Cell proliferation and apoptosis in the primary enamel knot measured by flow cytometry of laser microdissected samples

Cap-to-bell stage molar tooth morphogenesis occurs through proliferation-independent sulcus sharpening and condensation-associated tension in the dental papilla

The anatomy of molar teeth is important both functionally for chewing food and in evolutionary studies as a well-preserved species marker in the fossil record. Molar teeth begin to develop their characteristic biting-surface shape of cusps (peaks) and sulci (valleys) at the bell stage, when corresponding folds in the dental epithelium become apparent. Theories about the developmental mechanisms of cusp and sulcus morphogenesis have hitherto largely focused on the non-proliferating nature of the secondary enamel knots (EKs) at the cusp tips. EKs have been thought to direct cusp/sulcus formation by stimulating proliferative growth of the surrounding epithelium which, being confined within a capsule of condensed mesenchyme, bends by mechanical buckling. Here we show, using explant inhibition and cut-and-recoil experiments, that cap-to-bell morphogenesis is largely proliferation-independent (sulcus sharpening entirely so) and that tension in the mesenchyme of the dental papilla, immediately sub-adjacent to the cusps, rather than compression by the mesenchyme surrounding the whole structure, is what holds the structure in shape. Fine mapping of the degree of condensation shows that it is highest in the mesenchyme of the dental papilla and becomes progressively more focused to the cusp regions, consistent with a key role in cusp shaping. Together these findings overturn the prevailing models of molar morphogenesis, including both cusp and sulcus formation.

Cell cycle of the enamel knot during tooth morphogenesis

Enamel knot (EK) is known to be a central organ in tooth development, especially for cusp patterning. To trace the exact position and movement among the inner dental epithelium (IDE) and EK cells, and to monitor the relationship between the EK and cusp patterning, it is essential that we understand the cell cycle status of the EK in early stages of tooth development. In this study, thymidine analogous (IdU, BrdU) staining was used to evaluate the cell cycle phase of the primary EK at the early casp stage (E13.0) and the gerbil embryo (E19) in a developing mouse embryo. The centerpiece of this study was to describe the cell cycle phasing and sequencing during proliferation in the IDE according to the expression of IdU and BrdU following their injection at calculated time points. The interval time between IdU injection and BrdU injection was set at 4 h. As a result, the cell cycle in the IDE of the mouse and gerbil was found to be synchronous. Conversely, the cell cycle in primary EKs of mice was much longer than that of the IDE. Therefore, the difference of cell cycle of the IDE and the EK is related to the diversity of cusp patterning and would provide a new insight into tooth morphogenesis.

Dynamics of caspase-3 activation and inhibition in embryonic micromasses evaluated by a photon-counting chemiluminescence approach

Caspases are key enzymatic components of the intracellular apoptotic machinery, and their role in mammalian systems is often studied using fluoromethylketone (FMK) inhibitors. Despite many advantages of such approach, efficiency of the inhibitor and membrane permeability speed are often questioned. This work therefore focuses on an exact evaluation of caspase-3 FMK inhibition dynamics in camptothecin-induced mesenchymal micromasses. Two parameters of caspase-3 FMK inhibitor were investigated: first, the stability of the inhibitory potential in the time course of cultivation and, simultaneously, the dynamics of caspase-3 FMK inhibition after camptothecin-induced apoptosis peak. A photon-counting chemiluminescence approach was applied for quantification of active caspase-3. The sensitivity of the photon-counting method allowed for evaluation of active caspase-3 concentration in femtogram amounts per cell. The inhibitor penetrated the cells within the first minute after its application, and the peak of caspase-3 started to decline to the blank level after 30 min. The inhibitory effect of the FMK inhibitor was unchanged during the entire 48 h of cultivation.

Apoptotic signaling in mouse odontogenesis

Apoptosis is an important morphogenetic event in embryogenesis as well as during postnatal life. In the last 2 decades, apoptosis in tooth development (odontogenesis) has been investigated with gradually increasing focus on the mechanisms and signaling pathways involved. The molecular machinery responsible for apoptosis exhibits a high degree of conservation but also organ and tissue specific patterns. This review aims to discuss recent knowledge about apoptotic signaling networks during odontogenesis, concentrating on the mouse, which is often used as a model organism for human dentistry. Apoptosis accompanies the entire development of the tooth and corresponding remodeling of the surrounding bony tissue. It is most evident in its role in the elimination of signaling centers within developing teeth, removal of vestigal tooth germs, and in odontoblast and ameloblast organization during tooth mineralization. Dental apoptosis is caspase dependent and proceeds via mitochondrial mediated cell death with possible amplification by Fas-FasL signaling modulated by Bcl-2 family members.