Apical cell escape from the neuroepithelium and cell transformation during terminal lip fusion in the house shrew embryo

Development of deciduous and permanent dentitions in the upper jaw of the house shrew (Suncus murinus)

The diphyodont tooth replacement in mammals is characterized by a single replacement of a deciduous dentition by a permanent dentition. Despite its significance in mammalian biology and paleontology, little is known about the developmental mechanisms regulating the diphyodont replacement. Because the mouse never replaces its teeth, this study used the house shrew, Suncus murinus, as a model to investigate the control of the diphyodont replacement of a deciduous dentition by successions and additions of permanent teeth. Using morphological and gene expression analyses of serial sections, we have demonstrated the development of the upper dentition of the house shrew. In this species, the deciduous tooth germs are formed but soon become vestigial, whereas the successional and accessional (molar) germs are subsequently formed and developed. There are distinct Shh expression domains in the deciduous, successional, and accessional tooth germs, and those of the latter two germs are identified from the appearance of their primary enamel knots. The developmental sequence of tooth germs in the house shrew indicates that two adjacent primary enamel knots of the successional and accessional germs do not develop simultaneously, but with a constant time lag. We suggest that this mode of tooth succession and accession can be explained by a sequential inhibitory cascade model in which the timing of initiation and the spacing of tooth development are determined by the inhibition from the primary enamel knots of developmentally preceding adjacent tooth germs.

Differential expression of N-CAM, vimentin and MAP1B during initial pathfinding of olfactory receptor neurons in the mouse embryo

Olfactory receptor neurons extend their primary axons from the nasal epithelium to the olfactory bulb primordium via the frontonasal mesenchyme. In the present study, expression of neuronal markers (vimentin and MAP1B) and N-CAM was immunohistochemically investigated in the development of the olfactory system in mouse embryos. Expression of vimentin and MAP1B was first observed at early day 10 of gestation (D10) in the posterosuperior region of the medial nasal epithelium, while N-CAM was initially detected in the mesenchyme adjacent to the vimentin- and MAP1B-positive nasal epithelium. As development proceeded, the localization of neuronal marker-positive cells was mostly included in the N-CAM positive region. In addition, we adopted in situ labelling with vital dye (DiI) to directly determine the localization of the olfactory nerve and N-CAM on the same sections. We demonstrated that most extending axons were located in the N-CAM positive region. These results suggest that the expression of N-CAM plays a crucial role in the initial pathfinding of the olfactory nerve.