- 1 Tooth as an ectodermal organ
- 2 early tooth development
- 3 root development
- 4 Continuously growing teeth
- 5 Examples of continuously growing teeth
- 6 References
Tooth as an ectodermal organ
Teeth are ectodermal organs, similar to hair, scales, nails, feathers, mammary glands. They develop as a combination of ectoderm and mesenchyme with a continuous and dynamic reciprocal signalling interaction between them. The epithelium and mesenchyme instruct and receive information which guides the morphogenesis of the ectodermal organ and identity.
early tooth development
The development of all ectodermal organs starts in a similar fashion and deviates during later stages in order to give rise to different adult structures. The first sign in the development of all ectodermal organs is an epithelial thickening during the initiation stage resulting in the formation of a placode. The epithelium of the placode buds into the mesenchyme (our outwards in the case of feathers) and forms a bud of epithelium surrounded by condensed dental mesenchyme during this bud stage. From this moment onwards the morphogenesis of different ectodermal organs becomes specific.
In teeth the bud stage is followed by the cap stage where the cervical loop is formed and the bell stage in which more extensive growth and folding occurs of the dental epithelium. This folding pattern will later transform into the occlusal pattern of cusps that is species specific and determines the function.
After crown formation the cervical loop of the molar undergoes a radical transformation. The stellate reticulum and stratum intermedium in between the inner and outer enamel epithelium disappears. What is left is a double layer of basal epithelium known as Hertwig’s epithelial root sheath (HERS). The epithelium above HERS fragments and forms a fenestrated network of epithelial cells known as the epithelial cell rests of Malassez (ERM). Through this network mesenchymal components can migrate from outside the tooth and form the periodontal ligament which attaches the root surface to the jaw bone. With the formation of the HERS no ameloblast differentiation occurs anymore and it this structure together with the ERM is thought to be typical of a root.
Continuously growing teeth
Continuously growing teeth are quite abundant in nature. The best studied ever-growing tooth is the rodent incisor.
Examples of continuously growing teeth
- The rodent incisor
- the molars of some rodent species.
- sloth teeth
- elephant tusks
The rodent incisor
The rodent incisor is one of the evolutionary adaptations that make rodents such a successful group. Two incisors in the upper jaw and two in the lower jaw are separated from the molars by a diastema region, an area without any teeth. They regenerate from the apical end and and wear down at the distal tip. The ever-growing incisor can be subdivided into two areas, the crown analogue and the root analogue.
http://sciforums.spuriousmonkey.com/permanent/incisor-frontal.gif A frontal section of a mouse incisor. The top half shows a layer of ameloblasts typical of the crown analogue, the root analogue looks like a root.
The crown analogue is the labial half of the incisor. It is characterized by an enlarged cervical loop at the apical end. The cervical loop is the epithelial stem cell niche. The epithelial progeny of the crown analogue's cervical loop differentiates into ameloblasts that produce enamel.
The root analogue is the lingual half of the incisor. It's cervical loop is much smaller and the epithelium does not differentiate into ameloblasts, but instead forms a root sheath and fragments into epithelial cell rests of Malassez typical of root epithelium. The root analogue is covered in dentin and cementum like a normal root.
Functional significance of the subdivision into crown and root analogue
The crown analogue is covered with enamel which is much harder than the dentin/cementum of the root analogue. By constantly scraping the lower incisors against the upper incisors the root analogue is worn down more than the crown analogue sharpening the tooth. The system in fact generates a self-sharpening and regenerating set of chisels.