Spatial and temporal distribution of sonic hedgehog mRNA in the embryonic mouse mandible by reverse transcription/polymerase chain reaction and in situ hybridization analysis

Hedgehog signaling regulates dental papilla formation and tooth size during zebrafish odontogenesis

BACKGROUND

Intercellular communication by the hedgehog cell signaling pathway is necessary for tooth development throughout the vertebrates, but it remains unclear which specific developmental signals control cell behavior at different stages of odontogenesis. To address this issue, we have manipulated hedgehog activity during zebrafish tooth development and visualized the results using confocal microscopy.

RESULTS

We first established that reporter lines for dlx2b, fli1, NF-κB, and prdm1a are markers for specific subsets of tooth germ tissues. We then blocked hedgehog signaling with cyclopamine and observed a reduction or elimination of the cranial neural crest derived dental papilla, which normally contains the cells that later give rise to dentin-producing odontoblasts. Upon further investigation, we observed that the dental papilla begins to form and then regresses in the absence of hedgehog signaling, through a mechanism unrelated to cell proliferation or apoptosis. We also found evidence of an isometric reduction in tooth size that correlates with the time of earliest hedgehog inhibition.

CONCLUSIONS

We hypothesize that these results reveal a previously uncharacterized function of hedgehog signaling during tooth morphogenesis, regulating the number of cells in the dental papilla and thereby controlling tooth size.

Ameliorative effects of thyme and calendula extracts alone or in combination against aflatoxins-induced oxidative stress and genotoxicity in rat liver

The aims of the current work were to evaluate the hepatoprotective effect of calendula flowers and/or thyme leave extracts on aflatoxins (AFs)-induced oxidative stress, genotoxicity and alteration of p53 bax and bcl2 gene expressions. Eighty male Sprague-Dawley rats were divided into eight equal groups including: the control group, the group fed AFs-contaminated diet (2.5 mg/kg diet) for 5 weeks, the groups treated orally with thyme and/or calendula extract (0.5 g/kg b.w) for 6 weeks and the groups pretreated orally with thyme and/or calendula extract 1 week before and during AFs treatment for further 5 weeks. Blood, liver and bone marrow samples were collected for biochemical analysis, gene expression, DNA fragmentation and micronucleus assay. The results showed that AFs induced significant alterations in oxidative stress markers, increased serum AFP and inflammatory cytokine, percentage of DNA fragmentation, the expression of pro-apoptotic gene p53 and bax accompanied with a decrease in the expression of bcl2. Animals treated with the extracts 1 week before AFs treatment showed a significant decrease in oxidative damage markers, micronucleated cells, DNA fragmentation and modulation of the expression of pro-apoptotic genes. These results suggested that both calendula and thyme extracts had anti-genotoxic effects due to their higher content of total phenolic compounds.

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.

Dynamic interactions and the evolutionary genetics of dental patterning

The mammalian dentition is a segmental, or periodically arranged, organ system whose components are arrayed in specific number and in regionally differentiated locations along the linear axes of the jaws. This arrangement evolved from simpler dentitions comprised of many single-cusp teeth of relatively indeterminate number. The different types of mammalian teeth have subsequently evolved as largely independent units. The experimentally documented developmental autonomy of dental primordia shows that the basic dental pattern is established early in embryogenesis. An understanding of how genetic patterning processes may work must be consistent with the different modes of development, and partially independent evolution, of the upper and lower dentition in mammals. The periodic nature of the location, number, and morphological structure of teeth suggests that processes involving the quantitative interaction of diffusible signaling factors may be involved. Several extracellular signaling molecules and their interactions have been identified that may be responsible for locating teeth along the jaws and for the formation of the incisor field. Similarly, the wavelike expression of signaling factors within developing teeth suggests that dynamic interactions among those factors may be responsible for crown patterns. These factors seem to be similar among different tooth types, but the extent to which crown differences can be explained strictly in terms of variation in the parameters of interactions among the same genes, as opposed to tooth-type-specific combinatorial codes of gene expression, is not yet known. There is evidence that combinatorial expression of intracellular transcription factors, including homeobox gene families, may establish domains within the jaws in which different tooth types are able to develop. An evolutionary perspective can be important for our understanding of dental patterning and the designing of appropriate experimental approaches, but dental patterns also raise basic unresolved questions about the nature of the evolutionary assumptions made in developmental genetics.

Mouse Otlx2/RIEG expression in the odontogenic epithelium precedes tooth initiation and requires mesenchyme-derived signals for its maintenance

The mouse Otlx2 gene is a new member of the paired-like family of homeobox genes whose human homologue, RIEG, is involved in Rieger syndrome, an autosomal-dominant disorder. One of the cardinal features of Rieger syndrome is dental hypoplasia, indicating that Otlx2/RIEG activity is essential for normal tooth development. Here, we analyzed the expression of Otlx2 during mouse tooth development and studied its regulation in dental explants. Otlx2 expression distinguishes stomatodeal from other ectoderm as early as Embryonic Day 8.5, well before tooth initiation. Thereafter, its craniofacial expression becomes restricted to the tooth-forming areas and to the epithelial components of molar and incisor primordia. Although Otlx2 induction precedes the specification of odontogenic mesenchyme, tissue recombination experiments show that the maintenance of its expression requires signals from the mesenchyme and that dental mesenchyme has the capacity to induce ectopic expression of Otlx2 in nondental epithelium. Finally, we compare Otlx2 expression with that of the recently identified homeodomain transcription factor Barx1 expressed in molar mesenchyme. Their strictly complementary expression patterns in the epithelial and mesenchymal components suggest that both genes participate in the reciprocal tissue interactions which are a hallmark of odontogenesis.

Patterning of the mammalian dentition in development and evolution

The mammalian dentition is a segmented organ system with shape differences among its serially homologous elements (individual teeth). It is believed to have evolved from simpler precursors with greater similarities in shape among teeth, and a wealth of descriptive data exist on changes to the dentition that have occurred within mammals. Recent progress has been made in determining the genetic basis of the processes that form an individual tooth, but patterning of the dentition as a whole (i.e. the number, location and shape of the teeth) is less well understood. In contrast to similarly organized systems, such as the vertebral column and limb, Hox genes are not involved in specifying differences among elements. Nevertheless, recent work on a variety of systems is providing clues to the transcription factors and extracellular signalling molecules involved.

Spatial and temporal distribution of Indian hedgehog mRNA in the embryonic mouse mandible

Hedgehog genes are involved in pattern formation during embryonic development. A recent report showed that Sonic hedgehog is expressed in the mouse mandible in the presumptive incisor region. In the present study, Indian hedgehog (Ihh) transcripts were present from gestational day 9 to 14 in the mouse mandible (reverse transcription/polymerase chain reaction analysis). Ihh mRNA was present in the dental lamina in both incisor and molar regions and in the developing whiskers (in-situ hybridization). Ihh may be involved in the site-specific proliferation of mandibular epithelium during the formation of the dental lamina. This is consistent with the observation that endogenous synthesis of retinoic acid is necessary for the initiation of odontogenesis and that retinoic acid induces hedgehog expression.