic and possessed a vomeronasal organ. However, vomeronasal organ in the taepole of B. gargarizans and R. chensinensis may lack olfactory function and possess olfactory function after metamorphosis.6 The ossification sequence of the chondrocranium in the Chinese forest frog R. chensinensis is similar with that of Mexican Spadefoot S. multiplicata, except the septomaxilla, the premaxilla and the maxilla, while the ossification of the postcranial skeleton, which is similar with that of P. adspersus, occurs earlier than that of, S. multiplicata. Variation of ossification sequence in frogs may attribute to phylogenetic status.7 Our phylogeny tree supports Orton's hypothesis that the anuran larval were divided into 4 types based on the chondrocranium morphology. The larval morphology and the ontogeny characters of anuran larval are valuable for decoding anuran phylogeny.