Notch signaling defines an evolutionarily conserved cell communication mechanism, which enables neighboring cells to adopt different fates. Furthermore, Notch signaling may create boundaries that direct both the growth and patterning of the developing organs. Here we report on the expression of Notch receptors during the development of rodent incisors. Before the aquisition of their characteristic shape, incisors rotate antero–posteriorly and become asymmetric at their labial–lingual axis. Notch2 is expressed only in the anterior part of the developing incisors, well before their rotation, while Notch2 expression was symmetrical in the developing molars. This is the first demonstration of an asymmetric gene expression pattern during the rotation of the rodent incisors.

The molecular mechanisms governing the decision between molariform and incisiform patterns of rodent dentition are not yet known. Transcription factors are regulators of regionally specific morphogenesis and key co-ordinators of gene activity during developmental processes. Here, we analysed the expression of several transcription factors during mouse tooth development. Otlx2/Rieg is a homeobox gene involved in Rieger syndrome, a human disorder characterized by dental hypoplasia. Otlx2/Rieg expression distinguishes stomatodeal epithelium well before tooth initiation, and thereafter its expression becomes restricted to the epithelia of both molar and incisor primordia. The recently identified homeodomain transcription factor Barx1 is first expressed in mesenchyme of the first branchial arch, but during advanced developmental stages the gene is exclusively expressed in the mesenchyme of molar primordia. Finally, the Sry-related transcription factor Sox9 is expressed in epithelial components and to a lesser degree in condensed mesenchyme of the developing teeth. These results suggest that Otlx2/Rieg, Barx1, and Sox9 participate in the hierarchical cascade of factors involved in the regulation of tooth morphogenesis.

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.

The vertebrate nervous system forms by the specification of, successively, neuroepithelial regions and cell groups. One of the proposed major histogenic steps is the subdivision of the neural tube in compartments along its caudorostral and dorsoventral axis. This event is reflected, and may be directed, by the restricted expression of many transcription factors. Here, we report on the isolation of a new homeobox gene of the paired superclass, Otlx2, whose early expression pattern in the mesencephalon and prosencephalon is congruent with proposed neuromeric models of brain morphogenesis. In addition, its late embryonic and postnatal expression, in clear continuity with the earlier pattern, suggests a role in the neuronal differentiation and the histogenesis of several prosencephalic and mesencephalic areas. Finally, Otlx2 is expressed from the earliest morphogenetic events in the Rathke's pouch, the anlage of the adenohypophysis, an expression site shared by a very close homologue, Otlx1/Ptx1/P-Otx.

In the process of cloning murine proteins capable of binding to a regulatory module of the Ncam promoter, we isolated a novel homeobox gene, Barx1, the first vertebrate member of the structural subclass defined by Drosophila BarH1. Here we report its sequence, chromosomal localisation and embryonic expression pattern. Barx1 was strongly expressed in restricted areas of head and neck mesenchyme and in the wall of the developing stomach and at weaker levels in the proximal fore- and hindlimbs. At embryonic day 10.5, expression in the head region is detected in spatially restricted areas of the first and second branchial arches, before any apparent cellular or morphological differentiation. Later in development, all expressing tissues in this region, which include the mesenchyme underlying the olfactory epithelium, the primary and secondary palate, the molar tooth papillae and the stroma of the submandibular gland, appear derived from ectomesenchyme of neural crest origin. At day 16.5, all locations other than the developing molars had become Barx1-negative. An intriguing feature is the restriction of Barx1 expression to the molars suggesting a role in the differentiation of molars from incisors. Barx1 already marks the future stomach region of the primitive gut at embryonic day 9.5 and is present in the mesenchymal wall of the stomach up to day 16.5. These results thus direct a search for its function to a number of inductive epithelial-mesenchymal interactions during craniofacial development and to stomach organogenesis.