Expression of SPARC and the osteopontin‐like protein during skeletal development in the cichlid fish Oreochromis mossambicus

Expression of secretory calcium-binding phosphoprotein (scpp) genes in medaka during the formation and replacement of pharyngeal teeth

BACKGROUND

Analyses of tooth families and tooth-forming units in medaka with regard to tooth replacement cycles and the localization of odontogenic stem cell niches in the pharyngeal dentition clearly indicate that continuous tooth replacement is maintained. The secretory calcium-binding phosphoprotein (scpp) gene cluster is involved in the formation of mineralized tissues, such as dental and bone tissues, and the genes encoding multiple SCPPs are conserved in fish, amphibians, reptiles, and mammals. In the present study, we examined the expression patterns of several scpp genes in the pharyngeal teeth of medaka to elucidate their roles during tooth formation and replacement.

METHODS

Himedaka (Japanese medaka, Oryzias latipes) of both sexes (body length: 28 to 33 mm) were used in this study. Real-time quantitative reverse transcription-polymerase chain reaction (PCR) (qPCR) data were evaluated using one-way analysis of variance for multi-group comparisons, and the significance of differences was determined by Tukey's comparison test. The expression of scpp genes was examined using in situ hybridization (ISH) with a digoxigenin-labeled, single-stranded antisense probe.

RESULTS

qPCR results showed that several scpp genes were strongly expressed in pharyngeal tissues. ISH analysis revealed specific expression of scpp1, scpp5, and sparc in tooth germ, and scpp5 was continually expressed in the odontoblasts of teeth attached to pedicles, but not in the osteoblasts of pedicles. In addition, many scpp genes were expressed in inner dental epithelium (ide), but not in odontoblasts, and scpp2 consistently showed epithelial-specific expression in the functional teeth. Taken together, these data indicate that specific expression of scpp2 and scpp5 may play a critical role in pharyngeal tooth formation in medaka.

CONCLUSION

We characterized changes in the expression patterns of scpp genes in medaka during the formation and replacement of pharyngeal teeth.

From zero to ossified: Larval skeletal ontogeny of the Neotropical Cichlid fish Cichlasoma dimerus

The identification of skeletal elements, the analysis of their developmental sequence, and the time of their appearance during larval development are essential to broaden the knowledge of each fish species and to recognize skeletal abnormalities that may affect further fish performance. Therefore, this study aimed to provide a general description of the development of the entire skeleton highlighting its variability in Cichlasoma dimerus. Larvae of C. dimersus were stained with alcian blue and alizarin red from hatching to 25 days posthatching. Skeletogenesis began with the endoskeletal disk and some cartilage structures from the caudal fin and the splachnocranium, while the first bony structure observed was the cleithrum. When larvae reached the free-swimming and exogenous feeding stage, mostly bones from the jaws, the branchial arches, and the opercle series evidenced some degree of ossification, suggesting that the ossification sequence of C. dimerus adjusts to physiological demands such as feeding and ventilation. The caudal region was the most variable regarding meristic counts and evidenced higher incidence of bone deformities. In conclusion, this work provides an overview of C. dimerus skeletogenesis and lays the groundwork for further studies on diverse topics, like developmental plasticity, rearing conditions, or phylogenetic relationships.

Divergent Expression of SPARC, SPARC-L, and SCPP Genes During Jawed Vertebrate Cartilage Mineralization

While cartilage is an ancient tissue found both in protostomes and deuterostomes, its mineralization evolved more recently, within the vertebrate lineage. SPARC, SPARC-L, and the SCPP members (Secretory Calcium-binding PhosphoProtein genes which evolved from SPARC-L) are major players of dentine and bone mineralization, but their involvement in the emergence of the vertebrate mineralized cartilage remains unclear. We performed in situ hybridization on mineralizing cartilaginous skeletal elements of the frog Xenopus tropicalis (Xt) and the shark Scyliorhinus canicula (Sc) to examine the expression of SPARC (present in both species), SPARC-L (present in Sc only) and the SCPP members (present in Xt only). We show that while mineralizing cartilage expresses SPARC (but not SPARC-L) in Sc, it expresses the SCPP genes (but not SPARC) in Xt, and propose two possible evolutionary scenarios to explain these opposite expression patterns. In spite of these genetic divergences, our data draw the attention on an overlooked and evolutionarily conserved peripheral cartilage subdomain expressing SPARC or the SCPP genes and exhibiting a high propensity to mineralize.

The Estrogen-Responsive Transcriptome of Female Secondary Sexual Traits in the Gulf Pipefish

Sexual dimorphism often results from hormonally regulated trait differences between the sexes. In sex-role-reversed vertebrates, females often have ornaments used in mating competition that are expected to be under hormonal control. Males of the sex-role-reversed Gulf pipefish (Syngnathus scovelli) develop female-typical traits when they are exposed to estrogens. We aimed to identify genes whose expression levels changed during the development and maintenance of female-specific ornaments. We performed RNA-sequencing on skin and muscle tissue in male Gulf pipefish with and without exposure to estrogen to investigate the transcriptome of the sexually dimorphic ornament of vertical iridescent bands found in females and estrogen-exposed males. We further compared differential gene expression patterns between males and females to generate a list of genes putatively involved in the female secondary sex traits of bands and body depth. A detailed analysis of estrogen-receptor binding sites demonstrates that estrogen-regulated genes tend to have nearby cis-regulatory elements. Our results identified a number of genes that differed between the sexes and confirmed that many of these were estrogen-responsive. These estrogen-regulated genes may be involved in the arrangement of chromatophores for color patterning, as well as in the growth of muscles to achieve the greater body depth typical of females in this species. In addition, anaerobic respiration and adipose tissue could be involved in the rigors of female courtship and mating competition. Overall, this study generates a number of interesting hypotheses regarding the genetic basis of a female ornament in a sex-role-reversed pipefish.

Matricellular Proteins: Functional Insights From Non-mammalian Animal Models

The extracellular matrix (ECM) has central roles in tissue integrity and remodeling throughout the life span of animals. While collagens are the most abundant structural components of ECM in most tissues, tissue-specific molecular complexity is contributed by ECM glycoproteins. The matricellular glycoproteins are categorized primarily according to functional criteria and represented predominantly by the thrombospondin, tenascin, SPARC/osteonectin, and CCN families. These proteins do not self-assemble into ECM fibrils; nevertheless, they shape ECM properties through interactions with structural ECM proteins, growth factors, and cells. Matricellular proteins also promote cell migration or morphological changes through adhesion-modulating or counter-adhesive actions on cell-ECM adhesions, intracellular signaling, and the actin cytoskeleton. Typically, matricellular proteins are most highly expressed during embryonic development. In adult tissues, expression is more limited unless activated by cues for dynamic tissue remodeling and cell motility, such as occur during inflammatory response and wound repair. Many insights in the complex roles of matricellular proteins have been obtained from studies of gene knockout mice. However, with the exception of chordate-specific tenascins, these are highly conserved proteins that are encoded in many animal phyla. This review will consider the increasing body of research on matricellular proteins in nonmammalian animal models. These models provide better access to the very earliest stages of embryonic development and opportunities to study biological processes such as limb and organ regeneration. In aggregate, this research is expanding concepts of the functions and mechanisms of action of matricellular proteins.

Functional bone histology of zebrafish reveals two types of endochondral ossification, different types of osteoblast clusters and a new bone type

The zebrafish is as an important vertebrate animal model system for studying developmental processes, gene functions and signalling pathways. It is also used as a model system for the understanding of human developmental diseases including those related to the skeleton. However, surprisingly little is known about normal zebrafish skeletogenesis and osteogenesis. As in most vertebrates, it is commonly known that the bones of adult zebrafish are cellular unlike that of some other teleosts. After careful histological analyses of each zebrafish adult bone, we identified several acellular bones, with no entrapped osteocytes in addition to several cellular bones. We show that both cellular and acellular bones can even occur within the same skeletal element and transitions between these two cell types can be found. Furthermore, we describe two types of osteoblast clusters during skeletogenesis and two different types of endochondral ossification. The epiphyseal plate, for example, lacks a zone of calcification and a degradation zone with osteoblasts. A new bone type that we term tubular bone was also identified. This bone is completely filled with adipose tissue, unlike spongy bones. This study provides important insight on how osteogenesis takes place in zebrafish, and especially on the transition from cellular to acellular bones. Overall, this study leads to a deeper understanding of the functional histological composition of adult zebrafish bones.

Formation of the inner ear during embryonic and larval development of the cichlid fish (Oreochromis mossambicus)

BACKGROUND

The vertebrate inner ear comprises mineralized elements, namely the otoliths (fishes) or the otoconia (mammals). These elements serve vestibular and auditory functions. The formation of otoconia and otoliths is described as a stepwise process, and in fish, it is generally divided into an aggregation of the otolith primordia from precursor particles and then a growth process that continues throughout life.

RESULTS

This study was undertaken to investigate the complex transition between these two steps. Therefore, we investigated the developmental profiles of several inner ear structural and calcium-binding proteins during the complete embryonic and larval development of the cichlid fish Oreochromis mossambicus in parallel with the morphology of inner ear and especially otoliths. We show that the formation of otoliths is a highly regulated temporal and spatial process which takes place throughout embryonic and larval development.

CONCLUSIONS

Based on our data we defined eight phases of otolith differentiation from the primordia to the mature otolith.