Dental epithelial histo-morphogenesis in the mouse: positional information versus cell history
Introduction
Reciprocal epithelial–mesenchymal interactions control all steps of odontogenesis. Crown morphogenesis (i.e. cusp number, size and position) is specific for each molar in the upper and lower jaws. Tissue reassociation experiments have shown that crown morphogenesis is controlled by the dental mesenchyme.1, 2
Epithelial histogenesis is already initiated at the bud stage since two cell types can be distinguished morphologically. Cells in contact with the basement membrane are elongated, while internal epithelial cells are small and round. Positional information might thus be involved in specifying the cell shape and cell fate as illustrated for other models.3, 4, 5, 6 At the early cap stage, epithelial histogenesis becomes much more complex when the inner dental epithelium (IDE) and outer dental epithelium (ODE), the stellate reticulum (SR) and the primary enamel knot (PEK) become distinct. The molecular mechanisms, which control dental epithelial histogenesis, involve cell–cell and cell–matrix interactions as well as diffusible signaling molecules.7, 8, 9, 10, 11, 12, 13, 14, 15 The aim of the present work was to determine whether the mesenchyme could control epithelial histogenesis. For this purpose, the dental mesenchyme from the cap stage (E14) was reassociated with pelleted single dental epithelial cells, which had lost all positional information, and cultured in vitro. In complementary experiments, dissociated single dental mesenchymal cells were pelleted and reassociated with epithelial cells to test whether the potentialities of the mesenchyme required any specific tissue organisation.
Section snippets
Materials and methods
ICR mice were mated overnight and the detection of the vaginal plug was determined as Embryonic day (E)0. The first lower molars (n = 2420) were dissected from embryos at E14 under a stereomicroscope (Leica MZ9).
Results
Mouse first lower molars were trypsin-dissociated in order to separate the mesenchyme from the epithelium at E14, when the PEK was formed and expressed transcripts for signaling molecules.16 Reassociations between the dental mesenchyme and dissociated single epithelial cells were cultured in vitro (Fig. 1). Complementary experiments were also performed by reassociating pelleted dissociated mesenchymal cells and pelleted dissociated epithelial cells (Fig. 1). In both experimental groups, the
Discussion
When epithelial cells were dissociated, the enamel organ already comprised four distinct cell populations (IDE, PEK, ODE and SR). Their specific fate may be determined by positional information,3, 4, 5 which can be specified by differential cell–cell interactions,8, 9, 15, 17 cell–matrix interactions7, 10, 11, 12, 14, 18 and signaling molecules.13, 19, 20 After trypsin treatment, the breakdown of the basement membrane and cell-surface molecules resulted in the loss of positional information.
Acknowledgement
The authors wish to thank Mr. A. Ackermann for histology.
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