Root Development

Cervical Loop

Leading edge of the IEE and OEE are not separated by other cel types

Peripheral region in a developing tooth where inner and outer enamel epithelium meet

Hetwig’s Epithelial Root Sheath (HERS)

As the cervical loop continues to grow around the dental papillae and extends deeper

The pattern of this determines the shape of the root

Grows faster in some regions as it extends over dental papillae, producing multiple roots on larger teeth

Key Stages of Root Development

Cervical Loop

Root Formation

Root Formation

After crown development is complete, the cervical loop proliferates to HERS, which outlines the shape and number of roots

Initiation of Root

Triggered when enamel and dentin formation reach the CEJ

Once crown formation is complete, epithelial cells of the IEE and OEE proliferate from the cervical loop of the enamel to form HERS

Role of HERS

Bilayer of IEE and OEE extends downward to define shape and number of roots

Grows apically and induce dentin formation

HERS after Dentin

Breaks down, allowing dental follicle cell to differentiate into cementoblasts, forming cementum, while other cells develop PDL

Cementoblasts

Form cementum

IEE in HERS

Induce the formation of odontoblasts in the roots, but when they form pre-dentin, the lack of signal from stellate reticulu means the differentiation of ameloblasts does not occur

Stellate Reticulum

Not present in HERs

Does no differentiate into ameloblasts after pre-dentin formation

Apoptosis

IEE and OEE undergo this to kill themselves

Epithelial Rests of Malassez (ERM)

IEE and OEE undergo apoptosis with the remaining cells being this

Play a role regeneration of damaged root tissues

Cementoblasts

After most epithelial cells are removed, neuromesnchymal stem cells in the dental ac contact pre-dentin, which induces them to differentiate into this

Cover the root dentin with thin layer

Cementocytes

Cementoblasts traped within the ECM

Root Dentin Formation

HERS induces dental papilla cells to become odontoblasts, which deposit root dentin

Cementogenesis

After dentin forms, HERS fragments, allowing dental follicle cells to contact dentin an differentiate into cementoblasts, which lay down cementum

PDL

Cells from the dental follicle also differentiate into fibroblasts to create this

Connects tooth to the alveolar bone

PDL Formation

After root development neuro-mesenchymal stem cells of the dental sac that are not in contact with dentin differentiate into fibroblasts and osteoblasts, which form this

Occurs during tooth eruption - after enamel and dentin formation

Sharpey’s Fibers

As HERS breaks apart and cementum is deposited on the root dentin, fibroblasts invade between the epithelial rests and deposit collagen fibers so that one end end becomes entrapped in the forming cementum

The other end of the fibers attaches to the forming alveolar bone

PDL Origin

Fibers originate from the dental follicle, a cluster of cells surrounding the developing tooth germ

As root develops, these cells differentiate into fibroblasts that produce collagen fibers, which eventually anchor into the cementum and alveolar bone

Fiber Orientation

Initially, fibers are arranged parallel to the root surface

As tooth erupts, these fibers reorganize to take an oblique formation, which helps them withstand compressive forces of chewing

Key Cells of PDL

Primarily composed of fibroblasts. which is responsible for the continous remodeling of the fibrous matrix

Sharpey’s Fibers

Ends of the PDL fibers that insert into the cementum and alveolar bone, anchoring the tooth

Intermediate Plexus

Area in the middle of the ligament space where fibers from the cementum side and bone side meet and fuse

Regeneration of PDL

Capacity to regenerate tissue

Highly adapted to its function of anchoring, shock absorption, and sensory input, providing a tough, vascularized attachment

Active Eruption

Movement of crowns into the oral cavity

Tooth Eruption

Growing roots encounter mineralized bone tissue, putting hydrostatic pressure on the dells of the dental organ

Stellate reticulum secretes RANKL to induce bone resorption (induces osteoclast differentiation), and PH, activating osteoclasts

Stellate Reticulum in Eruption

Secretes morphogens that induce bone resorption

Found at surface of developing crown enamel clearing a path for the tooth to erupt

RANKL

Induces osteoclasts differentiation

Parathyroid Hormone

Activates osteoclast activity

Root Growth with Eruption

Pressure on bone tissue in the alveolar sockets increases

Triggers release of bone-simulating morphogens (BMPs), which lead to bone deposition, pushing teeth otuward

Bite forces contribute to BMP secretion

Passive Eruption

Remodeling of the oral mucosa to allow teeth to erupt (gingiva)

Old tissue is removed to clear a path for a tooth to erupt

Oral mucosa must be connected to the tooth as it erupts, otherwise oral bacteria could enter the submucosa and trigger an infection or inflammatory response

Reduced Enamel Epithelium (REE)

Before tooth eruption begins the crown has 2 layers of epithelial cells on surface of enamel

Ameloblasts and OEE

Protective layer of enamel against CT contact, preventing root resorption, and aiding in tooth eruption via desmolytic enzymes and matures into junctional epithelium

REE during Eruption

Re-joins the oral epithelium, forming desmosomes and other cell-cell junctions with oral epithelium cells, while maintaining contact with enamel

Undergo apoptosis, but those that remain become junctional epithelium

Junctional Epithelium

Maintains hemidesmosomes contact on both sides of the tissue: enamel and submucosa

Attaches the gingiva to the tooth surface sealing against bacteria

Forms from the REE

Lack of REE

No new enamel can be produced by human cells after eruption

Gingiva cannot form junctional epithelium with dental implants

Anodontia

Failure in the initiation stage resulting in missing teeth

Inadequate release of morphogens by neuro-mesenchymal stem cells, mutations in the morphogen receptor proteins, teratogens interfere with binding of morphogen to receptor activating secondary messenger

Partial Anodontia (Hypodontia)

One or few teeth missing

Most commonly - Max lateral, 3rd molar, mandibular 2nd pre-molar

Hyperdontia

Formation of extra teeth, occurs when the initiation of the tooth germ occurs where it shouldn’t

Most common: Central incisors (mesiodens), distal to the maxillary 3rd molar, and the premolar region

Mesiodens

Hyperdontia in central incisors

Distomolar

Hyperdontia in the maxillary 3rd

Perimolar

Hyperdontia in the premolar region

Gemination

Occurs when a single tooth germ is partially divided in two

Occurs if growing tooth germ bumps into a small dense region in dental arch

Healthy condition - teeth grow within loose mesenchyme tissue, which later differentiates into osseous tissue and calcifies

Coming from a single tooth germ, is larger than average but has a single pulp cavity

Tooth Fusion

Two tooth germs into a single germ causing a larger than average tooth to develop

Macrodontia with hypodontia

Teeth have two separate pulp cavities occurs when initiation of two tooth germs occurs too close to one another, or when external pressure forces them closer together

Two seperate buds that grow into a larger structure

Primary Failure of Eruption (PEE)

Partial or complete failure of a tooth to erupt (impacted tooth) despite a healthy eruption pathway

Dentigerous Cyst

Impacted tooth can produce a cyst around the crown

Tooth attempts to digest its way through the oral epithelium, fluid accumulates between the crown and REE

Cyst may continue to grow in size, causing pain and trauma to the jawbone

Extraction of the tooth will solve the issue, on orthodontic facilitation of tooth eruption may be an option as well

Rarely, the REE in a dentingerous cyst develops into a benign tumor know as an ameloblastoma