the development of the tooth takes place from the dental lamina in the following stages.
Bud stage
Cap stage
Early Bell stage
Advanced bell stage
BUD STAGE
In this stage, the dental lamina is separated from the underlying ectomesenchymal cells by the basement membrane.
Changes in the structures
Enamel Organ
It consists of low columnar cells in the periphery and polygonal cells in the central portion.
due to the increased migration of neural crest cells and the proliferation of ectomesenchymal cells the cellular density adjacent to the dental lamina increases.
this process is called as condensation of ectomesenchyme
Dental papilla
The condensed ectomesenchymal cells immediately under the dental lamina give rise to the future dental papilla.
Dental Follicle
the condensed ectomesenchymal cells below the dental lamina are called the dental follicle.
CLINICAL SIGNIFICACE
MACRODONTIA AND MICRODONTIA - Abnormally large tooth or abnormally small tooth formed due to abnormal proliferation of ectomesenchymal cells
CAP STAGE
The enamel organ is cap-shaped. the change in the shape of the enamel organ takes place due to the uneven proliferation of enamel organ cells.
Enamel organ
The enamel organ becomes cap-shaped. The cells of the enamel organ differentiate into inner enamel epithelium and outer enamel epithelium.
outer enamel epithelium.
present in the convex portion of the cap
contains cuboidal shaped cells
Inner enamel epithelium.
present in the inner cap concavity.
contains columnar cells
Stellate reticulum cells
The central polygonal cells seen in the bud stage start secreting glycosaminoglycans.
Glycosaminoglycans are hydrophilic in nature, they pull the water content present in the cells to the cell space.
this results in the formation of the star-shaped stellate reticular cells.
The mucoid fluid collected in the enamel organ is rich in albumin and provides shock absorbent effect.
Transient structures
The cells at the center of the enamel organ are densely packed to form an enamel knot.
The enamel knot projects towards the dental papilla.
vertical extensions of the enamel knot occur, when the vertical extension of enamel knot meets the outer enamel epithelium it is called the enamel septum. the enamel septum divides the stellate reticulum cells into two parts.
The depression where vertical extensions meet the outer enamel epithelium is called enamel navel.
Dental papilla
dental papilla appears more crowded and shows signs of vascular development. Dental sac appears condensed and fibrous.
CLINICAL SIGNIFICANCE
Enamel knot formation :
Gene knockout studies suggest that tooth development is arrested if there is failure in the enamel knot formation.
secondary enamel knot formation takes place in the future cusp, this directs the morphology of crown in multi-cuspid tooth.
Dens invaginates:
formation of tooth within the tooth due to abnormal invagination of enamel organ is called dens invaginates.
EARLY BELL STAGE
As the enamel organ further continues its growth, it takes a bell shape. both morpho differentiation and histodifferentiation of crown take place in this stage.
Enamel Organ
The enamel organ contains 4 different types of
Inner enamel epithelium
Stratum intermedium
Stellate reticulum cells
Outer enamel epithelium
Inner enamel epithelium
it is made up of a single layer of tall columnar cells called ameloblasts.
these ameloblast cells secrete enamel.
they are connected to one another and to the stratum intermedium by desmosomes.
Stratum intermedium.
a layer of squamous cells seen between the inner enamel epithelium and stellate reticulum cells.
they are connected to other cells through desmosomes and gap junctions.
they are associated with protein synthesis
they transport nutrients to ameloblast cells in advanced stages.
they exhibit high enzyme alkaline phosphatase activity.
they contain high cell organelles such as the Golgi apparatus, acid mucopolysaccharides, and glycogen deposits.
Stellate reticulum cells
star-shaped cells connected to the outer enamel epithelium and the stratum intermedium.
after a layer of dentin is laid down, the inner enamel epithelium is deprived of nutrition.
the stellate reticulum cells present above the inner enamel epithelium collapses to bring capillaries of dental follicles closer.
Outer enamel epithelium
is made up of cuboidal cells.
that are connected to other cells by junctional complexes.
Dental papilla
dental papilla consists of undifferentiated ectomesenchymal cells.
the acellular zone is present between the dental papilla and enamel organ, it is devoid of cells.
the dental papilla and enamel organ are separated by the basal lamina.
basal lamina before the differentiation of odontoblast cells is called membrane preformative.
the extracellular space is occupied by the fine fibrils made of collagen fibers, that come from dental lamina.
The peripheral undifferentiated cells of dental lamina grow into columnar cells and differentiate into odontoblast cells.
in the absence of inner enamel epithelium differentiation of odontoblasts does not occur.
Dental follicle.
more collagen fibers are present.
the dental follicle is more distinguished from the dental papilla.
fibers of dental follicles later developed to become the fibers of the periodontal ligament.
ADVANCED BELL STAGE
The advanced bell stage starts with the beginning of mineralization.
Enamel organ.
Enamel organs in the advanced bell stage consist of the following.
outer enamel epithelium.
stellate reticulum cells.
stratum intermedium.
ameloblast cells
Outer enamel epithelium.
the outer enamel epithelium remains as cuboidal-shaped cells. they protect the enamel organ.
Stellate reticulum and stratum intermedium.
A layer of dentin is first laid in the advanced bell stage. this layer restricts the nutrition supply to the inner enamel epithelium cells.
To provide nutritional supply the layer of stellate reticulum and stratum intermedium cells collapse to bring the nutrition supply from the capillaries of the dental follicle.
There is no difference seen between stratum intermedium and stellate reticulum cells.
Ameloblast cells.
After the formation of odontoblast cells. the layer of inner enamel epithelium differentiates to form ameloblast cells.
These cells secrete enamel after a layer of dentin is laid.
Dental Papilla
The dental papilla becomes highly vascular with peripheral odontoblast cells secrete dentine matrix through reciprocal induction.
Mineralization and Reciprocal induction
peripheral ectomesenchymal cells of dental papilla differentiate into odontoblast under the influence of inner enamel epithelium cells
After the odontoblast cells are formed, inner enamel epithelial cells differentiate to form ameloblast cells under the influence of odontoblast cells.
the odontoblast cells lay a layer of dentin starting from the region of the future cusp.
then the dentine layer is completely laid
after a layer of dentin, the ameloblast cells lay a layer of enamel matrix on top of the dentin layer.
further, there is the deposition of dentin and enamel matrix in a periodic manner.
the deposition of enamel does not take place without the deposition of a layer of dentin this is called the reciprocal induction
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