ATLAS OF ORAL HISTOLOGY

SECOND EDITION

Harikrishnan Prasad, BDS, MDS

(Oral Pathol & Microbiol)
Professor, Department of Oral and Maxillofacial Pathology, KSR Institute of
Dental Science and Research, Tiruchengode, INDIA

Krishnamurthy Anuthama, BDS,

MDS (Oral Pathol & Microbiol)
Consultant Oral Pathologist, Herndon, Virginia, United States of America
Table of Contents

Cover image

Title page

Copyright

Dedication

Preface to the Second Edition

Preface to the First Edition

List of videos

1. Introduction

Preparing tissues for microscopic study

Staining

Microscopy
 Points to remember

Useful hints

2. Development of tooth

Bud stage (figs 2.1 and 2.2)

Cap stage (figs 2.3–2.5)

Early bell stage (figs 2.6 and 2.7)

Advanced bell stage (figs 2.8 and 2.9)

Hertwig epithelial root sheath and cementum formation (figs 2.10

and 2.11)

Some important terminologies

3. Enamel

Enamel rods (figs 3.1 and 3.2)

Striae of retzius (figs 3.3, 3.4, and 3.5)

Enamel lamellae (figs 3.5, 3.6, and

3.7) Enamel tufts (figs 3.5, 3.6, and

3.7) Enamel spindles (figs 3.8, 3.9)

Gnarled enamel (figs 3.10 and 3.11)

Hunter–Schreger bands (figs 3.12, 3.13, and 3.14)

Useful hints
4. Dentin

Primary and secondary dentin (figs 4.1 and 4.2)

Dead tracts (figs 4.3 and 4.4)

Tertiary dentin (fig. 4.4)

Interglobular dentin (figs 4.5–4.7)

Tomes’ granular layer (figs 4.8 and 4.9)

Branching of dentinal tubules (figs 4.10–4.12)

Useful hints

5. Pulp

Zones of the pulp (figs 5.1 and 5.2)

Pulp stones (figs 5.3 and 5.4)

Useful hints

6. Cementum

Acellular cementum (figs 6.1 and 6.2)

Cellular cementum (figs 6.3 and 6.4)

Incremental lines of salter (figs 6.5 and 6.6)

Cementoenamel junction (figs 6.7–6.14)

Useful hints
7. Periodontal ligament

Principal fiber groups of periodontal ligament (figs 7.1–7.4)

Cementicles (fig. 7.5)

Useful hints

8. Bone

Compact bone (figs 8.1–8.4)

Cancellous bone (figs 8.5 and 8.6)

Useful hints

9. Salivary glands

Serous salivary glands (figs 9.1–9.3)

Mucous salivary glands (figs 9.4–9.6)

Mixed salivary glands (figs 9.7–9.9)

Ductal system of salivary glands

Useful hints

10. Oral mucous membrane

Keratinized stratified squamous epithelium (figs 10.1–10.5)

Nonkeratinized stratified squamous epithelium (figs 10.6 and 10.7)

Keratinocytes and nonkeratinocytes (figs 10.8 and 10.9)

 Papillae of the tongue

Dentogingival junction (figs 10.16 and 10.17)

Useful hints

11. Maxillary sinus

Histology of the sinus lining (figs 11.1 and 11.2)

Goblet cells (figs 11.3–11.5)

Useful hints
Copyright

RELX India Pvt. Ltd.

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Atlas of Oral Histology, 2e, Prasad Harikrishnan, Krishnamurthy

Anuthama

Copyright © 2019, 2015 by RELX India Pvt. Ltd.

ISBN: 978-81-312-5483-7
e-ISBN: 978-81-312-5484-4

Package ISBN: 978-81-312-5475-2

Previous edition copyrighted, 2015

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Dedication

Dedicated to

My beloved teachers from the Department of Oral Pathology and

Microbiology Mahatma Gandhi Postgraduate Institute of Dental
Sciences, Puducherry

Harikrishnan Prasad

Dedicated to

My dear parents, Usha and Krishnamurthy, my dear husband,

Abinandan and to my guru, Dr Pratibha Ramani

Krishnamurthy Anuthama
Preface to the Second Edition

Harikrishnan Prasad, Krishnamurthy Anuthama

We are humbled and thankful for the support and encouragement we

received after the first edition of Atlas of Oral Histology was
published 4 years ago. It has motivated us to put in more effort to
ensure that this book would find good use among students and
faculty alike. Some phase contrast microscopy and polarizing
microscopy images have been added. Every chapter now includes a
new section that provides Useful Points to Remember. A few errors
that had escaped our scrutiny in the first edition have now been
corrected.
A major addition to this edition is the audiovisual guide for oral
histology slides. We have made an attempt to show the various
microscopic structures related to oral histology through these videos.
An audio narration of the videos and on-screen annotations also point
out the structures being described, so that students find it easy to
follow and understand the concept. We hope the readers find this
useful.
Thank you.
Preface to the First Edition
Harikrishnan Prasad, Krishnamurthy Anuthama

“One look is worth a thousand words”, it is said. And

it is so true. Seeing and understanding has always
proven to be more effective to retain information than
reading and memorizing. It is with this idea that we
started preparing the manuscript for this atlas. Oral
histology, as a subject, can be very simple and very
complex at the same time. Young undergraduate
students, who are introduced to oral histology for the
first time, can feel overwhelmed with the amount of
information, especially so when they have to imagine
everything. Although textbooks carry numerous good
photomicrographs, it is very difficult to understand
which is what in these pictures, unless the student is
guided by a good teacher who can patiently explain
each. Our atlas aims to be such a good teacher. Almost
every photomicrograph in this atlas is accompanied
with a schematic diagram that makes identifying
different features easy. We have closely adhered to
Orban’s Oral Histology and Embryology in terms of
content arrangement. We would advise our readers to
use this atlas in conjunction with the textbook to
understand better.

We have tried to incorporate as many pictures as

possible at this point of time. Being in its first edition,
we feel there is sufficient scope for improvement,
especially in terms of quantity of content. Readers are
the best critics, and comments, criticisms or
suggestions are always welcome. They will only help
us improve and make this book better. It is our hope
that staff and students alike will benefit from this book.

Thank you.
List of videos

Dev of tooth.mp4 Chapter 2 - Page 10 - Advanced bell stage

Enamel full.mp4 Video for full Chapter 3 - Enamel
Dentin full.mp4 Video for full Chapter 4 - Dentin
Pulp and alveolar Video for full Chapter 5 - Pulp (same video file also to be used for
bone.mp4 chapter 8)
Cementum.mp4 Video for full Chapter 6 - Cementum
Periodontal Chapter 7 - Page 47 - Principal fiber groups of periodontal ligament
ligament.mp4
Pulp and alveolar Video for full Chapter 8 - Bone (same video file as used for chapter
bone.mp4 5)
Salivary glands.mp4 Video for full Chapter 9 - Salivary glands
Oral mucous Video for full Chapter 10 - Oral mucous membrane
membrane.mp4
Maxillary sinus.mp4 Video for full Chapter 11 - Maxillary sinus
CHAPTER 1

Introduction
Oral histology encompasses the microscopic study of tissues that form
the oral cavity. It is the basis on which our knowledge of the
physiology of oral cavity, and the pathologies that afflict it, are built
upon. Therefore, an understanding of the histology of oral tissues
becomes very significant.

Preparing tissues for microscopic

study
Oral cavity is made up of both hard tissues and soft tissues. Each of
these tissue types requires a specific method of preparation, so that it
can be viewed under a microscope.

Soft tissues
Soft tissues do not contain hard mineralized components. Hence, they
can be easily cut with a knife. However, to maintain their architecture,
they are subjected to a series of processes before being cut into thin
sections.
After removal for examination, the soft tissue is first fixed to
prevent degradation and decomposition. Neutral buffered formalin
(10% concentration) is the routinely used fixative for this purpose.
This is followed by complete removal of its water content and
replacement of the same by alcohol. To achieve this, the tissue is
immersed in a series of increasing grades of ethyl alcohol, so that
water in the tissue is gradually replaced by the alcohol. The next step
involves removal of the alcohol in the tissue and its replacement by
xylene. At the end of this step, the fixed tissue now contains no trace
of water in it; instead it is filled with xylene.
Following this, the tissue is immersed in molten paraffin wax,
which will replace the xylene completely. This step completes tissue
processing. The end result is that we now have a tissue that contains
wax instead of water; therefore, it is rigid and firm enough to be cut
into thin sections using an instrument called microtome. The
microtome allows sections as thin as 4 µm (4/1000 of a millimeter) to
be cut. These sections are placed on glass slides, the wax removed by
heat, and then subjected to different staining processes.

Hard tissues
Different methods are employed to study hard tissues like bone and
teeth because these cannot be cut into thin sections using routine
microtomes. The simplest method to study hard tissues is using
ground sections. Another frequently used method is decalcified
sections.

I. Ground section
Ground sections are made by grinding the specimen into thin slices
that can allow light to pass through them. Initially grinding is done
on a lathe or similar mechanical device. Later on, it can be done
manually on an abrasive stone (like Arkansas stone), and finally
polished on fine sandpapers. Such sections, which are about 25–50
micrometers thick, are then dehydrated and mounted directly on
glass slides using a mounting medium and then observed under the
microscope. It has to be stressed however that the thinner the ground
section, the better it is to appreciate many structures without much
overlap. One major
disadvantage of ground sections is that most of the tooth or bone is
wasted during the grinding process. Ground sections are useful for
visualizing the mineralized components and hypomineralized
structures of hard tissues. Pulp, however, cannot be seen in ground
sections.

II. Decalcified section

Hard mineralized structures can also be studied by making
decalcified sections. This works on the premise that most mineralized
structures also have a substantial organic component. When the
mineral portion is removed, these tissues attain properties similar to
soft tissues and can be treated akin to them. Bone, dentin, and
cementum have a considerable amount of organic matter and hence
can be studied after decalcification. Enamel however, being made up
of by 96% inorganic substance will be completely lost during
decalcification process.
Decalcification is usually done using acids like nitric acid, formic
acid and ethylenediaminetetraacetic acid (EDTA). Depending on the
acid used, the decalcification process can take anywhere between few
days and several weeks. Once decalcification is complete, the tissue
loses its hardness. It can then be processed and sections can be made
just like any other soft tissue. Decalcified sections are used to study
hard tissues, as mentioned earlier. Dental pulp, a soft tissue that is
safely enclosed inside dentin, also can be studied by decalcified
sections only. Although the pulp can be removed separately and
processed like a soft tissue, its tiny volume usually makes this
extremely difficult.

III. Sectioning using hard tissue microtome

Specialized microtomes that can cut hard tissues like bone and teeth
into thin sections are available in the market. These offer many
advantages over other methods of studying hard tissues. However,
they are very expensive and not readily available.

Staining
In general, tissues have very little contrast when viewed unstained. To
impart contrast to the tissue, and thereby identify and observe the
different structures and cells, sections from soft tissues and decalcified
hard tissues are subjected to staining. The commonest used
histological stain is hematoxylin and eosin (H&E). Hematoxylin is a
basic dye that gives a blue colour to acidic structures like nucleus and
rough endoplasmic reticulum. Eosin, being acidic in nature, stains
basic structures like cytoplasm and imparts a pink color. Other special
stains can also be used to selectively appreciate and identify specific
cells and tissues like skeletal muscle, elastic fibers, basement
membrane and microorganisms.

Microscopy
Study of histology necessitates the use of specialized equipment
called microscopes to magnify the tissues several hundred or
thousand times. Routine compound light microscope uses a system of
lenses and light source to achieve this. Usually light is allowed to pass
through the specimen (transmitted light). Certain structures are better
visible when using reflected light in which light is allowed to reflect
from the top of the specimen being studied. Various other types of
microscopes offering specific advantages are also available. This atlas
includes photomicrographs obtained from compound light
microscopes only, unless otherwise specified.

Points to remember
There are a few important points that need to be carefully considered
while viewing histological slides under the microscope:

• Setting up the microscope properly is a very essential step that

is frequently overlooked. Even a high-end microscope that has
been improperly set up will perform worse than a properly set
basic microscope.
• The various components of the microscope, especially the ones
that come in the light path, and the slides have to be clean and
free of dust.
• Transmitted light source is usually built-in, although some basic
microscopes require external light source. In such cases, be sure
to orient the reflecting mirror in such a way that adequate light
passes through the specimen being observed.
• For viewing structures under reflected light, transmitted light
source has to be switched off. If the ambient light in the room is
inadequate, additional light using a torch or a mobile phone
flash can be shone on top of the slide, and viewed.
• Although one structure is being highlighted upon in each
photomicrograph in this atlas, it is to be remembered than any
particular field can show multiple structures of interest.
Students appearing in undergraduate practical examinations
are usually expected to identify all such features and label them
in the diagrams.

Useful hints
• Commonly used fixative for tissues—10% neutral buffered
formalin.
• A microtome is used to cut tissues into thin sections.
• Hard tissues can be studied using ground sections, decalcified
sections, or sections obtained by hard tissue microtomy.
• Ground sections can be helpful to study the histology of enamel,
dentin, cementum, bone, and other hard tissues. Staining is not
 needed.
• Much of the tooth or bone is lost during grinding for ground
sections.
• Decalcified sections are useful to study the histology of dentin,
cementum, bone, and dental pulp. Enamel cannot be observed
with decalcified sections.
• Routinely used stain in histology and pathology is H&E stain.
• Hematoxylin—basic dye—stains acidic structures like nucleus,
RNA.
• Eosin—acidic dye—stains basic structures like cytoplasm and
its organelles.
CHAPTER 2

Development of tooth
In early fetal life, basal cells in some areas of the primitive oral cavity
proliferate more rapidly and result in the formation of a primary
epithelial band in each arch. This band later divides into a buccal
vestibular lamina and a lingual dental lamina. It is from this dental
lamina that the ectodermal portions of teeth develop.
Each tooth arises from a tooth germ, which is made up of three
parts: enamel organ, dental papilla, and dental sac. The enamel organ
is purely ectodermal in nature and derives from dental lamina. As the
name indicates, enamel organ plays the primary role in enamel
formation. Dental papilla is mesenchymal in origin, and gives rise to
dentin and pulp. Dental sac or dental follicle helps in the formation of
cementum, alveolar bone, and periodontal ligament.

Various stages in the development of tooth can be appreciated,

based on the morphology or shape of the enamel organ.
Bud stage (figs 2.1 and 2.2)
• Due to increased proliferation, tooth buds form at specific areas
of the dental lamina corresponding to the location of future
deciduous teeth.
• These buds grow downward into the underlying
ectomesenchyme.
• Two types of cells can be recognized at this stage: outer low
columnar cells and inner tightly packed polygonal cells.
• Cells of the ectomesenchyme surrounding the bud begin to
come close together and condense.

FIGURE 2.1 Development of tooth at bud stage (H&E

stain). Source: (Image courtesy: Dept of Oral Pathology, Saveetha Dental
College and Hospital, Chennai).
 FIGURE 2.2 Schematic representation of bud stage of tooth
development.

Cap stage (figs 2.3–2.5)

• The cells of the tooth bud do not grow equally. Few cells
proliferate at a faster rate than the rest, and this results in
a change of shape in the bud.
• The enamel organ assumes a cap shape, with a convex exterior
surface, and an invaginated or concave interior surface.
• The cells lining the concave surface also become elongated and
obtain a columnar shape. These are the inner enamel epithelial
(IEE) cells.
• The cells on the exterior are low cuboidal in shape and
constitute the outer enamel epithelium (OEE).
• Meanwhile, the central cells of enamel organ begin to separate
 from each other due to extracellular fluid accumulation and
become star shaped. They are attached to each other only at
their desmosomes. Hence, they are called stellate reticulum
(stellate – star shaped; reticulum – interconnected network).
• Sometimes, the inner enamel epithelial cells in the center of the
enamel organ become very closely packed. This dense structure
is called enamel knot, and is considered to provide signaling
that determines and controls the morphology of the crown.
• In this stage, condensed ectomesenchyme located within the
concavity of inner enamel epithelium is called dental
papilla.
• Dental papilla is demarcated from the rest of the
ectomesenchyme by accumulation of collagen fibers, which are
oriented almost in the form of a circle enclosing the dental
papilla. This is the dental sac.

FIGURE 2.3 Development of tooth at cap stage (H&E

stain). Source: (Image courtesy: Dept of Oral Pathology, Manipal College
of Dental Sciences, Mangalore).
FIGURE 2.4 Schematic representation of cap stage of tooth
development.
 FIGURE 2.5 Enamel knot (H&E stain). Source: (Image courtesy: Dept of
Oral Pathology, Manipal College of Dental Sciences, Mangalore).

Early bell stage (figs 2.6 and 2.7)

• As the cells continue to proliferate at different rates, the
invagination of IEE deepens and begins to assume the shape
of the future crown.
• OEE and stellate reticulum are seen similar to cap stage.
• Between stellate reticulum and IEE, two to three layers of
tightly packed flattened cells are noticed. This is called the
stratum intermedium.
• The region where the IEE is continuous with the OEE is called
cervical loop.
• The inner enamel epithelial cells, especially near the tip of the
future crown, begin to differentiate into ameloblasts. The cells
become more columnar in shape, and a reversal of polarity of
the nucleus is noticed.
• The cells of the dental papilla in the periphery, near the
ameloblasts, begin to differentiate into odontoblasts.
Elsewhere, a small acellular zone is seen separating the dental
papilla from IEE.
• Dental sac is more prominently noticed.
• The continuity of the tooth bud with the dental lamina
gradually begins to disintegrate.
 FIGURE 2.6 Development of tooth at early bell stage (H&E
stain). Source: (Image courtesy: Dept of Oral Pathology, SDM College of
Dental Sciences, Dharwad).
 FIGURE 2.7 Schematic representation of early bell stage of tooth
development.

Advanced bell stage (figs 2.8 and 2.9)

• The tooth bud is completely separated from the dental lamina.
• Cells of IEE and dental papilla differentiate into ameloblasts
and odontoblasts, respectively.
• Odontoblasts begin depositing the matrix for dentin initially.
After this, the ameloblasts begin depositing enamel.
• Deposition of dentin begins at the region of cusp tips, and then
proceeds in a downward and inward direction.
• Enamel deposition begins at the region of cusp tips, and
proceeds in a downward and outward direction.
• Stellate reticulum begins to collapse, and OEE is brought closer
to the ameloblasts.
 FIGURE 2.8 (A) Development of tooth at advanced bell stage (H&E
stain). (B) Higher magnification of the enamel organ showing enamel
and dentin formation. (C) Higher magnification of the cervical region
of the newly forming tooth.
 FIGURE 2.9 Schematic representation of the advanced bell stage of
tooth development.

Hertwig epithelial root sheath and

cementum formation (figs 2.10 and
2.11)
• The cervical loop of enamel organ plays an important role in
root formation.
• Once enamel and dentin formation reach the future cervical
line, the cervical loop begins proliferating in a downward
direction.
• However, this downgrowth is made up of only outer and inner
enamel epithelial cells. Stellate reticulum and stratum
intermedium are absent. This structure is called Hertwig
epithelial root sheath (HERS).
• HERS determines the shape of the roots. The cells of HERS
induce the dental papilla to differentiate into odontoblasts and
deposit root dentin.
• Following this, HERS disintegrates, and dental follicle cells
come in contact with root dentin. They then differentiate to
cementoblasts and deposit cementum.
• HERS at the most apical end shows a horizontal extension,
called epithelial diaphragm. This structure determines the
number of roots and location of the apical foramen.
• Remnants of HERS are called cell rests of Malassez.

FIGURE 2.10 Hertwig epithelial root sheath (HERS) (H&E

stain). Source: (Image courtesy: Dept of Oral Pathology, Manipal College
 of Dental Sciences, Mangalore).

FIGURE 2.11 Schematic representation showing Hertwig epithelial

root sheath and cementum formation.

Some important terminologies

Tooth germ: It is the term used to collectively denote the various
cells that together give rise to the different components of the
tooth. It is organized into enamel organ, dental papilla, and
dental sac.
Dental lamina: It is the lingual portion of the primary epithelial
band that later gives rise to the future primary tooth germs.
Vestibular lamina: It is the buccal portion of the primary epithelial
band that later gives rise to the future buccal vestibule.
Successional lamina: It is the lingual extension from the dental
lamina that gives rise to the tooth germs of the future permanent
succedaneous teeth.
Ectomesenchyme: It is a type of connective tissue which contains
neural crest cells, and which is especially important during
embryogenesis in head and neck region. It is thought to develop
from the ectoderm. It helps in the formation of bone, cartilage,
and connective tissue of cranial region, as well as tooth dentin,
cementum, and alveolar bone.
Dental papilla: It is the condensation of the ectomesenchyme that
is present within the invagination of the tooth germ, seen
prominently during bell stage of tooth development. It is
surrounded by the IEE and the dental follicle/sac. Cells in the
periphery near the ameloblasts differentiate into odontoblasts
which secrete dentin.
Dental sac: It is the condensed ectomesenchyme with circular
collagen fibres that enclose the enamel organ and dental papilla
together, thus separating them from the rest of the
ectomesenchyme. It gives rise to cementum, periodontal
ligament, and alveolar bone.
Outer enamel epithelium: This refers to the cells that make up the
outer convex surface of the enamel organ. The cells of OEE are
usually cuboidal in shape.
Inner enamel epithelium: This refers to the cells that make up the
inner concave surface (the invaginated portion) of the enamel
organ. These cells are usually low columnar in shape. They
transform into ameloblasts in the late bell stage, and secrete the
enamel matrix.
Stellate reticulum: The central cells of enamel organ that form a
network of star-shaped cells due to intercellular fluid
accumulation are called stellate reticulum. They are seen
between OEE and IEE in cap stage, and between OEE and
stratum intermedium in bell stages.
Stratum intermedium: Two/three layers of flattened cells seen
between stellate reticulum and IEE in the bell stages are called
stratum intermedium. They are rich in acid phosphatase enzyme.
Cervical loop: The region where the OEE folds and becomes the
IEE is called cervical loop (refer Figs 2.6 and 2.7).
HERS: Hertwig epithelial root sheath is a downward extension
of cervical loop, made up of only two layers of cells, namely,
OEE and IEE. The HERS determines the shape of roots.
Epithelial diaphragm: Horizontal extension of the HERS near the
apical end is called the epithelial diaphragm. This structure
corresponds to the future apical foramen region of the teeth, and
determines the number of roots.
CHAPTER 3

Enamel

Enamel is the hardest tissue in the body, with 96% of it being made
up of inorganic content. As a result of its predominantly mineralized
nature, enamel can be studied under the light microscope only using
ground sections. Decalcification will result in complete loss of enamel,
and hence decalcified sections are of little value.
Various light microscopic structures are appreciable in enamel. For
visualizing these structures, a transmitted light source is almost
always used, unless mentioned otherwise. Some of the obvious light
microscopic structures of enamel include:

1. Enamel rods
2. Striae of Retzius
3. Neonatal line
4. Enamel lamellae
5. Enamel tufts
6. Enamel spindles
7. Gnarled enamel
8. Hunter–Schreger bands

Enamel rods (figs 3.1 and 3.2)

• Hydroxyapatite crystals in the enamel are arranged in the form
of enamel rods.
• These rods are cylindrical structures extending from
dentinoenamel junction (DEJ) to enamel surface.
• They are generally arranged perpendicular to the underlying
dentin.
• During formation, around 4µ of enamel is deposited each day.
Dark lines can be noticed between these 4µ deposits, which
are called cross-striations.

FIGURE 3.1 Ground section of tooth showing enamel, dentinoenamel

junction, and dentin. Dense enamel rods can be seen extending from the
dentinal end to the outer surface in a wavy course.
 FIGURE 3.2 Schematic diagram showing the orientation of enamel
rods in relation to dentinoenamel junction. Inset shows the direction of
striae of Retzius in relation to cross-striations in the rods.

Striae of retzius (figs 3.3, 3.4, and 3.5)

• Dark brown lines representing a 6- to 11-day rhythm of enamel
deposition are also seen in enamel. These are called
incremental lines of Retzius (striae of Retzius).
• In cross-sections of teeth, these striae are noticed as concentric
 rings around the dentin. In longitudinal sections, they are seen
surrounding the tip of the dentin.
• An accentuated incremental line that separates the prenatal and
postnatal enamel is called neonatal line.
• These neonatal lines can be observed only in teeth that begin
mineralization of crown in utero. Hence, they are seen in all
primary teeth and in permanent first molars only.

FIGURE 3.3 Ground section of a molar crown showing incremental

lines of Retzius. Note the concentric arrangement of these lines around
the tip of the cusp.
FIGURE 3.4 Schematic diagram showing striae of Retzius and
neonatal line in enamel.
 FIGURE 3.5 Enamel observed under phase contrast microscopy.
Notice the concentric arrangement of striae of Retzius. Enamel lamellae
and tufts are also visible.

Enamel lamellae (figs 3.5, 3.6, and 3.7)

• Enamel lamellae are linear hypocalcified structures seen in
enamel, resembling thin leaf-like structures.
• These are seen extending from the enamel surface to the DEJ to
varying depths, sometimes even crossing into the dentin.
• These are of three types—A, B, and C.
• Type A is made up of poorly calcified rods.
• Type B contains degenerated cells.
• Type C is filled with organic matter from saliva.
• Lamellae can be easily confused with cracks that arise during
preparation of ground sections. If such ground sections are
decalcified carefully, the cracks will disappear, while lamellae
persist.
• Lamellae act as pathways for entry of bacteria resulting in
dental caries.

FIGURE 3.6 Ground section showing multiple enamel tufts and an

enamel lamella.
 FIGURE 3.7 Schematic diagram showing enamel lamella and tufts.

Enamel tufts (figs 3.5, 3.6, and 3.7)

• These are hypocalcified structures that arise from the DEJ and
extend to a short thickness of enamel.
• These resemble tufts of grass in ground sections.
• These are actually ribbon-like structures made up of
hypocalcified enamel rods that arise from the DEJ in different
planes and curve in different directions.
• These are better seen in cross-sections of teeth.
Enamel spindles (figs 3.8, 3.9)
• Enamel spindles are hypocalcified structures that result due to
extension of odontoblastic processes beyond the DEJ into the
enamel.
• These are seen in more numbers near the cusp tips.
• They are seen as small, dark extensions, sometimes having
blunt knob-like ends.

FIGURE 3.8 Ground section of a tooth near the region of cusp tip
showing multiple enamel spindles.
 FIGURE 3.9 Schematic diagram showing enamel spindles.

Gnarled enamel (figs 3.10 and 3.11)

• Enamel in the region of the cusp tips appears to have a more
complex arrangement of the rods.
• The rods show intertwining and wavy course, probably an
optical phenomenon, due to their increased numbers in a
very small area.
• It is thought that this arrangement helps in withstanding the
masticatory forces.
FIGURE 3.10 Ground section of a tooth in the cusp tip region usually
shows a wavy and intertwining course of enamel rods, also called
gnarled enamel.
 FIGURE 3.11 Schematic diagram of gnarled enamel.

Hunter–Schreger bands (figs 3.12, 3.13,

and 3.14)
• It is an optical phenomenon that can be visualized in oblique
reflected light.
• Alternating dark and light bands are visible in enamel in
longitudinal sections of the teeth.
• The dark bands are called diazones, and light bands are called
parazones.
• It is considered that this optical appearance is a result of change
in the direction of enamel rods.

FIGURE 3.12 Ground section of a tooth observed under reflected

light. Note the alternate dark and light bands in the enamel. These
represent the Hunter–Schreger bands.
FIGURE 3.13 Schematic diagram showing Hunter–Schreger bands.
 FIGURE 3.14 Enamel viewed under polarizing light. Alternating dark
and light Hunter–Schreger bands are visible very clearly.

Useful hints
• Enamel is the hardest tissue in the body, and can be studied
using ground sections.
• Enamel rods make up the structural unit of enamel. These are
formed by the regular arrangement of hydroxyapatite
crystals.
• Enamel is deposited in increments. The dark brown lines
separating each increment are called incremental lines of
Retzius.
• The prominent incremental line separating enamel formed
before birth and enamel formed after birth is called
neonatal
 line. It is seen in all deciduous teeth and in permanent first
molars only.
• DEJ is the junction where enamel and dentin meet. It has a
scalloped appearance.
• Hypomineralized or hypocalcified structures of enamel include
the following: enamel lamellae, enamel tufts, and enamel
spindles.
• Hunter–Schreger bands are an optical phenomenon, seen best
under oblique reflected light microscopy.
CHAPTER 4

Dentin

Dentin is a mineralized tissue that forms the bulk of the tooth. It is

made up of 65% inorganic substance in the form of hydroxyapatite
crystals. Unlike enamel, dentin is not a completely solid component.
Rather, hollow cylindrical structures called dentinal tubules are seen
throughout its thickness. These tubules contain odontoblastic cell
process and dentinal fluid. Peritubular dentin is seen surrounding the
walls of dentinal tubules. Between adjacent peritubular dentin,
intertubular dentin is present. This forms the major portion of dentin.
Adjacent to the pulp, a small layer of unmineralized dentin called
predentin, is also noticed.
Dentin can be studied using both ground sections and decalcified
sections.

Primary and secondary dentin (figs 4.1

and 4.2)
• Dentin can be classified into two types based on whether it is
formed before or after root completion.
• Dentin formed before root completion is called primary dentin.
• Dentinal tubules are seen arranged compactly and showing S-
shaped curvature in the primary dentin.
• Secondary dentin is formed after root completion.
• It contains lesser number of dentinal tubules compared to
primary dentin.
• A bend or angle in the dentinal tubules is noticed in the region
where primary and secondary dentin meet.

FIGURE 4.1 Ground section of tooth showing primary and secondary

dentin. Note the angle formed at the junction where primary and secondary
dentin meet.
 FIGURE 4.2 Schematic diagram showing primary and secondary
dentin.

Dead tracts (figs 4.3 and 4.4)

• Sometimes odontoblastic processes inside dentinal tubules
disintegrate due to dental caries, attrition, etc.
• Such tubules are instead filled with air, and in ground sections
they appear dark in transmitted light and white in reflected
light. These are called dead tracts.
 FIGURE 4.3 (A) Ground section of tooth under transmitted light
showing dead tract and tertiary dentin. Note the incisal edge of the tooth
appears abraded, along with exposure of dentin. (B) Ground section of
tooth under reflected light. Dead tract appears light here.
 FIGURE 4.4 Schematic diagram showing dead tract and tertiary
dentin.

Tertiary dentin (fig. 4.4)

• It is deposited as a healing response to some injury to the tooth.
• It is seen localized at the site of injury, as a result of deposition
of dentin by the surviving odontoblasts or newly
differentiated
 odontoblasts.
• It appears to have fewer and more convoluted dentinal tubules.

Interglobular dentin (figs 4.5–4.7)

• The peripheral portion of circumpulpal dentin, just below the
mantle dentin, shows mineralization in the form of globules.
• These globules fail to fuse occasionally, resulting in small
hypomineralized areas called interglobular dentin.
• It is seen most commonly in cervical and middle thirds of the
crown.
• Dentinal tubules are unaltered and pass through the
interglobular dentin undisturbed.

FIGURE 4.5 Ground section of dentin near the dentinoenamel junction

showing interglobular dentin.
FIGURE 4.6 Schematic diagram showing interglobular dentin.
 FIGURE 4.7 Interglobular dentin as observed under phase contrast
microscopy. The interglobular dentin appears refractile, and dentinal
tubules can be observed passing uninterrupted through the interglobular
dentin.

Tomes’ granular layer (figs 4.8 and 4.9)

• It is seen in radicular dentin just adjacent to the
cementodentinal junction.
• It appears as a dark, granular zone in transmitted light.
• It is thought to occur due to looping of dentinal tubules over
themselves during early root dentin formation.

FIGURE 4.8 Ground section of tooth. This field shows radicular dentin
and adjacent cellular cementum. Dark granules in the dentin near the
cementodentinal junction represent Tomes’ granular layer.
 FIGURE 4.9 Schematic diagram showing Tomes’ granular layer.

Branching of dentinal tubules (figs 4.10–

4.12)
• Dentinal tubules show branching, especially near the outer
surface.
• These terminal branches have been implicated in dentin
hypersensitivity.
• Terminal branches are more common in root dentin.
• Dentinal tubules also communicate with each other through
lateral branches that can occur anywhere along the course of
the tubule.

FIGURE 4.10 Ground section of tooth showing terminal branching of

dentinal tubules.
FIGURE 4.11 Numerous dentinal tubules are seen branching near the
dentinoenamel junction.
 FIGURE 4.12 Dentinal tubules showing fine lateral branches.

Useful hints
• Dentin makes up the major portion of the tooth, both in the
crown and the root. It is less mineralized than enamel.
• Dentin can be studied using both ground sections and
decalcified sections.
• Dentin is made up of millions of hollow dentinal tubules.
Imagine them to be a bunch of closely arranged flexible
drinking straws. The hollow portions of the straws correspond
to the dentinal tubules, and these hollow structures each
contain the odontoblastic process and the dentinal fluid. The
space between adjacent straws would then be the intertubular
dentin, while the plastic wall of the straw would be the
peritubular
 dentin.
• Dentin structure can be studied in two ways. The dentin can be
cut along the long axis of the tubules, so that we can follow and
appreciate the tubule from the pulpal surface to the
dentinoenamel junction. When we study dentin in this aspect,
we can appreciate the wavy course of the tubules, including the
primary curvature (S-shaped) and secondary curvatures. We
can also identify the primary dentin (which is further divided
into mantle dentin and circumpulpal dentin), secondary
dentin, and tertiary dentin. Other relevant structures like
interglobular dentin and Tomes’ granular layer can also be
noticed.
• Dentin can also be cut across, perpendicular to the long axes of
its dentinal tubules. When we see dentin from this aspect, we
can identify intertubular dentin and peritubular dentin, and
the dentinal tubule.
CHAPTER 5

Pulp

Pulp is the only soft tissue component of teeth, found in the center
within a space in the dentin called pulp cavity. The pulp cavity in the
crown, called pulp chamber, contains the coronal pulp. Radicular
pulp is present in the root portion of pulp cavity known as root canal.
Pulp is a loose connective tissue which is richly vascular and also
innervated. The vitality of a tooth is determined only by the viability
of the pulp. Pulp is probably the only soft tissue that is better studied
by decalcified sections, because it is safely located within dentin.

Zones of the pulp (figs 5.1 and 5.2)

• There are four recognizable areas or zones in the pulp. Starting
from the periphery (dentinal side), these are
• Odontogenic or odontoblastic zone
• Cell-free zone
• Cell-rich zone
• Core of the pulp
• The odontogenic zone contains the cell bodies of the
odontoblasts, arranged parallel to each other,
immediately subjacent to the predentin. Throughout life,
odontoblasts constantly secrete predentin that calcifies
into dentin.
• Cell-free zone, immediately beneath the odontoblasts, is a zone
that is relatively free of cells, and contains only the ground
substance. It is also called the cell-free zone of Weil. The
purpose of this zone is to provide space for the moving
 odontoblasts, as dentin production occurs.
• Cell-rich zone, as the name indicates, is more cellular in nature.
More number of fibroblasts and undifferentiated mesenchymal
cells are noticed in this zone.
• The pulp core is the central portion of pulp, and contains the
main trunk and branches of the blood vessels and nerve fibers
that supply the pulp. In addition, various cells including the
inflammatory cells are also noticed in pulp core.

FIGURE 5.1 Decalcified section of tooth showing the dentin–pulp

interface (H&E stain).
 FIGURE 5.2 Schematic representation of the various zones of pulp.

Pulp stones (figs 5.3 and 5.4)

• Pulp stones, also called denticles, are small nodular mineralized
structures in the pulp that can be seen as an age change.
• They can be classified as true or false denticles, depending on
their microscopic structure:
• True denticles contain dentinal tubules with
odontoblastic processes.
• False denticles are seen as concentric calcifications
 without any regular architecture.
• Both true and false denticles can also be classified into three
types based on their location in pulp:
• Free denticles are completely surrounded by pulp on all
sides.
• Attached denticles are partly within dentin and partly
within pulp.
• Embedded denticles are seen completely within the
dentin.
• It is considered that all denticles arise as free denticles initially.
As secondary dentin keeps depositing, they gradually become
attached, and then later completely embedded in dentin.
• Sometimes, the calcifications may be more diffuse and
widespread throughout the pulp tissue. Eventually, such diffuse
calcifications might lead to an almost complete obliteration of
the pulp canals.

FIGURE 5.3 Decalcified section showing free false pulp stones in the
pulp (H&E stain).
 FIGURE 5.4 Schematic representation of free false pulp stones.

Useful hints
• Pulp is the only soft tissue component of the tooth. It provides
vitality to the tooth.
• Pulp is highly vascular and richly innervated.
• Pulp is better studied by using decalcified sections of teeth.
• The odontogenic zone of pulp constantly produces predentin
throughout life that mineralizes to form secondary dentin.
• Pulp can show various age-related degenerative changes like
fibrosis, discrete calcifications (denticles) and diffuse
calcifications.
CHAPTER 6

Cementum

Cementum is a thin mineralized tissue that covers the roots of

teeth. It serves as an attachment for the periodontal ligament fibers,
thereby helping to hold the teeth in their sockets. It is avascular and
noninnervated. The thickness of cementum gradually increases from
the cervical line to the apical region. Cementum is made up of 45–50%
inorganic substance in the form of hydroxyapatite crystals.

Acellular cementum (figs 6.1 and 6.2)

• Acellular cementum is otherwise called primary cementum.
• This is a form of cementum in which entrapped cementocytes
are not present.
• It is more commonly seen in the cervical third of the roots.
• Acellular cementum is slowly deposited and the Sharpey’s
fibers in it are well mineralized.
FIGURE 6.1 Ground section of tooth showing acellular cementum and
adjacent dentin.
 FIGURE 6.2 Schematic diagram of acellular cementum.

Cellular cementum (figs 6.3 and 6.4)

• Cellular cementum is otherwise called secondary cementum.
• Numerous cementocytes are seen entrapped in lacunae. The
lacunae have many extensions called canaliculi that house the
cell processes of cementocytes. These canaliculi are directed
toward the outer surface of the cementum (toward
periodontal
 ligament).
• This type of cementum is more frequently seen in the apical
third of root.
• It is deposited at a faster rate; therefore, the Sharpey’s fibers in it
are partially mineralized.

FIGURE 6.3 Ground section of apical portion of tooth root showing

cellular cementum.
 FIGURE 6.4 Schematic diagram showing cellular cementum. Inset
shows the orientation of canaliculi in cementocytes.

Incremental lines of salter (figs 6.5 and

6.6)
• Careful observation of cementum under the microscope reveals
 lines parallel to its surface, which represent its periodic
deposition. These are called incremental lines of Salter.
• These are seen separating the cementum into layers.
• Contrary to the incremental lines of enamel and dentin,
incremental lines of cementum are hypermineralized areas
with less collagen.
• Counting the number of incremental lines can be a useful
method to estimate the age.

FIGURE 6.5 Ground section of tooth showing incremental lines of

Salter in cementum. Note that these lines are almost parallel to the
surface of the cementum.
 FIGURE 6.6 Schematic diagram showing incremental lines of Salter in
cementum. The horizontal band-like structures in cementum represent
cracks. They can be mistaken for Sharpey’s fibers, but Sharpey’s fibers
usually do not extend up to the dentinal surface.

Cementoenamel junction (figs 6.7–6.14)

• Cementum and enamel meet at the cervical line of the tooth.
• Depending on their relationship to each other, three major types
of cementoenamel junctions (CEJ) are now recognized:
i. Cementum overlapping enamel (Figs 6.7 and 6.8)
 ii. Edge-to-edge/butt/knife-edge type junction (Figs 6.9
and 6.10)
iii. Gap junction (Figs 6.11 and 6.12)
• Cementum overlapping enamel is by far the most common
type, accounting for about 60% in all the teeth.
• This probably occurs because the reduced enamel epithelium in
the cervical region degenerates, resulting in connective tissue in
that region to contact the enamel and differentiate into
cementoblasts forming cementum.
• Edge-to-edge junction is noticed in around 30% of teeth.
• Gap junction is seen in about 10% of teeth. It is believed that this
occurs because enamel epithelium in the cervical region remains
viable without undergoing degeneration for a longer period. As
a result, cementum is not deposited in that region.
• Recently, rare possibility of enamel overlapping cementum
(Figs 6.13 and 6.14) has also been acknowledged, although
the mechanism behind this phenomenon is not fully
understood.
FIGURE 6.7 Ground section showing overlap type of cementoenamel
junction.
FIGURE 6.8 Schematic representation of cementum overlapping
enamel type of junction.
FIGURE 6.9 Cementum and enamel meeting at a butt type/edge-to-
edge type of junction.
FIGURE 6.10 Schematic representation of butt/edge-to-edge type of
cementoenamel junction.
FIGURE 6.11 Gap type of cementoenamel junction.
FIGURE 6.12 Schematic representation of gap type of cementoenamel
junction. Note that enamel and cementum do not meet in this type.
FIGURE 6.13 Ground section showing a rare type of cementoenamel
junction where enamel overlaps cementum.
 FIGURE 6.14 Schematic representation of cementoenamel junction
where enamel overlaps cementum. This type of junction is quite rare in
incidence.

Useful hints
• Cementum is an avascular and noninnervated structure
covering the roots of teeth.
• It can be studied using ground sections or decalcified sections.
• It serves to provide attachment to periodontal ligament fibers.
Without cementum, a tooth will not stay in the socket for a
long time and will exfoliate very soon.
• The portions of periodontal ligament fibers that are embedded
into cementum are called Sharpey’s fibers. These are extrinsic
fibers (originate outside cementum).
• Cementum also has intrinsic fibers, which are short collagen
fibers produced by cementoblasts during cementum deposition.
• Cementum can be classified into different types based on the
presence or absence of cementocytes, and the presence or
absence of extrinsic and intrinsic fibers.
• Incremental lines of Salter are hypermineralized areas, in
contrast to the incremental lines of other structures. They
run parallel to the cementum surface.
CHAPTER 7

Periodontal ligament

Periodontal ligament is a soft connective tissue that helps in

retaining a tooth to its socket in the alveolar bone through bundles of
collagen fibers. It is attached on one side to the cementum of the tooth,
and on the other side, it is inserted into the alveolar bone. It is seen
occupying a thin space around the roots of teeth, sometimes referred
to as the periodontal space.

Principal fiber groups of periodontal

ligament (figs 7.1–7.4)
• Five distinct fiber groups can be observed in periodontal
ligament. These are
• Alveolar crest group
• Horizontal group
• Oblique group
• Apical group
• Interradicular group
• Alveolar crest bundles extend from the crest of the alveolar
bone to the cementum near the cementoenamel junction.
They have an obliquely upward course from bone to tooth.
• Fibers of the horizontal group run through the shortest course
from bone to cementum. These are almost at right angles to
both the cemental surface and the bone.
• Oblique fiber groups are most numerous and play a major role
in resisting occlusal forces. These run an obliquely downward
 course from bone to cementum, with the cemental insertion
being more apical than bone attachment.
• Apical group fibers are seen extending from around the tip of
the root to the bone. These are not seen in teeth with
incomplete roots, and probably play a role in protecting the
blood vessels and nerves entering the apical foramen (Figs 7.3
and 7.4).
• Interradicular fibers are seen only in multirooted teeth. These
extend from the cementum in the root furcation to the tip of
the interradicular septum of bone.
• Dentoperiosteal fibers, a group of gingival fibers, are also seen
in Figs 7.1 and 7.2. These extend from the cementum of the
tooth to the outer surface (periosteum) of the alveolar bone.

FIGURE 7.1 Decalcified section of tooth and adjacent alveolar bone

showing the various principal fiber groups of periodontal ligament
 (H&E stain).

FIGURE 7.2 Schematic representation of various principal fiber

groups of periodontal ligament, showing their orientation and
relationship with each other and with the adjacent tissues.
 FIGURE 7.3 Decalcified section of the apical portion of tooth root
showing the apical group of periodontal ligament fibers (H&E stain).
 FIGURE 7.4 Schematic representation of the apical group of
periodontal ligament fibers.

Cementicles (fig. 7.5)

• Cementicles are calcified structures noticed sometimes within
the periodontal ligament.
• These are considered to be a regressive change noticed more
frequently in older individuals.
• The exact nature and origin of these structures are unknown. It
 is considered that these masses arise as a result of degeneration
and calcification of epithelial rests of Malassez.

FIGURE 7.5 Decalcified section showing a mass of calcified material

(cementicle) located freely in the periodontal ligament (H&E stain).

Useful hints
• The periodontal ligament is a dense band of connective tissue
that connects the tooth to the alveolar bone.
• Within this connective tissue, collagen fibres are arranged and
 organized into five principal fibre groups.
• Each principal fiber group has a specific location and direction
within the periodontal ligament.
• The terminal ends of the principal fibre groups that are inserted
into the bone or cementum are called Sharpey’s fibers.
• The periodontal ligament acts like a thick gel that supports the
tooth in the socket and absorbs forces.
CHAPTER 8

Bone

Alveolar bone is the part of the jaw that supports and holds the
roots of teeth. Although functionally different, the histology of
alveolar bone is pretty much similar to bone elsewhere. Alveolar bone
can be arbitrarily divided into two parts: alveolar bone proper and
supporting alveolar bone.
Alveolar bone proper forms that part of the jaw which houses the
sockets for tooth roots. It consists of bundle bone and lamellar bone.
Bundle bone is the portion of socket into which the principal fibers of
periodontal ligament are inserted. Lamellated bone immediately
surrounds the bundle bone. Supporting alveolar bone lies beneath the
alveolar bone proper and is made up of outer cortical plates and inner
spongy bone.
Histologically, the cortical plates of jaws are made up of compact
bone. The spongy bone between these cortical plates is made up of
cancellous bone. The difference between these two types of bones
(compact and cancellous) lies in their internal structure.

Compact bone (figs 8.1–8.4)

• Compact bone is usually found at the outer surface of bones.
• It is dense and compactly arranged without any spaces or gaps.
• Basic functional units called osteons are seen throughout.
• An osteon or Haversian system is composed of a central
Haversian canal and surrounding concentric lamellae.
• Haversian canal usually contains blood vessels and nerve fibers.
Sometimes, two adjacent Haversian canals are connected by a
Volkmann’s canal.
• The external and internal surfaces of the bone also contain
circumferential lamellae.
• Osteocytes are seen within lacunae that contain numerous
extensions called canaliculi. Through these canaliculi, cell
processes of osteocytes communicate with each other and
with the exterior.

FIGURE 8.1 Ground section of compact bone showing different types

of lamellae.
FIGURE 8.2 Schematic diagram showing different types of lamellae
that form the compact bone.
FIGURE 8.3 Decalcified section of compact bone (H&E stain).
 FIGURE 8.4 Schematic diagram showing appearance of compact bone
in decalcified section.

Cancellous bone (figs 8.5 and 8.6)

• Cancellous bone is made up of small spicules or pieces of bone
called trabeculae.
• Trabeculae branch and unite and form a delicate network in the
center of bone.
• Spaces between these trabeculae are called marrow spaces.
 These spaces are richly vascular and form the major site of
hematopoiesis in young children.
• As age progresses, marrow becomes more fatty in nature.

FIGURE 8.5 Decalcified section of cancellous bone (H&E stain).

 FIGURE 8.6 Schematic representation of cancellous bone.

Useful hints
• Bone is a hard, mineralized structure that can be studied using
ground sections or decalcified sections.
• Alveolar bone or alveolar process is that part of the maxilla and
mandible that contains the tooth sockets.
• The alveolar bone undergoes resorption once the teeth are lost.
• The alveolar bone is organized as follows:
Lamellar Bunt;%
Oular cortical Inner apongy
aoñ e bona plates

Histofc•gi6ally, CacodloU8
”it is c‹xnpact bone,
inserted in "it bone. histok›gicaIIy

Has iamellaa Haa traLaculaa

and oateons
and mariow
spaces
CHAPTER 9

Salivary glands

Salivary glands are compound exocrine glands located within and

around the oral cavity. These have epithelial secretory units called
acini, and ducts lined by epithelium that serve to modify and conduct
the secretions into the oral cavity. These parenchymal structures are
supported by connective tissue that encapsulates the gland and also
divides it into small lobes and lobules.
Salivary glands can be classified based on their function or
histological appearance into:

1. Serous salivary glands

2. Mucous salivary glands
3. Mixed salivary glands

Serous salivary glands (figs 9.1–9.3)

• These are glands that are predominantly made up of serous
salivary acini.
• The cells of the serous acini are pyramidal in shape, and have a
round nucleus that is present in the basal third region of the
cell.
• Serous cells, by virtue of their protein content, stain more
eosinophilic.
• The acinar cells also contain small eosinophilic granules, which
appear more dense and concentrated toward the tip of the cell.
These are the zymogen granules.
• Interlobular and intralobular ducts (discussed later) are also
 noticed.
• Parotid gland is a predominantly serous salivary gland.
• The only minor salivary gland that is serous in nature is von
Ebner salivary gland.

FIGURE 9.1 Serous salivary gland under low magnification (H&E

stain). Note the distribution of various intralobular and interlobular ducts.
 FIGURE 9.2 Serous salivary gland under higher magnification (H&E
stain). Note the shape of nuclei in the acinar cells and the staining property
of acini.
 FIGURE 9.3 Schematic diagram of a serous salivary gland. Zymogen
granules, although visible here, are rarely visible under light microscope.

Mucous salivary glands (figs 9.4–9.6)

• Mucous type of salivary acini are predominantly noticed in
these glands.
• Mucous acinar cells are also pyramidal in shape. However, the
nucleus is flat and appears pushed toward the bottom of the
cell.
• The cytoplasm of acinar cells rarely take up any stain, and
appear pale or empty on routine hematoxylin and eosin
(H&E)
 stained sections.
• Interlobular and intralobular ducts are present.
• Sublingual salivary glands are predominantly mucous in
nature.
• All minor salivary glands, except von Ebner glands are also of
mucous type.

FIGURE 9.4 Mucous salivary gland under low magnification (H&E

stain). Note the overall pale staining property of acini.
FIGURE 9.5 Mucous salivary gland under higher magnification (H&E
stain). The acini appear empty and pale staining, while the nuclei are
apposed to the basal surface of acinar cells.
 FIGURE 9.6 Schematic diagram showing the histology of mucous
salivary gland. Nuclei in the acinar cells are flat and pushed toward the
basal surface.

Mixed salivary glands (figs 9.7–9.9)

• Mixed salivary glands contain both serous and mucous type of
acini.
• A few serous cells are seen arranged as a crescent-shaped
structure on top of some mucous acini. These are called serous
demilunes (demilunes of Giannuzzi) (demi—half; lune—
moon).
• It is now known that demilunes are artifacts that arise due to
traditional tissue preparation methods.
• Tissues prepared using liquid nitrogen and osmium tetroxide
show serous and mucous cells aligned normally within the
acinus.
• In conventional tissue preparations, mucous cells swell and
push the serous cells outside, resulting in the demilune
appearance.
• Submandibular salivary gland is a mixed salivary gland.

FIGURE 9.7 Mixed salivary gland under low magnification (H&E stain).
Note the distribution of two different types of acini with markedly varying
staining property.
 FIGURE 9.8 Mixed salivary gland under higher magnification (H&E
stain). Note the presence of serous demilunes, which are characteristic of
mixed glands.
 FIGURE 9.9 Schematic diagram of the histology of mixed salivary
gland.

Ductal system of salivary glands

Salivary glands contain a system of ducts which acts as canals in
which saliva flows from the secretory units (acini) to open into the
oral cavity. In addition, some parts of the ductal system also modify
the salivary composition.

Intercalated ducts (figs 9.10 and 9.13)

• Intercalated ducts are usually found in an intralobular location.
• These are the smallest in the ductal system, and carry secretions
from the acini to the striated ducts.
• Intercalated ducts are lined by a layer of low cuboidal cells.

FIGURE 9.10 High magnification of an intercalated duct (H&E stain).

 Note the small lumen surrounded by cuboidal cells with spherical nuclei at
their center.

Striated ducts (figs 9.11 and 9.13)

• Striated ducts form the major component of the ductal system,
and are mostly located intralobularly.
• These conduct saliva from the intercalated ducts to the
excretory ducts, and also play an important role in
modifying salivary composition.
• These are lined by columnar cells with centrally placed nuclei.
• The basal portion of these cells contains fine striations which
give them their name.
• Electron microscopy reveals that these striations are deep
infoldings of the cell membrane in the basal region, with
numerous mitochondria in between.

FIGURE 9.11 High magnification of a striated duct (H&E stain). Lumen

 is surrounded by columnar cells. Note the position of the nuclei in these
cells. The basal portion below the nucleus is made up of striations due to
surface infoldings that can be appreciated under electron microscope.

Excretory ducts (figs 9.12 and 9.13)

• Excretory ducts are the terminal structures of the ductal system
which ultimately open into the oral cavity.
• These are usually seen in the interlobular connective tissue
septa.
• These are lined by stratified cuboidal, stratified columnar,
pseudostratified columnar, or stratified squamous epithelium.
• These ducts help pour the secretions into the oral cavity, and
also contribute to salivary modification to a lesser extent.

FIGURE 9.12 A large excretory duct can be easily appreciated in the

fibrous septum, even under low magnification (H&E stain).
 FIGURE 9.13 Schematic diagram showing the histology of the
different types of ducts in salivary glands.

Useful hints
• Salivary glands are compound, tubuloacinar, exocrine glands
that produce saliva.
• They can be classified based on their location, size, or type of
secretion.
• Serous salivary glands produce a watery secretion that is rich in
proteins and contains little carbohydrates.
• Mucous salivary glands produce a thick, viscous secretion that
is rich in carbohydrate and poor in proteins.
• The morphology of the terminal secretory units (acini) varies
according to the type of secretion it produces.
• Ducts of salivary glands help to carry the secretions from acini
to the oral cavity. They also modify the composition of saliva.
• Minor salivary glands are distributed in most parts of the oral
cavity in the submucosa. Attached gingiva and the anterior part
of hard palate do not contain minor salivary glands.
CHAPTER 10

Oral mucous membrane

The oral mucous membrane covers all the surfaces of the oral
cavity and performs several important functions in addition to
protecting the underlying structures. It aids in mastication,
swallowing, speech, and taste sensation. Histologically, oral mucosa
comprises epithelium and lamina propria. Epithelium is of stratified
squamous type, and may be keratinized or nonkeratinized. Lamina
propria is the connective tissue seen immediately beneath the
epithelium. Deeper to this, the connective tissue that contains
structures like glands and adipocytes is called submucosa. Submucosa
helps in attaching the mucosa to underlying structures like muscle or
bone. Although it is present in most parts of the oral cavity,
submucosa is absent in some areas like attached gingiva and
midpalatine raphe.

Keratinized stratified squamous

epithelium (figs 10.1–10.5)
• Keratinized oral epithelium has several histological features that
differentiate it from nonkeratinized epithelium.
• Four distinct layers can be appreciated histologically:
• Stratum basale (or basal layer) is the single layer of cells
that is seen in apposition with the basement membrane.
It is made up of cuboidal cells, which actively
proliferate and move toward the surface.
 • Stratum spinosum (or spinous layer) is made up of
polygonal cells that gradually become larger as they
progress toward the surface. These cells are also called
acanthocytes. During routine tissue processing and
preparation for microscopy, the cells shrink slightly
and their intercellular bridges (desmosomes) become
visible easily; hence the name.
• Stratum granulosum (or granular cell layer) contains two
to three layers of flattened cells that have very
characteristic dark basophilic granules. These are the
keratohyaline granules, which play an important role in
keratin formation.
• Stratum corneum (or corneal layer or keratin layer) is the
most superficial layer made up of flattened cells that are
devoid of almost all organelles. Two types of keratin can
be distinguished: orthokeratin and parakeratin.
• Orthokeratin is made up of flattened cells that contain no nuclei
in the corneal layer. Orthokeratinized epithelium has a
prominent granular cell layer (Figs 10.1 and 10.2). It is mostly
seen in the hard palate (Fig. 10.3).
• Parakeratin is made up of flattened cells that contain few
pyknotic nuclei. The granular cell layer cannot be appreciated
clearly in parakeratinized epithelium in light microscopy
(Figs
10.4 and 10.5). Parakeratinized epithelium is noticed commonly
in the gingiva.
FIGURE 10.1 Orthokeratinized stratified squamous epithelium (H&E
stain).

FIGURE 10.2 Schematic diagram of orthokeratinized stratified

squamous epithelium.
FIGURE 10.3 Orthokeratinized stratified squamous epithelium as
noticed in the posterior glandular zone of hard palate (H&E stain).
 FIGURE 10.4 Parakeratinized stratified squamous epithelium (H&E
stain). Note the presence of pyknotic nuclei in the corneal layer. Granular
layer is not so prominent.
 FIGURE 10.5 Schematic diagram of parakeratinized stratified
squamous epithelium.

Nonkeratinized stratified squamous

epithelium (figs 10.6 and 10.7)
• The stratification in nonkeratinized epithelium is less distinct.
• A single layer of cuboidal cells (stratum basale or basal layer) is
evident.
• Above this, there are polygonal cells that gradually flatten
toward the surface. These cells can be divided arbitrarily
into
two layers:
• Stratum intermedium (or intermediate layer) is made up
of predominantly polygonal cells.
• Stratum superficiale (or superficial layer) contains
flattened cells that have nuclei.

FIGURE 10.6 Nonkeratinized stratified squamous epithelium (H&E

stain).
 FIGURE 10.7 Schematic diagram of nonkeratinized stratified
squamous epithelium.

Keratinocytes and nonkeratinocytes

(figs 10.8 and 10.9)
• Almost all cells of both keratinized and nonkeratinized
epithelium contain cytokeratin proteins. These cells are
collectively referred to as keratinocytes (cells
containing cytokeratin).
• Few cells in oral epithelium, however, do not contain
cytokeratin. These are called nonkeratinocytes.
• Nonkeratinocytes include melanocytes, Merkel cells,
Langerhans cells, and inflammatory cells.
• Melanocytes can be identified by their distinct color due to
melanin pigment. These are mostly seen in basal layer.
• Langerhans cells are antigen-presenting cells that play a role in
immunity. These are seen as clear cells in the more superficial
layers of the epithelium.
• Merkel cells are commonly noticed in the basal layer. These are
cells of neural origin that probably mediate the pressure
sensation. These are also identified as clear cells that do not take
up any cytoplasmic stain.
 FIGURE 10.8 (A) Melanocytes in oral epithelium seen in low
magnification (H&E stain). (B) Higher magnification showing a single
melanocyte in the basal layer (H&E stain). Note the numerous cell
processes that extend between adjacent cells in the epithelium.
 FIGURE 10.9 Numerous nonkeratinocytes noticed in oral epithelium
(H&E stain). These are observed as clear cells and could represent Merkel
cell or Langerhans cell.

Papillae of the tongue

Dorsal surface of the tongue has numerous tiny projections called
papillae which give it the rough texture. Papillae can be easily
observed by the naked eye especially when the tongue is dry. They
are of three major types:

Filiform papilla (figs 10.10 and 10.11)

• Filiform means thread-like. Filiform papillae are pointed,
conical structures in the anterior two-third of the tongue, and
are arranged in numerous rows parallel to the sulcus
terminalis.
• These are the most numerous among all papillae.
• Histologically, these are made up of stratified squamous
keratinized epithelium, containing a connective tissue core.
The tip of the papilla usually shows more keratin.
• These papillae do not contain taste buds.

FIGURE 10.10 Section through anterior tongue showing filiform

papillae (H&E stain). Note the abundant keratinization of the papillae and
lack of any taste buds.
 FIGURE 10.11 Schematic diagram showing the histology of filiform
papillae.

Fungiform papilla (figs 10.12 and 10.13)

• Fungiform means mushroom-like. Fungiform papillae are seen
distributed between filiform papillae and are seen mostly near
the tip and sides of the tongue.
• These are covered by a thin stratified squamous epithelium that
appears nonkeratinized, especially on its surface. As a result,
the vascularity of the connective tissue shows through, and the
papilla appears reddish in color.
• Few taste buds are seen on the dorsal surface.
 FIGURE 10.12 Section showing fungiform papilla (H&E stain).

FIGURE 10.13 Schematic diagram showing the histology of fungiform

 papillae.

Circumvallate papilla (figs 10.14 and 10.15)

• Circumvallate means surrounded by a walled trench or
depression.
• Circumvallate papillae (or vallate papillae) are characteristically
seen just in front of the sulcus terminalis, on either side of the
midline. These are about 8–10 in number.
• Each papilla is surrounded by a circular depression or furrow in
the mucous membrane.
• Epithelium is of stratified squamous type. The dorsal surface
shows keratinization occasionally. The lateral surfaces of the
papilla are nonkeratinized and contain numerous taste buds.
• Connective tissue core shows many secondary papillae,
especially toward the dorsal surface.
• Few minor serous salivary glands are noticed in the submucosa
beneath these papillae. These glands are the only minor
salivary glands that are serous in nature, and they open their
secretions into the trough of the circumvallate papillae. These
are called von Ebner glands.
FIGURE 10.14 Section showing a circumvallate papilla (H&E stain).
Note the presence of numerous taste buds along the lateral walls. Image
courtesy: Dept of Oral Pathology, Saveetha Dental College and Hospital,
Chennai.
 FIGURE 10.15 Schematic diagram showing the histology of a
circumvallate papilla.

Dentogingival junction (figs 10.16 and

10.17)
• Dentogingival junction (DGJ) is the junction where the gingiva
gets attached to the tooth.
• A shallow gingival sulcus lined by nonkeratinized stratified
squamous epithelium is seen surrounding the tooth on all
sides.
• Below the floor of this sulcus, the epithelium is attached to the
tooth enamel or cementum. This is called the junctional
epithelium.
• Junctional epithelium does not show the process of
keratinization.
• DGJ is initially seen completely on the enamel in newly erupted
teeth. It gradually shifts toward a more apical direction as age
 progresses. (The schematic diagram in Fig. 10.17 shows DGJ that
is partly on enamel and partly on cementum.).
• The junctional epithelium is unique in that it contains two sets
of basal lamina and basal layers of cells—one in apposition with
the tooth surface and the other toward the lamina propria of
gingiva.
• The lamina propria of gingiva shows the gingival groups of
fibers (dentogingival fibers, dentoperiosteal fibers, etc.).

FIGURE 10.16 Decalcified section showing the dentogingival junction

(H&E stain). Source: (Image courtesy: Dept of Oral Pathology, SDM
College of Dental Sciences, Dharwad).
 FIGURE 10.17 Schematic diagram showing the dentogingival junction.

Useful hints
• Oral mucous membrane is the soft tissue that covers all surfaces
of the oral cavity. It is organized as follows:
• Epithelium of the oral cavity is usually stratified squamous in
type. It may be keratinized or nonkeratinized in nature.
• Almost all epithelial cells in the oral cavity contain cytokeratin
filaments. Therefore, they are called keratinocytes.
• Few cells of the epithelium do not contain cytokeratin; hence,
they do not have the ability to keratinize. They are called
nonkeratinocytes, e.g., melanocytes, Merkel cells,
Langerhans cells, inflammatory cells.
• The oral mucosa shows structural modifications according to
the function it performs.
• Some unique structural changes include papillae of tongue,
vermilion border of lip, and DGJ.
• Papillae are projections on the dorsal surface of tongue, some of
which contain taste buds.
• Vermilion border is the transitional zone of the lip, where the
skin of face gradually changes and continues as the mucosa of
the lip. It is easily remembered as the lipstick zone (the part of
the lips where lipstick is applied). This zone has very thin
epithelium, and does not have salivary glands or sweat
glands or sebaceous glands in the submucosa. Therefore, it is
susceptible to dry out very easily.
• DGJ is unique because it is the only site where epithelium
attaches to a hard tissue directly. It has two basal laminas, one
on tooth surface side and other on the connective tissue side.
DGJ plays a significant role in tooth eruption, and in the health
of the gingiva and the periodontal tissues.
CHAPTER 11

Maxillary sinus

The maxillary sinus, also known as antrum of Highmore, is the

largest among the paranasal air sinuses. Although a structure entirely
within the maxilla and having no direct communication with the oral
cavity, it is of interest in oral histology because of its close association
with the roots of maxillary premolars and molars. The sinus is entirely
lined by mucosa that in most places is firmly bound to the underlying
bone, with little or no submucosa.

Histology of the sinus lining (figs 11.1

and 11.2)
• The sinus lining is made up of a thin epithelium of
pseudostratified ciliated columnar type and lamina propria.
• Cilia help to propel mucus, microorganisms, or debris from the
surface of the sinus lining toward the nasal cavity through
their wave-like beating.
• In addition to columnar and ciliated cells, the epithelium also
contains columnar nonciliated cells, basal type of cells, and
goblet cells (described in the following section).
• The lamina propria is made up of connective tissue cells and
fibers, along with blood vessels.
• Subepithelial glands of serous or mucous type are noticed in the
lamina propria. These glands pour their secretions on the
surface of the sinus lining through ducts.
FIGURE 11.1 Section showing the maxillary sinus lining made up of
pseudostratified ciliated columnar epithelium (H&E stain).

FIGURE 11.2 Schematic diagram showing the histology of maxillary

sinus mucosa.
Goblet cells (figs 11.3–11.5)
• Goblet cells are simple unicellular intraepithelial glands that are
interspersed among the other cells of the sinus epithelium.
• These are called so because of their unique shape with a wide
apical region and a narrow stalk-like basal region, resembling
a wine glass.
• These cells secrete mucin, which first accumulates in the apical
region of the cell resulting in their distention and widening,
before being discharged through exocytosis.
• The slender basal region rests on the basement membrane, and
contains nucleus and other organelles.
FIGURE 11.3 Higher magnification of the sinus lining showing the
pseudostratified appearance clearly (H&E stain). Goblet cells and
surface cilia are also appreciable in the picture.

FIGURE 11.4 Periodic acid–Schiff-stained section showing the

maxillary sinus lining. Note the distinctly visible goblet cells with their
unique shape.
 FIGURE 11.5 Periodic acid–Schiff-stained section of maxillary sinus
lining at higher magnification, distinctly showing the goblet cells.

Useful hints
• The maxillary sinus is the largest among all paranasal air
sinuses.
• It is seen in close relation to the roots of maxillary posterior
teeth (most commonly the first molar and second premolar).
• Pathologies affecting maxillary posterior teeth might involve the
maxillary sinus too, due to the close association.