Unravelling the blood supply to the zebrafish pharyngeal jaws and teeth

Continuous tooth replacement: what can teleost fish teach us?

Most tooth-bearing non-mammalian vertebrates have the capacity to replace their teeth throughout life. This capacity was lost in mammals, which replace their teeth only once at most. Not surprisingly, continuous tooth replacement has attracted much attention. Classical morphological studies (e.g. to analyse patterns of replacement) are now being complemented by molecular studies that investigate the expression of genes involved in tooth formation. This review focuses on ray-finned fish (actinopterygians), which have teeth often distributed throughout the mouth and pharynx, and more specifically on teleost fish, the largest group of extant vertebrates. First we highlight the diversity in tooth distribution and in tooth replacement patterns. Replacement tooth formation can start from a distinct (usually discontinuous and transient) dental lamina, but also in the absence of a successional lamina, e.g. from the surface epithelium of the oropharynx or from the outer dental epithelium of a predecessor tooth. The relationship of a replacement tooth to its predecessor is closely related to whether replacement is the result of a prepattern or occurs on demand. As replacement teeth do not necessarily have the same molecular signature as first-generation teeth, the question of the actual trigger for tooth replacement is discussed. Much emphasis has been laid in the past on the potential role of epithelial stem cells in initiating tooth replacement. The outcome of such studies has been equivocal, possibly related to the taxa investigated, and the permanent or transient nature of the dental lamina. Alternatively, replacement may result from local proliferation of undifferentiated progenitors, stimulated by hitherto unknown, perhaps mesenchymal, factors. So far, the role of the neurovascular link in continuous tooth replacement has been poorly investigated, despite the presence of a rich vascularisation surrounding actinopterygian (as well as chondrichthyan) teeth and despite a complete arrest of tooth replacement after nerve resection. Lastly, tooth replacement is possibly co-opted as a process to expand the number of teeth in a dentition ontogenetically whilst conserving features of the primary dentition. That neither a dental lamina, nor stem cells appear to be required for tooth replacement places teleosts in an advantageous position as models for tooth regeneration in humans, where the dental lamina regresses and epithelial stem cells are considered lost.

Innervation and osteoclast distribution in the inferior pharyngeal jaw of the cichlid Nile tilapia (Oreochromis niloticus)

In addition to an oral jaw, cichlids have a pharyngeal jaw, which is used for crushing and processing captured prey. The teeth and morphology of the pharyngeal jaw bones adapt to changes in prey in response to changes in the growing environment. This study aimed to explore the possible involvement of the peripheral nervous system in remodeling the cichlid pharyngeal jaw by examining the innervation of the inferior pharyngeal jaw in the Nile tilapia, Oreochromis niloticus. Vagal innervation was identified in the Nile tilapia inferior pharyngeal jaw. Double staining with tartrate-resistant acid phosphatase and immunostaining with the neuronal markers, protein gene product 9.5, and acetylated tubulin, revealed that osteoclasts, which play an important role in remodeling, were distributed in the vicinity of the nerves and were in apposition with the nerve terminals. This contact between peripheral nerves and osteoclasts suggests that the peripheral nervous system may play a role in remodeling the inferior pharyngeal jaw in cichlids.

Dimethenamid promotes oxidative stress and apoptosis leading to cardiovascular, hepatic, and pancreatic toxicities in zebrafish embryo

Dimethenamid, one of the acetamide herbicides, is widely used on soybeans and corns to inhibit weed growth. Although other acetamide herbicides have been reported to have several toxicities in non-target organisms including developmental toxicity, the toxicity of dimethenamid has not yet been studied. In this research, we utilized the zebrafish animal model to verify the developmental toxicity of dimethenamid. It not only led to morphological abnormalities in zebrafish larvae but also reduced their viability. ROS production and inflammation responses were promoted in zebrafish larvae. Also, uncontrolled apoptosis occurred when the gene expression level related to the cell cycle and apoptosis was altered by dimethenamid. These changes resulted in toxicities in the cardiovascular system, liver, and pancreas are observed in transgenic zebrafish models including fli1a:EGFP and L-fabp:dsRed;elastase:GFP. Dimethenamid triggered morphological defects in the heart and vasculature by altering the mRNA levels related to cardiovascular development. The liver and pancreas were also damaged through not only the changes of their morphology but also through the dysregulation in their function related to metabolic activity. This study shows the developmental defects induced by dimethenamid in zebrafish larvae and the possibility of toxicity in other non-target organisms.

Developmental defects induced by thiabendazole are mediated via apoptosis, oxidative stress and alteration in PI3K/Akt and MAPK pathways in zebrafish

Thiabendazole, a benzimidazole fungicide, is widely used to prevent yield loss in agricultural land by inhibiting plant diseases derived from fungi. As thiabendazole has a stable benzimidazole ring structure, it remains in the environment for an extended period, and its toxic effects on non-target organisms have been reported, indicating the possibility that it could threaten public health. However, little research has been conducted to elucidate the comprehensive mechanisms of its developmental toxicity. Therefore, we used zebrafish, a representative toxicological model that can predict toxicity in aquatic organisms and mammals, to demonstrate the developmental toxicity of thiabendazole. Various morphological malformations were observed, including decreased body length, eye size, and increased heart and yolk sac edema. Apoptosis, reactive oxygen species (ROS) production, and inflammatory response were also triggered by thiabendazole exposure in zebrafish larvae. Furthermore, PI3K/Akt and MAPK signaling pathways important for appropriate organogenesis were significantly changed by thiabendazole. These results led to toxicity in various organs and a reduction in the expression of related genes, including cardiovascular toxicity, neurotoxicity, and hepatic and pancreatic toxicity, which were detected in flk1:eGFP, olig2:dsRED, and L-fabp:dsRed;elastase:GFP transgenic zebrafish models, respectively. Overall, this study partly determined the developmental toxicity of thiabendazole in zebrafish and provided evidence of the environmental hazards of this fungicide.

Triadimenol promotes the production of reactive oxygen species and apoptosis with cardiotoxicity and developmental abnormalities in zebrafish

Various types of fungicides, especially triazole fungicides, are used to prevent fungal diseases on farmlands. However, the developmental toxicity of one of the triazole fungicides, triadimenol, remains unclear. Therefore, we used the zebrafish animal model, a representative toxicological model, to investigate it. Triadimenol induced morphological alterations in the eyes and body length along with yolk sac and heart edema. It also stimulated the production of reactive oxygen species and expression of inflammation-related genes and caused apoptosis in the anterior regions of zebrafish, especially in the heart. The phosphorylation levels of Akt, ERK, JNK, and p38 proteins involved in the PI3K and MAPK pathways, which are important for the development process, were also reduced by triadimenol. These changes led to malformation of the heart and vascular structures, as observed in the flk1:eGFP transgenic zebrafish models and a reduction in the heart rate. In addition, the expression of genes associated with cardiac and vascular development was also reduced. Therefore, we elucidated the mechanisms associated with triadimenol toxicity that leads to various abnormalities and developmental toxicity in zebrafish.

High-Resolution Histology for Craniofacial Studies on Zebrafish and Other Teleost Models

In the era of molecular biology, identification of cells and even tissues mostly relies on the presence of fluorescent tags, or of "marker gene" expression. We list a number of caveats and present a protocol for embedding, sectioning, and staining semithin plastic sections. The method is neither new nor innovative, but is meant to revive skills that tend to get lost.This easy-to-use and inexpensive protocol (1) yields high-resolution images in transmitted and polarized light, (2) can be utilized simultaneously for transmission electron microscopy, and (3) is applicable to any type of material (wild type, morphants, mutants, transgenic, or pharmacologically treated animals as well as all of their controls), provided the sample size is kept under a limit. Thus, we hope to encourage researchers to use microanatomy and histology to complement molecular studies investigating, e.g., gene function.

Zebrafish facial lymphatics develop through sequential addition of venous and non-venous progenitors

Lymphatic vessels are known to be derived from veins; however, recent lineage-tracing experiments propose that specific lymphatic networks may originate from both venous and non-venous sources. Despite this, direct evidence of a non-venous lymphatic progenitor is missing. Here, we show that the zebrafish facial lymphatic network is derived from three distinct progenitor populations that add sequentially to the developing facial lymphatic through a relay-like mechanism. We show that while two facial lymphatic progenitor populations are venous in origin, the third population, termed the ventral aorta lymphangioblast (VA-L), does not sprout from a vessel; instead, it arises from a migratory angioblast cell near the ventral aorta that initially lacks both venous and lymphatic markers, and contributes to the facial lymphatics and the hypobranchial artery. We propose that sequential addition of venous and non-venous progenitors allows the facial lymphatics to form in an area that is relatively devoid of veins. Overall, this study provides conclusive, live imaging-based evidence of a non-venous lymphatic progenitor and demonstrates that the origin and development of lymphatic vessels is context-dependent.

Making maxillary barbels with a proximal-distal gradient of Wnt signals in matrix-bound mesenchymal cells

The evolution of specific appendages is made possible by the ontogenetic deployment of general cell signaling pathways. Many fishes, amphibians and reptiles have unique skin appendages known as barbels, which are poorly understood at the cellular and molecular level. In this study, we examine the cell arrangements, cell division patterns, and gene expression profiles associated with the zebrafish maxillary barbel, or ZMB. The earliest cellular organization of the ZMB is an internal whorl of mesenchymal cells in the dermis of the maxilla; there is no epithelial placode, nor any axially-elongated epithelial cells as expected of an apical ectodermal ridge (AER). As the ZMB develops, cells in S-phase are at first distributed randomly throughout the appendage, gradually transitioning to a proliferative population concentrated at the distal end. By observing ZMB ontogenetic stages in a Wnt-responsive transgenic reporter line, TCFsiam, we identified a strongly fluorescent mesenchymal cell layer within these developing appendages. Using an in vitro explant culture technique on developing barbel tissues, we co-localized the fluorescent label in these cells with the mitotic marker EdU. Surprisingly, the labeled cells showed little proliferation, indicating a slow-cycling subpopulation. Transmission electron microscopy of the ZMB located these cells in a single, circumferential layer within the barbel's matrix core. Morphologically, these cells resemble fibroblasts or osteoblasts; in addition to their matrix-bound location, they are identified by their pancake-shaped nuclei, abundant rough endoplasmic reticulum, and cytoplasmic extensions into the surrounding extracellular matrix. Taken together, these features define a novel mesenchymal cell population in zebrafish, the "TCF(+) core cells." A working model of barbel development is proposed, in which these minimally mitotic mesodermal cells produce collagenous matrix in response to ectodermally-derived Wnt signals deployed in a proximal-distal gradient along the appendage. This documents a novel mechanism of vertebrate appendage outgrowth. Similar genetic signals and cell behaviors may be responsible for the independent and repeated evolution of barbel structures in other fish species.

The innervation of the zebrafish pharyngeal jaws and teeth

Zebrafish (Danio rerio) teeth are increasingly used as a model to study odontogenesis in non-mammalians. Using serial semi-thin section histology and immunohistochemistry, the nerves innervating the pharyngeal jaws and teeth have been identified. The last pair of branchial arches, which are non-gill bearing but which carry the teeth, are innervated by an internal branch of a post-trematic ramus of the vagal nerve. Another, external, branch is probably responsible for the motor innervation of the branchiomeric musculature. Nerve fibres appear in the pulp cavity of the teeth only late during cytodifferentiation, and are therefore likely not involved in early steps of tooth formation. The precise role of the nervous system during continuous tooth replacement remains to be determined. Nonetheless, this study provides the necessary morphological background information to address this question.

Blocking VEGF signaling delays development of replacement teeth in zebrafish

The dentition in zebrafish is extremely and richly vascularized, but the function of the vasculature, in view of the continuous replacement of the teeth, remains elusive. Through application of SU5416, a vascular endothelial growth factor receptor inhibitor, we studied the role of the blood vessels in the dentition of the zebrafish. We were unable to show an effect on the development of first-generation teeth as well as first tooth replacement. However, in juvenile fish, a delay was observed in the developmental state of the replacement tooth compared with what was expected based on the maturation state of the functional tooth. Furthermore, we observed a difference between treated and nontreated fish in the distance between blood vessels and developing replacement teeth. In conclusion, our results provide support for a nutritive, rather than an inductive, function of the vasculature in the process of tooth development and replacement.

Anatomy and ontogeny of a novel hemodynamic organ in zebrafish

The zebrafish maxillary barbel can protract and retract in response to stimuli, and appears connected to a prominent blood sinus on the lateral aspect of the maxillary bone. However, the mechanism of barbel movement is not described. Using whole-mount phalloidin staining of the sinus region, we observed long filamentous actin cables, suggesting highly organized vascular smooth muscle cells, surrounding an endothelial chamber. Although the chamber is variably filled by erythrocytes in vivo, cardiac injection of fluorescent dextrans shows that it consistently contains plasma. Full-thickness confocal imaging of dextran-injected adults containing EGFP(+) endothelial cells revealed a vascular complex with three compartments, here named the distal bulb, central chamber, and accessory chamber. The early ontogeny of all three compartments was confirmed in a whole-mount series of Tg(fli1a:EGFP) juveniles. In wild type adults, the fine structure of each chamber was studied using paraffin- and plastic-section histochemistry and transmission electron microscopy. The distal bulb and central chamber have smooth muscle coats with luminally-elongated septa, forming semi-detached blood-filled lacunae. The central chamber walls and septa are extensively innervated by small, unmyelinated axons, as confirmed by immunohistochemical detection of acetylated tubulin, a component of axonal cytoplasm. The accessory chamber appears neither innervated nor muscularized, but is an endothelial cul-de-sac with a thickened elastic adventitia, suggesting an extensible fluid reservoir. We propose that we have identified a new organ in zebrafish, the maxillary barbel blood sinus, whose neurovascular specializations may contribute to zebrafish sensory biology and appendage control.