Anatomy, Physiology, and Disorders of the Spectacle, Subspectacular Space, and Its Lacrimal Drainage System in Squamates

Various squamate species have completely fused eyelids that make up a transparent spectacle. The spectacle is a continuation of the integument that is renewed with each shedding cycle and creates a narrow subspectacular or corneospectacular space that is filled with lacrimal fluid. The latter is considered as the analogue of the conjunctival sac in other vertebrates. Almost all reptiles that have a spectacle lack a nictitating membrane, bursalis muscle, and lacrimal glands. The lacrimal fluid in the subspectacular space is secreted by the Harderian gland. The features of the spectacle and its lacrimal drainage system are an illustration of the enormous variation of the morphological adaptations that are seen in reptiles and one of the most distinguishable traits of snakes and most gecko species. Whereas ocular disease in squamates with a spectacle is infrequently seen in practice, disorders of the spectacle and the subspectacular space are commonly encountered. In order to apply an adequate diagnostic and therapeutic approach for these conditions, a sound knowledge and understanding of the anatomical and physiological peculiarities of the spectacle, subspectacular space, and lacrimal drainage system are fundamental.

Embryology of the naso-palatal complex in Gekkota based on detailed 3D analysis in Lepidodactylus lugubris and Eublepharis macularius

The vomeronasal organ (VNO), nasal cavity, lacrimal duct, choanal groove, and associated parts of the superficial (soft tissue) palate are called the naso-palatal complex. Despite the morphological diversity of the squamate noses, little is known about the embryological basis of this variation. Moreover, developmental data might be especially interesting in light of the morpho-molecular discordance of squamate phylogeny, since a 'molecular scenario' implies an occurrence of unexpected scale of homoplasy also in olfactory systems. In this study, we used X-ray microtomography and light microscopy to describe morphogenesis of the naso-palatal complex in two gekkotans: Lepidodactylus lugubris (Gekkonidae) and Eublepharis macularius (Eublepharidae). Our embryological data confirmed recent findings about the nature of some developmental processes in squamates, for example, involvement of the lateral nasal prominence in the formation of the choanal groove. Moreover, our study revealed previously unknown differences between the studied gekkotans and allows us to propose redefinition of the anterior concha of Sphenodon. Interpretation of some described conditions might be problematic in the phylogenetic context, since they represent unknown: squamate, nonophidian squamate, or gekkotan features.

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.

Embryology of the VNO and associated structures in the grass snake Natrix natrix (Squamata: Naticinae): a 3D perspective

BACKGROUND

Snakes are considered to be vomerolfaction specialists. They are members of one of the most diverse groups of vertebrates, Squamata. The vomeronasal organ and the associated structures (such as the lacrimal duct, choanal groove, lamina transversalis anterior and cupola Jacobsoni) of adult lizards and snakes have received much anatomical, histological, physiological and behavioural attention. However, only limited embryological investigation into these structures, constrained to some anatomical or cellular studies and brief surveys, has been carried out thus far. The purpose of this study was, first, to examine the embryonic development of the vomeronasal organ and the associated structures in the grass snake (Natrix natrix), using three-dimensional reconstructions based on histological studies, and, second, to compare the obtained results with those presented in known publications on other snakes and lizards.

RESULTS

Five major developmental processes were taken into consideration in this study: separation of the vomeronasal organ from the nasal cavity and its specialization, development of the mushroom body, formation of the lacrimal duct, development of the cupola Jacobsoni and its relation to the vomeronasal nerve, and specialization of the sensory epithelium. Our visualizations showed the VNO in relation to the nasal cavity, choanal groove, lacrimal duct and cupola Jacobsoni at different embryonic stages. We confirmed that the choanal groove disappears gradually, which indicates that this structure is absent in adult grass snakes. On our histological sections, we observed a gradual growth in the height of the columns of the vomeronasal sensory epithelium and widening of the spaces between them.

CONCLUSIONS

The main ophidian taxa (Scolecophidia, Henophidia and Caenophidia), just like other squamate clades, seem to be evolutionarily conservative at some levels with respect to the VNO and associated structures morphology. Thus, it was possible to homologize certain embryonic levels of the anatomical and histological complexity, observed in the grass snake, with adult conditions of certain groups of Squamata. This may reflect evolutionary shift in Squamata from visually oriented predators to vomerolfaction specialists. Our descriptions offer material useful for future comparative studies of Squamata, both at their anatomical and histological levels.

Through the looking glass: the spectacle in gymnophthalmid lizards

The anatomy and development of the eyelids in squamate reptiles are still relatively unknown, considering its variation within the group. The neotropical Gymnophthalmini are traditionally characterized by having lost the eyelids, but their structure is not well described. In this study, the embryonic development and the adult morphology of the gymnophthalmid eye, with special attention to the eyelids, the nictitating membrane, and the spectacle are described. The eye in some Gymnophthalmini is covered by a spectacle, formed by the embryonic fusion of the dorsal and ventral eyelids, a character possibly synapomorphic to the tribe. The genus Tretioscincus, which floats either as sister to all other Gymnophthalmini, or is nested within the group, is unique in showing functional and movable eyelids. Thus, the presence of functional eyelids can be either considered as the primitive condition for the gymnophthalmini or as a re-acquisition of the character, showing the importance of a well-established phylogenetic hypothesis for understanding morphological evolution.

Skull anatomy of the miniaturized gecko Sphaerodactylus roosevelti (Squamata: Gekkota)

A detailed description of the skull and jaw of the gecko Sphaerodactylus roosevelti is presented. The bones are described articulated and isolated with special consideration given to the type of suture among joining elements. S. roosevelti was compared with 109 gekkotan species to evaluate the osteological variation and to find characters for cladistic analysis. Changes in the skull associated with the miniaturization process are discussed within the sphaerodactylid geckos. A noticeable increase of overlapping sutures was observed in the snout of the smallest sphaerodactylids compared to other gekkotans. This pattern is convergent with that in miniaturized pygopodids and may be attributed to adaptations for decreasing mechanical resistance of the cranium during feeding or burrowing. New cranial characters support Sphaerodactylinae as a monophyletic group and should be useful for resolving questions such as their relationship with other gekkotans.

Alligator tears: a reevaluation of the lacrimal apparatus of the crocodilians

The Harderian gland is a poorly understood anterior ocular gland that occurs in most terrestrial vertebrates. Numerous extraorbital functions have been ascribed to the Harderian gland, principally based on its association with the nasolacrimal duct. Few studies have centered on archosaurs and the majority of those available focused solely on the Harderian gland of birds. Little is known about the lacrimal apparatus of the crocodilians. We examined the lacrimal apparatus of several specimens of Alligator mississippiensis anatomically, histologically, and histochemically and studied the embryogenesis of this system. The nasolacrimal duct possesses a distal secretory area, which is more convoluted than that of typical mammals or lepidosaurs. The alligator Harderian gland possesses a unique combination of characteristics found in lepidosaurs, birds, and mammals. Like that of both mammals and lepidosaurs, it is a large, tuboloacinar gland that appears to secrete both mucoprotein and lipids. However, the presence of blood vessels and immune cells is reminiscent of that of the avian Harderian gland. The immunogenesis of the alligator Harderian gland appears to be tied to the development of the vascular system. The presence of a distinct palpebral gland in the anterior aspect of the ventral eyelid is a feature unique to alligators. Based on position, this gland does not appear to be homologous to the anterior lacrimal gland of lepidosaurs. Lymphatic aggregations were also found in the palpebral gland. The presence of interstitial immune cells in the orbital glands of alligators suggests that the alligator lacrimal apparatus, like that of birds, may play a role in the head-associated lymphatic tissue system.

The Harderian gland of two species of snakes: Pseudonaja textilis (Elapidae) and Thamnophis sirtalis (Colubridae)

The reptilian Harderian gland is a poorly understood cephalic structure. Despite the recent assertion that in snakes it may function as part of the vomeronasal system, the Harderian gland has been described in few snake species. In this study we examined the gross anatomy, histology, and ultrastructure of the Harderian gland of two different advanced snake species (Colubroidea): Pseudonaja textilis (Elapidae) and Thamnophis sirtalis (Colubridae). In both species the Harderian gland is a large serous gland whose secretions pass directly into the vomeronasal organ via the nasolacrimal duct. Contrary to previous publications, the Harderian gland in both species studied possesses a specific duct system lined by mucous cells. However, the Harderian glands of these two species differ in shape, the histochemical nature of these mucous secretions, and the ultrastructure of the serous granules. In conclusion, though the Harderian glands of snakes are remarkably conserved morphologically, there is some interspecific variation.