Cartilaginous Skeleton Research Articles

Novos espinhos de nadadeiras de Chondrichthyes fósseis da Bacia do Parnaíba, Formação Pedra de Fogo (Permiano) no Estado do Tocantins, Brasil

New Chondrichthyes fin spines fossils from the Pedra de Fogo Formation (Permian) in State of Tocantins, Brazil. The Parnaíba Basin has a unique fossil record of Chondrichthyes that lived during the Permian period. Within this basin is the Pedra de Fogo Formation, dating from the Cisuralian, which is distributed in parts of the states of Pará, Tocantins, Piauí, Maranhão, Ceará, and Bahia. Due to their cartilaginous skeleton, Chondrichthyes skeletal remains are not prone to fossilization, with teeth, fin spines and dermal denticles being more commonly found. The main objective of this work is to describe new fins spines from the Parnaíba Basin, collected in the State of Tocantins and stored at the Museum of Archeology and Paleontology of the Federal University of Piauí. Six shark spines were identified as belonging to Ctenacanthiformes. A more specific classification at the genus and species level is still uncertain due to the difficulties with the definition of such taxa and incompleteness of the described remains. These occurrences contribute to future research in the area concerning about on fossil fin spines found in the Parnaíba Basin. Keywords: Ctenacanthus, Ctenacanthiformes, paleohistology, Paleozoic.

Deuterostome Ancestors and Chordate Origins.

The Deuterostomia are a monophyletic group, consisting of the Ambulacraria, with two phyla, Hemichordata and Echinodermata, and the phylum Chordata, containing the subphyla Cephalochordata (lancelets or Amphioxus), Tunicata (Urochordata) and Vertebrata. Hemichordates and echinoderms are sister groups and are critical for understanding the deuterostome ancestor and the origin and evolution of the chordates within the deuterostomes. Enteropneusta, worm-like hemichordates, share many chordate features as adults, including a post-anal tail, gill slits, and a Central Nervous System (CNS) that deploy similar developmental Genetic Regulatory Networks (GRNs). Genomic comparisons show that cephalochordates share synteny and a vermiform body plan similar to vertebrates, but phylogenomic analyses place tunicates as the sister group of vertebrates. Tunicates have a U-shaped gut and a very different adult body plan than the rest of the chordates, and all tunicates have small genomes and many gene losses, although the GRNs underlying specific tissues, such as notochord and muscle, are conserved. Echinoderms and vertebrates have extensive fossil records, with fewer specimens found for tunicates and enteropneusts, or worm-like hemichordates. The data is mounting that the deuterostome ancestor was a complex benthic worm, with gill slits, a cartilaginous skeleton, and a CNS. Two extant groups, echinoderms and tunicates, have evolved highly derived body plans, remarkably different than the deuterostome ancestor. We review the current genomic and GRN data on the different groups of deuterostomes' characters to re-evaluate different hypotheses of chordate origins. Notochord loss in echinoderms and hemichordates is as parsimonious as notochord gain in the chordates but has implications for the deuterostome ancestor. The chordate ancestor lost an ancestral nerve net, retained the central nervous system, and evolved neural crest cells.

Macroscopic, microscopic, and immunofluorescent characterization of the Greek tortoise (Testudo graeca graeca) oropharyngeal floor with concern to its feed adaptation as a herbivorous land reptile.

The current investigation focuses on gross anatomy, light, and scanning electron microscopy (SEM) of the Testudo graeca oropharyngeal floor, with particular reference to the immunofluorescence technique to examine its tongue. The T. graeca oropharyngeal floor showed many anatomical structures: the lower rhamphotheca, paralingual ridge, lower alveolar ridge, tongue, laryngeal mound, and glottis. The lower rhamphotheca appeared as a V-shaped jaw line with a highly serrated edge and a median tomium (beak). SEM observations of the lingual apex and the lingual body showed rectangular and conical filiform papillae with porous surfaces and taste pores. Meanwhile, the lingual root had two wings that carried papillae with different shapes: dagger-shaped, conical, bifurcated, and leaf-like papillae, and these papillae lacked taste pores. The laryngeal mound had openings for the laryngeal mucus gland and its secretions. Light microscopy findings showed mucous glands in the propria submucosa and near the mucosal surface of the lingual apex. The lingual root had lingual papillae and two hyaline cartilaginous skeletons between skeletal muscles, and the lingual papillae were elongated filiform, rectangular filiform papillae, and fungiform papillae. The lamina propria constituted the core of the lingual papillae and the mucous gland, they had a positive reaction with the periodic acid schiff (PAS) reagent. The apical surface of the fungiform papillae had taste pores. Under immunofluorescence, the vimentin was detected in taste bud cells, and synaptophysin reacted to the taste buds and nerve bundles. The current study of the Greek tortoise oropharyngeal floor investigated its herbivorous eating habits using its serrated lower rhamphotheca, a large tongue with differently shaped papillae, and numerous mucous glands. RESEARCH HIGHLIGHTS: The Greek tortoise (T. graeca graeca) oropharyngeal floor showed many anatomical structures: lower rhamphotheca, paralingual ridge, lower alveolar ridge, tongue, laryngeal mound, and glottis. SEM and light microscopy observations of the tongue revealed varied types and shapes of lingual papillae with a porous surface on the tongue apex (rectangular or conical filiform papillae), on the tongue body (filiform and fungiform papillae), and on the tongue root (dagger-shaped, conical, bifurcated, and leaf-like papillae). Light microscopy findings: the lamina propria constituted the core of the lingual papillae and had numerous mucous glands that had a slightly magenta-red color with PAS reagent. The apical surface of the fungiform papillae had taste pores. Vimentin and synaptophysin gave a reaction to the taste buds.

Rise and diversification of chondrichthyans in the Paleozoic

AbstractThe Paleozoic represents a key time interval in the origins and early diversification of chondrichthyans (cartilaginous fishes), but their diversity and macroevolution are largely obscured by heterogenous spatial and temporal sampling. The predominantly cartilaginous skeletons of chondrichthyans pose an additional limitation on their preservation potential and hence on the quality of their fossil record. Here, we use a newly compiled genus-level dataset and the application of sampling standardization methods to analyze global total-chondrichthyan diversity dynamics through time from their first appearance in the Ordovician through to the end of the Permian. Subsampled estimates of chondrichthyan genus richness were initially low in the Ordovician and Silurian but increased substantially in the Early Devonian. Richness reached its maximum in the middle Carboniferous before dropping across the Carboniferous/Permian boundary and gradually decreasing throughout the Permian. Sampling is higher in both the Devonian and Carboniferous compared with the Silurian and most of the Permian stages. Shark-like scales from the Ordovician are too limited to allow for some of the subsampling techniques. Our results detect two Paleozoic radiations in chondrichthyan diversity: the first in the earliest Devonian, led by acanthodians (stem-group chondrichthyans), which then decline rapidly by the Late Devonian, and the second in the earliest Carboniferous, led by holocephalans, which increase greatly in richness across the Devonian/Carboniferous boundary. Dispersal of chondrichthyans, specifically holocephalans, into deeper-water environments may reflect a niche expansion following the faunal displacement in the aftermath of the Hangenberg extinction event at the end of the Devonian.

Preliminary evaluation of fish cartilage as a promising biomaterial in cartilage tissue engineering

Fish cartilage is known as a valuable source of natural biomaterials due to its unique composition and properties. It contains a variety of bioactive components that contribute to its potential applications in different domains such as tissue engineering. The present work aimed to consider the properties of backbone cartilage from fish with a cartilaginous skeleton, including elasmobranch (reticulate whipray: Himantura uarnak and milk shark: Rhizoprionodon acutus) and sturgeon (beluga: Huso huso). The histomorphometric findings showed that the number of chondrocytes was significantly higher in reticulate whipray and milk shark compared to beluga (p < 0.05). The highest GAGs content was recorded in reticulate whipray cartilage compared to the other two species (p < 0.05). The cartilage from reticulate whipray and beluga showed higher collagen content than milk shark cartilage (p < 0.05), and the immunohistochemical assay for type II collagen (Col II) showed higher amounts of this component in reticulate whipray compared to the other two species. Young’s modulus of the cartilage from reticulate whipray was significantly higher than that of milk shark and beluga (p < 0.05), while no significant difference was recorded between Young’s modulus of the cartilage from milk shark and beluga. The gene expression of ACAN, Col II, and Sox9 showed that the cartilage-ECM from three species was able to induce chondrocyte differentiation from human adipose tissue-derived stem cells (hASCs). From these results, it can be concluded that the cartilage from three species, especially reticulate whipray, enjoys the appropriate biological properties and provides a basis for promoting its applications in the field of cartilage tissue engineering.

The effect of tessellation on stiffness in the hyoid arch of elasmobranchs.

Tessellated cartilage forms much of the skeleton of sharks and rays, in contrast to most other aquatic vertebrates who possess a skeleton of bone. Interestingly, many species of sharks and rays also regularly generate exceptionally high forces in the execution of day-to-day activities, such as when feeding on bony fish, mammals, and hard-shelled invertebrates. Tessellated cartilage differs from other types of cartilage in that they are covered by an outer layer of small mineralized tiles (tesserae) that are connected by fibrous connective tissue. Tesserae, therefore, are hypothesized to play a role in stiffening the cartilaginous skeleton for food capture and other activities that require the generation of high forces. In this study, the hyomandibula and ceratohyal cartilages, which support the jaw and throat regions of sharks and rays, were tested under compressive load in a material testing system to determine the contribution of tesserae to stiffness. Previous hypotheses suggest an abrupt upward shift in the slope of the stress-strain curve in tessellated materials due to collision of tesserae. Young's Modulus (E) was calculated and used to evaluate cartilage stiffness in a range of elasmobranch species. Our results revealed that there was an abrupt shift in Young's Modulus for elements loaded in compression. We postulate that this shift, characterized by an inflection point in the stress-strain curve, is the result of the tesserae approaching one another and compressing the intervening fibrous tissue, supporting the hypothesis that tesserae function to stiffen these cartilages under compressive loading regimes. Using published data for nontessellated cartilage for comparison, we show that this shift is, as expected, unique to tessellated cartilage.

Loss of noggin1, a classic embryonic inducer gene, in elasmobranchs

Secreted proteins of the Noggin family serve as pivotal regulators of early development and cell differentiation in all multicellular animals, including vertebrates. Noggin1 was identified first among all Noggins. Moreover, it was described as the first known embryonic inducer specifically secreted by the Spemann organizer and capable of inducing a secondary body axis when expressed ectopically. In the classical default model of neural induction, Noggin1 is presented as an antagonist of BMP signalling, playing a role as a neural inducer. Additionally, Noggin1 is involved in the dorsalization of embryonic mesoderm and later controls the differentiation of various tissues, including muscles, bones, and neural crest derivatives. Hitherto, noggin1 was found in all studied vertebrates. Here, we report the loss of noggin1 in elasmobranchs (sharks, rays and skates), which is a unique case among vertebrates. noggin2 and noggin4 retained in this group and studied in the embryos of the grey bamboo shark Chiloscyllium griseum revealed similarities in expression patterns and functional properties with their orthologues described in other vertebrates. The loss of noggin1 in elasmobranchs may be associated with histological features of the formation of their unique internal cartilaginous skeleton, although additional research is required to establish functional connections between these events.

Morphological comparison of the larynx and trachea of Chelonia mydas (Linnaeus, 1758), Caiman yacare (Daudin, 1802) and Caiman latirostris (Daudin, 1802).

The larynx is in the lower respiratory tract and has the function of protecting the airways, controlling, and modulating breathing, assisting the circulatory system, and vocalizing. This study aims to describe the anatomy and histology of the skeleton of the larynx and trachea of the species Chelonia mydas, Caiman yacare and Caiman latirostris. The study was conducted at the Federal University of Espírito Santo (UFES), using nine specimens of Ch. mydas, 20 of Ca. yacare and four of Ca. latirostris. Samples of the larynx and trachea were collected, fixed, and sent for dissection of the structures and subsequent macroscopic analysis. For histology, samples were processed by the routine paraffin embedding method and stained with hematoxylin-eosin and Verhoeff. For the three species, two arytenoid cartilages, a cricoid cartilage, a hyoid apparatus composed of a base and two horns were found. In Ch. mydas, two structures called thyroid wings were observed, not found in crocodilians. The trachea of crocodilians presented incomplete tracheal rings and musculature, while the trachea of Ch. mydas presented complete tracheal rings. Histologically, the entire cartilaginous skeleton of the larynx of the three species, as well as the tracheal rings, are constituted by hyaline cartilage.

Pathological study of a traumatic anthropogenic injury in the skeleton of a spiny butterfly ray (Gymnura altavela).

External injuries in elasmobranchs are frequent findings, either due to inter- or intraspecific interactions or as a result of interaction with human activities. However, the resilience of these species to traumatic injury remains poorly understood. This work provides an insight into the clinical presentation, diagnostic imaging, and pathological features of a severe traumatic injury to the cartilaginous skeleton of a spiny butterfly ray (Gymnura altavela). An adult female was found lethargic in the bottom of the coast of Gran Canaria, with an external incised-contused traumatic lesion of 2 cm diameter in the scapulocoracoid cartilage. It was captured and transferred to the Poema del Mar Aquarium for its clinical evaluation and treatment. Despite these efforts, the animal eventually died and was transfer to the Institute of Animal Health and Food Safety (IUSA) for its pathological diagnosis, including a Computed Tomography (CT) study and necropsy. The animal presented a marked reduction in hematocrit and hepatosomatic index due a chronic debilitation process. The CT scan revealed a destructive lesion with irregular margins at the level of the right scapulocoracoid cartilage. The main pathological findings were the disorganization of the tesserae layer, appearing as whitish square to rectangular geometric pieces separated from the cartilaginous core. Histologically, these pieces of tesserae were separated from the unmineralized cartilage core and displaced from the adjacent perichondrium, where inflammatory cells infiltrate. Edema and hemorrhages were also observed. This study reports the first comprehensive description of skeleton trauma in a spiny butterfly ray, including the clinical presentation, diagnostic imaging and the anatomopathological features.

Femoral posture during embryonic and early fetal development: An analysis using landmarks on the cartilaginous skeletons of ex vivo human specimens.

The pre-axial border medially moves between the fetal and early postnatal periods, and the foot sole can be placed on the ground. Nonetheless, the precise timeline when this posture is achieved remains poorly understood. The hip joint is the most freely movable joint in the lower limbs and largely determines the lower-limb posture. The present study aimed to establish a timeline of lower-limb development using a precise measurement of femoral posture. Magnetic resonance images of 157 human embryonic samples (Carnegie stages [CS] 19-23) and 18 fetal samples (crown rump length: 37.2-225 mm) from the Kyoto Collection were obtained. Three-dimensional coordinates of eight selected landmarks in the lower limbs and pelvis were used to calculate the femoral posture. Hip flexion was approximately 14° at CS19 and gradually increased to approximately 65° at CS23; the flexion angle ranged from 90° to 120° during the fetal period. Hip joint abduction was approximately 78° at CS19 and gradually decreased to approximately 27° at CS23; the average angle was approximately 13° during the fetal period. Lateral rotation was greater than 90° at CS19 and CS21 and decreased to approximately 65° at CS23; the average angle was approximately 43° during the fetal period. During the embryonic period, three posture parameters (namely, flexion, abduction, and lateral rotation of the hip) were linearly correlated with each other, suggesting that the femoral posture at each stage was three-dimensionally constant and exhibited gradual and smooth change according to growth. During the fetal period, these parameters varied among individuals, with no obvious trend. Our study has merits in that lengths and angles were measured on anatomical landmarks of the skeletal system. Our obtained data may contribute to understanding development from anatomical aspects and provide valuable insights for clinical application.

Using Fish Remains to Trace Dynamic Exchanges Between Ecology and Economy in Fiji

Island nations in the developing world are some of the communities most at-risk to the effects of climate change and are under increasing pressure from globalized seafood markets. Indigenous Fijians have a stark understanding of environmental change because of their economic and dietary reliance on marine resources, including shark fisheries and tourism. Sharks are important apex predators with deep cultural significance in Fiji and they are thus useful species to focus on when investigating historical ecology. However, they are difficult to study; sharks have cartilaginous skeletons, making full body fossils rare, and their behavior can make them difficult to survey with traditional methods. Sharks are also covered in dermal denticles which they shed throughout their lives and which compose some of the most extensive and oldest fossil types. All fish, including sharks, also shed their teeth, and together these microfossils are called ichthyoliths. Scientists in the Caribbean have developed innovative techniques to use ichthyoliths to illuminate the importance of parrotfish to coral reef ecology and trace historical and pre-anthropogenic shark populations. However, this microfossil approach has not yet reached Fiji. We developed a trait-based character coding scheme to describe denticle morphology based on both modern denticles and fossil denticles and discuss our work to expand current denticle reference collections. When combined with ecological factors, morphological analysis can identify temporal periods and spatial regions of importance in both modern and paleo-ocean ecosystem dynamics and aid historical ecologists in describing shark communities of the past. We are embarking on a research project to collect cores from Fiji to examine parrotfish and shark ecology through time. Here we summarize the methods we will use and how we have tailored them to our study region and invite input from the conservation paleobiology community on our study design.

In vitro and in vivo gene transfer in the cloudy catshark Scyliorhinus torazame.

Cartilaginous fishes have various unique physiological features such as a cartilaginous skeleton and a urea-based osmoregulation strategy for adaptation to their marine environment. Also, because they are a sister group of bony vertebrates, understanding their unique features is important from an evolutionary perspective. However, genetic engineering based on gene functions as well as cellular behavior has not been effectively utilized in cartilaginous fishes. This is partly because their reproductive strategy involves internal fertilization, which results in difficulty in microinjection into fertilized eggs at the early developmental stage. Here, to identify efficient gene transfer methods in cartilaginous fishes, we examined the effects of various methods both in vitro and in vivo using the cloudy catshark, a candidate model cartilaginous fish species. In all methods, green fluorescent protein (GFP) expression was used to evaluate exogenous gene transfer. First, we examined gene transfer into primary cultured cells from cloudy catshark embryos by lipofection, polyethylenimine (PEI) transfection, adenovirus infection, baculovirus infection, and electroporation. Among the methods tested, lipofection, electroporation, and baculovirus infection enabled the successful transfer of exogenous genes into primary cultured cells. We then attempted in vivo transfection into cloudy catshark embryos by electroporation and baculovirus infection. Although baculovirus-injected groups did not show GFP fluorescence, electroporation successfully introduced GFP into muscle cells. Furthermore, we succeeded in GFP transfer into adult tissues by electroporation. The in vitro and in vivo gene transfer methods that worked in this study may open ways for genetic manipulation including knockout experiments and cellular lineage analysis in cartilaginous fishes.

Courtship and Reproduction of the Whitetip Reef Shark Triaenodon obesus (Carcharhiniformes: Carcharhinidae) in an Ex Situ Environment, with a Description of the Late Embryonic Developmental Stage.

Elasmobranchs represent a group of species under considerable anthropic pressure because of the scale of industrial and artisanal fisheries and the loss of essential areas for nursery and feeding, which are causing substantial population losses around the world. Reproduction in an ex situ environment enables a healthy population to be built and maintained in networks of public aquariums, increasing our knowledge of elasmobranch reproductive biology and offering the opportunity for reintroductions in areas where native populations have been removed. The study reports two successful pregnancies of the whitetip reef shark Triaenodon obesus, considered a vulnerable species by the International Union for the Conservation of Nature. Copulation and gestation data are provided, including ultrasound recordings of the late stage of embryo development. Ultrasonography was performed with the GE Logiq and convex transducer and revealed a fetus with defined fins and organogenesis, with definition of eyes, gills, liver, a heart with individualized chambers, partially defined kidneys, and a well-defined spiral intestine. A cartilaginous skeleton forming a posterior acoustic shadow was detailed, as well as a moving fetus with a biparietal diameter of 6.47 cm and a heart rate of 62 Beats Per Minute on spectral Doppler. This is the first successful reproduction of T. obesus in an aquarium in Brazil.

Laryngeal Radioanatomy

The larynx, a specialized segment of the airways, is the essential organ of phonation, breathing and protection of the lower airways when swallowing by a complex anatomic structures organized around a cartilaginous skeleton. We realized a prospective study on three months, ranging from March 01 to May 31 2020, of patients referred to our service for cervical CT scan or MRI exploration in different contextes. We retained the normal exams. For scanographic exploration we used a multi-slice CT scan consisting of 64 rows of detectors. Helical acquisition in native axial slices 0.627 mm thick from the base of the skull to the arch of the aorta was performed in indifferent breathing after asking the patient not to swallow. Iodinated contrast product was injected according to the administration protocol in a single bolus of 90 ml at an average flow rate of 1.5 ml / s and to perform the acquisition with 50 second delay. A dynamic phonation maneuver (saying “eee” continuously and evenly) was used to analyse the true and false vocal cords. On MRI exploration of the larynx, we used a high-field device (1.5Tesla) with a semi-circular specific surface antenna, the phase oriented in an anteroposterior direction, the 3DT1 spin echo sequence without fat signal saturation was performed. In ultrasound, we used literature data. Knowledge of laryngeal radioanatomy contributes to the diagnosis of laryngeal cancers which occupy an important place in all cancers of the upper aerodigestive tract. They can be diagnosed early and the refinement of their endoscopic exploration and imaging allows them to make a precise extension assessment.

Skeletal and soft tissue completeness of the acanthodian fossil record

AbstractAcanthodians are a poorly understood paraphyletic grade of extinct Palaeozoic fishes. They play an increasingly prominent role in our understanding of vertebrate evolution as part of the chondrichthyan stem‐group even though their evolutionary history is scarce. The limited preservation of their mostly cartilaginous skeleton largely results in a bias towards isolated remains such as fin spines and scales. Here, we quantify the quality of the acanthodian fossil record by using a variation of the Skeletal Completeness Metric (SCM), an approach that calculates how complete the skeletons of individuals are compared to their theoretical complete skeleton. A novel Soft Tissue Completeness Metric (STCM) is introduced to estimate the percentage of soft body tissue preserved as an alternate measurement of completeness. Completeness scores for &gt;1600 specimens comprising &gt;300 taxa obtained from museum collection visits and literature surveys were assembled into a database. Acanthodian completeness peaks in the Lower–Middle Devonian, Pennsylvanian, and earliest Permian. There is no correlation between acanthodian taxonomic richness and completeness. Acanthodians show a significantly lower completeness distribution than many tetrapod groups, but a similarly low distribution to bats. Skeletons deposited in freshwater are significantly more complete than in marine environments where sea level significantly negatively correlates with observed completeness. Our assessment reveals only weak spatial biases influencing the acanthodian fossil record while environmental biases are much higher. This quantified evaluation of acanthodians provides a foundation for further assessments of the likely influence of character absences from morphological datasets on estimates of early chondrichthyan and, therefore, early gnathostome evolution.

Cranial morphology of the orectolobiform shark, Chiloscyllium punctatum Müller &amp;amp; Henle, 1838

Elasmobranchs, comprising sharks, skates, and rays, have a long evolutionary history extending back into the Palaeozoic. They are characterized by various unique traits including a predominantly cartilaginous skeleton, superficial prismatic phosphatic layer, and permanent tooth replacement. Moreover, they exhibit a more or less marked sexual dimorphism. Especially the morphology of the chondrocranium and the elements of the whole cranial region of extant and extinct chondrichthyans can provide valuable information about corresponding functions, e.g. the feeding apparatus might reflect the diet of the animals. However, studies on sexual dimorphisms are lacking in orectolobiform sharks, therefore, little is known about possible sexual dimorphic characters in the cranial region in this group. For this reason, we present in this study a comprehensive morphological description of the cranial region of the brownbanded bamboo shark Chiloscyllium punctatum Müller & Henle, 1838, with a special focus on its sexual dimorphic characters. Our results reveal clear morphological differences in both sexes of the examined C. punctatum specimens, particularly in the chondrocranium and the mandibular arch. The female specimen shows a comparatively more robust and compact morphology of the chondrocranium. This pattern is also evident in the mandibular arch, especially in the palatoquadrate. The present study is the first to describe the morphology of an orectolobiform shark species in detail using both manual dissection and micro-CT data. The resulting data furthermore provide a starting point for pending studies and are intended to be a first step in a series of comparative studies on the morphology of the cranial region of orectolobiform sharks, including the determination of possible sexual dimorphic characteristics.

Morphology of Palaeospondylus shows affinity to tetrapod ancestors.

Palaeospondylus gunni, from the Middle Devonian period, is one of the most enigmatic fossil vertebrates, and its phylogenetic position has remained unclear since its discovery in Scotland in 1890(ref.1). The fossil's strange set of morphological features has made comparisons with known vertebrate morphotype diversity difficult. Here we use synchrotron radiation X-ray micro-computed tomography to show that Palaeospondylus was a sarcopterygian, and most probably a stem-tetrapod. The skeleton of Palaeospondylus consisted solely of endoskeletal elements in which hypertrophied chondrocyte cell lacunae, osteoids and a small fraction of perichondral bones developed. Despite the complete lack of teeth and dermal bones, the neurocranium of Palaeospondylus resembles those of stem-tetrapod Eusthenopteron2 and Panderichthys3, and phylogenetic analyses place Palaeospondylus in between them. Because the unique features of Palaeospondylus, such as the cartilaginous skeleton and the absence of paired appendages, are present in the larva of crown tetrapods, our study highlights an unanticipated heterochronic evolution at the root of tetrapods.

Sequence of chondrocranial development in basal anurans\u2014Let\u2019s make a cranium

BackgroundThe craniofacial skeleton is an evolutionary innovation of vertebrates. Due to its complexity and importance to protect the brain and aid in essential functions (e.g., feeding), its development requires a precisely tuned sequence of chondrification and/or ossification events. The comparison of sequential patterns of cartilage formation bears important insights into the evolution of development. Discoglossus scovazzi is a basal anuran species. The comparison of its chondrocranium (cartilaginous neuro- & viscerocranium) development with other basal anurans (Xenopus laevis, Bombina orientalis) will help establishing the ancestral pattern of chondrification sequences in anurans and will serve as basis for further studies to reconstruct ancestral conditions in amphibians, tetrapods, and vertebrates. Furthermore, evolutionary patterns in anurans can be studied in the light of adaptations once the ancestral sequence is established.ResultsWe present a comprehensive overview on the chondrocranium development of D. scovazzi. With clearing and staining, histology and 3D reconstructions we tracked the chondrification of 44 elements from the first mesenchymal Anlagen to the premetamorphic cartilaginous head skeleton and illustrate the sequential changes of the skull. We identified several anuran and discoglossoid traits of cartilage development. In D. scovazzi the mandibular, hyoid, and first branchial arch Anlagen develop first followed by stepwise addition of the branchial arches II, III, and IV. Nonetheless, there is no strict anterior to posterior chondrification pattern within the viscerocranium of D. scovazzi. Single hyoid arch elements chondrify after elements of the branchial arch and mandibular arch elements chondrify after elements of the branchial arch I.ConclusionsIn Osteichthyes, neurocranial elements develop in anterior to posterior direction. In the anurans investigated so far, as well as in D. scovazzi, the posterior parts of the neurocranium extend anteriorly, while the anterior parts of the neurocranium, extend posteriorly until both parts meet and fuse. Anuran cartilaginous development differs in at least two crucial traits from other gnathostomes which further supports the urgent need for more developmental investigations among this clade to understand the evolution of cartilage development in vertebrates.

Microscopical observations on the regenerating tail of tsinling dwarf skink (Scincella tsinlingensis)

Although the key steps of tail regeneration in lizards are well understood, further investigations involving skinks can provide the field of regeneration research with additional information. In order to characterize the cytoarchitecture of tail regeneration in Scincella tsinlingensis, an endemic species in China, its histological events and growth trends are investigated. The rate of tail regeneration varies with the season: it proceeds faster in summer and autumn than it does in winter and spring. Tail regeneration of S. tsinlingensis is summarized as wound healing, blastema formation, cell differentiation and tail growth, which can be subdivided into seven stages. Wound healing following tail loss, begins with an obvious outgrowth undergoing re-epithelialization. Numerous proliferating mesenchymal-like cells aggregate near the distal end of the severed spinal cord to form the blastema. The expanding blastema is invaded by blood vessels, nerves and ependyma. A cartilaginous skeleton is formed around the ependymal tube and the muscle starts to differentiate. The keratinization of epidermis coincides with scale formation. Pigmentation eventually occurs in the regenerated tail. Tail regeneration in S. tsinlingensis is an epimorphic kind of regeneration that is also known as blastema-mediated. Structure and composition of the regenerated tail, including its cytoarchitecture, represent a conserved pattern of regeneration also known from other lizards.

Microscopic Characterization of the Mucous Cells and Their Mucin Secretions in the Alimentary Canal of the Blackmouth Catshark Galeus melastomus (Chondrichthyes: Elasmobranchii)

Sharks belong to the most primitive group of jawed vertebrates and have some special structural and functional features such as a cartilaginous skeleton, a spiral intestinal valve, and a rectal gland for osmoregulation. In January 2020, ten specimens of Galeus melastomus, the Blackmouth catshark, were collected from the Gulf of Asinara (North Sardinia, Italy) and the entire alimentary canal was studied using histochemical reactions to characterize the mucous cell types. In the alimentary canal of G. melastomus, mucous cells mainly secrete a mixture of acidic and neutral mucins. Of the acidic mucins, only the carboxylated type was present in mucous cells of the stomach, while the sulfated type predominated in the esophagus and the intestines. The use of lectins revealed a distribution of sugar residues in mucins related to cellular activities of the different regions of the catshark alimentary canal. The current study is the first report to characterize the intestinal mucous cells of G. melastomus and to provide quantitative data on their different populations in the alimentary canal.