The rat small eye strain (rSey) lacks eyes and nose in the homozygote, and is similar to the mouse Sey strain with mutations in the Pax-6 gene. We isolated Pax-6 cDNA clones from an rSey homozygote library, and found an internal deletion of about 600 basepairs in the serine/threonine-rich domain. At the genomic level, a single base (G) insertion in an exon generates an abnormal 5' donor splice site, thereby producing the truncated mRNA. Anterior midbrain crest cells in the homozygous rSey embryos reached the eye rudiments but did not migrate any further to the nasal rudiments, suggesting that the Pax-6 gene is involved in conducting migration of neural crest cells from the anterior midbrain.
Teeth are formed by reciprocal interactions between the epithelium and mesenchyme in the first pharyngeal arch. Although the contribution of midbrain and hindbrain crest cells to the first pharyngeal arch has been previously examined in rodent embryos, no direct evidence exists that these cells are actually involved in the dental mesenchyme. In order to elucidate the contribution of the cranial neural crest cells in tooth formation, we first identified the emigration sites and stages providing the crest cells that migrate to the presumed tooth-forming region of the mandibular prominence. Focal labeling with DiI was performed at the midbrain and anterior hindbrain crests in rat embryos, and the labeled embryos were cultured for 30 or 60 hr. The resultant migration patterns indicated that posterior midbrain crest cells emigrating by the end of the 4-somite stage predominantly migrated to the region where tooth buds normally develop. Second, we established a new type of long-term culture system in which whole embryo culture is followed by a mandibular organ culture. Using this system, rat embryos were maintained from the early-somite stage and the molars in the explants were able to reach the bud stage within 8 days. Finally, to ascertain if posterior midbrain crest cells emigrating by the end of the 4-somite stage were involved in the dental mesenchyme, these cells were labeled with DiI and processed for the long-term culture. Labeled crest cells were clearly detectable in the dental mesenchyme. These findings indicate that the early-emigrating posterior midbrain crest cells contribute to mandibular molar tooth development in rat embryos.
The precise border of somites formed during mouse somitogenesis is defined by a Tbx6 expression domain, which is established by Mesp2-mediated Tbx6 suppression in the anterior part of the presomitic mesoderm (PSM). Ripply2, a target of Mesp2, is proposed to be involved in this down-regulation because Ripply2 deficiency causes an anterior expansion of the Tbx6 domain, resembling the Mesp2-null phenotype. However, it is unclear whether Ripply2 acts on Tbx6 independently or in association with Mesp2. To address this question, we generated three sets of transgenic mice with the following Ripply2 expression patterns: (1) overexpression in the endogenous expression domain, (2) expression instead of Mesp2 (Ripply2-knockin), and (3) ectopic expression in the entire PSM. We found accelerated Tbx6 degradation in the embryos showing Ripply2 overexpression. In the Ripply2-knockin embryos, the anterior limit of Tbx6 domain was generated by Ripply2 even in the absence of Mesp2. Ectopic Ripply2 expression along the entire PSM suppressed Tbx6 and induced Sox2-positive neural tube formation at the bilateral domain, resembling the Tbx6-null phenotype. This phenotype resulted from Tbx6 protein and not mRNA elimination, suggesting the post-translational down-regulation of Tbx6 by Ripply2. Taken together, our results demonstrate that Ripply2 represses Tbx6 in a Mesp2-independent manner, which contributes to the accurate segmental border formation.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.