Lec.1 Tooth development د0سحرغانم
Tooth development :(odontogenesis)
is the complex process by which teeth form from embryonic cells, grow, and erupt into the mouth.
DENTAL LAMINA
Dental lamina represent the first sign of tooth development.
Two or 3 weeks after the rupture of the buccopharyngeal membrane,
when the embryo is about 6 weeks old, certain areas of basal cells of the oral ectoderm proliferate more rapidly than cells of the adjacent areas.
This leads to the formation of the :
Primary epithelial band which is a band of epithelium that has invaded the underlying ectomesenchyme along each of the horseshoe-shaped future dental arches.
At about 7th week the primary epithelial band divides into:
an inner (lingual) process called Dental lamina
and an outer (buccal)process called Vestibular lamina.
The dental laminae serve as the primordium for the ectodermal portion of the deciduous teeth.
Later, during the development of the jaws, the permanent molars arise directly from a distal extension of the dental lamina.
The development of the first permanent molar is initiated at the fourth month in utero.
The second molar is initiated at about the first year after birth, the third molar at the fourth or fifth years.
The distal proliferation of the dental lamina is responsible for the location of the germs of the permanent molars in the ramus of the mandible and the tuberosity of the maxilla.
The successors of the deciduous teeth develop from a lingual extension of the free end of the dental lamina opposite to the enamel organ of each deciduous tooth.
The lingual extension ofthe dental lamina is named the successional lamina and develops from the fifth month in utero (permanent central incisor)
to the tenth month of age (second premolar).
Fate of dental lamina :
It is evident that the total activity of the dental lamina extends over a period of at least 5 years.However, the dental lamina may still be active in the third molar region after it has disappeared elsewhere, As the teeth continue to develop, they lose their connection with the dental lamina.
Remnants of the dental lamina persist as epithelial pearls or islands within the jaw as well as in the gingiva.
These are referred to as cell rest of Serrae.
Vestibular lamina:
Labial and buccal to the dental lamina in each dental arch, another epithelial thickening develops independently and somewhat later. It is the vestibular lamina, also termed the lip furrow band.Proliferation of vestibular lamina leads to the formation of vestibule
Cells rapidly enlarge & then degenerate to form a cleft which becomes the vestibule between the cheek & tooth-bearing area.
Stages of Human tooth development:
Tooth development is in continuous stages.
This stages morphological stages depended on shape of epithelial part of the tooth germ.(enamel organ)
Histology of important stages of tooth development.
Tooth development is commonly divided into the following stages:
1. Initiation stage .2. Bud stage .3. Cap stage. 4. Bell stage
a.Early bell stage b.Advanced. bell stage
1.Initiation Stage:One of the earliest signs in the formation of a tooth that can be seen microscopically is the distinction between the vestibular lamina and the dental lamina. The dental lamina connects the developing tooth bud to the epithelial layer of the mouth for a significant time. This is regarded as the initiation stage.
2.Bud stage
Epithelium of dental lamina is separated from the underlying ectomesenchymeBy abasement membrane.With differentiation of each dental lamina ,round swelling arises at different points in each jaw.
These represent the future position of deciduous teeth and are primordia of enamel organs, the tooth buds.
Thus,development of the tooth germs are initiated and cells continue to proliferate faster than adjacent cells. Main function of certain epithelial cells of tooth bud is to form enamel –hence enamel organ.
In the bud stage, the enamel organ consists of peripherally located low columnar cells and centrally located polygonal cells.
Many cells of tooth bud and surrounding mesenchyme, under go mitosis,due to this and migration of neural crest cells into the area, ectomesenchyme cells surrounding the tooth bud condense.
The ectomesenchymal condensation subajacent to the enamel organ is the dental papilla.
Condensed ectomesenchyme surrounding the tooth bud and dental papilla is called dental sac.
Dental papilla -------------gives rise to dentin and tooth pulp.
Dental sac---------------gives rise to cementum ,periodontal ligment and alveoliar bone.
The dental lamina and bud stage:
the dental laminae are seen early in week 6 as U-shaped thickenings or buds of the oral epithelium (surface ectoderm)
Localized proliferation of cells in the dental lamina forms round or oval swellings, the tooth buds, which grow into the mesenchyme
The tooth buds develop into the deciduous or milk teeth (shed during childhood). There are 10 tooth buds in each jaw, one for each tooth
The tooth buds for the permanent teeth, with deciduous predecessors, are seen in the 10-week fetus, developing from deeper continuations of the dental lamina They lie on the tongue or lingual side of the deciduous buds
Tooth buds for the permanent teeth appear at different ages during the fetal period except for the second and third permanent molars, which appear after birth, at about 4 months and 5 years, respectively
The permanent molars with no deciduous predecessors develop as buds from backward extensions of the dental lamina.
3.Cap stage
As the tooth bud continues to proliferate, it does not expand uniformly into a larger sphere. Instead, unequal growth in different parts of the tooth bud leads to the cap stage, which is characterized by a shallow invagination on the deep surface of the bud.
Outer enamel (dental)epithelium
The peripheral cells of the cap stage are cuboidal, cover the convexity
of the “cap,” and are called the outer enamel (dental) epithelium.
Inner enamel (dental) epithelium
The cells in the concavity of the “cap” become tall, columnar cells and represent the inner enamel (dental) epithelium.
The outer enamel epithelium is separated from the dental sac,and the inner enamel epithelium from the dental papilla, by adelicate basement membrane. Hemidesmosomes anchor the cells to the basal lamina. The enamel organ may be seen to have a double attachment of dental lamina to the overlying oral epithelium enclosing ectomesenchyme called enamel niche between them. This
appearance is due to a funnel-shaped depression of the dental lamina.
Stellate reticulum (enamel pulp)
Polygonal cells located in the center of the epithelial enamel organ, between the outer and inner enamel epithelium begin to separate as more inter cellular fluid
is produced and form acellular network called the Stellate reticulum
As a result the polygonal cells become star shaped but maintain contact with each other by their cytoplasmic process. As these star shaped cells form a cellular network, they are called the stellate reticulum.
the spaces between the cells are filled with mucoid fluid rich in albumin. due to water being drawn into the enamel organ from the surrounding dental papilla as a result of osmotic force exerted by glycosaminoglycans contained in the ground substance.
This gives the stellate reticulum a cushion like consistency and acts as a shock absorber that may support and protect the delicate enamel-forming cells.
The cells in the center of the enamel organ are densely packed and form the Enamel knot .
This knot projects in part toward the underlying dental papilla, so that the center of the epithelial invagination shows a slightly knob like enlargement that is bordered by the labial and lingual enamel grooves.
At the same time a vertical extension of the enamel knot, called the Enamel cord occurs.uuj
When the enamel cord extends to meet the outer enamel epithelium it is termed as Enamel septum, for it would divide the stellate reticulum into two parts.
The outer enamel epithelium at the point of meeting shows a small depression and this is termed Enamel navel as it resembles the umbilicus. These are temporary structures (transitory structures) that disappear before enamel formation begins.
The function of the enamel knot and cord may act as a reservoir of dividing cells for the growing enamel organ.
Recent studies have shown that enamel knot acts as a signaling center as many
important growth factors are expressed by the cells of the enamel knot and thus they play an important part in determining the shape of the tooth.
Dental papilla
Under the organizing influence of the proliferating epithelium of the enamel organ, the ectomesenchyme (neural crest cells) that is partially enclosed by the invaginated portion of the inner enamel epithelium proliferates.
It condenses to form the dental papilla, which is the formative organ of the dentin and the primordium of the pulp.
The dental papilla shows active budding of capillaries and mitotic figures, and its peripheral cells adjacent to the inner enamel epithelium enlarge and later differentiate into the odontoblasts.
Dental sac (dental follicle)
Concomitant with the development of the enamel organ and the dental papilla, there is a marginal condensation. Gradually, in this zone, a denser and more fibrous layer develops,which is the primitive dental sac in the ectomesenchyme surrounding the enamel organ and dental papilla.