Assignment #1
My Favorite Tissue: Teeth
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Introduction
During childhood humans have 20 deciduous teeth (also called
primary of milk teeth). The first tooth erupts six to seven months after birth
and are complete by 2 years of age. They are shed between 6 and 12 years of age
as they are replaced by permanent teeth.
Adult humans normally have 32 permanent teeth arranged in
two bilaterally symmetric arches in the maxillary and mandibular bones (Mescher,
2009). The mouth is divided up into 4
quadrants with each containing 8 teeth (2 incisors, 1 canine, two premolars and
3 permanent molars)
Function of teeth
Teeth play a role in the digestive system within the body. Teeth
are designed for chewing which breaks down foods into smaller pieces to help
allow for better absorption of nutrients. The dentition of teeth vary depending
on their specific function. The incisors have been modified for biting (thin
and long) and the molars (thick and relatively flat with grooves) are for
grinding food.
Dental Anatomy
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Figure 3: Internal components of the tooth(4).
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The main components of the tooth are:
- Enamel
- Dentin
- Pulp cavity
- Cementum
- Periodontal membrane (ligaments)
- Gingiva
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Figure 4: Decalcified section of enamel
showing rods (1).
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- The hardest component of the human body (98%
hydroxyapatite, mainly calcium and phosphate in the form of apatite crystals)
- Covers only the crown of the tooth
- Composed of rods or prisms, rod sheaths
- Produced by ameloblasts
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| Figure 5: Histological section showing production of dentin and layers of growth (1). |
Dentin
- 30% organic matter and water and 70% inorganic
material
- Organic substances consist of collagenous
fibrils (arranged in random network) and
ground substance of mucopolysaccharides
- Inorganic component consists of hydroxyapatite
crystals that a plate shaped and much smaller than those in enamel
- Produced by odontoblasts
Pulp cavity
- - Pulp of the tooth is derived from mesenchyme of embryonic dental
papilla and it fills the pulp cavity (pulp chamber and root canals)
- A layer of epithelial-like, columnar cells
called odontoblasts (derived from mesenchyme) underlie dentin and are
responsible for dentin formation
- - Contains
sensory nerves that respond to stimuli such as heat, cold and pressure
- - Lymphocytes and macrophages and leukocytes are
present to aid in repair of the pulp following irritation
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Figure 6: Histological section showing
periodontal ligament, cementum,
dentin and alveolar bone (1).
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Cementum
- Covers dentin of the root of the tooth
- Attaches the tooth to the periodontal membrane
- Similar histologically to bone with coarse
budles of collagen fibrils in a calcified matrix
Periodontal membrane
- Modified periosteum of alveolar bone and is a
dense fibrous connective tissue.
- Supports the gingiva at the neck of the tooth
- Bundles of collagenous fibers connect alveolar
bone and cementum
- Functions as the suspensory ligament of the
tooth
Gingiva (Gums)
- Oral mucous membrane that
surrounds the tooth and connects to periosteum of alveolar bone
- Connective tissue underlying the stratified
squamous epithelium consists of bundles of collagenous fibers and a rich vascular
network of capillaries which is responsible for the pink color of the gums
- Is initially attached to the enamel but
gradually recedes as it exposes the crown of the tooth
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From the lining of the oral cavity tooth buds are formed and develop into a tooth. Tooth bud consists of three components:
- Enamel organ – derived from oral ectoderm and produces tooth enamel
- Dental papilla – derived from mesenchyme and produces the tooth pulp and dentin
- Dental sac – derived from mesenchyme and produces cementum and periodontal ligament
Stages of tooth development
(Shown for
a lower central incisor)
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Figure 8: Tooth development stages of a lower central incisor. (3)
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Figure 8A: Tooth development:
Dental Lamina formation (3)
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Tooth development begins during the sixth week of embryonic
life (Bhaskar, 1976). The oral ectoderm give rise to the oral epithelium
followed by the formation of dental lamina by the proliferation of cells. This
is a band of epithelium that outlines the future dental arches along the jaws.
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Round swellings arise from the dental lamina (at 10
different points) to form the enamel organ and tooth bud. Unequal growth and
differentiation of the bud leads to the formation of an invagination on the deep
surface of bud. (7-8 weeks intrauterine)
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Figure 8C: Tooth development:
Early bell stage of deciduous tooth (3)
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This bell shaped epithelial bud is called the enamel organ,
which sits ontop of the dental papilla which is embedded in the dental sac (a
layer of connective tissue). Extension
of the dental lamina will lead to the formation of the permant tooth. (10 weeks intrauterine)
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| Figure 9: Section showing enamel and dentin producing cells within the enamel organ and dental papilla (2). |
Cells within the enamel organ separate by intercellular
spaces which are filled with mucoid fluid rich albumin (this region is refered
to as the stellate reticulum). (Bhaskar, 1976). Peripheral epithelial cells
form a layer on either side of the stellate reticulum. The outer enamel
epithelium has smaller cells and the inner enamel epithelium has taller
columnar cells. These are ameloblasts which are responsible for enamel
formation. The peripheral cells of the dental papilla (next to the inner enamel
epithelium) differentiate into odontoblasts which are responsible for dentin
formation. Dentin formation precedes
enamel formation.
Cap of dentin has formed at the tip of the dental papilla
and tooth bud as disconnected from the dental lamina by mesenchymal
invasion. (16 weeks intrauterine)
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Figure 8E: Tooth development:
Deciduous tooth crown complete
and permanent tooth development starting (3)
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Crown of deciduous tooth is complete with enamel formation
and the permanent tooth is in the bell stage. (Birth)
Root development occurs shortly before tooth eruption, progressing as the crown of the tooth emerges through the gingiva (Bhaskar, 1976).. The enamel organ forms the Hertwig’s epithelial root sheath, molding the shape of the roots and initiating dentin formation. Note that roots do not contain an enamel layer.
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| Figure 8F: Tooth development: Deciduous tooht erupting and permanent tooth developing |
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Figure 8G: Tooth development:
Resorption of deciduous tooth roots (3)
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As the permanent tooth starts eruption the resorption of the
roots of the deciduous tooth occurs. When deciduous tooth is shed it consists
only of crown (unless tooth is pulled before permanent tooth has fully
erupted). (6-7 years)
Odontoclasts are responsible for the resorption of the
roots. (Bhaskar, 1976). They are most commonly found on the surface of the
roots inrelation to the the assending permanent tooth. Demineralization of the
dentin in the roots occurs. The pressure from the permanent teeth also aids in
this process.
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Figure 8H: Tooth development:
Permanent tooth erupting (3)
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Permanent tooth is now erupting (7-8 years)
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Figure 8I: Tooth development:
Permanent tooth attrition (3)
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Permanent tooth in attrition (reduction in size). Enamel and dentin layers are thinner. You can clearly see recession in the neck and roots of the tooth as well as secondary dentin forming. Lines around the root of the tooth are the periodontal ligament. (After 20 years).
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| Figure 9: Development of the Human Dentition (Shows the changes in dentition over time and the development stages for the different types of teeth ie molar, incisor)(5) |
Dental Caries (Cavities)
Dental caries are one of the most prevalent chronic diseases
of people worldwide and individuals are susceptible to it through out their
lifetime (Selwitz et al., 2007). They are defined as localized destruction of
hard tissue resulting from the demineralization of tooth tissue due to acidic byproducts from
the bacterial fermentation of dietary carbohydrates. This process is initiated within the
bacterial biofilms (dental plaque) that covers the surfaces of the teeth.
Endogenous bacteria such as Streptococcus
mutans and Lactobacillus spp
present within biofilm metabolize fermentable sugars producing weak organic
acids (Selwitz et al., 2007). This acid causes the pH to drop and when it
reaches a critical level demineralization of the tooth tissues occurs.
Cavitation occurs with the diffusion of calcium, phosphate and carbonate out of
the tooth. This process however can be reversed in its early stage with the
uptake of calcium, phosphate and fluoride. Fluoride acts as a catalyst for the
diffusion of calcium and phosphate back into the tooth allowing for
remineralization. First signs of demineralization are the presence of white
spot lesions.
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In this figure this individual failed to thoroughly clean
their teeth while undergoing orthodontic treatment. When braces were removed permanent white
rings are visible on each tooth surrounding where the brackets would have been.
This is due to decalcification.
As bacteria penetrate further into the tooth tissue becomes
softer and cavities (holes) start to form.
In this figure this individual has extensive tooth decay.
The holes are darker then the rest of the tooth since the enamel layer as been
eaten away by the acids and the dentin layer is now visible (which is
characteristically darker in color).
In both the crown and root surfaces of the tooth of primary
and permanent teeth decay can be found. Along the root surface cavities form
faster as the hard enamel surface is not present. Dentin is much softer so
erosion occurs much quicker.
Some factors that affect the development of dental caries:
- The shape and positioning of the teeth can make
cleaning more difficult leading to more plaque build up. Pits and cracks in the
teeth make it easier for bacteria to infiltrate.
- Saliva provides a rinsing action for the teeth
and neutralizes the acids produced by the bacteria helping slow down this
process. Individuals with reduced saliva production results from head and neck
irradiation or medications are more prone to rapid progressive caries.
- Foods containing fermentable carbohydrates such
as sweets, pasta, rice, potato chips, fruits which increases acid formation in
plaque.
- Baby bottle caries or nursing caries are caused
by prolonged exposure to drinks containing sugars. When infants are put to
sleep with a bottle of formula or juice that can be a particular problem as
saliva flow is greatly reduced during sleep allowing for sugars to sit on teeth
for extended periods of time allowing for tooth decay to develop.
Treatment:
Dental caries significantly weaken the tooth, to
prolong the life of the tooth restoration is usually completed. This involves
removing all of the soft decalcified tissue to prevent further decay and
filling in with either a composite of amalgam material. This however is not a
permanent fix as overtime these materials break down and chip due to forces
from mastication. A more permanent procedure would be cover the crown of the
tooth with a cap. This protects and strengthens the tooth. Crowns can be made
from a variety of different materials such as gold, porcelain or other metals.
Porcelain fused metal crowns are popular as the porcelain gives the tooth a
natural appearance while the metal provides extra support (important for
molars).
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Some ways to prevent dental caries:
- Regularly brushing and flossing teeth
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| Brush and Floss |
- Regular dental cleanings and checkups helps keep
tartar (next stage after plaque) build up under control as well as catching
dental caries in their early stages before there is extensive damage to the
teeth.
- Fluoride will strengthen the enamel making it
more resistant to decay. Fluoride can be found in some city water
systems (although many are choosing to remove fluoride), many tooth pastes and
mouth rinses. Topical fluoride treatments are often given to children in office
(more concentrated than toothpastes and mouth rinses). Although it is a bit of
a controversial topic fluoride drops can be given to children between the ages
of 6 months and 3 years to strengthen the enamel during the development of the
permanent teeth!
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| Fluoride trays |
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| Fluoride drops |
Fun Fact:
Eating cheese can prevent tooth decay by increasing saliva
flow and neutralizing acidity!
References:
1) Bhaskar, S.N.
(1976) Orban’s Oral Histology and
embryology: Saint Louis: The C.V Mosby Company.
2) Di Fiore, M.S. (1974) Atlas of Human Histology.Philadelphia:
Lea & Febiger.
3) Leeson, C.R., Leeson, T.S.
(1976) Histology. Philadelphia: W.B.
Saunders Company.
4 )Mescher, A. (2009) Junqueira’s
Basic Histology Text & Atlas, 12ed. Unites States of America.
McGraw-Hill Companies.
5) Schour
I., Massler M. 1944. Development of the Human Dentition. Chicago: American
Dental Assoc. 350 p.
6) Selwitz, R.H., Ismail, A,I., Pitts, N.B. (2007) Dental
caries. Lancet. 369:51-59
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