Oral Mucosa
Oral mucosa is the moist lining of the oral cavity. The mucous lining of oral
cavity shares some features with skin as well as the mucosa lining the gastro
intestinal tract.
Functions of Oral Mucosa
 Protection of underlying structures against mechanical trauma that
may result from heavy masticatory stress or from hard food. It also
acts as a protective barrier against invasion of microorganisms and
various bacterial products and toxins.
 Sensory – Oral mucosa has a special sensory function i.e. taste
perception due to the presence of taste buds. Oral mucosa also has
receptors that respond to pain, temperature and touch.
 Secretory function –The presence of minor salivary glands within the
mucosa aids in secretory function.
 Thermal regulation – Heat regulation of the body is one of the
functions of oral mucosa which is mainly seen in animals especially
dogs.
Classification of oral mucosa
1. Based on function
 Lining mucosa – lines the inner aspect of lips and cheeks, soft palate,
floor of mouth, ventral aspect of tongue, alveolar mucosa, vestibule,
faucial pillars etc.
 Masticatory mucosa – lines the hard palate and gingiva. This mucosa
is subjected to considerable friction during mastication.
 Specialized mucosa – lines dorsal aspect of tongue and this mucosa
shares the characteristics of both masticatory mucosa and gustatory
mucosa.
2. Based on type of epithelium covering the mucosa
 Keratinized mucosa – is found in the region of hard palate, gingiva,
vermilion border of lip and some papillae of tongue
 Non keratinized mucosa – is found in the region of lining mucosa and
certain areas lining the dorsal aspect of tongue.
Structure of oral mucosa
Oral mucosa resemble skin in its structure and is composed of two
components: epithelium and connective tissue. The interface between
epithelium and connective tissue is not flat rather is irregular. Epithelium has
many irregular projections that interdigitate with similar projections from
connective tissue. The epithelial projections are called rete ridges, rete pegs
or epithelial ridges and connective tissue projections are called papillae. The
epithelium and the connective tissue are separated by a basement membrane
of 1-2 microns thickness. The irregular epithelial - connective tissue junction
increases the surface area of contact between these two components which
helps in better adhesion and better transport of nutrients and other materials
between the two. The number and configuration of rete ridges vary in
different regions of oral mucosa.
Connective tissue component of oral mucosa
Connective tissue of oral mucosa is called as lamina propria. The loose
connective tissue below the lamina propria is continuous with it and is called
sub mucosa.
Lamina propria, seen subjacent to the epithelium is arbitrarily divided
in to papillary portion occupying the region of papillae and reticular portion
found beneath the papillary portion. Papillary portion contains loose
connective tissue with many capillary loops and nerves. Few nerves fibers
from here also enter in to the epithelium and remain as free nerve endings,
perceiving sensations such as cold heat, touch, pain and taste. In reticular
region connective tissue is less cellular and denser with fibers having more
parallel arrangement to the epithelial surface. Lamina propria shows all the
normal connective tissue components which include cells such as fibroblasts
and defense cells, extra cellular components including collagen fibers,
elastic fibers, oxytalan fibers and ground substance. Reticular portion is
always present, but papillary portion can vary depending on the presence
and absence of rete ridges.
Submucosa is the deeper connective tissue seen beneath the mucosa.
The submucosa comprises of loose connective tissue. In addition to the
normal connective tissue components submucosa contains large blood
vessels and nerves, minor salivary glands, fat cells etc. Submucosa is
divided into compartments by bundles of vertically arranged collagen fibers
extending from the lamina propria to fascia of the muscle or periosteum.
These bands of collagen along with elastic fibers attach the mucosa to the
underlying structures and therefore prevent the folding of mucosa which
might otherwise become entrapped between the teeth.
Submucosa is absent in gingiva and some regions of hard palate. In
these regions lamina propria is directly bound to periosteum of underlying
bone. This type of attachment is called mucoperiosteal attachment and this
makes mucosa tough, immovable and tightly bound to the bone.
Oral epithelium
The covering epithelium of the oral mucous membrane is stratified
squamous variety. The cells are tightly bound to each other and arranged to
form different layers or strata. The integrity of oral epithelium is maintained
by a system of continuous renewal mechanism. Old cells are continuously
lost from the surface by a process termed as desquamation and are replaced
by new cells formed by the process of mitotic division.
Basal cells of the epithelium can undergo mitotic division giving rise to
two daughter cells. One of the daughter cell remains in the progenitor
compartment while the other cell enters in to the maturing compartment. The
cells entering into the maturing compartment undergo further differentiation
and maturation as it passes through different layers till it reaches the surface
layer from where these are desquamated. The maturation of oral epithelium
follows two patterns, keratinization or nonkeratinization. Different types of
maturation pattern are observed in different regions of oral mucosa.
Histological structure of oral epithelium
Microscopically oral epithelium shows different layers which vary in
keratinized and nonkeratinized epithelium. Majority of cells of both
keratinized and nonkeratinized epithelium have the capacity to produce
keratin and therefore called as keratinocytes. These cells show some
common features unique to epithelial cells which include presence of keratin
tonofilaments as a component of cytoskeleton and intercellular attachment in
the form of desmosomes.
Keratinized epithelium
Light microscopic appearance – Four different layers are seen in
keratinized oral epithelium
1.Stratum basale / Basal cell layer – This layer is composed of single layer
of cuboidal or columnar cells that rest on basement membrane. Basal and
parabasal cells have the capacity to undergo mitotic division. So these cell
layers are also called as proliferative or germinative layer (stratum
germinativum). Basal cells have basophilic cytoplasm and centrally placed
nucleus which is hyper chromatic and relatively larger occupying 1/3rd of
cytoplasm. The nucleus is arranged perpendicular to basement membrane.
2. Stratum spinosum / prickle cell layer is seen above basal layer and
composed of several rows of polyhedral cells. As the cells pass from basal
layer to prickle cell layer there is considerable decrease in basophilia,
making the boundary between these layers distinct. Cells are larger than
basal cells and have centrally placed round or ovoid nucleus. The nuclear
cytoplasmic ratio of spinous cells is 1: 6. This layer is also called prickle cell
layer because in histological sections cells have a spiny or prickly
appearance. While tissue processing, cells shrink away from each other
remaining in contact only in the areas of inter cellular attachment, resulting
in a prickly appearance. In stratum spinosum as the cell mature and move
superficially they increase in size and become more flattened with flattened
nucleus in a plane parallel to the surface.
3. Stratum granulosum/granular cell layer – This layer is composed of few
layers of flattened cells seen immediately above stratum spinosum. The
cytoplasm of the cells in this layer is filled with basophilic granules called
keratohyline granules and hence the name stratum granulosum. The
nucleus of these cells are flattened with long axis parallel to the outer surface
of epithelium.
4.Stratum cornium/ cornified layer – This is the most superficial layer
found in keratinized epithelium and is composed of keratin squames which
are larger and flatter than the cells of stratum granulosum. This layer appears
as eosinophilic amorphous layer in histologic sections. As the cells reach the
cornified layer nucleus undergoes degeneration. If the nucleus is completely
absent in surface layer, the pattern of maturation is called as
orthokeratinization. If pyknotic nucleus is retained in all or some squames it
is called as parakeratinization. Parakeratinized epithelium mainly is seen in
gingiva. In parakeratinized epithelium the keratohyaline granules in stratum
granulosum is less prominent.
Ultra structural or Electron microscopic features
Basal cells – are least differentiated cells of the epithelium. These cells
contain nucleus occupying 1/3rd of the cells with evenly distributed
chromatin and 2-3 nucleoli. Basal cells are involved in protein synthesis and
therefore cytoplasm has rich cellular organelles like rough endoplasmic
reticulum, mitochondria, golgi complex, few lysosomes etc.
Basal cell layer has two populations of cells. One group of cells is serrated
with protoplasmic processes at basal region and is heavily packed with
tonofilaments. These cells are adapted for attachment. Second population of
cells are the stem cells which undergo division and provide cells for
maturing compartment. The basal cells are attached to each other by
desmosomes and to the basement membrane by hemidesmosomes. These
cells also contain tonofilaments like any other epithelial cell but are few in
number.
Prickle cells
Over all size of the cell and nucleus increases as it passes to spinous cell
layer. Nucleus has evenly distributed chromatin with 2-3 nucleoli.
Cytoplasm is rich in organelles for protein synthesis. The concentration of
the tonofilaments increases and gets arranged to form bundles. The cells are
attached to each other by desmosomes. The number of desmosomes and
width of intercellular space is more in keratinized epithelium. The size of
desmosome is wider in prickle cell layer than basal cell layer. As the cell
passes to upper prickle layers the desmosomes become smaller.
Cells in the upper part of prickle cell layer show new cytoplasmic
organelles called Odland bodies. These are also known as membrane coating
granules, cytoplasmic lamellated body, keratinosomes, microgranules or
cementosomes. (Odland bodies are also present in non keratinized
epithelium but structurally different).
In keratinized epithelium Odland bodies appear as ovoid membrane bound
organelles of 0.25microns length containing a series of parallel internal
lamellae consisting of alternate electron lucent and electron dense bands.
These organelles may be derived from golgi bodies. The size of the Odland
bodies do not increase but the density increases as the cell passes to more
superficial layer and also these structures move closer of superficial cell
membrane.
Granular layer cells
In this layer the size of the cells still increases. The cells are flatter with long
axis parallel to the epithelial surface. Nucleus is also flattened and shows
pyknotic changes. The cells still retain the capacity for protein synthesis,
only to a lesser extent. This is indicated by decrease in number of
cytoplasmic organelles. Although the cells show a decrease in cytoplasmic
components, the amount of tonofilaments is found to be more The cell
surfaces become more regular and closely approximated with each other.
Odland bodies are also present in these cells where they fuse with the
superficial cell membrane and discharge the contents in to the intercellular
spaces. This discharged material provide lipid rich permeability barrier that
limits the movements of substances through intercellular spaces.
Desmosomes maintain their structure in this layer while the intercellular
contact layer of desmosomes becomes more condensed. Cytoplasm of
stratum granulosum cells also shows keratohyaline granules. In keratinized
epithelium these are variable in size ranging from 0.1-1.5um. Their size and
number increases as the cell moves through the granular layer.
Keratohyaline granules are usually angular or irregular and they are usually
associated with ribosomes suggesting they are synthesized by ribosomes.
Keratohyaline granules contain sulphur rich proteins fillagrin and loricrin
which provide an embedding matrix for the tonofilaments and therefore help
in aggregating the tonofilaments. They also contain a protein involucrin
which provide constituents for the cell membrane thickening and makes is
resistant to chemical solvents.
Stratum corneum
Ultrastructurally stratum corneum is composed of cells resembling
hexagonal disc called squames. Large amount of bundles of keratin
tonofilaments are found to be embedded in a matrix that is contained in a
thick envelop. Keratin is tough insoluble protein which more or less
completely fills the interior of shrunken cells. Cellular organelles are almost
completely lost. The nucleus may be completely lost or remain pyknotic.
The cell membrane is thickened. Desmosomes can be still recognized but
they become less distinct. As the cell passes to the superficial layer
desmosomes tend to degenerate resulting in desquamation of cells.
Desquamation
The physiological process of shedding off of the superficial cells of
epithelium is called as desquamation. Mechanism of desquamation is not
fully understood. The possible mechanisms include.
a.
Release of hydrolytic enzymes from membrane coating granules
causing
destruction of desmosomes leading to desquamation.
b.
Inter cellular junctions have a physiological life span after which there
will be
rapid break down leading to desquamation.
Nonkeratinized epithelium
Light microscopically three different layers are seen in nonkeratinized oral
epithelium
1.Stratum basale / Basal cell layer – This layer is similar to that of basal
layer of keratinized epithelium and is composed of single layer of cuboidal
or columnar cells immediately adjacent to basement membrane. Basal cells
have centrally placed nucleus which is hyper chromatic and relatively larger
occupying 1/3rd of cytoplasm. The cytoplasm of these cells shows significant
basophilia due to high RNA content.
2.Stratum intermedium - This layer is composed of several rows of
polyhedral cells located above basal layer. The cytoplasm of these cells
takes up eosinophilic stain therefore this layer can be easily differentiated
from basal cells exhibiting basophilic cytoplasm. Cells are larger than basal
cells and have centrally placed round nucleus. The nuclear cytoplasmic ratio
of spinous cells is 1: 6. In contrast to stratum spinosum of keratinized
epithelium, the cells of this layer are closely apposed to each other and
prickly appearance is not distinct. In stratum spinosum as the cells mature
and are moved superficially they increase in size and become more flattened
with flattened nucleus in a plane parallel to the surface.
3.Stratum superficiale– This is the most superficial layer found in
nonkeratinized epithelium and is composed of few layers of flattened cells
The nucleus of these cell are flattened with long axis parallel to the outer
surface of epithelium. These cells ultimately undergo desquamation.
Ultra structural or Electron microscopic features
Basal cells – Ultrastructurally basal cells of nonkeratinized epithelium
resemble the basal cells of keratinized epithelium in all respects. These are
least differentiated cells of the epithelium. These cells contain nucleus
occupying 1/3rd of the cells with evenly distributed chromatin and 2-3
nucleoli. These cells are involved in protein synthesis and therefore
cytoplasm has rich cellular organelles like rough endoplasmic reticulum,
mitochondria, golgi complex, few lysosomes etc.
Basal cell layer has two populations of cells, serrated cells with protoplasmic
processes at basal region and cytoplasm heavily packed with tonofilaments.
These cells are adapted for attachment. Second population of cells is stem
cells which undergo division and provide cells for maturing compartment.
The basal cells are attached to each other by desmosomes and to the
basement membrane by hemidesmosomes. These cells also contain
tonofilaments like any other epithelial cell but are few in number.
Stratum intermedium
Over all size of the cell and nucleus increases as it passes to stratum
intermedium. Relative increase in size of the cell and nucleus is more in
nonkeratinized epithelium than in keratinized epithelium. Nucleus has
evenly distributed chromatin with 2-3 nucleoli. Cytoplasm is rich in
organelles for protein synthesis. The concentration of tonofilaments is more
than that in basal cells but in contrast to the cells of stratum spinosum,
tonofilaments are found in unbundled form. The cells are attached to each
other by desmosomes. The number of desmosomes and width of intercellular
space is less in nonkeratinized epithelium. The size of desmosome is wider
in stratum intermedium than basal cell layer but as the cells move more
superfially the number of desmosomes becomes lesser.
Superficial cells of stratum intermedium show new cytoplasmic organelles
called Odland bodies which are structurally different from that of the
keratinized epithelium
In non keratinized epithelium Odland bodies appear as spherical membrane
bound organelles of 0.2microns diameter. These structures have an electron
dense core from which delicate radiating strands are observed. The size of
the Odland bodies do not increase but their density increases as the cell
passes to more superficial layers and also these structures move closer of
superficial cell membrane.
Stratum superficiale
In this layer the size of the cells still increases. The cells are flatter with long
axis parallel to the epithelial surface. Nucleus is also flattened and shows
pyknotic changes. The cytoplasmic organelles decrease in number indicating
a lesser capacity to produce protein. Although the cells show a decrease in
cytoplasmic components, the amount of tonofilaments is found to be more
but in unbundled form. The cell surfaces become more regular and closely
approximated with each other. Desmosomes decrease in size and number
and inter cellular space becomes wider and irregular and maintain their
structure in this layer while the intercellular contact layer of desmosomes
become more condensed.
In contrast to superficial layer of keratinized epithelium, the superficial cells
of nonkeratinized epithelium show nucleus and various cytoplasmic
organelles. These cells undergo desquamation
Differences Between lining and masticatory mucosa
Masticatory Mucosa/ Keratinized
mucosa
Tough and tightly
underlying structures
bound
Lining Mucosa / non keratinized
mucosa
to loosely attached to the underlying
structures
Non stretchable
Stretchable to adapt to the contraction and
relaxation of underlying muscles.
Lamina propria is dense
Lamina propria is less dense
Sub mucosa may or may not be Distinct sub mucosa is present which vary
present.
Some
regions
show in thickness
mucoperiosteal attachment.
Epithelium
connective
tissue Rete ridges are short and irregular
interface is very irregular with long
and narrow rete ridges interdigitating
with connective tissue papillae.
Covering epithelium is keratinized Covering epithelium is non keratinized
stratified squamous epithelium
stratified squamous epithelium
Four distinct layers are seen in Only three layers are seen
epithelium
Stratum basale
Stratum spinosum
Stratum granulosam
Stratum corneum
Stratum basale
Stratum intermedium
Stratum superficiale
Epithelial thickness is less
Turn over rate of epithelium slower
Stratum spinosum
 Cells
have
prickly
appearance
 Number of desmosomes is
more.
 Percentage
of
cell
membrane occupied by
desmosomes is more
 Intercellular space is more
prominent
 Cytokeratin present are
1,6,10,16
 Relative size of the cells in
this layer is less
 The cell surfaces are less
closely applied
 Tonofilaments
are
in
bundles
 Odland bodies are ovoid
with alternating electron
dense and lucent areas
Structure granulosum with distinct
keratohyaline granules are seen
Superficial layer is composed of
keratin flakes
Superficial cells do not have nucleus
or cytoplasmic organelles
Intercellular Junctions
Epithelium is thicker
Relatively faster
 Prickly appearance is not distinct
 Number of desmosomes is lesser
than that of masticatory mucosa
 Percentage of cell membrane
occupied by desmosomes is less
 Intercellular space is less prominent
 Cytokeratin present an 3,14,19
 Cells are larger
 The cell surfaces are more closely
applied
 Tonifilaments are in unbundled form
 Odland bodies are round in shape
with central electron dense core and
radiating lines
No layer called stratum granulosum is
found, no keratohyaline granules.
Superficial layer is composed of flattened
cells
Surface cells contain nucleus and
cytoplasmic organelles
Inter cellular junctions are cell junctions that bind the cells to one another.
Three different types of junctions may be seen between the epithelial cells,
which include desmosomes, tight junctions and gap junctions.
Desmosomes – are the most characteristic and most numerous type of inter
cellular junctions seen in epithelial cells. Ultra structurally demosomes
present as a circular or ovoid area of 0.2 –0.5microns in which plasma
membranes of adjacent cells remain in juxta position to each other with a
distance of 25-30mm. This space between the plasma membrane contains an
electron dense lamina called intercellular contact layer. This layer is
composed of protein particles of 5microns diameter which is arranged in a
row.
On the cytoplasmic side plasma membrane of each of the adjoining cells
show a thickening called attachment plaque and this structure contain the
protein desmoplakin and plakoglobin. The tonofilaments present in
cytoplasm of each cell run into attachment plaque and loop out again. The
tonofilaments are not attached to the plasma membrane. This arrangement of
tonofilaments helps to dissipate physical forces from attachment site
throughout the cell. There are a separate group of smaller filaments
containing protein cadherins (desmogleins and desmocolin) attaches the
tonofilaments to plasma membrane, penetrate the cell membrane. These
filaments are called as transversing filaments and they traverse the inter
cellular region to extend into the inter cellular contact layer. The
transversing filaments from both cells come and attach to the inter cellular
contact layer retaining the attachment between the cells.
In gap junctions adjacent cell membranes run parallel to each other with a
gap of 2-5microns. In these areas some channels are present that allow
communication between cells.
Tight junctions are characterized by fusion between adjacent plasma
membranes without any intervening space and act as diffusion barriers.
The junctions that allow cytoplasmic compartments of adjacent cells to
communicate are special adaptation of mucous membrane channels and are
called gap junctions.
Hemidesmosomes
These are specific type of attachments seen between basal cells and
basement membrane. These attachments are called as hemidesmosomes
because the structure is equivalent to half of a desmosome. The
hemidesmosomes have one attachment plaque in the basal plasma membrane
of basal cells. The traversing filaments extending from this attachment
plaque enter in to the basal lamina to provide attachment between epithelium
and connective tissue.
Basement membrane and Basal lamina complex
The epithelium connective tissue interface is irregular and the epithelium is
separated from connective tissue by a distinct homogeneous structure less
layer of 1-2 microns thickness called basement membrane. The basement
membrane acts as a barrier which controls the movement of various
materials from epithelium to connective tissue and vice versa. The term
basement membrane is given based on light microscopic structure and it
does not appear distinct in hematoxylin and eosin stained sections. Special
stains like periodic acid Schiff stain or silver stains can be used to
demonstrate it.
Electron microscopically this basement membrane is composed of two
distinct layers:lamina lucida and lamina densa. Therefore based on electron
microscopic structure the term basal lamina is used to denote this structure.
The layer of basal lamina adjacent to the basal cell is around 45nm thick and
appears electron lucent. This layer is called as lamina lucida. Lamina lucida
contain laminin and bullous pemphigoid antigen .Beneath the lamina lucida
an electron dense layer of 55nm thickness is seen which is termed as lamina
densa. Smaller diameter collagen fibers( type VII) are found, forming loops
with both ends attached to the lamina densa. These fibers are called
anchoring fibrils. Collagen fibers from connective tissue pass through these
and loop around to form a strong attachment between epithelium and
connective tissue. Type IV collagen fibers are also found in lamina densa
which show a chicken wire pattern. This layer also contains proteoglycans
such as heparin sulfate and chondroitin sulfate. The proteoglycans control
the passage of ions across basement membrane. Both layers of basal lamina
and the anchoring fibrils are together called as basal lamina complex. The
basal lamina is of epithelial origin while the anchoring fibrils are of
connective tissue origin.
Functions of basement membrane
1. Structural attachment i.e. providing attachment between epithelium
and connective tissue.
2. Compartmentalisation – basement membrane isolates the epithelium
from connective tissue.
3. Filtration- transport of materials to and from the connective tissue is
regulated by basement membrane.
4. Tissue scaffolding - basement membrane act as a scaffold during
regeneration of epithelium.
5. Polarity induction – Epithelial cells gets organized in to normal
layered arrangement only if they are supported by a basement
membrane.
Non keratinocytes
In the oral epithelium 90% of the cells are keratinocytes which have the
capability of producing keratin. Another 10% of the cells belong to a group
called non keratinocytes. They are melanocytes, Langerhans cells, Merkel
cells and inflammatory cells.These cells do not produce keratin and except
for Merkel cells do not posses desmosomal junctions or tonofilaments.
a.
Melanocytes – Melanocytes are dentritic cells scattered among the
basal cells of epithelium and these are the melanin producing cells. The
origin of these cells is from neural crest cells which migrate to ectoderm by
8-11 weeks of intrauterine life which have the capacity to replicate through
out post natal life though at a much slower rate than keratinocytes. These
cells have a cell body containing the nucleus located at basal region and
multiple long processes extending between the keratinocytes of stratum
spinosum. The melanocytes neither contain tonofilaments nor posses
desmosomal attachment. Because of absence of desmosomal attachment the
cell tend to shrink against nucleus while tissue processing. Hence these cells
appear as clear cells in between the basal cells. Since they are seen in basal
layer as clear cells, melanocytes are called low level clear cells. The
melanocytes contain characteristic electron dense cytoplasmic organelles
called melanosomes that contain melanin pigments. Production of melanin
depends on melanocyte stimulating hormone. The variation in pigmentation
seen in different individuals depends on the activity of melanocytes and not
on number of melanocytes.
The melanocytes help to impart color to skin and mucosa and also protect
against u-v. light.
Melanocytes can be demonstrated using special stains like silver stain and
also by DOPA reaction.
b.
Langerhans cells – are dentritic cells present in the epithelium of skin
and mucosa. These cells have a cell body harboring the nucleus and long
processes extending between the prickle cell layers. Langerhans cells do not
have desmosomal attachment and tonofilaments. These cells also appear as
clear cells in an H & E stained histological section because of shrinkage of
cells. Because of their location in upper layer of epithelium compared to
melanocytes, Langerhans cells are called as high level clear cells. These
cells cannot be revealed in routine H & E stain. They can be demonstrated
by histochemical, immuno fluorescent or immuno histochemical techniques
which reveal the cell surface antigen or ATPase reaction
Election microscopically Langerhans cells show a characteristic racquet or
flask shaped cytoplasmic organelle called Birbeck granules or Langerhans
granules.
The origin of Langerhans cells is from bone marrow and they are immuno
competent cells. They trap the antigens entering the mucosa, process it and
present it to the immune system. They are referred to as antigen presenting
cells.
Merkel cells
These are modified keratinocytes located in the basal layer of oral
epithelium. In contrast to other non keratinocytes these Merkel cells are
nondentritic cells which form occasional desmosomal attachment with
neighbouring epithelial cells and contain some tonofilaments. Because of
few desmosomal attachments these cells do not appear as clear cells in
histological sections. Electron microscopically these cells show cytoplasmic
granules with dense core resembling neurosecretory granules. Presence of
these granules and the close association of these cells with nerve endings
suggest the possible role of sensory function of Merkel cells. The Merkel
cells are considered as pressure sensitive cells responding to touch.
There is a controversy regarding origin of Merkel cells. One opinion is that
these cells could be neural crest origin while few others consider that they
are formed by the division sof keratinocytes like cells.
Inflammatory cells
Inflammatory cells like lymphocytes are also present in the epithelium.
These cells are of bone marrow origin. Since these cells move from
connective tissue to epithelium and also back, they can be seen at different
levels of epithelium. Lymphocytes appear as round cells with nucleus
occupying the major part of the cell with little cytoplasm. They can also be
demonstrated by immuno histochemical techniques that demonstrate the
surface markers(OKT-3) of these cells. Lymphocytes perform defense
function.
Structural variations of oral mucosa
Oral mucosa is composed of epithelium (either keratinized or non
keratinized) and connective tissue. Yet as an adaptation to the function to be
performed, different parts of oral mucosa show some variations.
Lining mucosa
Lining mucosa includes mucosa lining the cheeks, lips, alveolar mucosa,
vestibule, floor of the mouth, ventral aspect of tongue, soft palate etc. The
lining mucosa is non keratinized mucosa.
Characteristic features of lip and cheek mucosa
Lip and cheek are lined by non keratinized mucosa. Epithelial ridges seen at
the interface between the epithelium and connective tissue are small and
irregular and interdigitate with few, short irregular connective tissue
papillae. Lamina propria is thick and has less dense collagen fibers. Lip and
cheek mucosa has a distinct submucosa that contains mixed salivary glands,
fat cells, muscle etc. Mucosa is stretchable and is well adapted to contraction
and relaxation of underlying musculature.
Vertical band of collagen fibers with elastic fibers are found extending from
lamina propria to fascia covering the underlying muscle which provide the
attachment between the mucosa and muscle. Thus the folding of mucosa is
prevented while muscle relaxation and avoid mucosa being caught between
the teeth.
Vestibular mucosa and alveolar mucosa
Vestibular mucosa lines the vestibule; a ‘v’ shaped sulcus separating the
alveolar mucosa from cheek and lip. Vestibular mucosa is in continuation
with alveolar mucosa which lines parts of the alveolar bone. Alveolar
mucosa appears reddish and extends up to mucogingival junction which
separates it from gingival mucosa. In contrast to cheek and lip mucosa which
is tightly bound to the underlying muscle, this alveolar and vestibular
mucosa is loosely attached to the underlying structures. This permits the
easy movement of lip and cheek. Median and lateral labial frena are seen as
folds of mucous membrane containing loose connective tissue.
Alveolar mucosa is thin with a thin non keratinized epithelium lining it. The
epithelial connective tissue junction is relatively flat with small rete ridges
and connective tissue papillae which may even be absent at times. Alveolar
mucosa is loosely attached to underlying bone by a loose connective tissue
that contains minor salivary gland.
Ventral surface of tongue and floor of the mouth
Floor of the mouth is a small horseshoe shaped region beneath the movable
part of the tongue. Mucosa lining the floor of the mouth and ventral surface
of the tongue share many common features. Mucosa is thin with a non
keratinized epithelium. The epithelial rete ridges and connective tissue
papillae are short. Connective tissue shows rich blood supply which is
particularly prominent in floor of the mouth. Submucosa of floor of the
mouth contains adipose tissue and minor salivary glands. In the ventral
aspect of tongue submucosa may be very thin or even absent where the
mucosa will be highly bound to the underlying musculature. The thin
epithelial lining and rich blood supply permit the rapid absorption of
medicines administered sublingually.
Soft palate
Soft palate is lined by non keratinized stratified squamous epithelium.
Epithelium may show presence of few taste buds. Lamina propria is highly
vascular because of which soft palate appears reddish clinically. The
epithelium connective tissue interface is irregular with thick and short rete
ridges and connective tissue papillae. A distinct layer of elastic fibers are
found forming a lamina between lamina propria and submucosa. The
submucosa is composed of diffuse loose connective tissue containing
numerous minor salivary glands.
Vermilion border of lip ( transitional zone)
This zone is the transitional zone between the skin covering the outer
surface of the lip and the labial mucosa lining the inner aspect. The
skin is composed of keratinized stratified squamous epithelium with
all appendages like hair follicles, sweat glands and sebaceous glands.
The labial mucosa is lined by non keratinized stratified squamous
epithelium. The connective tissue beneath the labial mucosa shows
minor salivary glands. The central portion of lip shows orbicularis oris
muscle. The transitional zone has a thin lining epithelium with mild
keratinization on the surface. There are many long connective tissue
papillae reaching high in to epithelium carrying many capillary loops.
This makes it more red compared to labial mucosa. Underlying
connective tissue is characteristically devoid of glands which causes
the mucosa to dry up.
Gingiva
Gingiva is the part of the oral mucosa that covers the alveolar process and
surrounds the neck of the tooth. The gingiva relatively tightly bound to the
buccal and lingual plates of alveolar process and extends from the dentogingival junction to the alveolar mucosa.
Macroscopic structure of gingiva
Gingiva is pink in colour with some degree of melanin pigmentation.
Anatomically gingiva can be divided into three parts; marginal gingiva,
interdental papilla and attached gingiva.
Marginal Gingiva – is the unattached portion of gingiva that forms the
border which surrounds the teeth in a collar like fashion. Marginal gingiva
follows a scalloped line on the facial and lingual surface of the teeth. The
contour depends on shape and alignment of teeth. Marginal gingiva forms
the soft tissue wall of the gingival sulcus and is separated from attached
gingiva by a free gingival groove which runs parallel to the terminal edge of
the gingiva at a distance of 0.5-1.5mm almost at the level of bottom of
gingival sulcus.
Gingival sulcus – is a shallow crevice or ‘V’ shaped space present around
the tooth bounded by tooth surface on one side and marginal gingiva on
other side. The depth of this sulcus varies from 0.5 to 3mms with an average
of 1.8mm. More than 3mms is considered as pathological and is called as a
pocket.
Interdental Papilla
The part of the gingiva that fills the interdental space between two adjacent
teeth is called interdental papilla. Interdental papilla appears pyramidal or
triangular from the facial and lingual aspect with its lateral borders and tip
formed by a continuation of marginal gingiva of adjacent teeth. Three
dimensionally the anterior inter dental gingiva is described to have a
pyramidal shape with facial and lingual gingiva tapering towards the inter
dental area. In the posterior region inter dental gingiva has a ‘tent’ shape.
This shape is formed because the interdental papillae present on lingual and
buccal sides of each interproximal space are connected by depressed central
area. The facial and lingual portions of papillae forms the high points and a
concave or valley like area fits below the contact area. This valley like area
is called ‘col’ which is lined by non keratinized epithelium. This col is
considered as a weak point in gingiva and is more prone to periodontal
diseases. When the adjacent teeth are not in contact, the inter dental gingiva
appears smooth with round surface, firmly adherent to inter dental bone.
Attached gingiva
The firm, resilient immobile portion of the gingiva which is tightly bound to
the alveolar bone is called attached gingiva. This extends from free gingival
groove to mucogingival junction by which it is separated from alveolar
mucosa. On the palatal aspect attached gingiva blends with palatal mucosa.
The width of attached gingiva varies in different regions. In maxilla it ranges
from 3.5-4.5mm while in mandible it is 3.3-3.9mm. The surface of the
attached gingiva is irregular with elevations and depression. The orange peal
appearance created by these elevations and depression is described as
stippling. These are considered as functional adaptations to mechanical
stress and may be caused by traction on mucosa by underlying fibrous
attachment to the bone. Loss of stippling is one of the initial signs of
gingival inflammation. The pattern and extent of stippling varies in different
regions of the mouth; being less prominent on lingual than on facial surface.
Males tend to have more stippling than females. Gingiva may show slight
vertical depression between the alveolar bone eminences of adjacent teeth.
These are called inter dental grooves.
Microscopic structure of gingiva
Structurally gingiva is composed of parakeratinized stratified squamous
epithelium and connective tissue.
Covering epithelium shows structural variations in different regions.
Accordingly it can be categorized as epithelium covering oral region of
gingiva (outer portion), sulcular epithelium and junctional epithelium.
Oral region – epithelium lining the oral region of gingiva is keratinized or
parakeratinized stratified squamous epithelium which has four distinct layers
i.e. stratum basale, stratum spinosum, stratum granulosum and stratum
cornium. Microscopically a shallow ‘V’ shaped notch on the surface
corresponding to a heavy epithelial ridge which represent the free
gingingival groove. In the region of attached gingiva stippling is reflected by
alternate rounded protuberances and depressions on the surface. The
depressions correspond to the center of heavy epithelial ridges.
The epithelium connective tissue interface is irregular with long narrow rete
ridges inter- digitating with long connective tissue papillae. This long
branching rete ridges help in microscopic identification of gingiva from
other parts of oral mucosa. Gingival epithelium is parakeratinized in 75% of
population.
Sulcular epithelium
Sulcular epithelium lines the gingival sulcus and it extend from the coronal
limit of junctional epithelium to the crest of gingival margin. Sulcular
epithelium is composed of thin layer of non keratinized epithelium. The
junction between epithelium and connective tissue is flat without rete ridge
formation. Lack of keratinization is thought to be due to inflammation of
connective tissue
Junctional Epithelium
The part of the gingival epithelium that is attached to the cervical part of the
tooth and therefore forming a junction between the tooth and gingiva is
called junctional epithelium.
Junctional epithelium is stratified squamous epithelium which appears as a
triangular strip with 15-30 cell layer thickness at the cervical portion (floor
of sulcus) and 3-4 cell layer thickness at the apical margin. Junctional
epithelium is composed of flattened cells which are arranged parallel to the
tooth surface. The cells have lesser number of desmosomal junctions and
more intercellular spaces helping in migration of polymorpho nuclear
leucocytes into the epithelium and to the sulcus. The epithelium connective
tissue interface is flat. One of the most important features which make it
differ from other epithelium is presence of basal lamina on both sides; at the
junction of epithelium and connective tissue and also on the surface adjacent
to the tooth. This basal lamina on the surface is attached to the tooth by
hemidesmosomes. The junctional epithelium also shows high turnover rate.
The cells from the basal layer migrate to within 2-3 layers of junctional
epithelium and join a migratory root in a coronal direction and finally
exfoliate at the gingival sulcus.
Gingival connective tissue
The connective tissue beneath the gingival epithelium is lamina proprina
with papillary and reticular layer. The connective tissue of gingiva consists
of dense collagenous tissue arranged in bundles of fibers which play a very
important role in maintaining integrity of the supporting apparatus of tooth.
These fiber groups are referred to as the secondary fibers of periodontal
ligament or gingival ligament or gingival fibers of periodontal ligament. In
addition to collagen fibers oxytalan fibers and elastic fibers are also present
in gingival connective tissue.
The gingival fibers include 1.
Dento-gingival fibers – These fibers extend from the cervical
portion of the cementum to the lamina propria of gingiva.
2.
Dento-periosteal fibers extending from cervical part of
cementum to the periosteum of the alveolar crest and the vestibular
and oral surface of the alveolar bone.
3.
Alveologingival fibers extend from the crest of the alveolar
bone to the lamina propria of gingiva.
4.
Circular fibers – these fibers are arranged in the gingival
connective tissue, encircling the neck of tooth like a collar. These
fibers are also known as the marginal ligament and they play an
important role in maintaining a tightly fitting gingival collar.
Trans-septal fibers which are also called interdental ligament is also
found in gingival connective tissue as accessory fibers extending inter
proximally between adjacent teeth, from cementum of one tooth to
cementum of adjacent tooth over the interdental bony alveolar crest.
Lamina propria of oral gingival epithelium is firmly attached to periosteum
of the alveolar bone by course collagen bundles. This type of attachment is
called mucoperiosteum. Subucosa is absent in gingiva therefore no large
blood vessels or minor salivary glands are observed in gingiva. The lamina
propria of sulcular and junctional epithelium is different from that of oral
gingival epithelium. Connective tissue in this region is delicate with
presence of inflammatory cells.
Palate
The palate forms the roof of the oral cavity and is divided in to immovable
hard palate anteriorly and the movable soft palate posteriorly. The hard
palate has a hard bony support while soft palate has only fibrous tissue.
The mucosa covering the hard palate differs in microscopic and macroscopic
structure in different regions.
Macroscopic structure of hard palate
Palate can be divided into different zones.
1.
Gingival zone – consists of peripheral portion of hard palate found
adjacent to teeth.
2.
Palatine raphae – a narrow zone in the midline of hard palate
extending from incisive papilla posteriorly. This zone appears depressed
compared to adjacent areas.
3.
Incisive papilla – Incisive papilla is an oval prominence seen at the
extreme anterior region of palate immediately behind the maxillary central
incisors covering the oral opening of incisive canal.
4. Anterolateral region between raphae and gingiva containing much of fat
tissue in submucosa
5. Posterolateral region between raphae and gingiva containing mainly
minor salivary glands in sub mucosa.
Palatine rugae Radiating outwards from the palatine raphae in the anterior
region of hard palate are irregular transverse palatine ridges referred to as
palatine rugae. These ridges may have a role in suckling in infants and also
may be helping in backward movement of food during mastication.
Fovia palatina:- is an elongated depression of few millimeters depth in
post part of palate on either side of midline.
Microscopic structure of hard palate
The hard palate is covered by keratinized mucosa. The keratinized stratified
squamous lining the mucosa has four different layers, stratum basale,
stratum spinosum, stratum granulosum and stratum corneum. As a functional
adaptation, to bear with masticatory stress, the cells show more dense
tonofilaments, increased number and length of desmosomes etc. The
epithelium connective tissue interface is irregular with many long regular
epithelial ridges interdigitating with connective tissue papillae.
The lamina propria is dense throughout the hard palate and is thicker in
anterior region than in posterior region. In the region of rugue the connective
tissue core is dense with interwoven collagen fibers. Incisive or palatine
papilla is also composed of dense connective tissue. This contains the
remnants of nasopalatine duct which is lined by pseudo stratified squamous
epithelium. Small islands of hyaline cartilage may be seen around the duct
opening.
Structure of sub mucosa varies in different regions of palate. Sub mucosa is
absent in the peripheral zone of palate adjacent to the teeth i.e. the gingival
zone and in the mid palatine raphae. In these regions, the lamina propria is
tightly bound to the periosteum of bone which is referred to as
mucoperiosteal attachment.
In between the gingival zone and mid palatine raphae, the palate has distinct
submucosa. The sub mucosa is thicker in the posterior region than anterior
region. The submucosa in the anterior part of the hard palate is filled with
adipose tissue and in posterior region with mucous glands. Therefore
anterolateral part of hard palate is referred to as fatty zone and posterolateral
part the glandular zone.
In spite of thick submucosa in certain regions, the mucosa of the hard palate
is tightly fixed to the underlying bone and is immobile. This is achieved by
dense vertical band of connective tissue which attaches mucosa firmly to the
periosteum of palatal bone. These dense bands of connective tissue are at
right angle to surface and divide the submucosa in to compartments.
The wedge shaped area where the alveolar process joins to the horizontal
plate of hard palate contains loose connective tissue which carry large
vessels are nerves. The thickness of this loose connective tissue gradually
increases from anterior region of palate to posterior region.
Tongue
Tongue is a muscular organ situated in the floor of the mouth which play
important role in speech, mastication, deglutition, taste sensation etc.
Macroscopic features
Dorsum of the tongue is convex in all directions. A ‘V’ shaped sulcus
divides the dorsal aspectof the tongue into anterior 2/3 rd, body or oral part
and posterior 1/3rd, base or pharyngeal part. A small pit is seen where the
two arms of ‘V’ meet. It is called foramen ceacum representing the opening
of thyroglossal duet. Anterior 2/3rd of the tongue is also called as papillary
part because the mucosa has numerous papillae which give it a velvety
appearance. The most numerous papillae are fine pointed, cone shaped
filiform papillae that the widely distributed on the dorsal surface. These
papillae make the surface of the tongue rough and help in crushing the food
particles while pressing against hard palate. Numerous fungiform papillae
are seen also distributed between the filiform papillae on the dorsal aspect
mainly on the tip and lateral margins. Fungiform papillae are seen as red
round, projections. Anterior to sulcus terminates 8-12 large papillae called
circumvallate papillae are seen. Circumvallate papillae are partly
submerged and do not project above the surface of tongue and are
surrounded by a circular groove. Margins of the papillae project above the
surface. In the posterior region of anterior 2/3rd of tongue, on the lateral
margin foliate papillae are seen which consists of series of folds forming
clefts. These foliate papillae are rudimentary in humans.
Posterior 1/3 of the tongue has an irregular surface with round projection,
the lingual follicles containing lymphoid component. Therefore posterior
1/3rd of the tongue is also called lymphoid region. The mucous membrane
lining the posterior 1/3rd does not show papillae and is relatively smoother.
Inferior surface or ventral aspect of the tongue is covered with smooth
mucous membrane papillae are not seen on this aspect of tongue. The
inferior surface is attached to the floor of the mouth by a loose lingual
frenum. On either side of lingual frenum prominent lingual veins are seen.
Lateral part of inferior surface shows the presence of two folds called
plicafimbriata which runs forward and medially to the tip of the tongue.
Microscopic structure of tongue
Papillae of the tongue
Filiform papillae
Filiform ( hair like) papillae are seen as hair like or thread like
projection on the dorsal aspect of the tongue. Filiform papilla in a
histological section is seen as cone shaped structure lined by stratified
squamous epithelium with thick keratin on the surface. Central core
of connective tissue supports the blood vessels. Taste buds are not
seen in these papillae.
Fungiform papillae
Fungiform (fungus like) papillae are mushroom shaped structure
projecting above the surface of the tongue and located between the
filiform papillae. The epithelium covering the fungiform papillae is
thin nonkeratinized stratified squamous epithelium. The superficial
surface of the papillae contains few taste buds. The supporting
connective tissue shows collagen fibers, fibroblasts and rich capillary
network. Fungiform papillae appear reddish in color because of the
capillary network visible through relatively thin nonkeratinized
epithelium
Circumvallate papillae
The circum vallate (walled) papillae are seen in the anterior two third
just anterior to sulcus terminals. These are 10 -12 in number. The
superficial surface of these papillae is at the level of surface of tongue
and a ‘V’ shaped sulcus is present all around the papillae separating
them from the adjacent portion of tongue. The lining epithelium is
keratinized stratified squamous epithelium at the superficial surface
and non keratinized on the lateral surface of circumvallate papillae.
Taste buds are seen only on the lateral surface. Central portion is
occupied by the connective tissue. The characteristic feature of this
papilla is presence of serous minor salivary glands (von Ebner’s
gland) in the connective tissue beneath it. These glands secrete
watery saliva into the 'V' shaped trough around the papillae to flush
out the food debris.
Taste buds
Taste buds are specialized sense organs that can perceive the taste sensation.
They are mainly located in papillae of tongue i.e. superficial surface of
fungiform papillae, lateral walls of circum vallate papillae and in the cleft
walls of foliate papillae. In addition taste buds are also seen in posterior part
of palate, uvula, epiglottis, pharyngeal region etc.
Taste buds are barrel shaped structures composed of 30-50 spindle
shaped, modified epithelial cells that extend from basement membrane to
epithelial surface. The taste buds measure around 50-80 microns in height
and 30-50 microns in diameter. At the epithelial surface the tapered end of
all cells end in a small opening of 2-5 microns called taste pore through with
the cells communicate to exterior.
Based on the morphological features 4 different types of cells can be seen in
taste buds.
Type I cells – (dark cells)
Then are long narrow cells which make up the major population (60%) of
cells. The base of the cells rests on basement membrane and apex end as a
long finger like microvilli in the taste pore. These cells have dark nucleus,
rich cytoplasmic organelles and large dense cored vesicles in apical
cytoplasm.
Type II cells – light cells
They are more or less regularly oval shaped cells with electron-lucent
cytoplasm having few organelles and large round or oval light stained
nuclei. Around 30-40% of the cells of taste bud belong to this group. These
cells also extend from basement membrane to taste pore where they end in
short microvilli.
Type III cells
These cells are some what similar to type III cells in morphology. They
make up only 5-15% of cells. These cells end in a narrow club shaped
projection in the taste pore. In contrast to the type II cells, type III cells have
numerous dense covered vesicles concentrated in the basal region.
Type IV cells - Basal cells
These cells rest on basement membrane, but do not extend to the taste pore.
These cells have been considered of undifferentiated precursor cells which
can give rise to all three different types of cells.
Difference in opinion exists about taste receptor cells. Some authors
consider the type III cells are taste receptors cells because of the electron
dense vesicles at the basal region and their close proximity to the nerve
endings. They consider other cells as supporting cells.
But others are of the opinion that the type I, II & III cells are transitional
form of a single cell and all the types could act as chemoreceptor
transduction cells because synapses have been observe on all three cells.
The nerve fibers enter the taste buds by penetrating the basal lamina, within
the taste bud they undergo extensive branching and contact with the taste
and cells.
Taste buds of fungiform papillae of tip of tongue have receptors for sweet,
and that of lateral borders of tongue have receptors for salty taste. Bitter
taste is perceived by taste receptors of circum vallate papillae and the sour
taste by vallate papillae.
Dentogingival Junction
The junction between the tooth and gingiva is called dentogingival junction.
The junctional epithelium has an important role in this. The epithelium i.e.
the junctional epithelium that is attached to the tooth to form a dentogingival
junction is called attachment epithelium and the mode by which this
epithelium is attached to the tooth is called epithelial attachment.
Formation of Dentogingival Junction
Once the enamel formation is completed the ameloblasts secretes
protenaceous martial on to the surface of newly formed enamel which is
structurally similar to basal lamina. This structure is called primary enamel
cuticle. Once the enamel organ transforms into reduced enamel epithelium(
REE) it gets attached to the surface of enamel with the help of this basal
lamina through hemidesmosomes.
During the process of eruption the connective tissue between the REE and
oral epithelium degenerate followed by proliferation of oral epithelium and
REE. These layers ultimately fuse together to form a solid plug of
epithelium. The central cells of this plug degenerate forming a canal through
which the tooth emerges in to the oral cavity. As the tooth move to the
occlusal plane the epithelium covering the enamel surface shortens. Even
after the tooth reaches the occlusal plane 1/3rd of the tooth is still covered by
epithelium. Once the tip of the cusp emerges in to the oral cavity the part of
reduced enamel epithelium attached to the tooth is called primary attachment
epithelium and is in continuation with oral epithelium. The reduced enamel
epithelium gradually shortens to expose the crown of the tooth completely
and is slowly replaced by the oral epithelium. The attachment epithelium
derived from oral epithelium is referred to as secondary attachment
epithelium. The actual movement of the tooth to occlusal plane is called
active eruption and the exposure of the crown by the apical migration of the
covering epithelium without actual movement of the tooth is called passive
eruption.
Shift of Dentogingival junction
Once the dentogingival junction is established the attachment epithelium
shows a gradual migration in apical direction exposing more tooth surface
into the oral cavity. This shift can be discussed under four stages.
First Stage
During this stage the attachment epithelium is completely attached to enamel
with its apical end at the cemento enamel junction. The bottom of gingival
sulcus is located on enamel surface. This level of attachment is seen between
20-30 years.
2nd Stage
In this stage the attachment epithelium migrates apically and is attached
partly on to enamel and partly on cemental surface and apical end is on
cementum. The bottom of gingival sulcus is on enamel itself. This stage is
seen at 40 years.
3rd Stage
As the apical migration of attachment epithelium progresses gradually in 3 rd
stage it becomes completely attached on to the cementum surface with
bottom of gingival sulcus at cemento enamel junction. At this stage the
complete anatomic crown is exposed to oral cavity.
4th Stage
In this stage the attachment epithelium still migrates apically on the surface
of cementum and the bottom of gingival sulcus is located on the cemental
surface exposing even a part of root. At this stage clinical crown is longer
than anatomic crown.
First two stages are physiological while the 3 rd, 4th may be physiological or
pathological.
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