Olfactory and vomeronasal systems of caecilians (Amphibia: Gymnophiona)

Turbinal bones are still one of the last frontiers of the tetrapod skull: hypotheses, challenges and perspectives

Turbinals are bony or cartilaginous structures that are present in the nasal cavity of most tetrapods. They are involved in key functions such as olfaction, heat, and moisture conservation, as well as protection of the respiratory tract. Despite recent studies that challenged long-standing hypotheses about their physiological and genomic correlation, turbinals remain largely unexplored, particularly for non-mammalian species. Herein, we review and synthesise the current knowledge of turbinals using an integrative approach that includes comparative anatomy, physiology, histology and genomics. In addition, we provide synonyms and correspondences of tetrapod turbinals from about 80 publications. This work represents a first step towards drawing hypotheses of homology for the whole clade, and provides a strong basis to develop new research avenues.

Triassic stem caecilian supports dissorophoid origin of living amphibians

Living amphibians (Lissamphibia) include frogs and salamanders (Batrachia) and the limbless worm-like caecilians (Gymnophiona). The estimated Palaeozoic era gymnophionan-batrachian molecular divergence¹ suggests a major gap in the record of crown lissamphibians prior to their earliest fossil occurrences in the Triassic period^2-6. Recent studies find a monophyletic Batrachia within dissorophoid temnospondyls^7-10, but the absence of pre-Jurassic period caecilian fossils^11,12 has made their relationships to batrachians and affinities to Palaeozoic tetrapods controversial^1,8,13,14. Here we report the geologically oldest stem caecilian-a crown lissamphibian from the Late Triassic epoch of Arizona, USA-extending the caecilian record by around 35 million years. These fossils illuminate the tempo and mode of early caecilian morphological and functional evolution, demonstrating a delayed acquisition of musculoskeletal features associated with fossoriality in living caecilians, including the dual jaw closure mechanism^15,16, reduced orbits¹⁷ and the tentacular organ¹⁸. The provenance of these fossils suggests a Pangaean equatorial origin for caecilians, implying that living caecilian biogeography reflects conserved aspects of caecilian function and physiology¹⁹, in combination with vicariance patterns driven by plate tectonics²⁰. These fossils reveal a combination of features that is unique to caecilians alongside features that are shared with batrachian and dissorophoid temnospondyls, providing new and compelling evidence supporting a single origin of living amphibians within dissorophoid temnospondyls.

Olfaction across the water-air interface in anuran amphibians

Extant anuran amphibians originate from an evolutionary intersection eventually leading to fully terrestrial tetrapods. In many ways, they have to deal with exposure to both terrestrial and aquatic environments: (i) phylogenetically, as derivatives of the first tetrapod group that conquered the terrestrial environment in evolution; (ii) ontogenetically, with a development that includes aquatic and terrestrial stages connected via metamorphic remodeling; and (iii) individually, with common changes in habitat during the life cycle. Our knowledge about the structural organization and function of the amphibian olfactory system and its relevance still lags behind findings on mammals. It is a formidable challenge to reveal underlying general principles of circuity-related, cellular, and molecular properties that are beneficial for an optimized sense of smell in water and air. Recent findings in structural organization coupled with behavioral observations could help to understand the importance of the sense of smell in this evolutionarily important animal group. We describe the structure of the peripheral olfactory organ, the olfactory bulb, and higher olfactory centers on a tissue, cellular, and molecular levels. Differences and similarities between the olfactory systems of anurans and other vertebrates are reviewed. Special emphasis lies on adaptations that are connected to the distinct demands of olfaction in water and air environment. These particular adaptations are discussed in light of evolutionary trends, ontogenetic development, and ecological demands.

Morphology and histology of the male reproductive tract of Caecilia thompsoni (amphibia: Gymnophiona)

We studied the male reproductive tract of individuals of different body sizes of Caecilia thompsoni to describe morphological characteristics in comparison to other Gymnophiona. The reproductive tract consists of paired testes segmented into chains of primary and secondary lobes, sperm ducts that empty to Wolffian ducts, the cloaca that receives the Wolffian ducts and possesses a phallodeum. Müllerian ducts are present and develop into paired glands that empty into the cloacal urodeum. Testicular secondary lobes contain lobules with cysts of the entire germinal cell line, whereas primary lobes, in the terminal ends of the chains, only have spermagonia, Sertoli cells, and connective tissue. The smallest individual examined (21 cm body length) was immature and only possessed a few testicular primary lobes. Once the individuals reach sexual maturity, the morphological characteristics are quite consistent at macroscopic and histological level among males of very different body sizes. The histological features of the Wolffian and Müllerian glands suggest a complementary secretory role between the two ducts. In the cloaca we found the propulsor muscle, venous sinuses, and blind sacs in the phallodeum, which differentiate C. thompsoni from other species of the genus. Despite these slight differences, the general morphological characteristics, both macroscopic and microscopic, of the reproductive tracts of adult males of C. thompsoni follow the pattern known for the reproductively active males of Gymnophiona.

Development of the squamate naso-palatal complex: detailed 3D analysis of the vomeronasal organ and nasal cavity in the brown anole Anolis sagrei (Squamata: Iguania)

Background

Despite the diverse morphology of the adult squamate naso-palatal complex - consisting of the nasal cavity, vomeronasal organ (VNO), choanal groove, lacrimal duct and superficial palate - little is known about the embryology of these structures. Moreover, there are no comprehensive studies concerning development of the nasal cavity and VNO in relation to the superficial palate. In this investigation, we used X-ray microtomography and histological sections to describe embryonic development of the naso-palatal complex of iguanian lizard, the brown anole (Anolis sagrei). The purpose of the study was to describe the mechanism of formation of adult morphology in this species, which combines the peculiar anole features with typical iguanian conditions. Considering the uncertain phylogenetic position of the Iguania within Squamata, embryological data and future comparative studies may shed new light on the evolution of this large squamate clade.

Results

Development of the naso-palatal complex was divided into three phases: early, middle and late. In the early developmental phase, the vomeronasal pit originates from medial outpocketing of the nasal pit, when the facial prominences are weakly developed. In the middle developmental phase, the following events can be noted: the formation of the frontonasal mass, separation of the vestibulum, appearance of the lacrimal duct, and formation of the choanal groove, which leads to separation of the VNO from the nasal cavity. In late development, the nasal cavity and the VNO attain their adult morphology. The lacrimal duct establishes an extensive connection with the choanal groove, which eventually becomes largely separated from the oral cavity.

Conclusions

Unlike in other tetrapods, the primordium of the lacrimal duct in the brown anole develops largely beyond the nasolacrimal groove. In contrast to previous studies on squamates, the maxillary prominence is found to participate in the initial fusion with the frontonasal mass. Moreover, formation of the choanal groove occurs due to the fusion of the vomerine cushion to the subconchal fold, rather than to the choanal fold. The loss or significant reduction of the lateral nasal concha is secondary. Some features of anole adult morphology, such as the closure of the choanal groove, may constitute adaptations to vomeronasal chemoreception.

Quantitative comparative analysis of the nasal chemosensory organs of anurans during larval development and metamorphosis highlights the relative importance of chemosensory subsystems in the group

The anuran peripheral olfactory system is composed of a number of subsystems, represented by distinct neuroepithelia. These include the main olfactory epithelium and vomeronasal organ (found in most tetrapods) and three specialized epithelia of anurans: the buccal-exposed olfactory epithelium of larvae, and the olfactory recess and middle chamber epithelium of postmetamorphic animals. To better characterize the developmental changes in these subsystems across the life cycle, morphometric changes of the nasal chemosensory organs during larval development and metamorphosis were analyzed in three different anuran species (Rhinella arenarum, Hypsiboas pulchellus, and Xenopus laevis). We calculated the volume of the nasal chemosensory organs by measuring the neuroepithelial area from serial histological sections at four different stages. In larvae, the vomeronasal organ was relatively reduced in R. arenarum compared with the other two species; the buccal-exposed olfactory epithelium was absent in X. laevis, and best developed in H. pulchellus. In postmetamorphic animals, the olfactory epithelium (air-sensitive organ) was relatively bigger in terrestrial species (R. arenarum and H. pulchellus), whereas the vomeronasal and the middle chamber epithelia (water-sensitive organs) was best developed in X. laevis. A small olfactory recess (likely homologous with the middle chamber epithelium) was found in R. arenarum juveniles, but not in H. pulchellus. These results support the association of the vomeronasal and middle chamber epithelia with aquatic olfaction, as seen by their enhanced development in the secondarily aquatic juveniles of X. laevis. They also support a role for the larval buccal-exposed olfactory epithelium in assessment of oral contents: it was absent in X. laevis, an obligate suspension feeder, while present in the two grazing species. These initial quantitative results give, for the first time, insight into the functional importance of the peripheral olfactory subsystems across the anuran life cycle.

Tract-tracing study of the extrabulbar olfactory projections in the brain of some teleosts

The extrabulbar olfactory projections (EBOP) is a collection of nerve fibers that originate from primary olfactory receptor neurons. These fibers penetrate into the brain, bypassing the olfactory bulbs (OBs). While the presence of an EBOP has been well established in teleosts, here we morphologically characterize the EBOP structure in four species each with a different morphological relationship of OB with the ventral telencephalic area. Tract-tracing methods (carbocyanine DiI/DIA and biocytin) were used. FMRFamide immunoreactive nervus terminalis (NT) components were also visualized to define any neuroanatomical relationship between the NT and EBOP. Unilateral DiI/DiA application to the olfactory chamber stained the entire olfactory epithelium, olfactory nerve fibers, and ipsilateral olfactory bulb. Labeled primary olfactory fibers running ventromedially as extrabulbar primary olfactory projections reached various regions of the secondary prosencephalon. Only in Moenkhausia sanctaefilomenae (no olfactory peduncle) did lipophilic tracer-labeled fibers reach the ipsilateral mesencephalon. The combination of tracing techniques and FMRFamide immunohistochemistry revealed a substantial overlap of the label along the olfactory pathways as well as in the anterior secondary prosencephalon. However, FMRFamide immunoreactivity was never colocalized in the same cellular or fiber component as visualized using tracer molecules. Our results showed a certain uniformity in the neuroanatomy and extension of EBOP in all four species, independent of the pedunculate feature of the OBs. The present study also provided additional evidence to support the view that EBOP and FMRFamide immunoreactive components of the NT are separate anatomical entities.

The anuran vocal sac: a tool for multimodal signalling

Although in anurans the predominant mode of intra- and intersexual communication is vocalization, modalities used in addition to or instead of acoustic signals range from seismic and visual to chemical. In some cases, signals of more than one modality are produced through or by the anuran vocal sac. However, its role beyond acoustics has been neglected for some time and nonacoustic cues such as vocal sac movement have traditionally been seen as an epiphenomenon of sound production. The diversity in vocal sac coloration and shape found in different species is striking and recently its visual properties have been given a more important role in signalling. Chemosignals seem to be the dominant communication mode in newts, salamanders and caecilians and certainly play a role in the aquatic life phase of anurans, but airborne chemical signalling has received less attention. There is, however, increasing evidence that at least some terrestrial anuran species integrate acoustic, visual and chemical cues in species recognition and mate choice and a few secondarily mute anuran species seem to fully rely on volatile chemical cues produced in glands on the vocal sac. Within vertebrates, frogs in particular are suitable organisms for investigating multimodal communication by means of experiments, since they are tolerant of disturbance by observers and can be easily manipulated under natural conditions. Thus, the anuran vocal sac might be of great interest not only to herpetologists, but also to behavioural biologists studying communication systems.

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Our view of anuran communication has changed drastically during the last few decades.

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Anurans use diverse communication strategies often directly related to the vocal sac.

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Distinctive vocal sac shapes, sizes and colours suggest functional diversity.

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Evidence increases that the vocal sac also plays a role in chemical signalling.

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The anuran vocal sac may be of great interest for studies on communication systems.

The braincase of Eocaecilia micropodia (Lissamphibia, Gymnophiona) and the origin of Caecilians

The scant fossil record of caecilians has obscured the origin and evolution of this lissamphibian group. Eocaecilia micropodia from the Lower Jurassic of North America remains the only stem-group caecilian with an almost complete skull preserved. However, this taxon has been controversial, engendering re-evaluation of traits considered to be plesiomorphic for extant caecilians. Both the validity of the placement of E. micropodia as a stem caecilian and estimates of the plesiomorphic condition of extant caecilians have been questioned. In order to address these issues, the braincase of E. micropodia was examined via micro-computed tomography. The braincase is considered to be a more reliable phylogenetic indicator than peripheral regions of the skull. These data reveal significant new information, including the possession of an ossified nasal septum, ossified anterior wall of the sphenethmoid, long anterolateral processes on the sphenethmoid, and paired olfactory nerve foramina, which are known only to occur in extant caecilians; the latter are possibly related to the evolution of the tentacle, a caecilian autapomorphy. A phylogenetic analysis that included 64 non-amniote taxa and 308 characters represents the first extensive test of the phylogenetic affinities of E. micropodia. The results place E. micropodia securely on the stem of extant caecilians, representing a clade within Temnospondyli that is the sister taxon to batrachians plus Gerobatrachus. Ancestral character state reconstruction confirms the braincase of E. micropodia to be largely representative of the plesiomorphic condition of extant caecilians. Additionally, the results refine the context within which the evolution of the caecilian form can be evaluated. The robust construction and pattern of the dermal skull of E. micropodia is interpreted as symplesiomorphic with advanced dissorophoid temnospondyls, rather than being autapomorphic in its robust construction. Together these data increase confidence in incorporating E. micropodia into discussions of caecilian evolution.

Development of the olfactory and vomeronasal organs in Discoglossus pictus (Discoglossidae, Anura)

Using histological techniques and computer-aided three-dimensional reconstructions of histological serial sections, we studied the development of the olfactory and vomeronasal organs in the discoglossid frog Discoglossus pictus. The olfactory epithelium in larval D. pictus represents one continuous unit of tissue not divided into two separate portions. However, a small pouch of olfactory epithelium (the "ventromedial diverticulum") is embedded into the roof of the buccal cavity, anteromedial to the internal naris. The lateral appendix is present in D. pictus through the entire larval period and disappears during the onset of metamorphosis. The disappearance of the lateral appendix at this time suggests that it is a typical larval organ related to aquatic life. The vomeronasal organ develops during hindlimb development, which is comparatively late for anurans. The development of the vomeronasal organ in D. pictus follows the same general developmental pattern recognized for neobatrachians. As with most anurans, the vomeronasal glands appear later than the vomeronasal organ. After metamorphosis, the olfactory organ of adult D. pictus is composed of a series of three interconnected chambers: the cavum principale, cavum medium, and cavum inferius. We suggest that the ventromedial diverticulum at the anterior border of the internal naris of larval D. pictus might be homologous with the ventral olfactory epithelium of bufonids and with the similar diverticulum of Alytes.

The nasolacrimal duct of anuran amphibians: suggestions on its functional role in vomeronasal perception

Tear secretions discharged by the Harderian gland are suggested to function as a solvent for molecules sensed by the vomeronasal organ (VNO) in anurans. It has been assumed that chemical stimuli are absorbed at the surface of the eye to be carried - together with the lacrimal fluid - into the nasal cavity via the nasolacrimal duct. In the study presented herein, we examined the intranasal anatomy of 10 different anuran species to analyse the opening region of the nasolacrimal duct and its functional relationship with the VNO and the external naris. In addition, vital staining of the nasal cavities was conducted. Our results indicate that stimuli reaching the VNO are more likely to be ingested through the nostril than via the eye. In many cases the intranasal orifice of the nasolacrimal duct shows a close proximity to the external naris and simultaneously we observed a noticeable distance to the VNO. We suggest that the secretions of the Harderian gland are carried to the external naris by the nasolacrimal duct, where they bind chemical stimuli that are subsequently actively transported into the VNO. In some of the investigated species the opening region of the tear duct was situated in a more caudal part of the nasal cavity and closer to the VNO. In these cases a conspicuous system of channels can be found, which is suspected to carry the intruding medium of smell from the nostril to the nasolacrimal aperture.

Cytoarchitecture of the accessory olfactory bulb in the salamander Plethodon shermani

Plethodontid terrestrial salamanders are emerging models in the study of the evolution of chemical communication in vertebrates. Their vomeronasal system is well defined. It comprises sensory neurons in the epithelium of the vomeronasal organ, whose axons form the vomeronasal nerve projecting to the accessory olfactory bulb (AOB), which in turn projects to the vomeronasal amygdala through the accessory olfactory tract. A detailed description of the cellular elements of the urodele AOB is lacking. Neuronal morphology in the AOB was studied by means of biocytin intracellular injections and retrograde tract tracing in the salamander Plethodon shermani. The AOB exhibits the characteristic lamination of olfactory bulbs, except that it displays a mixed periglomerular and mitral somata layer superficially. Mitral cells are the only AOB neurons projecting to the vomeronasal amygdala. Each mitral cell sends multiple axonal branches, generally through both dorsal and ventral portions of the accessory olfactory tract. Some mitral cells additionally send axon collaterals in the white matter immediately ventral to the AOB. AOB interneurons are divided into superficial periglomerular and deep granule cells, each category exhibiting morphological variety. Some neurons in the granule cell layer of the AOB or the region ventral to the AOB have dendritic trees that cover both regions. The present study is the first to highlight the full anatomical extent of single AOB neurons and surprisingly suggests that the ventrolateral telencephalon found below the AOB is part of the salamander vomeronasal system.

Ontogeny of the nasolacrimal duct in primates: functional and phylogenetic implications

The ontogeny of the nasolacrimal ducts (NLD) and canals (NLC) are investigated in strepsirrhine and haplorhine primates. Developmental series of serially sectioned fetal, perinatal and adult specimens, in combination with juvenile and adult skulls subjected to high-resolution computed tomography, reveal that the vertical NLC and NLD of adult tarsiers and anthropoids are produced by the degeneration of a more horizontal anterior arm of the NLD that is present only transiently in haplorhines, but is maintained throughout life in strepsirrhines. This degeneration manifests as an 'unzipping' of the anterior arm by means of progressive enlargement (in a rostral direction) of a caudally placed opening of the NLD (at the base of the vertical NLC), followed by breakdown of the resulting epithelial groove. The similar mode by which the anterior arm of the membranous NLD degenerates in tarsiers and anthropoids strongly suggests that the conditions in these two taxa are homologous, and provides additional evidence for a monophyletic Haplorhini. The functional relationship between the nasolacrimal duct and the vomeronasal organ is reviewed in light of this evidence, and it is suggested that these changes in the haplorhine NLD were functionally linked to the development of anatomical haplorhinism of the oronasal complex.

Olfactory metamorphosis in the Coastal Giant Salamander (Dicamptodon tenebrosus)

This study examined the gross morphology and ultrastructure of the olfactory organ of larvae, neotenic adults, and terrestrial adults of the Coastal Giant Salamander (Dicamptodon tenebrosus). The olfactory organ of all aquatic animals (larvae and neotenes) is similar in structure, forming a tube extending from the external naris to the choana. A nonsensory vestibule leads into the main olfactory cavity. The epithelium of the main olfactory cavity is thrown into a series of transverse valleys and ridges, with at least six dorsal and nine ventral valleys lined with olfactory epithelium, and separated by ridges of respiratory epithelium. The ridges enlarge with growth, forming large flaps extending into the lumen in neotenes. The vomeronasal organ is a diverticulum off the ventrolateral side of the main olfactory cavity. In terrestrial animals, by contrast, the vestibule has been lost. The main olfactory cavity has become much broader and dorsoventrally compressed. The prominent transverse ridges are lost, although small diagonal ridges of respiratory epithelium are found in the lateral region of the ventral olfactory epithelium. The posterior and posteromedial wall of the main olfactory cavity is composed of respiratory epithelium, in contrast to the olfactory epithelium found here in aquatic forms. The vomeronasal organ remains similar to that in large larvae, but is now connected to the mouth by a groove that extends back through the choana onto the palate. Bowman's glands are present in the main olfactory cavity at all stages, but are most abundant and best developed in terrestrial adults. They are lacking in the lateral olfactory epithelium of the main olfactory cavity. At the ultrastructural level, in aquatic animals receptor cells of the main olfactory cavity can have cilia, short microvilli, a mix of the two, or long microvilli. Supporting cells are of two types: secretory supporting cells with small, electron-dense secretory granules, and ciliated supporting cells. Receptor cells of the vomeronasal organ are exclusively microvillar, but supporting cells are secretory or ciliated, as in the main olfactory cavity. After metamorphosis two distinct types of sensory epithelium occur in the main olfactory cavity. The predominant epithelium, covering most of the roof and the medial part of the floor, is characterized by supporting cells with large, electron-lucent vesicles. The epithelium on the lateral floor of the main olfactory cavity, by contrast, resembles that of aquatic animals. Both types have both microvillar and ciliated receptor cells. No important changes are noted in cell types of the vomeronasal organ after metamorphosis. A literature survey suggests that some features of the metamorphic changes described here are characteristic of all salamanders, while others appear unique to D. tenebrosus.

Characterization and distribution of neuropeptide Y in the brain of a caecilian amphibian

Neuropeptide Y (NPY) from the brain of an amphibian from the order Gymnophiona (the caecilian, Typhlonectes natans) was characterized. We cloned a 790 base pair cDNA encoding the caecilian NPY precursor. The open reading frame consisted of 291 bases, indicating an NPY precursor of 97 amino acids. Both deduced and isolated NPY primary structures were Tyr-Pro-Ser-Lys-Pro-Asp-Asn-Pro-Gly-Glu(10)-Asp-Ala-Pro-Ala-Glu-Asp-Met-Ala-Lys-Tyr(20)-Tyr-Ser-Ala-Leu-Arg-His-Tyr-Ile-Asn-Leu(30)-Ile-Thr-Arg-Gln-Arg-Tyr. NH2. In caecilian brain, we observed NPY immunoreactive cells within the medial pallium, basal forebrain, preoptic area, midbrain tegmentum and trigeminal nucleus. The prevalence of preoptic and hypothalamic terminal field staining supports the hypothesis that NPY controls pituitary function in this caecilian.

Septomaxilla of nonmammalian synapsids: soft-tissue correlates and a new functional interpretation. DELETE

The function of the septomaxilla of nonmammalian synapsids has long been problematic. Distinctive features of this bone, including a prominent intranarial process and a septomaxillary canal and foramen, are characteristic of pelycosaurs and nonmammalian therapsids, but are lost in their mammalian descendants. Numerous contradictory reconstructions have been proposed for the soft anatomy associated with the septomaxilla of nonmammalian synapsids. This review supports the following conclusions: 1) No particular correlation exists between the septomaxilla and the vomeronasal organ (VNO), and the most likely location for the VNO is on the dorsal surface of the palatal process of the vomer; 2) The most likely occupant of the septomaxillary canal is the nasolacrimal duct, which opened either anterior or medial to the intranarial process, near the opening of the VNO duct; and 3) The occupant of the septomaxillary foramen remains uncertain. These conclusions suggest that the functional significance of the septomaxilla in the nonmammalian synapsids is tied to that of the nasolacrimal duct. The association of this duct and the VNO in these animals resembles the condition in Recent amphibians and lepidosaurs, in which the nasolacrimal duct supplies orbital fluids to the VNO, apparently to enhance vomeronasal function. The peculiar shape of the synapsid septomaxilla may have served to collect vomeronasal odor molecules. The changes of the septomaxilla in early mammals, and its nearly complete loss in extant mammals, are probably correlated with a dissociation of the nasolacrimal duct and VNO, and functional changes in both structures.

Vomeronasal organ in bats and primates: extremes of structural variability and its phylogenetic implications

The mere appearance of a tubular, epithelially-covered, bilateral structure, no matter how minuscule, on the anteroventral nasal septum of tetrapods, is generally called the vomeronasal organ (of Jacobson). However, considering the functionality of this chemosensory structure, the presence of a non-cilated (microvillar) neuroepithelium (and not just any odd type of epithelium) encased in a variously shaped vomeronasal cartilage, along with vomeronasal nerve bundles and above all an accessory olfactory bulb connected to the limbic vomeronasal amygdala, are the absolute essential neurostructural characteristics and anatomic requirement for a functional VNO and the accessory olfactory system in any tetrapod. The distribution of the vomeronasal organ is reported here in two mammalian orders: Chiroptera and Primates. An impressive data pool on the vomeronasal organ of bats is now available, pointing to the fact that at this time bats may be the only group in which this organ system is extremely variable, ranging from total absence (even in the embryo) to spectacular development with numerous intervening stages in different chiropteran species. Of the eighteen bat families, only one family of New World leaf-nosed bats, family Phyllostomidae, exhibits functional vomeronasal organs. The vespertilionid bat Miniopterus, and the mormoopid bat Pteronotus, present exceptions to this rule. Among Primates, very few species have been rigorously studied. As a result, developmental variability of the vomeronasal organ is almost unknown; either the vomeronasal organ is well developed (such as in New World monkeys) or absent (as in Old World monkeys and great apes) in the adult. The concept whether adult humans or embryonic and fetal forms are endowed with this so-called sixth sense, is a controversial one and is under intense study in our laboratory and by others. The general phylogenetic implications based on our cladistic analysis of bats are that the vomeronasal organ complex has evolved several times. Among the prosimians and platyrrhine primates, the organ is well developed, although to a varying degree. Among catarrhine primates, its loss has occurred only once, as it is generally absent in the adult forms.

Ultrastructure of the olfactory organ in the clawed frog, Xenopus laevis, during larval development and metamorphosis

Development of the olfactory epithelia of the African clawed frog, Xenopus laevis, was studied by scanning and transmission electron microscopy. Stages examined ranged from hatching through the end of metamorphosis. The larval olfactory organ consists of two chambers, the principal cavity and the vomeronasal organ (VNO). A third sensory chamber, the middle cavity, arises during metamorphosis. In larvae, the principal cavity is exposed to water-borne odorants, but after metamorphosis it is exposed to airborne odorants. The middle cavity and the VNO are always exposed to waterborne odorants. Electron microscopy reveals that in larvae, principal cavity receptor cells are of two types, ciliated and microvillar. Principal cavity supporting cells are also of two types, ciliated and secretory (with small, electron-lucent granules). After metamorphosis, the principal cavity contains only ciliated receptor cells and secretory supporting cells, and the cilia on the receptor cells are longer than in larvae. Supporting cell secretory granules are now large and electron-dense. In contrast, the middle cavity epithelium contains the same cell types seen in the larval principal cavity. The VNO has microvillar receptor cells and ciliated supporting cells throughout life. The cellular process by which the principal cavity epithelium changes during metamorphosis is not entirely clear. Morphological evidence from this study suggests that both microvillar and ciliated receptor cells die, to be replaced by newly generated cells. In addition, ciliated supporting cells also appear to die, whereas there is evidence that secretory supporting cells transdifferentiate into the adult type. In summary, significant developmental additions and neural plasticity are involved in remodeling the olfactory epithelium in Xenopus at metamorphosis.

Species, sex, and seasonal differences in VNO size

Differences in the sizes of sensory and neural structures are used as an indication of differences in the function of those structures. Large VNOs often are interpreted to mean that this sense is particularly important in the life history of the animal. They also are assumed to be associated with more primitive animals. I examined VNO sizes across mammalian, reptilian, and amphibian lineages while attempting to account for total body size, because VNO and total body sizes are related. Most descriptions of VNO size and development are not quantified and often ambiguous. Large VNOs in a lineage should not be interpreted necessarily as primitive. Comparisons across smaller taxonomic ranges are easier to interpret. Plethodontid salamanders are a diverse set of species for which VNO descriptions show trends in size associated with habitat, sex, and season. Semiaquatic species tend to have proportionately larger VNOs than terrestrial species, males have larger organs than females, and VNOs can show increases and decreases in size that may be associated with seasonal activities. Salamanders may use their VNOs to locate and identify mates, as part of the courtship sequences, or to identify and assess neighboring territory holders.

The soft-tissue components of the vomeronasal organ in pigs, cows and horses

The soft-tissue components of the vomeronasal organ of the pig, the cow and the horse were studied with the aid of dissection, microdissection, and light microscopy and immunohistochemistry of series of transverse sections. In horses, the rostral end of the incisive duct was blind: thus, unlike in pigs and cows, there was no communication between the vomeronasal organ and the oral cavity. In all three species, the central part of the vomeronasal duct bore the 'typical' respiratory/ receptor epithelium lining on its lateral and medical walls. The rostral part of the duct was characterized by stratified columnar epithelium, while more caudal parts bore simple columnar type. The patterns of distribution of glands, blood vessels and nerves were closely associated with the patterns of distribution of duct linings. The distribution of soft-tissue components in pigs was less clearly defined than in cows and horses. Of the three species, nerves were detected in the rostral half of the vomeronasal parenchyma only in the horse.

Distribution of gonadotropin-releasing hormone immunoreactivity in the brain of Ichthyophis beddomei (Amphibia: Gymnophiona)

From a comparative viewpoint, we have investigated the presence and neuroanatomical distribution of gonadotropin-releasing hormone (GnRH)-immunoreactive material in the brain of a gymnophione amphibian, Ichthyophis beddomei. Immunocytochemical analysis of the adult brain and terminal nerves in both sexes shows the presence of neurons and fibers containing mammalian GnRH (mGnRH)- and chicken GnRH-II (cGnRH-II)-like peptides. With respect to GnRH-immunoreactive material, there are two distinct neuronal systems in the brain: one containing mGnRH, which is located in the forebrain and terminal nerve, and the other containing cGnRH-II, which is restricted to the midbrain tegmentum. Basically, this distribution pattern parallels that of many species of anurans and a urodele. Whereas the presence of cGnRH-II-immunoreactive fibers in the dorsal pallium of L. beddomei is a feature in common with a urodele amphibian, the total absence of cGnRH-II-like material in the median eminence is unique to this species. It is suggested here that the distribution profile of GnRH-like material within the brain and terminal nerve of I. beddomei represents a primitive pattern.

Differentiation processes in the amphibian brain with special emphasis on heterochronies

Amphibians and caecilians exhibit a great variety of adult morphologies, life histories, and developmental strategies (biphasic development, direct development, viviparity, and neoteny). While early brain development and the differentiation of neural tissues in the three amphibian orders follow a basic pattern, differences exist in the onset and offset as well as the rate of growth and differentiation processes. These differences are described within a phylogenetic framework, and special emphasis is laid on the relationship between altered ontogenies and phylogenetic diversity. We concentrate on ontogenetic differentiation processes in the motor, olfactory, and visual system. We discuss the morphological consequences of secondary simplification of the brain in the context of paedomorphosis, which has happened several times independently among amphibians and consists in the abbreviation or truncation of late developmental processes. We deal with the cellular and molecular basis of brain development and the consequences for the adult nervous system in representative species of the three amphibian orders. Our analysis reveals that differences in brain morphology are largely due to heterochrony (i.e., the desynchronization of ontogenetic processes), a phenomenon that in turn is related to changes in genome sizes and life histories.

Ultrastructure of the olfactory organ of the newt, Cynops pyrrhogaster

The ultrastructure of the nasal sacs of the Japanese newt, Cynops pyrrhogaster, was studied by scanning and transmission electron microscopy. The paired nasal sacs of the newt are dorsoventrally flattened with a lateral nasal sinus off the main cavity of each sac. Throughout each sac is a series of ridges and grooves. In the main cavity, sensory epithelium with ciliated and microvillous receptor cells lines the grooves, and a thin, ciliated non-sensory epithelium lines the ridges. Secretory glands are present in the lamina propria. In the lateral nasal sinus, the ridges are lined with a thick, non-ciliated sensory epithelium that lacks glands. This region resembles and may function as a primitive vomeronasal organ.

Phylogeny of the vomeronasal system and of receptor cell types in the olfactory and vomeronasal epithelia of vertebrates

In this paper, the evolutionary origin of the vomeronasal system as a discrete sensory system separate from olfaction is examined. The presence of a discrete vomeronasal system appears to be a derived character in tetrapods, and its presence in larval amphibians indicates that the system did not arise as a terrestrial adaptation. The vomeronasal system has been lost independently in several taxa, including crocodilians, some bats, cetaceans, and some primates. The presence of microvillar receptor cells in the vomeronasal epithelium appears to be the ancestral condition for tetrapods, and alternative hypotheses concerning the ancestral condition for receptor cell types in the vertebrate olfactory epithelium are discussed. Finally, the possibility that the vomeronasal system is present in some fishes in a form that has not been recognized is discussed in relation to the phylogenetic distribution of receptor cell types in vertebrates.