Investing layers of Teeth / oral surgery courses

INDIAN DENTAL ACADEMY
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References
O Cellular, molecular, and genetic determinants of tooth
eruption; G.E. Wise, S. Frazier-bowers and R.N.
D'souza ; CROBM 2002 13: 323
O Cellular and chemical events during enamel maturation ;
C.E. Smith ; CROBM 1998 9: 128
O The mechanisms and mediators of tooth eruption –
Models for developmental biologists ; Sandy c. MARKS,
jr., Jeffrey p. Gorski and Gary e Wise

O Current concepts of the biology of tooth eruption ;
Jeffrey P. Gorski and Sandy C. Marks, jr ; CROBM 1992
3: 18
O Tooth eruption: theories and facts ; Sandy c. Marks, jr.
And Hubert e. Schroeder; The anatomical record
245~374-393 (1996)
O Tencate’s oral histology, development, structure and
function. Sixth edition.
O Textbook of oral anatomy, histology and embryology.
Third edition. B.K berkovitz.

Investing layer
O Throughout the life the crown of the tooth is
covered, more precisely invested by various types
of layers before and after the eruption of tooth
O So based on this, investing layers are of two types
OPre - Eruptive investing layer of teeth
OPost - Eruptive investing layer of teeth

Pre eruptive investing layers of teeth
O Before the tooth is erupted in the oral cavity the
crown of the tooth is covered by
O Vestiges of enamel organ
O Coronal part of the dental follicle
O Overlying mucous membrane

Post eruptive investing
layers of teeth
O After the eruption, all the structure of the pre
eruptive investing layer of the crown of the tooth
may get lost by attrition or erosion
O This is been replaced by the
O Salivary pellicle
O Plaque Enamel integument
O Junctional epithelium

Pre eruptive investing layers of
teeth
OVestiges of enamel organ

What is Enamel Organ?
O At certain points along the dental lamina, the
ectodermal cell multiply rapidly and form little
knobs that grows in to the underlying mesenchyme
O Each of this down growth from dental lamina
represents beginning of the formation of Enamel
organ.

Enamel organ
H & E section of Enamel organ
form from the down growth of
dental alamina
Dental lamina
Oral ectoderm

Constituents of an
Enamel organ
The enamel organ is developed from the
ectodermal component i.e. the oral ectoderm and
consist of :
O inner enamel epithelium
O outer enamel epithelium
O stratum intermedium,
O stellate reticulum.

O In the bell stage of enamel organ (tooth
development) the histodifferentiation of the cells
of enamel organ begins and leads to the
formation of
O inner enamel epithelium
O outer enamel epithelium
O stratum intermedium,
O stellate reticulum
O These four layer can be identified under light
microscopy.

Inner enamel epithelium
O Inner enamel epithelium constitute of single
layer of cell
O Which differentiate in to tall columnar; enamel
forming cell, called Ameloblast.

Outer enamel epithelium
O Consist of relatively flat – low cuboidal cells
O It laid down in to folds during enamel formation
and later stages of tooth formation.
O To supply nourishment to avascular Enamel
organ

Stellate reticulum
O Polygonal cells located in the center of epithelial
enamel organ
O Between the outer and inner enamel epithelium
O Contain glycosaminoglycan : thus attract water
by osmotic pressure

Stellate reticulum
O And cell then acquires the shape of star shape
but maintain contact by cytoplasmic process
O It act as shock absorber for developing tooth
and enamel forming cell

Studies show that, the paracrine signalling through
molecule such as
O parathyroid-hormone-related protein (PTHrP)
O interleukin -1a, (IL-1a)
produced in the stellate reticulum adjacent to the
follicle, may play a role in regulating eruption.

O These molecules from stellate reticulum
enhance the expression of either
O CSF-1 (colony stimulating factor – one )
Or
O MCP-1(monocyte chemotactic protein- one)
in the dental follicle

Stratum intermedium
O Few layer of squamous cell form the stratum
intermedium between inner enamel epithelium
and stellate reticulum
O This layers cell contain highly developed cell
organelles acid mucopolysaccharide and
glycogen deposit indicate a high degree of
metabolic activity.
O This is absent in region that outline root portion
of tooth

Reduced enamel epithelium
OReduced enamel epithelium is a
continuous lining over the surface of un
erupted crown after the formation of
completed enamel.

How REE forms:
O When ameloblast finish the formation of enamel
matrix, they leave a thin membrane on the
surface of enamel called as primary enamel
cuticle.
O (figure of formation of enamel cuticle)

O The ameloblast shorten after the enamel cuticle
has been formed and the epithelial enamel organ
( stellate reticulum , outer enamel epithelium and
stratum-intermedium ) is reduced to a few layers
of flat cuboidal cells which are then called
Reduced enamel epithelium

Section of reduced enamel epithelium.
BV, blood vessel; CT/DF, connective tissue of the dental follicle; EC/REE, external cells of
the reduced enamel epithelium; ES, enamel space; RA, reduced ameloblasts

O Thus the reduced enamel epithelium
consists of two major cellular layers:
A. Layer of “Reduced ameloblasts”
O Ameloblast cells those that are in contact
with the enamel through primary enamel
cuticle.
O But are no longer able to undergo cell
division
O Once the enamel is completely mineralized,
these ameloblasts shrink from columnar to
cuboidal or flattened cells

B. The external cells of the reduced enamel
epithelium that consist mostly the cells of
Epithelial enamel organ
i.e
O Mostly of stratum intermedium cells,
O And possibly cellular remnants of the
O Stellate reticulum
O Outer enamel epithelium

Secretion of reduced enamel epithelium
O Marks and cahill in 1987
O In his bio chemical analysis of dental follicle
including the enamel organ of developing tooth
during intra osseous eruption found that..
O Dental folicle found that most common
sialoprotien is one with a molecular weight (MW)
of 95000.
O So called DF-95

O Gorski et al., in 1994 demonstrate that in these
intra alveolar erupted tooth follicle preparations
DF-95 is localized to the reduced enamel
epithelium.
O This considered as a biochemical evidence that
this reduced epithelium is involved in and may
initiate eruption.

O At the onset of eruption DF-95 content is
reduced
O And this reduction is equal by exactly the amount
of three new sialoproteins of MW 20-25,000,
which appear at this time
DF-95
New
Sialoprotein

O In the study of Woessner and Cahill, in 1974 ,
It has been shown that dental follicles contain
O collagenolytic activity
and
O The follicular content of the
metalloproteinases (i.e.collagenase and
stromelysin) is reduced during eruption
O Suggesting a role for these enzymes in
tissue
turnover.

Also Nanci et al., & Smith et al., in 1989 in their
study found that
O In the enamel organ during tooth development
Proteases have been identified
O Activation of these proteases at the completion
of crown formation could cause fragmentation of
DF-95 and initiate eruption by release of
metalloproteinases from the dental follicle.

O Figure of df- 95 fragmentation

O By doing various studies in situ hybridization,
reverse-transcription/ polymerase chain-
reaction (RT-PCR), and immunostaining,
O Wise et al., in his studies have demonstrated
that the putative eruption genes and their
products are localized primarily in either the
O Dental follicle
or
O Stellate reticulum (SR),

O Paracrine signaling from the molecules in the
SR affects gene expression of the molecules in
the DF
O The receptor for IL-1a (IL-1R) is localized in the
DF, and its gene expression is enhanced either
by
O A ligand present in the SR,
O TGF-ß1, or
O EGF (molecule present in the DF)

O The presence of this receptor in the DF enables
IL-1a to act on the DF to enhance CSF-1 gene
expression (Wise and Lin, 1994)
O IL-1a also enhances MCP-1 gene expression as
well
as MCP-1 synthesis and secretion in the DF.

RECRUITMENT OF OSTEOCLAST PRECURSORS
O The signaling cascades that initiate eruption must
trigger the influx of mononuclear cells into the DF.
O Two molecules, CSF-1 and MCP-1, are prime
candidates for recruiting the osteoclast
precursors (mononuclear cells) into the DF.

O CSF-1 is required for the formation of
osteoclasts from monocytes (mononuclear cells)
O CSF-1 is also is a chemo-attractant for
monocytes
O Monocyte chemotactic protein-1 (MCP-1) is a
well-known chemokine for monocytes and is
found in a variety of cell types

O CSF-1 and MCP-1 are secreted molecules that
act as chemokines to recruit mononuclear cells
to the DF
O The CSF-1 and MCP-1 secreted by the DF cells
are chemotactic for monocytes

O Que and Wise, 1998 in their study found that many
of the eruption molecules, in turn, have been shown
to enhance the gene expression of MCP-1 and the
gene expression of CSF-1,
O Which include
O PTHrP
O IL-1 a secreted from the Stellate
reticulum cell of REE
O TGF-1B
O EGF ------------ secreted from the Dental follicle
itself

O CSF-1 protein has an autocrine effect on its gene
expression
O Thus, once CSF-1 is induced to be expressed, the
autocrine effect could lead to a burst of CSF-1
being
produced to recruit the mononuclear cells.

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