Tooth Development 3

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بسم الله الرحمن الرحيم

IN THE NAME OF ALLAH THE MOST GRACIOUS THE MOST MERCIFUL

Niagra Falls

DEVELOPMENT AND GROWTH OF TEETH

Prof. Abdelhamied Y. Saad, BDS, MS (Egypt), Ph.D. (U.S.A.)

• The stomodeum is lined with ectoderm.

• It is separated from the blind upper end of the foregut by bucco-pharyngeal membrane.

• This membrane ruptures at about 27 days.

The ectoderm consists of:

1. A basal layer of columnar cells.

2. A surface layer of flattened cells.

• Most of the C.T. cells underlying the oral ectoderm are neural crest cells or ectomesenchyme in origin.

Dental lamina appears after 37 days of development

• It is continuous band of thickened epithelium in the presumptive upper and lower jaws.

• It is roughly horse-shoe shaped structure corresponding to the position of the future deciduous teeth.

• Each tooth develops from a tooth germ that forms from the lining of the oral cavity.

A tooth germ consists of 3 parts:

1. An enamel organ; derived from the oral ectoderm and produces the enamel.

2. A dental papilla, derived from the ectomesenchyme and produces dentin and pulp.

3. A dental sac (tooth follicle); derived from the ectomesenchyme and produces cementum, PDL, and alveolar bone.

Developmental Stages

• Bud stage

• Cap stage

• Bell stage

Bud Stage

Cap Stage

E. navel

Function of Enamel Knot

1. It may act as signaling center, producing a unique set of signaling molecules for tooth development.

Function of Enamel Knot

2. Acts as an organizing center that initiates cuspal morphogenesis and crown pattern formation.

Fate of Enamel Knot

• It undergoes programmed cellular death (apoptosis) at the onset of the early bell stage.

Bell Stage

• Early

• Late (advanced)

Early Bell Stage Morpho-differentiationProliferation

HistodifferentiationInduction

outer dental epithelium

inner dental epithelium

stellate reticulum

Dental papilla

Dental sac “follicle”

main dental lamina

lateral dental lamina

successional dental lamina

cervical loop

Fine Structure of the Enamel Organ at the Bell Stage

• It is relatively uncomplicated but must be understood to know the ultrastructural changes occurring in the formation of the dental hard tissue, enamel and dentin.

• The enamel organ is supported by a basal lamina around its periphery.

• The outer enamel epithelial cells are cuboidal and have a high nuclear / cytoplasmic ratio (little cytoplasm).

• Their cytoplasm contains:(1) free ribosomes,(2) few endoplasmic reticulum,(3) some mitochondria, and(4) a few scattered tonofilaments.

• Adjacent cells are joined by junctional complexes.

• The stellate reticulum cells are connected to each other, to the cells of the outer enamel epithelium, and to stratum intermedium by desmosomes.

• Their cytoplasm contains all the usual cytoplasmic organelles, but these are sparsely distributed.

• The cells of the stratum intermedium are connected to each other and to the cells of the stellate reticulum and inner enamel epithelium by desmosomes.

• Their cytoplasm also contains the usual complement of organelles and tonofilaments.

• The inner enamel epithelial cells have a centrally placed nucleus and a cytoplasm that contains:

(1) Free ribosomes.

(2) A few scattered rough endoplasmic reticulum.

(3) Mitochondria evenly dispersed.

(4) Some tonofilaments.

(5) A Golgi complex situated towards the stratum

intermedium.

(6) A high glycogen content.

• Before forming the first dentin, cells of the enamel organ and, in particular, those of the inner enamel epithelium receive nourishment from two sources:

1. Blood vessels located in the dental papilla.

2. Vessels situated along the periphery of the outer enamel epithelium.

• When the dentin is formed, it cuts off the papillary source of nutrients.

• This occurs at a time when the cells of the inner enamel epithelium are about to secrete enamel.

• An apparent collapse of the stellat reticulum will occur and the ameloblasts will be approximated to the blood vessels lying outside the outer enamel epithelium.

Both dentinogenesis and amelogenesis have begun.

Note collapse of the stellate reticulum & folding of IEE.

• Until this point the ameloblasts meet their metabolic requirements by:

1. Using the glycogen stored in their cytoplasm, and

2. Probably also by using some of the extracellular components of the stellat reticulum.

• Epithelial- ectomesenchymal inductive interaction during normal odontogenesis lead to:

1. Cytodifferentiation of dentin and enamel forming cells, as well as

2. To subsequent dental hard tissue formation.

• It has been demonstrated that extracellular matrix composed of:

1. Collagen,2. Glycoproteins,3. Glycosaminoglycans, and4. Additional macro-molecules may

represent an important factor in mediating developmental events of odontogenesis by inducing an organizing influence between epithelial and ectomesenchyme.

Function of the Dental Lamina

1. Initiation of the entire deciduous dentition.

2. Initiation of the successors of deciduous dentition.

3. Formation of permanent molars by distal extension.

Fate of the Dental Lamina

• During the cap stage, the dental lamina maintains a broad connection with the enamel organ.


• In the bell stage, it begins to break up by mesenchymal invasion, which first penetrates its central portion and divides it into lateral lamina and the dental lamina proper.


• The mesenchymal invasion is at first incomplete and does not perforate the dental lamina.

(Disintegrated)


• The dental lamina proper (successional lamina) proliferates only at its deeper margin, which becomes free and situated lingually to the enamel organ and forms the primordium of the permanent tooth.


• The epithelial connection of the enamel organ with the oral epithelium is severed by the proliferating mesoderm.

• Fragmentation of the dental lamina results in the formation of discrete cluster of epithelial cells that normally degenerate and are resorbed.

• If any persist, they may form small cysts (eruption cysts) over the developing tooth and delay eruption.


• Remnants of the dental lamina persist within the jaw and gingiva as epithelial pearls or islands called epithelial rests of Serres.

Functions of Enamel Organ

1. It has a morphogenetic function that helps to determine the crown pattern.

2. It has an inductive role in initiating coronal and root dentinogenesis and therefore determines the size, shape, and number of roots.

3. It has a formative function in that its cells elaborate enamel.

Functions of Enamel Organ

4. It has a protective function that prevent exposure of the crown to surrounding connective tissue before eruption.

5. It permits tooth eruption.

6. It assists in establishing the dentogingival junction.

Some investigators prefer the term dental organ instead of enamel organ because:

1. It is responsible for determining the shape of the crown.

2. Initiating dentin formation.

3. Establishing the dentogingival junction.

4. Forming enamel.

Vestibular Lamina

• Labial and buccal to the dental lamina another epithelial thickening develops independently and somewhat later. It is the vestibular lamina (lip furrow band).

Vestibular lamina

Main dental lamina

• It subsequently hollows and disappears and forms the oral vestibule (oral sulcus) between the alveolar portion of the jaw and the lips and cheeks.

Root Formation

Multi-rooted Teeth

Epithelial-Mesenchymal Relations • In the developing embryo a functional

relationship exists between epithelium and the mesenchyme that supports it.

• The proper interplay between these tissues are essential for orderly development.

Therefore:

1. Teeth cannot form until ectomesenchyme establishes contact with the epithelium.

2. The shape of the tooth is determined by folding of the dental epithelium, and this folding is dictated by the mesenchyme of the dental papilla.

3. Odontoblast differentiation requires the presence of epithelium.

4. The anatomy of the dentogingival junction (attachment epithelium) is determined by its supporting connective tissue.

… and the list goes on.

How Cells And Tissues Communicate With Each Other• Communication is possible between cells

through either:

1. Direct cell-to-cell contact.

2. The transmission of molecules (cytokines) synthesized and secreted by one cell and then captured by surface receptors of another cells. These molecules which, once captured, can influence cell behavior and phenotype.

• When cells come into contact with each other, junctional arrangements are established that can take several forms such as desmosomes, tight junction, and gap junction.

• Most are concerned with adherence; but one form of junction, the gap junction, permits direct communication between cells which occurs frequently between embryonic cells of the same population.

• This suggests that they likely have some role to play in development.

• No evidence exists, however, for the occurrence of

cell-to-cell contact between epithelial and ectomesenchymal cells during early stages of tooth development.

• Indeed, the presence of a basal lamina (consisting of two layers; a lamina lucida and a lamina densa) between the two tissues preclude direct contact.

The main constituents of the basal lamina are:

• Type IV collagen.

• The adhesive glycoproteins laminin and fibronectin.

• The proteoglycan heparan sulfate.

• The basal lamina supports and binds the dental epithelium and in addition, according to some investigators, contributes to epithelial-mesenchymal communication.

It does this in two ways:

1. It acts as a dynamic sieve that can control the passage of extracellular molecules (or isolated basal lamina produced by inner enamel epithelium) between the two tissues. These molecules when combined with the surface receptors of dental papillary cells causes differentiation of odontoblasts.

2. It provides a spatial configuration to which mesenchymal cells can react.

• Most teratogenic agents interfere with, and adversely effect, normal odontogenesis by:

(1) inhibiting and retarding polysaccharide and glycoproteins synthesis, as well as

(2) arresting, in general the formation of extracellular matrix.

Histophysiology and Clinical Cosiderations

• Initiation

• Tooth may develop in abnormal location (ovary or hypophysis).

• A lack of initiation results in anodontia (partial or complete).

• Abnormal initiation results in the development of single or mutiple supernumerary teeth.

• Proliferation

• A disturbance in proliferaton has different effects, according to:

1. The time of occurrence.

2. The stage of development.

• Histodifferention

• In vitamine A deficiency the ameloblasts fail to differentiate properly resulting in formation of osteodentin.

• Morphodifferention

• Endocrine disturbances affect the size or form of the crown if it occurs during morphodifferentiation (in utero or in the 1st year of life).

• This disturbance may affect the form and size of the tooth without impairing the function of the ameloblasts or odontoblasts.

• Hypopituitarism and hypothyroidism results in small clinical crown and retarded eruption.

Disturbance may also results in:

1. New parts may be differentiated (supernumerary cusps or roots).

2. Twinning may occur.

3. A suppression of parts may occur (loss of cusps or roots).

4. A peg or malformed tooth (Hutchinson’s incisors in individual born with congenital syphilis).

• Apposition

• Disturbance during:

1. Matrix formation E. or D. hypoplasia.

2. Mineralization E. or D. hypocalcification.

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Tooth Development 3