Development of cartilage and bone tissues of the anterior part of the mandible in cichlid fish: a light and TEM study

Zebrafish endochondral growth zones as they relate to human bone size, shape and disease

Research on the genetic mechanisms underlying human skeletal development and disease have largely relied on studies in mice. However, recently the zebrafish has emerged as a popular model for skeletal research. Despite anatomical differences such as a lack of long bones in their limbs and no hematopoietic bone marrow, both the cell types in cartilage and bone as well as the genetic pathways that regulate their development are remarkably conserved between teleost fish and humans. Here we review recent studies that highlight this conservation, focusing specifically on the cartilaginous growth zones (GZs) of endochondral bones. GZs can be unidirectional such as the growth plates (GPs) of long bones in tetrapod limbs or bidirectional, such as in the synchondroses of the mammalian skull base. In addition to endochondral growth, GZs play key roles in cartilage maturation and replacement by bone. Recent studies in zebrafish suggest key roles for cartilage polarity in GZ function, surprisingly early establishment of signaling systems that regulate cartilage during embryonic development, and important roles for cartilage proliferation rather than hypertrophy in bone size. Despite anatomical differences, there are now many zebrafish models for human skeletal disorders including mutations in genes that cause defects in cartilage associated with endochondral GZs. These point to conserved developmental mechanisms, some of which operate both in cranial GZs and limb GPs, as well as others that act earlier or in parallel to known GP regulators. Experimental advantages of zebrafish for genetic screens, high resolution live imaging and drug screens, set the stage for many novel insights into causes and potential therapies for human endochondral bone diseases.

Cells at the Edge: The Dentin-Bone Interface in Zebrafish Teeth

Bone-producing osteoblasts and dentin-producing odontoblasts are closely related cell types, a result from their shared evolutionary history in the ancient dermal skeleton. In mammals, the two cell types can be distinguished based on histological characters and the cells’ position in the pulp cavity or in the tripartite periodontal complex. Different from mammals, teleost fish feature a broad diversity in tooth attachment modes, ranging from fibrous attachment to firm ankylosis to the underlying bone. The connection between dentin and jaw bone is often mediated by a collar of mineralized tissue, a part of the dental unit that has been termed “bone of attachment”. Its nature (bone, dentin, or an intermediate tissue type) is still debated. Likewise, there is a debate about the nature of the cells secreting this tissue: osteoblasts, odontoblasts, or yet another (intermediate) type of scleroblast. Here, we use expression of the P/Q rich secretory calcium-binding phosphoprotein 5 (scpp5) to characterize the cells lining the so-called bone of attachment in the zebrafish dentition. scpp5 is expressed in late cytodifferentiation stage odontoblasts but not in the cells depositing the “bone of attachment”. nor in bona fide osteoblasts lining the supporting pharyngeal jaw bone. Together with the presence of the osteoblast marker Zns-5, and the absence of covering epithelium, this links the cells depositing the “bone of attachment” to osteoblasts rather than to odontoblasts. The presence of dentinal tubule-like cell extensions and the near absence of osteocytes, nevertheless distinguishes the “bone of attachment” from true bone. These results suggest that the “bone of attachment” in zebrafish has characters intermediate between bone and dentin, and, as a tissue, is better termed “dentinous bone”. In other teleosts, the tissue may adopt different properties. The data furthermore support the view that these two tissues are part of a continuum of mineralized tissues. Expression of scpp5 can be a valuable tool to investigate how differentiation pathways diverge between osteoblasts and odontoblasts in teleost models and help resolving the evolutionary history of tooth attachment structures in actinopterygians.

Miniaturization: How many cells are needed to build a tooth?

BACKGROUND

Organs that develop early in life, and are replaced by a larger version as the animal grows, often represent a miniature version of the adult organ. Teeth constituting the first functional dentition in small-sized teleost fish, such as medaka (Oryzias latipes), are examples of such miniature organs. With a dentin cone as small as the size of one human cell, or even smaller, these teeth raise the question how many dentin-producing cells (odontoblasts) are required to build such a tooth, and whether this number can be as little as one.

RESULTS

Based on detailed observations with transmission electron microscopy (TEM) and TEM-based 3D-reconstructions, we show that only one mesenchymal cell qualifies as a true odontoblast. A second mesenchymal cell potentially participates in dentin formation, but only at a late stage of tooth development. Moreover, the fate of these cells appears to be specified very early during tooth development.

CONCLUSIONS

Our observations indicate that in this system, one single odontoblast fulfills roles normally exerted by a large and communicating cell population. First-generation teeth in medaka thus provide an exciting model to study integration of multiple functions into a single cell.

Mucin histochemistry as a tool to assess rostral digestive tract health in a teleost model (Danio rerio)

The zebrafish (Danio rerio), a small agastric teleost fish, has become a popular species to model diseases of the gastrointestinal tract. Remarkably, its rostral digestive tract has been largely understudied. To allow for accurate reporting and to improve the translational potential of this research model, the present study first proposes terminology for the different regions of the rostral digestive tract in zebrafish. Based on markers (in particular mucin histochemistry) which are easy to apply in routine histopathology laboratories, five regions are identified, in line with descriptions for carp, a close relative of zebrafish. The mucin histochemical staining is specific and distinct in each region identified, and can be used as a tool to assess health of the rostral digestive tract. In addition, we identify a transitional zone between the oesophagus and intestinal bulb which may represent a remnant of a rudimentary stomach.

Multiple epithelia are required to develop teeth deep inside the pharynx

To explain the evolutionary origin of vertebrate teeth from odontodes, it has been proposed that competent epithelium spread into the oropharyngeal cavity via the mouth and other possible channels such as the gill slits [Huysseune et al., 2009, J. Anat. 214, 465-476]. Whether tooth formation deep inside the pharynx in extant vertebrates continues to require external epithelia has not been addressed so far. Using zebrafish we have previously demonstrated that cells derived from the periderm penetrate the oropharyngeal cavity via the mouth and via the endodermal pouches and connect to periderm-like cells that subsequently cover the entire endoderm-derived pharyngeal epithelium [Rosa et al., 2019, Sci. Rep. 9, 10082]. We now provide conclusive evidence that the epithelial component of pharyngeal teeth in zebrafish (the enamel organ) is derived from medial endoderm, as hitherto assumed based on position deep in the pharynx. Yet, dental morphogenesis starts only after the corresponding endodermal pouch (pouch 6) has made contact with the skin ectoderm, and only after periderm-like cells have covered the prospective tooth-forming endodermal epithelium. Manipulation of signaling pathways shown to adversely affect tooth development indicates they act downstream of these events. We demonstrate that pouch-ectoderm contact and the presence of a periderm-like layer are both required, but not sufficient, for tooth initiation in the pharynx. We conclude that the earliest interactions to generate pharyngeal teeth encompass those between different epithelial populations (skin ectoderm, endoderm, and periderm-like cells in zebrafish), in addition to the epithelial-mesenchymal interactions that govern the formation of all vertebrate teeth.

Periderm invasion contributes to epithelial formation in the teleost pharynx

The gnathostome pharyngeal cavity functions in food transport and respiration. In amniotes the mouth and nares are the only channels allowing direct contact between internal and external epithelia. In teleost fish, gill slits arise through opening of endodermal pouches and connect the pharynx to the exterior. Using transgenic zebrafish lines, cell tracing, live imaging and different markers, we investigated if pharyngeal openings enable epithelial invasion and how this modifies the pharyngeal epithelium. We conclude that in zebrafish the pharyngeal endoderm becomes overlain by cells with a peridermal phenotype. In a wave starting from pouch 2, peridermal cells from the outer skin layer invade the successive pouches until halfway their depth. Here the peridermal cells connect to a population of cells inside the pharyngeal cavity that express periderm markers, yet do not invade from outside. The latter population expands along the midline from anterior to posterior until the esophagus-gut boundary. Together, our results show a novel role for the periderm as an internal epithelium becomes adapted to function as an external surface.

Ultrastructural morphology of the envelope of Dover sole Solea solea eggs from fertilization until hatching with emphasis on sample preparation

This study is the first to describe the ultrastructural morphology of the envelope of Solea solea eggs from fertilisation until hatching. Defining the ultrastructural morphology of fish eggs is important for species identification and may assist in predicting the effect of external influences on these early life stages. In first instance, various fixation and embedding protocols were assessed to explore the morphology of the egg envelope, whereby the encountered difficulties were highlighted. The successful protocol for SEM proved to be combined fixation with 4% glutaraldehyde in 0.1M cacodylate buffer for minimum 4h with post-fixation of 2h with 1% OsO₄ in 0.1M cacodylate buffer. For TEM, puncturing the egg envelope during the first steps of the fixation protocol was necessary to allow the embedding medium to penetrate through the egg envelope. Based on both scanning and transmission electron microscopical examination, three distinct layers were discerned in the egg envelope. During the development of the fish embryo, a change in the outer structure of the egg was observed. Scanning electron microscopical examination of one day post-fertilisation eggs (DPF) revealed a homogeneous outer layer, displaying a large number of pores uniformly distributed on the surface of the egg envelope. Starting from 2 DPF parts of the outermost layer or two outer layers peeled off. The second deeper layer showed larger pores, with less defined edges. In the third innermost layer irregular indentations were noted. On transmission electron microscopy the first outermost layer of 1 DPF eggs clearly folded into the pores. The second layer was more electron dense, had a uniform appearance and did not cover the surface of the pores. The third innermost layer was much thicker and possessed indentations. A total number of 12 undulating zones were discriminated based on different degrees of electron density. Prior to hatching, the compact structure of the innermost layer was distorted by dispersed holes and tears.

Vertebral Development and Ossification in the Siberian Sturgeon (Acipenser Baerii), with New Insights on Bone Histology and Ultrastructure of Vertebral Elements and Scutes

In order to improve our knowledge on the vertebral development, structure and mineralization in Acipenseriformes, we undertook a study in a growth series of reared Siberian sturgeons (Acipenser baerii) using in toto clear and stain specimens, histological and ultrastructural observations, X-ray micro-tomography, and solid state NMR analyses. Scutes were also studied to compare the tissue structure and mineralization of endoskeletal and dermal skeletal elements. This study completes and clarifies previous investigations on vertebral development and architecture in sturgeons, and brings original data on the structure of (i) the perichondral bone that is progressively deposited around the vertebral elements during ontogeny, (ii) the typical cartilage composing these elements, and (iii) the scutes. In addition we provide data on the mineralization process, on the nature of the bone mineral phase, and on the growth dynamics of the vertebral elements. Anat Rec, 300:437-449, 2017. © 2016 Wiley Periodicals, Inc.

Small teleost fish provide new insights into human skeletal diseases

Small teleost fish such as zebrafish and medaka are increasingly studied as models for human skeletal diseases. Efficient new genome editing tools combined with advances in the analysis of skeletal phenotypes provide new insights into fundamental processes of skeletal development. The skeleton among vertebrates is a highly conserved organ system, but teleost fish and mammals have evolved unique traits or have lost particular skeletal elements in each lineage. Several unique features of the skeleton relate to the extremely small size of early fish embryos and the small size of adult fish used as models. A detailed analysis of the plethora of interesting skeletal phenotypes in zebrafish and medaka pushes available skeletal imaging techniques to their respective limits and promotes the development of new imaging techniques. Impressive numbers of zebrafish and medaka mutants with interesting skeletal phenotypes have been characterized, complemented by transgenic zebrafish and medaka lines. The advent of efficient genome editing tools, such as TALEN and CRISPR/Cas9, allows to introduce targeted deficiencies in genes of model teleosts to generate skeletal phenotypes that resemble human skeletal diseases. This review will also discuss other attractive aspects of the teleost skeleton. This includes the capacity for lifelong tooth replacement and for the regeneration of dermal skeletal elements, such as scales and fin rays, which further increases the value of zebrafish and medaka models for skeletal research.

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.

Wnt signaling during tooth replacement in zebrafish (Danio rerio): pitfalls and perspectives

The canonical (β-catenin dependent) Wnt signaling pathway has emerged as a likely candidate for regulating tooth replacement in continuously renewing dentitions. So far, the involvement of canonical Wnt signaling has been experimentally demonstrated predominantly in amniotes. These studies tend to show stimulation of tooth formation by activation of the Wnt pathway, and inhibition of tooth formation when blocking the pathway. Here, we report a strong and dynamic expression of the soluble Wnt inhibitor dickkopf1 (dkk1) in developing zebrafish (Danio rerio) tooth germs, suggesting an active repression of Wnt signaling during morphogenesis and cytodifferentiation of a tooth, and derepression of Wnt signaling during start of replacement tooth formation. To further analyse the role of Wnt signaling, we used different gain-of-function approaches. These yielded disjunct results, yet none of them indicating enhanced tooth replacement. Thus, masterblind (mbl) mutants, defective in axin1, mimic overexpression of Wnt, but display a normally patterned dentition in which teeth are replaced at the appropriate times and positions. Activating the pathway with LiCl had variable outcomes, either resulting in the absence, or the delayed formation, of first-generation teeth, or yielding a regular dentition with normal replacement, but no supernumerary teeth or accelerated tooth replacement. The failure so far to influence tooth replacement in the zebrafish by perturbing Wnt signaling is discussed in the light of (i) potential technical pitfalls related to dose- or time-dependency, (ii) the complexity of the canonical Wnt pathway, and (iii) species-specific differences in the nature and activity of pathway components. Finally, we emphasize the importance of in-depth knowledge of the wild-type pattern for reliable interpretations. It is hoped that our analysis can be inspiring to critically assess and elucidate the role of Wnt signaling in tooth development in polyphyodonts.

Unravelling the blood supply to the zebrafish pharyngeal jaws and teeth

We describe the vascular supply to the pharyngeal jaws and teeth in zebrafish, from larval stages to juveniles, using serial high quality semithin sections and 3D reconstructions. We have identified that the arterial blood supply to the last pair of branchial arches, which carries the teeth, issues from the hypobranchial artery. Surprisingly, the arteries supplying the pharyngeal jaws show an asymmetric branching pattern that is modified over ontogeny. Moreover, the blood vessel pattern that serves each jaw can best be described as a sinusoidal cavity encircling the bases of both the functional and replacement teeth. Capillaries branching from this sinusoidal cavity enter the pulp and constitute the intrinsic blood supply to the attached teeth. The role of these blood vessels during tooth development (whether instructive or nutritive) remains to be determined and requires further study. However, we have provided a firm morphological basis that will aid in the interpretation of experiments addressing this question.

Distinct patterns of notochord mineralization in zebrafish coincide with the localization of Osteocalcin isoform 1 during early vertebral centra formation

BACKGROUND

In chondrichthyans, basal osteichthyans and tetrapods, vertebral bodies have cartilaginous anlagen that subsequently mineralize (chondrichthyans) or ossify (osteichthyans). Chondrocytes that form the vertebral centra derive from somites. In teleost fish, vertebral centrum formation starts in the absence of cartilage, through direct mineralization of the notochord sheath. In a second step, the notochord is surrounded by somite-derived intramembranous bone. In several small teleost species, including zebrafish (Danio rerio), even haemal and neural arches form directly as intramembranous bone and only modified caudalmost arches remain cartilaginous. This study compares initial patterns of mineralization in different regions of the vertebral column in zebrafish. We ask if the absence or presence of cartilaginous arches influences the pattern of notochord sheath mineralization.

RESULTS

To reveal which cells are involved in mineralization of the notochord sheath we identify proliferating cells, we trace mineralization on the histological level and we analyze cell ultrastructure by TEM. Moreover, we localize proteins and genes that are typically expressed by skeletogenic cells such as Collagen type II, Alkaline phosphatase (ALP) and Osteocalcin (Oc). Mineralization of abdominal and caudal vertebrae starts with a complete ring within the notochord sheath and prior to the formation of the bony arches. In contrast, notochord mineralization of caudal fin centra starts with a broad ventral mineral deposition, associated with the bases of the modified cartilaginous arches. Similar, arch-related, patterns of mineralization occur in teleosts that maintain cartilaginous arches throughout the spine.Throughout the entire vertebral column, we were able to co-localize ALP-positive signal with chordacentrum mineralization sites, as well as Collagen II and Oc protein accumulation in the mineralizing notochord sheath. In the caudal fin region, ALP and Oc signals were clearly produced both by the notochord epithelium and cells outside the notochord, the cartilaginous arches. Based on immunostaining, real time PCR and oc2:gfp transgenic fish, we identify Oc in the mineralizing notochord sheath as osteocalcin isoform 1 (Oc1).

CONCLUSIONS

If notochord mineralization occurs prior to arch formation, mineralization of the notochord sheath is ring-shaped. If notochord mineralization occurs after cartilaginous arch formation, mineralization of the notochord sheath starts at the insertion point of the arches, with a basiventral origin. The presence of ALP and Oc1, not only in cells outside the notochord, but also in the notochord epithelium, suggests an active role of the notochord in the mineralization process. The same may apply to Col II-positive chondrocytes of the caudalmost haemal arches that show ALP activity and Oc1 accumulation, since these chondrocytes do not mineralize their own cartilage matrix. Even without cartilaginous preformed vertebral centra, the cartilaginous arches may have an inductive role in vertebral centrum formation, possibly contributing to the distinct mineralization patterns of zebrafish vertebral column and caudal fin vertebral fusion.

Conditional ablation of osteoblasts in medaka

Different from tetrapods, teleost vertebral centra form without prior establishment of a cartilaginous scaffold, in two steps: First, mineralization of the notochord sheath establishes the vertebral centra. Second, sclerotome derived mesenchymal cells migrate around the notochord sheath. These cells differentiate into osteoblasts and deposit bone onto the mineralized notochord sheath in a process of intramembranous bone formation. In contrast, most skeletal elements of the cranial skeleton arise by chondral bone formation, with remarkably similar mechanisms in fish and tetrapods. To further investigate the role of osteoblasts during formation of the cranial and axial skeleton, we generated a transgenic osx:CFP-NTR medaka line which enables conditional ablation of osterix expressing osteoblasts. By expressing a bacterial nitroreductase (NTR) fused to Cyan Fluorescent Protein (CFP) under control of the osterix promoter these cells become sensitive towards Metronidazole (Mtz). Mtz treatment of stable osx:CFP-NTR transgenic medaka for several consecutive days led to significant loss of osteoblasts by apoptosis. Live staining of mineralized bone matrix revealed reduced ossification in head skeletal elements such as cleithrum and operculum, as well as in the vertebral arches. Interestingly in Mtz treated larvae, intervertebral spaces were missing and the notochord sheath was often continuously mineralized resulting in the fusion of centra. We therefore propose a dual role for osx-positive osteoblasts in fish. Besides a role in bone deposition, we suggest an additional border function during mineralization of the chordal centra. After termination of Mtz treatment, osteoblasts gradually reappeared, indicating regenerative properties in this cell lineage. Taken together, the osx:CFP-NTR medaka line represents a valuable tool to study osteoblast function and regeneration at different stages of development in whole vertebrate specimens in vivo.

A comparative view on mechanisms and functions of skeletal remodelling in teleost fish, with special emphasis on osteoclasts and their function

Resorption and remodelling of skeletal tissues is required for development and growth, mechanical adaptation, repair, and mineral homeostasis of the vertebrate skeleton. Here we review for the first time the current knowledge about resorption and remodelling of the skeleton in teleost fish, the largest and most diverse group of extant vertebrates. Teleost species are increasingly used in aquaculture and as models in biomedical skeletal research. Thus, detailed knowledge is required to establish the differences and similarities between mammalian and teleost skeletal remodelling, and between distantly related species such as zebrafish (Danio rerio) and medaka (Oryzias latipes). The cellular mechanisms of differentiation and activation of osteoclasts and the functions of teleost skeletal remodelling are described. Several characteristics, related to skeletal remodelling, distinguish teleosts from mammals. These characteristics include (a) the absence of osteocytes in most species; (b) the absence of haematopoietic bone marrow tissue; (c) the abundance of small mononucleated osteoclasts performing non-lacunar (smooth) bone resorption, in addition to or instead of multinucleated osteoclasts; and (d) a phosphorus- rather than calcium-driven mineral homeostasis (mainly affecting the postcranial dermal skeleton). Furthermore, (e) skeletal resorption is often absent from particular sites, due to sparse or lacking endochondral ossification. Based on the mode of skeletal remodelling in early ontogeny of all teleosts and in later stages of development of teleosts with acellular bone we suggest a link between acellular bone and the predominance of mononucleated osteoclasts, on the one hand, and cellular bone and multinucleated osteoclasts on the other. The evolutionary origin of skeletal remodelling is discussed and whether mononucleated osteoclasts represent an ancestral type of resorbing cells. Revealing the differentiation and activation of teleost skeletal resorbing cells, in the absence of several factors that trigger mammalian osteoclast differentiation, is a current challenge. Understanding which characters of teleost bone remodelling are derived and which characters are conserved should enhance our understanding of the process in fish and may provide insights into alternative pathways of bone remodelling in mammals.

MicroRNAs and micromanaging the skeleton in disease, development and evolution

MicroRNAs (miRNAs) are short, non-protein-encoding RNAs that effect post-transcriptional gene regulation by targeting messenger RNAs. miRNAs are associated with specific human diseases and help regulate development. Here we review recent advances in understanding the roles of miRNAs in skeletal malformations, including cleft palate, and in the evolution of skeletal morphologies. We propose the hypothesis that evolutionary variation in miRNA expression patterns or structural variation in miRNA binding sites in messenger RNAs can help explain the evolution of craniofacial variation among species, the development of human craniofacial disease and physiological changes leading to osteopenia that increases with ageing.

Bone development in the jaw of Nile tilapia Oreochromis niloticus (Pisces: Cichlidae)

East African cichlids have evolved feeding apparatus morphologies adapted to their diverse feeding behaviors. The evolution of the oral jaw morphologies is accomplished by the diversity of bone formation during development. To further understand this evolutionary process, we examined the skeletal elements of the jaw and their temporal and sequential emergence, categorized by developmental stages, using the Nile tilapia Oreochromis niloticus as a model cichlid. We found that chondrogenesis started in Stage 17. The deposition of osteoid for the dermal bones commenced in Stage 18. The uptake of calcium dramatically shifted from the surface of larvae to the gills in Stage 20. The bone mineralization of the skeleton began in Stage 25. These data provide important information regarding the sequential events of craniofacial development in East African cichlids and lay the groundwork for studying the molecular mechanisms underlying adaptation of jaw structure to feeding behavior.

Skeletogenesis in the swell shark Cephaloscyllium ventriosum

Extant chondrichthyans possess a predominantly cartilaginous skeleton, even though primitive chondrichthyans produced bone. To gain insights into this peculiar skeletal evolution, and in particular to evaluate the extent to which chondrichthyan skeletogenesis retains features of an osteogenic programme, we performed a histological, histochemical and immunohistochemical analysis of the entire embryonic skeleton during development of the swell shark Cephaloscyllium ventriosum. Specifically, we compared staining properties among various mineralizing tissues, including neural arches of the vertebrae, dermal tissues supporting oral denticles and Meckel's cartilage of the lower jaw. Patterns of mineralization were predicted by spatially restricted alkaline phosphatase activity earlier in development. Regarding evidence for an osteogenic programme in extant sharks, a mineralized tissue in the perichondrium of C. ventriosum neural arches, and to a lesser extent a tissue supporting the oral denticle, displayed numerous properties of bone. Although we uncovered many differences between tissues in Meckel's cartilage and neural arches of C. ventriosum, both elements impart distinct tissue characteristics to the perichondral region. Considering the evolution of osteogenic processes, shark skeletogenesis may illuminate the transition from perichondrium to periosteum, which is a major bone-forming tissue during the process of endochondral ossification.

Establishment, maintenance and modifications of the lower jaw dentition of wild Atlantic salmon (Salmo salar L.) throughout its life cycle

In this paper we elucidate the pattern of initiation of the first teeth and the pattern of tooth replacement on the dentary of wild Atlantic salmon (Salmo salar L.), throughout nearly all stages of its life cycle, using serially sectioned heads and jaws, cleared and stained animals, and X-rays. The dentary teeth are set in one row. Tooth germs appear around hatching, first in odd positions, followed by even positions. From position 8 further backwards, teeth are added in adjacent positions. The first replacement teeth appear in animals of about 30 mm fork length. On the dentary of early life stages (alevins and fry), every position in the tooth row holds a functional (i.e. attached and erupted) tooth and a replacement tooth. The alternating pattern set up anteriorly in the dentary by the first-generation teeth changes in juveniles (parr) whereby teeth are in a similar functional (for the erupted teeth) or developmental stage (for the replacement teeth) every three positions. This pattern is also observed in marine animals during their marine life phase and in both sexes of adult animals prior to spawning (grilse and salmon), but every position now holds either a functional tooth or a mineralised replacement tooth. This is likely due to the fact that replacement tooth germs have to grow to a larger size before mineralisation starts. In the following spring, the dentary tooth pattern of animals that have survived spawning (kelts) is highly variable. The abundance of functional teeth in post-spawning animals nevertheless indicates that teeth are not lost over winter. We confirm the earlier reported lack of evidence for the existence of an edentulous life phase, preceding the appearance of so-called breeding teeth during upstream migration to the spawning grounds, and consider breeding teeth to be just another tooth generation in a regularly replacing dentition. This study shows how Atlantic salmon maintains a functional adaptive dentition throughout its complex life cycle.

Development of the embryo, larva and early juvenile of Nile tilapia Oreochromis niloticus (Pisces: Cichlidae). Developmental staging system

We described the developmental stages for the embryonic, larval and early juvenile periods of Nile tilapia Oreochromis niloticus to elucidate sequential events of craniofacial development. Craniofacial development of cichlids, especially differentiation and morphogenesis of the pharyngeal skeleton, progresses until about 30 days postfertilization (dpf). Because there is no comprehensive report describing the sequential processes of craniofacial development up to 30 dpf, we newly defined 32 stages using a numbered staging system. For embryonic development, we defined 18 stages (stages 1-18), which were grouped into seven periods named the zygote, cleavage, blastula, gastrula, segmentation, pharyngula and hatching periods. For larval development, we defined seven stages (stages 19-25), which were grouped into two periods, early larval and late larval. For juvenile development until 30 dpf, we defined seven stages (stages 26-32) in the early juvenile period. This developmental staging system for Nile tilapia O. niloticus will benefit researchers investigating skeletogenesis throughout tilapia ontogeny and will also facilitate comparative evolutionary developmental biology studies of haplochromine cichlids, which comprise the species flocks of Lakes Malawi and Victoria.

Evolution of novelty in the cichlid dentition

The shape of teeth occupies a central position in various biological disciplines, from paleo-ecology to molecular biology to cosmetic and reconstructive dentistry. Despite a long tradition of study in mammals, important questions remain regarding the genetic and developmental basis of differences in tooth shape. Here, we use natural mutants of cichlid fish from East Africa, which exhibit tremendous dental diversity, to help fill the gaps in our understanding of vertebrate odontogenesis. We employ an expanded genetic linkage map to demonstrate that cusp number segregates as a gene of major effect, which explains approximately 40% of the phenotypic variance, on cichlid chromosome 5. Furthermore, we examine patterns of Bmp4 expression in early odontogenesis to address and refine predictions of models linking tooth shape and tooth number. Mutations in the Bmp4 cistron do not control tooth shape in this mapping cross. Our data suggest that the evolution of novelty in the cichlid dentition is galvanized by a small number of genetic changes, echoing similar conclusions from recent studies of other vertebrate adaptive morphologies.

Developmental mechanisms underlying tooth patterning in continuously replacing osteichthyan dentitions

The dentition of osteichthyans presents an astonishing diversity with regard to the distribution of teeth in the oral cavity, tooth numbers, arrangements, shapes, and sizes. Taking examples from three unrelated teleosts--the most speciose group of osteichthyans--and from the literature, this study explores how the initial tooth pattern is set up, and how this relates to the establishment and maintenance (or modification) of the tooth replacement pattern. In teleosts, first-generation teeth (the very first teeth in ontogeny to develop at a particular locus) are commonly initiated in adjacent or in alternate (odd and even) positions. The mechanisms responsible for these divergent developmental patterns remain to be elucidated, in particular, whether they reflect a field or local type of control. However, patterns of adjacent or alternate tooth initiation, set up by the first-generation teeth, can easily turn into replacement patterns where new teeth are initiated simultaneously every second, or even every third position, by synchronizing the formation of new first-generation teeth to the formation of replacement teeth at older loci. Our observations suggest that, once established, the replacement pattern appears to be maintained, as a kind of "default" state. Variations and modifications in this pattern are nevertheless common and suggest that tooth replacement is under local control, exerted at the level of the initiation of replacement teeth. Further studies are needed to test the hypothesis that regular replacement patterns are more frequent in association with the plesiomorphic condition of extramedullary replacement (replacement on the surface of the dentigerous bone) and more rare in the derived condition of intramedullary replacement (replacement within the medullary cavity of the dentigerous bone).

Expression of thedlx gene family during formation of the cranial bones in the zebrafish (Danio rerio): Differential involvement in the visceral skeleton and braincase

We have used dlx genes to test the hypothesis of a separate developmental program for dermal and cartilage bones within the neuro- and splanchnocranium by comparing expression patterns of all eight dlx genes during cranial bone formation in zebrafish from 1 day postfertilization (dPF) to 15 dPF. dlx genes are expressed in the visceral skeleton but not during the formation of dermal or cartilage bones of the braincase. The spatiotemporal expression pattern of all the members of the dlx gene family, support the view that dlx genes impart cellular identity to the different arches, required to make arch-specific dermal bones. Expression patterns seemingly associated with cartilage (perichondral) bones of the arches, in contrast, are probably related to ongoing differentiation of the underlying cartilage rather than with differentiation of perichondral bones themselves. Whether dlx genes originally functioned in the visceral skeleton only, and whether their involvement in the formation of neurocranial bones (as in mammals) is secondary, awaits clarification.

The cusp of evolution and development: a model of cichlid tooth shape diversity

Tooth shape is a hallmark of repeated evolutionary radiations among cichlid fishes from East Africa. Cusp shape and number vary both within populations and among closely related species with different feeding behaviors and ecologies. Here, we use histology and scanning electron microscopy to chart the developmental trajectory of tooth shape differences in fishes from Lake Malawi. We demonstrate that species with bi- or tricuspid adult (replacement) teeth initially possess a first-generation unicuspid dentition. Notably, the timing of turnover from first-generation to replacement teeth differs among species and is correlated with feeding ecology. Next, we use field data for cichlid species with adult unicuspid, bicuspid, and tricuspid teeth to demonstrate a strong and positive relationship between the number of teeth in a row and tooth shape. We discuss cichlid tooth ontogeny in the context of morphogenetic models designed to explain the developmental basis of tooth shape variation in mammals. We suggest that the dramatic differences in cichlid dentitions can be explained by variation in the expression of common activators and inhibitors acting at multiple stages of odontogenesis.

Formation of dermal skeletal and dental tissues in fish: a comparative and evolutionary approach

Osteichthyan and chondrichthyan fish present an astonishing diversity of skeletal and dental tissues that are often difficult to classify into the standard textbook categories of bone, cartilage, dentine and enamel. To address the question of how the tissues of the dermal skeleton evolved from the ancestral situation and gave rise to the diversity actually encountered, we review previous data on the development of a number of dermal skeletal elements (odontodes, teeth and dermal denticles, cranial dermal bones, postcranial dermal plates and scutes, elasmoid and ganoid scales, and fin rays). A comparison of developmental stages at the tissue level usually allows us to identify skeletogenic cell populations as either odontogenic or osteogenic on the basis of the place of formation of their dermal papillae and of the way of deposition of their tissues. Our studies support the evolutionary affinities (1) between odontodes, teeth and denticles, (2) between the ganoid scales of polypterids and the elasmoid scales of teleosts, and (3) to a lesser degree between the different bony elements. There is now ample evidence to ascertain that the tissues of the elasmoid scale are derived from dental and not from bony tissues. This review demonstrates the advantage that can be taken from developmental studies, at the tissue level, to infer evolutionary relationships within the dermal skeleton in chondrichthyans and osteichthyans.

Chondrogenesis of the branchial skeleton in embryonic sea lamprey, Petromyzon marinus

This study provides concise temporal and spatial characteristics of branchial chondrogenesis in embryonic sea lamprey, Petromyzon marinus, using high resolution light microscopy, transmission electron, and immunoelectron microscopy. Prechondrogenic condensations representing the first branchial arch appeared first in the mid-region of the third pharyngeal arch at 13 days post-fertilization (pf). Cartilage differentiation, defined by the presence of the unique, fibrillar, non-collagenous matrix protein characteristic of branchial cartilage, was first observed at 14 days pf. Development of lamprey branchial cartilage appeared unusual compared to that in jawed fishes, in that precartilage condensations appear as a one-cell wide orderly stack of flattened cells that extend by the addition of one dorsal and one ventral condensation. Development of lamprey gill arches from three condensations that fuse to form a single skeletal element differs from the developing gill arches of jawed fishes, where more than one skeletal element forms from a single condensation. The initial orderly arrangement of cells in the lamprey branchial prechondrogenic condensations remains throughout development. Once chondrification of the condensations begins, the branchial arches start to grow. Initially, growth occurs as a result of matrix secretion and cell migration. Later in development, the arches grow mainly by cell proliferation and enlargement. This study defines the morphology and timing of lamprey branchial chondrogenesis. Studies of lamprey chondrogenesis provide not only insight into the developmental biology of a unique non-collagenous cartilage in a primitive vertebrate but also into the general evolution of the skeletal system in vertebrates.

Retinoic acid given at late embryonic stage depresses sonic hedgehog and Hoxd-4 expression in the pharyngeal area and induces skeletal malformation in flounder (Paralichthys olivaceus) embryos

During the development of pharyngeal cartilages, signal molecules, including sonic hedgehog (shh) and various growth factors, as well as Hox genes are expressed in the pharyngeal area. To elucidate whether shh and Hoxd-4 function in pharyngeal cartilage formation in teleost jaw and gill primordia, spatial and temporal patterns of shh expression in flounder (Paralichthys olivaceus) embryonic pharynx were examined. The effects of retinoic acid (RA) on shh and Hoxd-4 expression and the patterning of pharyngeal cartilages were analyzed. At the prim-5 stage, when cartilage precursor cells aggregate in the pharyngeal primordia, pharyngeal endoderm expressed shh in two domains, in portions of the mandibular and hyoid primordia and in the gill primordia. After a further 40 h, shh domains expanded at the posterior edge of the endoderm of each mandibular, hyoid and gill primordium, concurrent with the growth of the primordia. A new shh expression domain appeared at the endodermal border of the mouth. Retinoic acid treatment depressed shh and Hoxd-4 expression, and also reduced the amount of expansion of the shh expression domains. Pharyngeal cartilages that formed in these embryos were malformed; their growth direction was shifted posteriorly and size was reduced. This provides the possibility that shh and Hoxd-4 regulate the growth and direction of pharyngeal cartilage precursor cells and that RA disturbs their expression, causing skeletal malformation.

Histological identification of osteocytes in the allegedly acellular bone of the sea breams Acanthopagrus australis, Pagrus auratus and Rhabdosargus sarba (Sparidae, Perciformes, Teleostei)

The bone of advanced teleost fishes such as those of the family Sparidae is said to lack osteocytes or to be acellular. Acellularity has been determined by apparent lack of osteocyte lacunae. This study questions the validity of this criterion. Scanning electron and light microscopy of paraffin and resin sections were used to show that the sides of sea bream mandibles consist of laminar parallel-fibred bone that we call tubular bone, because it contains tubules, and localised regions of Sharpey fibre bone. Osteocytes lie along the walls of tubules that also contain collagen fibril bundles (T-fibres), or in the lumens of tubules that do not contain T-fibres. We show that the osteocytes are derived from osteoblasts. The T-fibre system is different from other fibre systems that have been described. The tubules enclose wide T-fibres (lenticular in cross-section, maximum width about 8 microns) that taper at their ends and continue as thin T-fibres (round in cross-section, about 2 microns wide). The T-fibres originate in the periosteum. In mature tubular bone, spaces of increasing size develop around the osteocytes. Osteocytes are released from the bone matrix and become postosteocytes or bone-lining cells. Secondary bone lines the largest spaces. In Sharpey fibre bone, small osteocytes in small lacunae (about 2 microns wide) are found in columns parallel to the Sharpey fibres. Large osteocytes are found in large round spaces and are much larger than comparable osteocytes in lacunae in the bone of the salmon Salmo salar. We conclude that an absence of visible or conventional osteocyte lacunae does not mean that the cells themselves are absent. There are cells and two types of collagen fibre bundle in the tubules. The cells are osteocytes derived from osteoblasts, and these osteocytes apparently resorb bone with the result that large amounts of bone are destroyed. "Acellular" tubular and Sharpey fibre bone are types of cellular bone that differ from each other and from conventional cellular bone.

Bone and cartilage resorption in relation to tooth development in the anterior part of the mandible in cichlid fish: a light and TEM study

This paper presents ultrastructural features of the contact region between particular tooth germs and Meckel's cartilage prior to, during, and after initial resorption of the perichondral bone and of the cartilage in the cichlids Hemichromis bimaculatus and Astatotilapia burtoni. Imminent resorption opposite such teeth is announced by the presence, in this region, of a particular cell type, considered to be a stage in the cytodifferentiation of osteoclasts. Slightly later, an osteoclast with typical ruffled border is seen to open a fenestra in the perichondral bone which surrounds Meckel's cartilage. Although the action of the osteoclast is directed primarily towards the bone, it may also affect, to a much lesser extent, the underlying uncalcified cartilage. Typically, fibroblast-like cells invade the resorption cavity along with the osteoclast; the tooth germ soon follows. Capillaries are seen to invade the cartilage only at a later stage when a large cavity has been established. It is proposed that the fibroblast-like cells may have a dual function: degradation of cartilage and deposition of new bone. Although these processes are normally limited to the area surrounding tooth germs at specific loci, tooth germs in other positions may sometimes be seen invade the cartilage. They do so either passively, because of the existence of such a cavity, or as a result of their own resorption-inducing activity. Whatever the mechanism, attachment bone is being deposited within the erosion cavity and on the surface of the exposed perichondral bone. The stimuli possibly eliciting resorption of Meckel's cartilage are discussed. It is hypothesized that pressure exerted by the growing tooth germ may stimulate the osteoblasts covering the bone surface and, in this way, provoke osteoclastic bone resorption.