Development of the tooth

Introduction

Tooth development (odontogenesis) is a long and quite organised process that begins early in embryonic life. It involves interaction between oral epithelium and the underlying ectomesenchyme (neural crest origin). These two tissues constantly “talk” to each other, guiding the formation of enamel, dentin, cementum and the periodontal structures. Although the steps sound complicated at first, they can be visualised like a small growing bud that slowly shapes into a mature tooth.

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Main Stages of Tooth Development

Traditionally, odontogenesis is divided into bud stage, cap stage, bell stage, and root formation. Each stage has characteristic shapes and cellular events.

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1. Bud Stage (around week 6)

The first sign is a thickening of the oral ectoderm called the dental lamina.

From this lamina, small epithelial buds grow downward into the mesenchyme.

• Looks like: tiny round epithelial bumps.
• No clear organisation yet.
• Ectomesenchyme around the bud starts accumulating.

This stage basically says: “The place of a future tooth is marked.

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2. Cap Stage

The epithelial bud enlarges and begins to fold, forming a cap shape over a ball of condensed mesenchyme.

The main structures here are:

• Enamel organ – the epithelial cap; will eventually produce enamel.
• 
  Dental papilla – condensed mesenchyme inside the cap; will give rise to dentin and pulp.
• Dental follicle (dental sac) – mesenchyme surrounding the whole thing; will form cementum, periodontal ligament, alveolar bone.

This stage is also where morphogenesis begins — the tooth crown shape starts being defined (molar vs. incisor, etc).

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3. Bell Stage

The enamel organ deepens and forms a bell-like shape. It now has four distinct layers, easy to spot in diagrams:

1. Outer enamel epithelium (OEE) – protective layer.
2. Inner enamel epithelium (IEE) – these cells later transform into ameloblasts that make enamel.
3. Stratum intermedium – supports ameloblasts.
4. 
   Stellate reticulum – star-shaped cells that create a soft, cushion-like space.

At this stage, histodifferentiation occurs:

• IEE → ameloblasts
• Dental papilla cells → odontoblasts

The future crown shape becomes even more precise.

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Formation of Hard Tissues

This is the key event where the tooth starts to become “hard”.

Dentin Formation (Dentinogenesis)

Odontoblasts at the border of the dental papilla begin secreting predentin, which later mineralises into dentin.

• Odontoblasts leave behind dentinal tubules as they retreat inward.
• Dentin always forms first before enamel.

Enamel Formation (Amelogenesis)

Ameloblasts form enamel only after dentin is present.

• They lay down enamel in rods/prisms.
• 
  Ameloblasts move outward, leaving enamel behind.
• Once the tooth erupts, enamel cannot regenerate because ameloblasts disappear.

A nice mental picture: dentin grows inward, enamel grows outward like two teams building walls back-to-back.

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Root Development

Root formation starts after the crown is mostly finished.

The cervical loop (where IEE and OEE meet) bends downward to form Hertwig’s epithelial root sheath (HERS).

Functions of HERS:

• 
  Guides root shape and number of roots.
• Stimulates the formation of root dentin by odontoblasts.
• Eventually breaks down to allow dental sac cells to contact the dentin and become cementoblasts.

Cementum Formation

• Cementoblasts deposit cementum on the root dentin.
• Some cementoblasts get trapped → become cementocytes (similar to osteocytes).

Periodontal Ligament Formation

Cells from the dental follicle produce collagen fibres that insert into cementum and alveolar bone, forming the PDL.

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Tooth Eruption

The eruptive phase begins once roots start forming. The tooth moves toward the oral cavity, guided partly by PDL traction and partly by bone remodelling. Before eruption, the enamel is covered by a thin layer called the reduced enamel epithelium, which later becomes the junctional epithelium around the tooth.

Bibriography

B83006 Histology and General Embryology 24/25 RNDr. Lucie Fraser Lantová, Ph.D.

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