Dental assisting/ hygiene assistant

Teeth develop from epithelial cells from the mu-

cosal lining of the oral cavity and cranial neural

crest-derived ectomesenchymal cells. These latter

cells originate at the ectodermal/neuroectoder-

mal junction of the developing brain, extending

rostrally from the caudal boundary of the hind-

brain and neural tube to the midbrain and cau-

dal forebrain. They give rise to most connective

tissues in the craniofacial region including the

bones of the calvarium, face, and jaws. Since these bones that are formed by mesenchymal cells have

an ectodermal/neuroectodermal ancestry, they

are also known as ectomesenchyme [4–7].

The process of tooth formation starts with the

formation of the dental lamina, a sheet of epithe- lial cells extending from the lining of the oral cav- ity into the underlying ectomesenchyme. In this

dental lamina, focal bud-like thickenings deter-

mine the sites of the future teeth, 20 for the de- ciduous dentition and 32 for the permanent one,

and together with a surrounding aggregation of

ectomesenchymal cells they represent the earliest

stage of the tooth germ (Figs. 1.1a, 1.2).

Subsequently, the epithelial bud transforms

into a cap and from that point on is called enamel organ. Due to the formation of a concavity along

the inner surface, the cap transforms into a bell. Ectomesenchymal cells lying within this concav-

ity form the dental papilla that will become the

dental pulp, the soft tissue core of the teeth. Other ectomesenchymal cells surround the enamel or-

gan and form the dental follicle, the fibrous bag

that invests the tooth germ and separates it from

the adjacent jaw bone. Within the bell-shaped

enamel organ, three different components are

discerned: the inner enamel epithelium facing

the dental papilla, the outer enamel epithelium

lying adjacent to the dental follicle, and the inter- vening loose stellate epithelium that is called the stellate reticulum (Figs. 1.1b, 1.3).

From the bell stage onwards, reciprocal in-

ductive events take place causing inner enamel

epithelium and adjacent dental papilla cells to

develop into enamel-forming ameloblasts and

dentin-producing odontoblasts. The differen-

tiation of dental papilla cells into odontoblasts

Fig. 1.1a–c Diagram showing consecutive stages of early tooth formation. a Bud stage, b Cap stage, and c Bell stage. Epithelium shown in blue, ectomesenchyme in pink, dentin in yellow, and enamel in grey 1

Chapter 1

Tooth Formation

1

requires a stimulus from the inner enamel epi-

thelium, whereas the terminal differentiation of

inner enamel epithelium into enamel-producing

ameloblasts cannot occur without the presence

of dentin. Therefore, dentin production occurs

before enamel production and this is recapitu-

lated in some odontogenic lesions that may dis-

play dentin formation but no enamel production,

whereas the converse is never seen.

The odontoblasts form a matrix of collagen fi-

bres called predentin that subsequently calcifies

to become dentin. During dentinogenesis, odon-

toblasts recede from the dentino-enamel junc-

tion leaving a cytoplasmic extension behind in

the deposited dentinal matrix. This explains why

dentin has a tubular architecture, each dentinal

tubule containing the cytoplasmic process of a

single odontoblast (Figs. 1.1c, 1.4a, b).

Deposition of enamel starts after a tiny amount

of dentin has been formed at the interface be-

tween future ameloblasts and odontoblasts. The

enamel matrix subsequently calcifies to consist of approximately 95% minerals. This high mineral

content explains why it does not withstand decal-

cification needed for histology.

While ameloblasts and odontoblasts are de-

positing enamel and dentin, inner and outer

enamel epithelial cells join and proliferate in a

downward way to encircle an increasing part of

the dental papilla, thus creating a tube that maps out the form and size of the root of the teeth.

This epithelial cuff is known as the epithelial root sheath or root sheath of Hertwig. In this root

sheath, the inner enamel epithelium does not

differentiate into enamel producing ameloblasts

anymore but still induces the dental papilla cells to become odontoblasts that have to form the

root dentin (Figs. 1.5, 1.6). Thereafter, Hertwig’s root sheath fragmentates. In this way, ectomes-

enchymal cells from the dental follicle gain ac-

cess to the root surface. These cells differentiate into cementoblasts and secrete cementoid on the

surface of the intermediate cementum laid down

before by them as an initial layer. Cementoid cal- cifies to become cementum. Whether cells from

Hertwig’s root sheath also contribute to initial ce- mentum formation is controversial [1, 3, 8].

Besides the formation of cementum, dental

follicle ectomesenchymal cells are also respon-

sible for the formation of the other periodontal

tissues: parts of the bony alveolar socket and the collagenous periodontal ligament that connects

the tooth with this socket.

Remnants of Hertwig’s root sheath form a per-

manent component of the periodontal ligament;

they are known as rests of Malassez and are the

source of some cystic jaw lesions. Moreover, these epithelial rests probably play a role in preventing contact between root surface and alveolar socket

bone, thus ensuring preservation of tooth mobil-

Fig. 1.2 Epithelial thickening of oral epithelium with underlying ectomesenchyme

Fig. 1.3 Cap stage of tooth germ showing enamel organ composed of loose stellate reticulum bordered by inner enamel epithelium facing the ectomesenchymal con-

densation that will develop into the dental papilla (see Fig. 1.4)

Tooth Formation Chapter 1

Fig. 1.4 a Bell stage of tooth germ. b At the tip of the dental papilla, deposition of enamel (purple) and dentin (pink) has started

Fig. 1.5 Tooth germ in its late development. The crown is completely formed. The root is still growing at its api- cal tip, cells from the dental papilla being recruited to become dentin-forming odontoblasts. At this site, inner enamel epithelium still induces development of odon- toblasts with subsequent dentin formation, but the sub- sequent differentiation of epithelium into ameloblasts forming enamel does not take place in root formation.

(Drawing by John A.M. de Groot)

Fig. 1.6 Apical part of tooth germ showing Hertwig’s epithelial root sheath. Cylindrical odontoblasts form dentin adjacent to the basal side of the opposing inner enamel epithelium

1

ity and inhibiting root resorption [2]. Other epi- thelial reminiscencies to the tooth development

lie more superficially in the jaw tissues; they are the epithelial rests of Serres, which originate from the dental lamina.

When the formation of the crown is complete,

the enamel organ atrophies. The stellate reticu-

lum disappears and inner and outer enamel epi-

thelium form an epithelial covering of the tooth

crown the so-called reduced enamel epithe-

lium which remains present until the tooth

erupts into the oral cavity. Fluid accumulation

between enamel surface and this epithelial in-

vestment may give origin to cystic lesions.

In humans, tooth formation starts already at the

6th week of embryonic life. It continues until early adulthood when the roots of the permanent 3rd

molars reach their completion. The various stages

of tooth development are clearly displayed by jaw

radiographs taken from children with a mixed

dentition. These radiographs show fully formed

deciduous teeth and permanent teeth in varying

stages of development. One should realise that

all above-described developmental stages may be

observed in the jaw at one and the same time as

odontogenesis takes place from early embryonic

life until early adolescence. To mention just one

example, inner and outer enamel epithelium may

show active proliferation at the developing root

tip while slightly more coronally, the root sheath dissolves and cementoblasts from the dental fol-

licle are lying down cementum, both these events

occurring within a distance of only 1 mm from

each other at the developing root surface.

References

1. Diekwisch TGH (2001) Developmental biology of

cementum. Int J Dev Biol 45:695–706

2. Fujiyama K, Yamashiro T, Fukunaga T et al. (2004) Denervation resulting in dento-alveolar ankylosis

associated with decreased Malassez epithelium. J

Dent Res 83:625–629

3. Luan X, Ito Y, Diekwisch TGH (2006) Evolu-

tion and development of Hertwig’s epithelial root

sheath. Developmental Dynamics 235:1167–1180

4. Miletich I, Sharpe PT (2004) Neural crest contribu- tion to mammalian tooth formation. Birth Defects

Research (Part C) 72:200–212

5. Sharpe PT (2001) Neural crest and tooth morpho- genesis. Adv Dent Res 15:4–7

6. Thesleff I, Keränen S, Jernvall J (2001) Enamel knots as signaling centers linking tooth morpho-

genesis and odontoblast differentiation. Adv Dent

Res 15:14–18

7. Tucker A, Sharpe P (2004) The cutting edge of

mammalian development. How the embryo makes

teeth. Nat Rev Genet 5:499–508

8. Zeichner-David M, Oishi K, Su Z et al. (2003)

Role of Hertwig’s epithelial root sheath cells in

tooth root development. Developmental Dynamics

228:651–663

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