Tooth Development (Odontogenesis) – Part 2

1. Initiation Stage:

This is the first stage of odontogenesis; it begins between the sixth and seventh week of prenatal development during embryonic period, where ondontogenesis of primary (baby) dentition starts.

At the beginning of the sixth week, the embryo’s primitive mouth is lined by ectoderm. The outer position of the ectoderm gives rise to oral epithelium which consists of horse shoe shaped band of tissue. There is a base membrane, which separates developing oral epithelium and mesenchyme.

During the later portion of the seventh week, the oral epithelium grows and produces a layer known as dental lamina. The dental lamina starts to form in the mid line for both arches and periodically develops.

2. Bud Stage:

This occurs at the beginning of the eighth week of prenatal development for the primary dentition.

This stage is mainly characterised by the appearance of a tooth bud without the clear arrangement of cells. The tooth bud itself is the group of the cells at the end of the dental lamina. The stage is named for the extensive proliferation into bds. Each of these buds from the dental lamina, together with the mesenchyme will develop into a tooth germ. The important point to note is that only the proliferation of the two tissues occurs during this stage.

3. Cap Stage:

This occurs for the primary dentition between the ninth and the tenth week of prenatal development during the foetal period. This stage is characterised by the continuation of the ingrowth of the oral epithelium into mesenchyme. This stage marks the beginning of histodifferentiation. At the end of this stage, the three embryological structures (the dental papilla, dental follicle and the enamel organ) are considered together to be the tooth germ.

4. Bell Stage:

This occurs for primary dentition between the eleventh and the twelfth week of prenatal development. It is characterised by the continuation of the process of proliferation, differentiation and morphogenesis. Differentiation produces four types of cells within the enamel/dental organ:

  1. Inner enamel epithelium
  2. Outer enamel epithelium
  3. Stellate reticulum
  4. Stratum intermedium

At the same time, the dental papilla undergoes differentiation and produces two types of cells:

  1. Outer cells of the dental papilla that forms the dentin secreting cells (odontoblasts).
  2. Central cells of the dental papilla which forms the primordium of the pulp

The dental sac (follicle) increases its collagen content and differentiates at a later stage than the enamel/dental organ and the dental papilla.

5. Apposition Stage:

In this stage, the enamel, cementum and dentin are secreted in layers. These tissues are initially secreted as a matrix that is partially calcified, yet serves as a framework for later calcification.

6. Maturation Stage (Crown Stage):

This stage is characterised by the completion of calcification. Hard tissues including enamel and dentin develop during this stage. Formation of dentin, known as dentinogenesis, is the first identifiable feature of this stage. The formation of dentin must always occur before the formation of enamel. The different stages of formation of dentin results in different types of dentin: mantle dentin, primary dentin, secondary dentin and tertiary dentin.

Odontoblasts (The dentin forming cells):

These are different from the mesenchyme of the dental papilla and it occurs after the differentiation of pre-AB begins. They begin secreting an organic matrix, closest to the area of the future cusp of a tooth, and this organic matrix contains collagen fibres. The odontoblasts are responsible for the odntoblasts process.

The odontoblasts process causes the secretion of hydroxyapatite crystals and the mineralisation of the matrix. This area is known as mantle dentin. Mantle dentin forms from the pre-existing ground substance of the dental papilla. Primary dentin forms through different process whereas the secondary dentin is formed after finishing of the root formation. Tertiary dentin (reparative dentin) forms as a result of stimuli for example dentin carries.

Enamel formation is known as Amelogenesis and occurs in the crown stage of the tooth development. Generally speaking, the enamel formation takes place in two stages: the secretion and maturation stages.

In the secretion stage, ameloblasts release enamel proteins that contribute to the enamel matrix. The appearance of this mineralised tissue occurs usually around the third or fourth month of pregnancy.

In the maturation stage, the ameloblasts transports some of the substances used in formation of enamel out of the enamel and so the function of ameloblasts also changes into the transportation of substances. Most of the minerals transported by ameloblasts in this stage are proteins used to complete mineralisation. By the end of this stage, the enamel has completed its mineralisation.



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