Biomineralization and Mineral Structures in Dental Tissues

Mineral

A solid with one specific chemical composition and a crystalline, highly ordered arrangement

Biomineral

A mineral produced by living organisms (e.g., apatite, calcium carbonate, silica) that functions in support, defense, or feeding

Biomineralization

The process by which organisms produce mineral to stiffen or harden tissues to withstand mechanical forces

Why are biominerals tougher than the mineral itself

Biominerals can be 1000x or more tougher than the pure mineral due to organic matrix organization and controlled crystal structure

Hydroxyapatite (HA/HAP)

The main biomineral in bone and teeth; formula Ca10(PO4)6(OH)2, hexagonal shape, crystal unit cell has 2 entities

Unit cell

The smallest amount of Ca, phosphate, and hydroxyl ions that form a stable, repeating arrangement in a crystal

Ion substitutions in HA

Mg/Na can substitute for Ca; F/Cl can substitute for hydroxyl; carbonate can substitute for hydroxyl and phosphate

Effect of fluoride substitution

Decreases solubility of hydroxyapatite crystals (makes them more resistant to acid)

Effect of carbonate substitution

Increases solubility of hydroxyapatite crystals

Effect of magnesium

Inhibits crystal growth

Crystalline vs polycrystalline vs amorphous

Crystalline = very regular; polycrystalline = more irregular; amorphous = transition state with no defined arrangement

4 General Principles of Biomineralization

1) Solubility of ions, 2) Saturation of ions, 3) Mineral/crystal initiation (nucleation), 4) Mineral/crystal growth (elongation)

Ionic Product (IP)

The product of ion concentrations in solution; compared to Ksp to determine mineralization direction

Solubility Product (Ksp)

The equilibrium constant for a mineral dissolving into its ions; depends on pH and ion concentration

IP = Ksp

Equilibrium (on the solubility isotherm line)

IP < Ksp

Demineralization (undersaturated; crystal will shrink/dissolve)

IP > Ksp

Remineralization (supersaturated; crystal can grow)

DCPD

Dicalcium Phosphate Dihydrate, CaHPO4·2H2O — most soluble calcium phosphate phase

OCP

Octacalcium Phosphate, Ca8H2(PO4)6·5H2O

TCP

Tricalcium Phosphate, Ca3(PO4)2

HAP solubility ranking

Hydroxyapatite has the highest calcium content and is the LEAST soluble (most stable) calcium phosphate phase

Sources of Ca and phosphate ions

GI tract (diet) into extracellular fluid, exchanged with bone via PTH/calcitonin/calcitriol, filtered by kidneys

Maturation stage ameloblasts

Ameloblasts that rapidly add mineral to enamel while gradually losing water and organic material; active for up to 4 years in human permanent teeth

Ruffle-ended ameloblasts

80% of maturation ameloblasts; pH 6.0 (acidic); release Ca2+

Smooth-ended ameloblasts

20% of maturation ameloblasts; pH 7.4; neutralize H+

NHE

Sodium/hydrogen ion exchanger; eliminates H+ generated during enamel mineralization

Carbonic Anhydrase II (CA2)

Catalyzes formation of bicarbonate (HCO3-) which is transported into the enamel matrix via AE2

Carbonic Anhydrase VI (CA6)

Secreted into enamel matrix; combines bicarbonate with H+ (generated by HA formation) to form CO2 and water

KLK4

Enzyme that cleaves enamel proteins during the maturation stage

CNNM4

Transporter that removes Mg2+ from the enamel matrix

Ion clustering pathway to mineral

Ionic Ca and phosphate → ion clusters → Amorphous Calcium Phosphate (ACP) → crystalline Hydroxyapatite (HAP)

Ca:P ratio progression

Cluster 1.5 → ACP 1.5 → HAP 1.67 (mature crystal has highest ratio)

Collagen fibril role in mineralization

Serves as a scaffold; "holes" and "gaps" between triple helices provide confined space for mineral deposition

Intrafibrillar mineralization

Crystal nucleation and growth aligned INSIDE the collagen fibril

Extrafibrillar mineralization

Crystal deposition in random arrangements OUTSIDE the collagen fibril

Noncollagenous proteins (NCPs) in mineralization

Bind calcium/phosphate ions and clusters on collagen surfaces, promoting nucleation of crystals

Matrix vesicles

Small (25-250 nm) membrane-bound structures that bud off osteoblasts, chondrocytes, and odontoblasts; rich in phospholipids; site of Ca/Pi accumulation and initial HA formation

Annexins

Channel proteins that bring Ca2+ into the matrix vesicle lumen

TNAP

Tissue-nonspecific alkaline phosphatase; generates inorganic phosphate (Pi) inside matrix vesicles

PHOSPHO1

Orphan phosphatase 1; generates Pi from phosphocholine inside matrix vesicles

ENPP1

Ectonucleotide pyrophosphatase/phosphodiesterase 1; converts ATP to PPi, contributing to Pi generation

Hypophosphatasia (HPP)

Genetic disease caused by mutation in ALPL gene (encodes TNAP); low alkaline phosphatase activity causes dental/skeletal defects; childhood form severe, adult form milder

HPP dental findings

Premature loss of deciduous teeth without periodontal disease, thin roots, more repair cementum, more root resorption

Nucleation

The first step of crystal formation; attraction of Ca2+ and PO4 3- ions cluster in a nucleus, reaching supersaturation, then precipitation

Crystal growth

Occurs from a nucleus via reaction of soluble ions at the crystal surface; crystal grows from the inside out

Acidic matrix proteins and crystal shape

Bind selectively to specific crystal faces (e.g., the a-axis/(100) face), preventing growth in that direction and forcing elongation along the c-axis

Secretory Calcium-binding Phosphoproteins (SCPP)

A gene family/cluster of non-collagenous, acidic and proline/glutamine-rich proteins that evolved from the SPARC gene; found in enamel, dentin, bone, cementum, milk, and saliva

SCPP shared structural features

Signal peptide (for secretion), calcium binding ability, exon-intron structure, phosphorylation on serine

SPARC

Basement membrane gene from which the SCPP gene cluster arose via whole genome duplication

DSPP

Dentin Sialophosphoprotein; single gene product cleaved into DPP, DSP, and DGP; mineralizes dentin only (not enamel)

DPP (Dentin Phosphoprotein)

Highly phosphorylated fragment of DSPP with DSS repeat sequence; strong calcium-binding affinity via phosphate/carboxyl groups

DSP (Dentin Sialoprotein)

Fragment of DSPP; localizes to dentin, cementum, pulp, and alveolar bone; concentrated around odontoblast processes

Peritubular dentin

Dentin matrix with less collagen, located around the dentinal tubules, high in DSP

Intertubular dentin

Dentin matrix between tubules; constitutes the bulk of dentin

IBSP / BSP

Integrin Bone Sialoprotein; acidic phosphorylated glycoprotein with 10 glutamic acids for Ca binding, RGD integrin-binding site; nucleates HA and anchors protein to cell surface before transfer to collagen

BSP localization

Acellular (AEFC) and cellular (CIFC) cementum and bone

OPN (Osteopontin)

Gene for cementum mineralization; phosphoprotein with RGD site and poly-aspartic acid (Asp)n sequence that INHIBITS HA crystal growth; also mediates cell attachment

OPN localization

Concentrated in acellular extrinsic fiber cementum (AEFC), especially near dentin and between inserted Sharpey's fibers

RGD sequence

Arginine-Glycine-Aspartate; integrin-binding cell attachment motif found in BSP and OPN

Integrin

Cell surface receptor that binds the RGD sequence, linking matrix to cell

AMELX (Amelogenin)

Main gene/protein (80-90%) of enamel matrix; hydrophobic, intrinsically disordered, self-assembles into 20 nm nanospheres; regulates crystal spacing

Ameloblastin (AMBN)

Enamel protein involved in initiation of mineralization

Enamelin (ENAM)

Enamel protein involved in initiation of mineralization

MMP20

Matrix metalloproteinase that cleaves enamel proteins during the secretory stage

Enamel ribbons

Long, closely packed crystal structures of enamel; grow as flattened hexagons (60-70 nm wide, 20-30 nm thick), organized by the Tomes' process of the ameloblast into rods and interrods

Tomes' process

Specialized secretory end of the ameloblast that organizes enamel crystal ribbons into rods and interrods

Dentin gene/protein example

DSPP - Dentin phosphoprotein

Bone gene/protein example

IBSP - Integrin Bone sialoprotein

Cementum gene/protein example

OPN - Osteopontin

Enamel gene/protein example

AMELX - Amelogenin

Evolutionary evidence for SCPP importance

Inactivating mutations in DSPP, AMELX, MMP20, AMTN, ENAM, AMBN are linked to loss of enamel or teeth across vertebrate lineages (e.g., aardvarks, sloths, baleen whales, toothless birds)

Amelogenesis Imperfecta

Human genetic condition from mutations in enamel genes causing pitted, discolored, hypoplastic, or rough enamel

Bone mineral shape

Plate-like crystals ~10 nm x 20 nm x 5 nm, arranged in random woven or lamellar sheet patterns

Enamel mineral composition

~95% hydroxyapatite; formed by epithelial cells (ameloblasts) rather than mesenchymal cells

Type of collagen in dentin, cementum, bone

Type I collagen mainly (bone also has Type III)