Biomineralisation of enamel and dentine

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Presentation Information

Presented by: Ramin FarahaniCourse: SDDM5115 – Biomineralisation of Enamel and Dentine

Learning Outcomes

Upon completion of this topic, you will be able to:

• Describe the molecular basis of biomineralization: Understand the intricate biological processes involved in mineral formation by living organisms.

• Identify key components and steps in the biomineralization of enamel and dentine: Recognize the essential proteins, ions, and cellular mechanisms that contribute to the formation of these dental tissues.

• Recognize major differences in the formation of enamel and dentine: Differentiate between the unique biochemical pathways and structural characteristics of these two types of mineralized dental tissues.

General Principles of Biomineralisation

Biomineralisation refers to the process through which living organisms produce minerals. This section introduces various foundational principles that underpin these biological processes and explores how they contribute to the function and structure of biological systems.

Definition of Biomineralization

Biomineralization is the set of processes by which organisms form minerals, which can be categorized as follows:

• Amorphous Forms: Such as amorphous silica and amorphous hydrous iron phosphate, which lack a defined crystalline structure.

• Crystallized Forms: These minerals acquire well-defined structured forms through biological processes, crucial for their functional roles in nature.

Example of Biomineralization

An example of biomineralization is observed in the holothurian (Molpadia), where granules of amorphous hydrous iron phosphate are present in the skin. The diameter of the largest granule measures approximately 200 microns, illustrating the diverse scales at which biomineralization can occur.

Summary of Biomineralization Characteristics

Biomineralization exhibits several notable features:

• Amorphous and Crystallized Forms: Both forms play significant roles in structural and functional applications within biological systems.

• Complex Morphology: The three-dimensional shapes of biominerals can be highly intricate, reflecting their biological roles and the environmental conditions under which they formed.

• Matrix-Mediated Processes: Many biominerals are formed through processes involving a biological matrix, which guides mineral growth and serves functional roles in living organisms.

Molecular Basis of Biomineralisation

This section introduces the molecular mechanisms involved in biomineralization, detailing interactions among proteins, ions, and other biological molecules crucial for mineral formation.

Crystal Structure Basics

Unit Cell: The smallest repeating unit of a crystal, which contains all interionic distances and angles necessary for describing the entire crystal structure. For example, the NaCl unit cell contains a total of 4 sodium (Na+) ions and 4 chloride (Cl–) ions arranged in a cubic lattice.

Crystal Growth Mechanism

The growth of crystals involves the stacking of unit cells, where ions interdigitate across adjacent unit cells. This process can be visualized as assembling "molecular LEGOs," where specific ions, such as calcium (Ca2+) and phosphate (P), share surfaces and contribute to crystal integrity.

Initiation of Nucleation

Heterogeneous Nucleation: This type of nucleation occurs when the template used for initiation differs from the components of the crystal itself. Initially, certain proteins align with surfaces, leading to the first ions binding. Initiator proteins are crucial for starting the nucleation process, facilitating the transition from unstructured precursors to organized crystal growth.

Process of Nucleation

Nucleation involves a layering process in the formation of crystals where initial ions bind together to form layers, ultimately resulting in complete crystal growth. Key initiators in this stage include phosphoryns and bone sialoprotein II, which facilitate the development of the crystal structure.

Crystal Growth Process

Crystal growth occurs within matrix vesicles containing critical proteins such as bone sialoprotein II and phosphoryns derived from osteoblasts and odontoblasts. These vesicles contain transmembrane calcium pumps (CaATPase) that aid in the transport of ions necessary for crystallite development. ATP hydrolysis provides the required energy for these processes, highlighting the energetic demands of biomineralization.

Focus on Dentine and Enamel Biomineralisation

This section focuses on understanding the unique mineralization processes involved in forming enamel and dentine, which are distinct yet critical components of the tooth structure. Each has specialized pathways that reflect their functional requirements and roles in dental health.

Mineralization in Bone and Dentin

In these processes, crystallites are initially deposited in cavities or parallels aligned with tropocollagen strands. Understanding the tropocollagen structure is essential, as it accommodates and influences the growth of the associated crystallites, impacting the mechanical properties and resilience of the tissues.

Enamel Mineralization Process

Key Proteins in Enamel Formation include:

• Enamelins: These proteins act as nucleators, facilitating the formation of enamel crystals and coating mature hydroxyapatite (HA) crystallites.

• Amelogenins: Crucial in regulating crystallite growth, these proteins undergo enzymatic digestion during the maturation stage of enamel development.

Functions of Enamel Proteins

An overview of enamel proteins during the formation process:

• Amelogenin: Synthesized and accumulated during secretion, playing a significant role in the subsequent structures.

• Nonamelogenins (enamalin): Undergo limited degradation yet remain significant for maintaining enamel integrity.

• Ameloblastin: Functions primarily in cell adhesion, but is subjected to rapid degradation following its role.

• Tuftelin: Localizes at the Dentino-Enamel Junction (DEJ), contributing to the structural integrity of dental tissues.

Enzymatic degradation of these proteins involves key enzymes such as enamelysin and metalloproteinase MMP20, both playing critical roles in the maturation and turnover of enamel proteins, influencing the overall quality and durability of the enamel layer.