The teeth must be attached to the bones of the jaws. Sound healthy teeth depend on three separate structures for support. The name periodontium (peri = around, odont = tooth) is given to those structures that surround and support the teeth, namely:
1. the periodontal membrane serves to attach the tooth to bone;
2. alveolar bone is the bone surrounding the roots of the teeth;
3. gingivae is the soft tissue or skin that covers and protects the neck of the crown and the periodontal structures.
Each of three structures can become affected by disease or other abnormal conditions with the result that the tooth loses its support, loosens, and may require removal. These teeth are lost because of gum diseases, or more correct periodontal diseases. There are just as many teeth lost for this reason as from tooth decay. Therefore, the dentist and his patient must pay as much attention to the tooth surrounding structures as they do to the teeth themselves.
Tooth Development
Introduction
The development of the tooth is an example of organogenesis that involves many biological processes. As there is a gradient of activity an advantage the tooth germ has over many other organ systems is the temporal relationships between cells utilizing different processes. principal processes are mitosis (cell division), differentiation (the acquisition of a different phenotype amongst cells of the same genotype) growth (an increase in size),apoptosis (cell deletion), morphogenesis (the establishment of shape), and mineralization (the deposition of calcium phosphate salts). By the production of extracellular matrices and the incorporation of inorganic material the outcome of these widely diverse processes is preserved. Subsequently the teeth move into functional relationships. For convenience, the process of tooth development is divided into a number of stages, e.g., dental lamina, bud, cap, bell and apposision stages, but it must be remembered, however, that the process is continuous. Furthermore the adjacent bone and soft tissues develop concurrently.
In describing the growth the convention is followed that the crown of the tooth is above and the root below, therefore, although cells or tissues are described as growing down it is the relative position that is being described.
The processes associated with tooth development are the same as for other organ systems but what is unusual about teeth is that they first appear during the embryonic period but unlike other organs the processes from induction through to emergence are repeated several times for each tooth from the foetal period through to early childhood (the third molars).
Early Events
The stromatodeum or primitive oral cavity is lined by primitive, ectodermally derived epithelium rich in glycogen. Tooth development commences as a condensation of cells beneath the primitive epithelium due to the migration of cells that have their origin from the margins of the ectodermal neural crest. These cells are considered to be similar but distinct from those of the primary germ layers and since they behave as mesenchymal cells they are called ectomesenchyme.
|
|
|
Interaction between the ectomesenchyme and the overlying ectoderm leads to the thickening of the latter to form the primary epithelial band in each jaw. This is a more or less horse-shoe shaped continuum in each jaw. The nature of these interactions remain unknown. As many developmental events require reciprocal interactions between ectodermal and mesenchymal cells the process is generally referred to as epithelial-mesenchymal interaction (EMI). The mechanism underlying these early developmental events is unclear, however, there is some evidence of direct hererotypic cell contacts during tooth development.
from the primary epithelial band a vestibular lamina and dental lamina develop. These laminae are regions of thickened ectoderm. The vestibular lamina which is situated buccally leads to the formation of the sulcus between the cheeks and the tooth bearing areas of the jaws. The dental lamina gives rise to the teeth. Often it is difficult to distinguish which is which, the dental lamina is the one closely associated with the condensation of ectomesenchyme.
Shortly after the dental lamina has formed mitotic activity in the primary epithelial band results in thickening in four discrete regions in each quadrant (in lamina). Each of these give rise to an epithelial downgrowth or bud into the ectomesenchyme. The epithelial downgrowth is distinctive and characterizes the bud stage. The tissue changes are not exclusive to teeth common to many situations where epithelium and mesenchyme interact to produce an organ, e.g., hair follicles and mucous glands. A fifth thickening forms later distal to the fourth resulting in epithelial buds at sites that correspond to the positions of the future deciduous teeth.
As the epithelial bud proliferates into the ectomesenchyme the deeper peripheral cells divide more rapidly than those in the center. This difference in mitotic activity results in the formation of concavity. The epithelial mass now resembles a small cap, hence named the cap stage. Up to this point the principal processes have been mitosis and epithelial-mesenchymal interaction but the cap stage marks the beginning of the formation of distinct layers within the epithelium.
Enamel Organ
From now on the epithelial ingrowth is called the enamel organ. As tooth development progresses the epithelial cells lining the concavity of the cap-like structure become progressively elongated, the central epithelial cells become separated from each other while the cells lining the outer convex surface remain cuboidal. Eventually a stage is reached where there are four distinct epithelial layers and the enamel organ resembles a bell.
The tall columnar epithelial cells lining the concavity are called the inner enamel epithelium (IEE) while the cuboidal cells on the convex outer surface are the outer enamel epithelium (OEE). At the periphery of the cap the cells of the inner enamel epithelium and the outer enamel epithelium are continuous. This junctional region is called the cervical loop and delineates the anatomical crown.
The mass between the inner and outer enamel epithelia is largely made up of polyhedral cells that appear to form a network of star-shaped cells - the stellate reticulum. Their appearance is due to the presence of intercellular fluid rich in acid mucopolysaccharides (glycolsaminoglycans). A fourth, less conspicuous, layer or flattened cells found between the stellate reticulum and the inner enamel epithelium is called the stratum intermedium.
|
|
|
As the epithelial mass increases in size and it envelops most of the condensed ectomesenchymal cells beneath it and now resembles a bell as shown in the model. With continued development the IEE adjacent to the ectomesenchymal cells will differentiate into enamel forming cells (ameloblasts) while the juxtaposed ectomesenchymal cells will differentiate into cells that form dentine (odontoblasts). The shape of the crown is traced by the IEE. Permanence is only obtained when mineralization occurs. Initially the two cell layers are adjacent to each other separated by the basement membrane but as each forms their respective extracellular matrix they become separated moving in opposite directions away from each other. two important developmental processes occur at the bell stage, the appearance of four distinct epithelial layers (histodifferentiation) and the development of form, i.e. determination of crown shape (morphodifferentiation).
The shape of the future crown is largely determined by the underlying ectomesenchyme. The mechanism by which this occurs is not known. A number of cusps appear first and once the crown has mineralized root development commences as the tooth erupts. A close relationship between the number of conglomerates of small vessels, crown form, and root form has been demonstrated. The exact relationship between vasulature and morphogenesis during tooth development remains a mystery but it is evident that the vasculature provides more than nutrient supply.
The ectomesenchyme beneath the epithelial downgtowth becomes partially enclosed by the enamel organ during the bell stage. The tissue enclosed by the epithelium is called the dental papilla while that continuous with it and surrounding the enamel organ is called the dental follicle. The cells of the dental papilla will give rise to odontoblasts, dentine and the dental pulp whereas those of the dental follicle will give rise to the supporting tissues of the tooth, the periodontium. Collectively the enamel organ, the dental papilla, and the dental follicle constitute the tooth germ. The outline of the crown is determined by the inner enamel epithelium moving into the stellate reticulum to approximate the outer enamel epithelium in the region of the cusp tips. Mineralization later commences in these regions.
At the ultrastructural level the enamel organ at the bell stage is relatively uncomplicated. All the cells show the unusual cytoplasmic organells such as free ribosomes, endoplasmic reticulum, mitochondria, and a few scattered tonofilamentes. As with other epithelia, modifications in the plasma membrane forming junctional complexes allow for communication between neighbouring cells and also with cells of the dental papilla. Desmosomal attachments are particularly evident among the cells of the stellate reticulum as these cells are widely separated and attached to their neighbours by fine cytoplasmic extentions.
The cells of the dental papilla also have a relatively uncomplicated ultrastructure with all the usual organells associated with secretory activity being separated from the enamel organ by the basal lamina.
During the bell stage the connection of the enamel organ with the overlying oral epithelium is lost. The epithelial cells of the dental lamina undergo apoptosis resulting in fragmentation forming discrete islands of epithelial cells called the epithelial glands of Serres that later disappear. As the permanent teeth develop in bone the connection with the overlying oral epithelium passes through the bone, by contrast the deciduous teeth develop in a bony trough. The bundles of collagen fibres adjacent to the dental lamina were considered by the early anatomists responsible for the eruption of teeth by the mechanism similar to the retraction of the testis, hence this tissue was called the gubernaculum. The gubarnacular canal is thought to provide a path along which the tooth moves from its site of development into the oral cavity. As the number of cusps of a tooth is dependent on the folding of the inner enamel epithelium it is possible that the force vectors from the vasculature (conglomerates) in the papilla act on the epithelium causing it to fold into the stellate reticulum and the latter provides the eruptive force for tooth eruption.
|
|
|
Some of the teeth of the permanent or secondary dentition also arise from the dental lamina as a result of lingual extentions of the dental lamina of each of the deciduous tooth germs to give rise to the permanent incisors, canines, and premolars. These teeth, therefore, form from a successional dental lamina. The permanent molars, however, arise from a distal extention from the second deciduous molar. As there is no deciduous predecessor it is called an accessional dental lamina.
