Number of olfactory receptor neurons in the Chinese soft-shelled turtle

Chondrocranial anatomy of Testudo hermanni (Testudinidae, Testudines) with a comparison to other turtles

Using histological cross-sections, the chondrocranium anatomy was reconstructed for two developmental stages of Hermann's tortoise (Testudo hermanni). The morphology differs from the chondrocrania of most other turtles by a process above the ectochoanal cartilage with Pelodiscus sinensis being the only other known species with such a structure. The anterior and posterior processes of the tectum synoticum are better developed than in most other turtles and an ascending process of the palatoquadrate is missing, which is otherwise only the case in pleurodiran turtles. The nasal region gets proportionally larger during development. We interpret the enlargement of the nasal capsules as an adaption to increase the surface area of the olfactory epithelium for better perception of volant odors. Elongation of the nasal capsules in trionychids, in contrast, is unlikely to be related to olfaction, while it is ambiguous in the case of Sternotherus odoratus. However, we have to conclude that research on chondrocranium anatomy is still at its beginning and more comprehensive detailed descriptions in relation to other parts of the anatomy are needed before providing broad-scale ecological and phylogenetic interpretations.

Principles of odor coding in vertebrates and artificial chemosensory systems

The biological olfactory system is the sensory system responsible for the detection of the chemical composition of the environment. Several attempts to mimic biological olfactory systems have led to various artificial olfactory systems using different technical approaches. Here we provide a parallel description of biological olfactory systems and their technical counterparts. We start with a presentation of the input to the systems, the stimuli, and treat the interface between the external world and the environment where receptor neurons or artificial chemosensors reside. We then delineate the functions of receptor neurons and chemosensors as well as their overall I-O relationships. Up to this point, our account of the systems goes along similar lines. The next processing steps differ considerably: while in biology the processing step following the receptor neurons is the "integration" and "processing" of receptor neuron outputs in the olfactory bulb, this step has various realizations in electronic noses. For a long period of time, the signal processing stages beyond the olfactory bulb, i.e., the higher olfactory centers were little studied. Only recently there has been a marked growth of studies tackling the information processing in these centers. In electronic noses, a third stage of processing has virtually never been considered. In this review, we provide an up-to-date overview of the current knowledge of both fields and, for the first time, attempt to tie them together. We hope it will be a breeding ground for better information, communication, and data exchange between very related but so far little connected fields.

Distribution of cells expressing vomeronasal receptors in the olfactory organ of turtles

Generally, the olfactory organ of vertebrates consists of the olfactory epithelium (OE) and the vomeronasal organ (VNO). The OE contains ciliated olfactory receptor neurons (ORNs), while the VNO contains microvillous ORNs. The ORNs in the OE express odorant receptors (ORs), while those in the VNO express type 1 and type 2 vomeronasal receptors (V1Rs and V2Rs). In turtles, the olfactory organ consists of the upper (UCE) and lower chamber epithelia (LCE). The UCE contains ciliated ORNs, while the LCE contains microvillous ORNs. Here we investigated the distribution of cells expressing vomeronasal receptors in the olfactory organ of turtles. The turtle vomeronasal receptors were encoded by two V1R genes and two V2R genes. Among them, V2R1 and V2R26 were mainly expressed in the LCE, while V1R3 was expressed both in the UCE and LCE. Notably, vomeronasal receptors were expressed by a limited number of ORNs, which was confirmed by the expression of the gene encoding TRPC2, an ion channel involved in the signal transduction of vomeronasal receptors. Furthermore, expression of ORs by the majority of ORNs was suggested by the expression of the gene encoding CNGA2, an ion channel involved in the signal transduction of ORs. Thus, olfaction of turtle seems to be mediated mainly by the ORs rather than the vomeronasal receptors. More importantly, the relationship between the fine structure of ORNs and the expression of olfactory receptors are not conserved among turtles and other vertebrates.