Squamate egg tooth development revisited using three-dimensional reconstructions of brown anole (Anolis sagrei, Squamata, Dactyloidae) dentition

Squamates as a model to understand key dental features of vertebrates

Thanks to their exceptional diversity, teeth are among the most distinctive features of vertebrates. Parameters such as tooth size, shape, number, identity, and implantation can have substantial implications for the ecology and certain social behaviors of toothed species. Despite decades of research primarily focused on mammalian dentition, particularly using the laboratory mouse model, squamate reptiles ("lizards" and snakes) offer a wide array of tooth types and dentition variations. This diversity, which includes differences in size, shape, function, and replacement capacity, provides invaluable opportunities for investigating these fundamental properties. The central bearded dragon (Pogona vitticeps), a popular pet species with well-established husbandry practices, is of particular interest. It features a broad spectrum of morphs and spontaneous mutants and exhibits a wide range of heterodont phenotypes, including variation in the size, shape, number, implantation, and renewal of teeth at both posterior and anterior positions. These characteristics position the species as a crucial model organism for developmental studies in tooth research and for gaining deeper insights into evolutionary patterns of vertebrate dentitions. In this article, we provide an overview of the current understanding of squamate dentition, its diversity, development, and replacement. Furthermore, we discuss the significant advantages offered by squamate species as model organisms for investigating the evolutionary and developmental aspects of vertebrate dentition.

Formation and Replacement of Bone and Tooth Mineralized Tissues in Green Iguanas (Iguana iguana) Revealed by In-Vivo Fluorescence Marking

Hard tissue formation patterns and rates reveal details of animal physiology, life history, and environment, but are understudied in reptiles. Here, we use fluorescence labels delivered in vivo and laser confocal scanning microscopy to study tooth and bone formation in a managed group of green iguanas (Iguana iguana, Linné 1758) kept for 1.5 years under experimentally controlled conditions and undergoing several dietary switches. We constrain rates of tooth elongation, which we observe to be slow when enamel is initially deposited (c. 9 µm/day), but then increases exponentially in the dentin root, reaching c. 55 µm/day or more after crown completion. We further constrain the total timing of tooth formation to ∼40-60 days, and observe highly variable timings of tooth resorption onset and replacement. Fluorescent labels clearly indicate cohorts of teeth recruited within Zahnreihen replacement waves, with faster sequential tooth recruitment and greater wave sizes posteriorly, where each wave initiates. Fluorescence further reveals enamel maturation after initial deposition. Rates of hard tissue formation in long bones range from 0.4 to 3.4 µm/day, correlating with animal weight gain and cortical bone recording the entire history of the experiment. We suggest additional labeling experiments to study hard tissue formation patterns in other reptiles, and propose strategies for chemical analyses of hard tissues in order to extract temporal information about past environments, behaviors, and diets from reptilian fossils throughout the Phanerozoic.

Getting out of a mammalian egg: the egg tooth and caruncle of the echidna

In the echidna, after development in utero, the egg is laid in the pouch and incubated for 10 days. During this time, the fetuses develop an egg tooth and caruncle to help them hatch. Using rare and unprecedented access to limited echidna pre- and post-hatching tissues, development of the egg tooth and caruncle were assessed by micro-CT, histology and immunofluorescence. Unlike therian tooth germs that develop by placode invagination, the echidna egg tooth developed by evagination, similar to the first teeth in some reptiles and fish. The egg tooth ankylosed to the premaxilla, rather than forming a tooth root with ligamentous attachment found in other mammals, with loss of the egg tooth associated with high levels of activity odontoclasts and apoptosis. The caruncle formed as a separate mineralisation from the adjacent nasal capsule, and as observed in birds and turtles, the nasal region epithelium on top of the nose expressed markers of cornification. Together, this highlights that the monotreme egg tooth shares many similarities with typical reptilian teeth, suggesting that this tooth has been conserved from a common ancestor of mammals and reptiles.

Ontogeny of the skull of the blind snake Amerotyphlops brongersmianus (Serpentes: Typhlopidae) brings new insights on snake cranial evolution

Blind snakes represent the most basal group of extant snakes and include fossorial species with unusual skeletal traits. Despite their known phylogenetic position, little is known about their ontogeny and what it might reveal about the origin of their skull anatomy. Here we describe for the first time the ontogenetic transformations of the skull of a blind snake, the typhlopid Amerotyphlops brongersmianus, including embryos and postnatal individuals. Furthermore, we provide data on the size changes relative to skull growth of the main elements of the gnathic complex. We observed that the skull of this blind snake undergoes considerable morphological change during late ontogeny. Additionally, we detected delayed development of some traits (closure of the skull roof, opisthotic-exoccipital suture, ossification of the posterior trabeculae) simultaneously with clearly peramorphic traits (development of the crista circumfenestralis, growth of the pterygoid bar). Our analysis suggests that the unique skull anatomy of blind snakes displays plesiomorphic and highly autapomorphic features, as an outcome of heterochronic processes and miniaturization, and is shaped by functional constraints related to a highly specialized feeding mechanism under the selective pressures of a fossorial lifestyle.

Changes in the avascular area of the meniscus using mesenchymal stem cells and growth plate chondrocytes in a pig model

Menisci are wedge-shaped cartilage discs that are divided into two parts: the avascular and vascular regions. They are formed by fibrocartilage tissue, which contains round cartilage-like cells and extracellular matrix. Meniscus injury in animals is a common orthopedic problem, but data on the natural healing process mainly deals with the vascular zone. The healing processes in the avascular zone of the meniscus are significantly limited. Thus, this study aimed to evaluate autologous growth plate chondrocytes' impact on the healing process of a damaged meniscus in the avascular zone based on a growing animal model. The study group consisted of 10 pigs at about three months of age. From each animal, chondrocytes from the iliac growth plate and from concentrated bone marrow were taken. Knee joints were divided into right (R) and left (L). The medial meniscus of the R knee joint was treated with a hyaluronic acid based scaffold incubated with bone marrow cells from marrow aspirates (nCHON). The medial meniscus of the L knee joint was treated with a hyaluronic acid based scaffold incubated with bone marrow cells from marrow aspirates supplemented with immature chondrocytes isolated from growth plates (wCHON). The meniscus was damaged in the avascular zone in both knee joints. Followingly, the damaged part of the meniscus was filled with a scaffold with cells from the concentrated bone marrow and from growth plate chondrocytes. In the control group, a scaffold with concentrated bone marrow cells was used. After three months the animals were euthanized and preparations (microscopic slides) were made from the meniscus' damaged part. A qualitative and quantitative analysis have been prepared. The wCHON group in comparison with the nCHON group showed a statistically significantly higher number of fusiform cells on the surface of the graft as well as better healing of the graft. In addition, the degree of vascularization was higher in specimens from the wCHON group than in the nCHON group. The results of our research on immature pig knees revealed that mesenchymal stem cell and growth plate chondrocytes could be treated as the cell source for meniscus reconstruction, and growth plate chondrocytes enhance healing processes in the avascular zone of the injured meniscus.

Role of Cell Death in Cellular Processes During Odontogenesis

The development of a tooth germ in a precise size, shape, and position in the jaw, involves meticulous regulation of cell proliferation and cell death. Apoptosis, as the most common type of programmed cell death during embryonic development, plays a number of key roles during odontogenesis, ranging from the budding of the oral epithelium during tooth initiation, to later tooth germ morphogenesis and removal of enamel knot signaling center. Here, we summarize recent knowledge about the distribution and function of apoptotic cells during odontogenesis in several vertebrate lineages, with a special focus on amniotes (mammals and reptiles). We discuss the regulatory roles that apoptosis plays on various cellular processes during odontogenesis. We also review apoptosis-associated molecular signaling during tooth development, including its relationship with the autophagic pathway. Lastly, we cover apoptotic pathway disruption, and alterations in apoptotic cell distribution in transgenic mouse models. These studies foster a deeper understanding how apoptotic cells affect cellular processes during normal odontogenesis, and how they contribute to dental disorders, which could lead to new avenues of treatment in the future.

Ultrastructural studies of developing egg tooth in grass snake Natrix natrix (Squamata, Serpentes) embryos, supported by X-ray microtomography analysis

The egg tooth development is similar to the development of all the other vertebrate teeth except earliest developmental stages because the egg tooth develops directly from the oral epithelium instead of the dental lamina similarly to null generation teeth. The developing egg tooth of Natrix natrix changes its curvature differently than the egg tooth of the other investigated unidentates due to the presence of the rostral groove. The developing grass snake egg tooth comprises dental pulp and the enamel organ. The fully differentiated enamel organ consists of outer enamel epithelium, stellate reticulum, and ameloblasts in its inner layer. The enamel organ directly in contact with the oral cavity is covered with periderm instead of outer enamel epithelium. Stellate reticulum cells in the grass snake egg tooth share intercellular spaces with the basal part of ameloblasts and are responsible for their nutrition. Ameloblasts during egg tooth differentiation pass through the following stages: presecretory, secretory, and mature. The ameloblasts from the grass snake egg tooth show the same cellular changes as reported during mammalian amelogenesis but are devoid of Tomes' processes. Odontoblasts of the developing grass snake egg tooth pass through the following classes: pre-odontoblasts, secretory odontoblasts, and ageing odontoblasts. They have highly differentiated secretory apparatus and in the course of their activity accumulate lipofuscin. Grass snake odontoblasts possess processes which are poor in organelles. In developing egg tooth cilia have been identified in odontoblasts, ameloblasts and cells of the stellate reticulum. Dental pulp cells remodel collagen matrix during growth of the grass snake egg tooth. They degenerate in a way previously not described in other teeth.

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.

Structural and ultrastructural studies on the developing vomeronasal sensory epithelium in the grass snake Natrix natrix (Squamata: Colubroidea)

The sensory olfactory epithelium and the vomeronasal sensory epithelium (VSE) are characterized by continuous turnover of the receptor cells during postnatal life and are capable of regeneration after injury. The VSE, like the entire vomeronasal organ, is generally well developed in squamates and is crucial for detection of pheromones and prey odors. Despite the numerous studies on embryonic development of the VSE in squamates, especially in snakes, an ultrastructural analysis, as far as we know, has never been performed. Therefore, we investigated the embryology of the VSE of the grass snake (Natrix natrix) using electron microscopy (SEM and TEM) and light microscopy. As was shown for adult snakes, the hypertrophied ophidian VSE may provide great resolution of changes in neuron morphology located at various epithelial levels. The results of this study suggest that different populations of stem/progenitor cells occur at the base of the ophidian VSE during embryonic development. One of them may be radial glia-like cells, described previously in mouse. The various structure and ultrastructure of neurons located at different parts of the VSE provide evidence for neuronal maturation and aging. Based on these results, a few nonmutually exclusive hypotheses explaining the formation of the peculiar columnar organization of the VSE in snakes were proposed.

Embryonic skull development in the gecko, Tarentola annularis (Squamata: Gekkota: Phyllodactylidae)

Tarentola annularis is a climbing gecko with a wide distribution in Africa north of the equator. In the present paper, we describe the development of the osteocranium of this lizard, from the first appearance of the cranial elements up to the point of hatching. This is based on a combination of histology and cleared and stained specimens. This is the first comprehensive account of gekkotan pre-hatching skull development based on a comprehensive series of embryos, rather than a few selected stages. Given that Gekkota is now widely regarded as representing the sister group to other squamates, this account helps to fill a significant gap in the literature. Moreover, as many authors have considered features of the gekkotan skull and skeleton to be indicative of paedomorphosis, it is important to know whether this hypothesis is supported by delays in the onset of cranial ossification. In fact, we found the sequence of cranial bone ossification to be broadly comparable to that of other squamates studied to date, with no significant lags in development.