Cell and molecular biology with genetics by almuzian ok ok
Cell and Molecular Biology with
Structures within the Periodontium
The periodontium is the connective tissue connecting the teeth to the jaws. It is
composed of the two fibrous tissues the periodontal ligament and the lamina
propria of the gingiva, as well as the mineralised tissues of the cementum and
the alveolar bone.
Mineralised tissue covering the roots of the teeth.
The cementum provides an attachment surface for the collagen fibres that
bind the tooth to surrounding structures.
Composition of the cementum
It is specialised CT with some similar characteristics with compact bone.
It consists of 45-50% inorganic substances (calcium and phosphate as
hydroxyapatite), 50-55% organic material (collagen type I and
proteoglycans) and water.
The cementum has two forms, cellular and acellular. The acellular
cementum can cover the root dentine from the cementoenamel junction to
the apex, but is often missing on the apical third of the root; this is why
the apical third resorbed the most by orthodontic forces.
Composition of PDL
• Collage fibres which are either gingival gp from tooth to gum or
dentoalveolar gp from cementum to lamina dura. PDL is composed of
80% type I collagen and 20% type II collagen
• Between these fiber bundle there is blood vessel and lymphatic v
• Extracellular matrix
• Cells as below.
Cells of PDL
1. Chondrocyte originate from mesenchymal tissues differentiate to produce
minerlaized bone matrix
2. PDL fibroblast originate from mesenchymal tissues differentiate to
3. Pulpal fibroblast originate from mesenchymal tissues differentiate to
produce pulp tissues and odontoblast
4. Gingival fibroblast originate from mesenchymal tissues differentiate to
produce gingival CT
5. Cementoblast originate from mesenchymal tissues differentiate to
6. Odontoblast originate from mesenchymal tissues differentiate to produce
7. Osteoblast originate from mesenchymal tissues differentiate to produce
minerlaized bone matrix
8. Osteoclast originate from heamopetic tissues differentiate to produce
9. Cementoclast originate from heamopetic tissues differentiate to produce
• Most bones have a basic architecture composed of outer cortical bone and
inner trabecular or cancellous bone.
• Cortical bone, which is 80% calcified, forms a rigid outer shell that resists
• The trabecular bone, only 20% calcified, provides its strength through a
complex system of internal struts, which follow the principal stress
trajectories for these sites. Spaces between the trabecular bone are filled
with bone marrow (haematopoietic cells).
1. Inorganic mineral like CA, Ph and carbonate that form hydroxyapatite,
2/3 of weight.
2. Organic part involving type I collagen (1/3 by weight)
3. Non-collagenous component consists of glycoproteins
4. Cells distributed between them.
Osteoprogenitor cells, or pre-osteoblasts
• They are bone stem cells derived from mesenchymal cells that eventually
differentiate into mature osteoblasts then steoblasts remain in the
mineralised osteoid and become osteocyte.
The main functions of the osteoblasts is
1. Respond to this stain Hematoxylin and eosin, or H&E, staining,
2. The production of type I collagen
3. Mature osteoblast will turn to osteocyte
4. The control of osteoclast function by the production of a soluble factor
that acts directly on the osteoclasts (OPG-RANKL-RANK)
5. In addition the osteoblasts also control resorption by forming a
physical barrier to the bone surface against the osteoclasts, and when
it stimulated by PTH it changes its shape to become more round and
allow osteoclast to be in a direct contact with bone and to start
resorption. Sandy, 1992.
6. Production of MMPs which are proteolytic enzymes which degrade
the organic matrix of the bone (because the non-organic component
removed by osteoclast while the organic part removed by the MMPs
that produced by osteoblast. Osteoblast starts secretion of protease
enzyme MMPs after interaction with osteoclast).
7. Part of osteoblast-osteocyte complex that maintain the integrity of the
bone matrix and Prevent its hyper mineralisation,
8. Connect to osteocyte to maintain bone integrity.
1. In bone, osteoclasts are found in pits in the bone surface which are called
resorption bays, or Howship's Lacunae. Osteoclasts are characterized by a
cytoplasm with a homogeneous, "foamy" appearance. This appearance is
due to a high concentration of vesicles and vacuoles. These vacuoles
include lysosomes filled with acid phophatase.
2. The osteoclast is a large multinucleate cell that is derived from blood
3. The principle function of osteoclasts is to mobilise mineralised bone
through a combination of enzyme hydrolyses and acid hydrolases such as
acid phosphatase. This is why serum acid phosphatase is an important
marker of bone disease.
4. Indeed, osteoclast has no surface receptor for hormone like parathyroid
hormone while osteoblast has. So osteoblast interact with hormone and
send signal to osteoclast to activate bone resorption (OPG-RANKL-
1. Osteocytes are osteoblasts that have become surrounded by the forming
2. They become smaller in size and lose their ability to form bone.
3. They communicate with one another and the surface osteoblasts via long
radiating processes joined at gap junctions.
4. The function of the osteoblast-osteocyte complex is to:
Osteocytes act as mechanoreceptors identifying the loads placed on the
individual bones (Mullender and Huiskes 1997).
Maintain the integrity of the bone matrix
Prevent its hyper mineralisation,
NB: Other cells that are found within bone include: periosteal fibroblasts,
chondrocytes, chondroblasts, epithelial cells, macrophages/monocytes,
erythrocytes, leukocytes, and platelets.
1. Collagen formed by osteoblasts is deposited in parallel or concentric
layers to produce mature, lamellar bone
2. Collagen not deposited in a parallel array but in a basket-like weave and
is called immature, primitive or woven bone
1. Cortical bone
2. Spongy bone
• Mechanical support for the muscle
• Site of muscle attachment for locomotion.
• Protection for vital organs.
• The marrow is a source of manufacture for blood cells.
• A metabolic reservoir for ions especially calcium and phosphate.
• Skeletal formation begins when mesenchymal cells migrate to the site of
skeletogenesis. The cells interact with epithelial cells, which triggers the
mesenchymal cells to undergo condensation. Condensed cells undergo
differentiation into chondrocytes, osteoblasts, adipocyte or myofibroblast.
• Core binding factor-1 (CBFA-1) is an important bone specific gene,
which is vital for mesenchymal differentiation into osteoblasts.
• Bone morphogenetic proteins (BMP's) are important in skeletal
patterning and skeletal cell differentiation.
Process of bone resorbs
• 1. hormone like PGE2 produced and bind to osteoblast
• osteoblast activate osteoclast by OPG-RANKL-RANK
• osteoblast secret MMP that remove organic part of the bone
• Osteoclast remove mineral part of bone
Intramembranous ossification is seen during embryonic development by direct
transformation of mesenchymal cells into osteoblasts. Intramembranous
ossification is seen in the cranial vault, some facial bones, parts of the mandible
and the clavicle.
Endochondral - Long bones develop by ossification of a cartilaginous
Chondrocytes differentiate from the mesenchyme in response to genes and
These chondrocytes deposit matrix proteins at the middle of limb bud in the
region known as the primary ossification centre
The mesenchymal cells differentiate into osteoblasts, as previously described,
and begin to secrete matrix proteins producing a primary bone collar around the
circumference of the bone.
The cartilaginous core within the bone collar is ossified to form a trabecular
network of bone. During ossification of the bone, blood vessels develop from
the limb vasculature and one of the vessels develops into the nutrient artery,
which provides nourishment for the developing bone.
A hyaline cartilaginous known as the perichondrium formed at the end of limb
bud WHICH represent future joints.
The bone collar and trabecular core make up the shaft of the limb known as the
diaphysis. At birth, the diaphysis is completely ossified whereas the epiphyses
(ends of the bone) are still cartilaginous. Postnatally, secondary ossification
centres form within the epiphysis that gradually ossify the cartilage.
A layer of cartilage known as the epiphyseal growth plate persists between the
epiphysis and the metaphysis (the growth end of the diaphysis) until growth has
finished (~20years). The growth plate allows the limb bones to grow
longitudinally by a mechanism of continued proliferation of chondrocytes.
Sutural Bone Growth
Sutural bone growth is found exclusively in the skull. It is a method of
bone growth rather than development.
Associated disorder: Craniosynostosis is the umbrella term used to
describe a collection of disorders that are caused by a premature fusion of
the cranial sutures. Some of the most common craniosynostosis
syndromes include Crouzon and Apert. Mutations in the gene coding for
fibroblast growth factor receptor (FGFR) 2 have been shown to be
responsible for these syndromes.
Factors affecting bone turn over
• Local Factors like
1. Prostaglandins - potent mediators of bone resorption. They can be found
in sites of inflammation,.
2. Cytokines - soluble mediators released from cells, which modulate
activity of the same cells, or other cells.
3. Interleukin-1 (IL-1) -potent stimulator of bone resorption, acting both
directly and by increasing prostaglandin synthesis. IL-1 is also an
inhibitor of bone formation.
4. Tumour necrosis factor (alpha, beta) (TNF) - stimulates bone resorption
and inhibits bone collagen and non-collagenous protein synthesis.
5. Growth Factors
6. Transforming growth factor B (TGFB)
7. Fibroblast growth factor (FGF)
8. Bone morphogenic proteins (BMP'S)
• Systemic Factors
1. Parathyroid hormone - released from the parathyroid gland in response to
low serum calcium, phosphate or vitamin D3.
2. Calcitonin - released from thyroid C-cells in response to high serum
calcium. Acts as an antagonist to the effects of PTH and inhibits bone
resorption by osteoclasts.
3. Thyroid hormones
6. Growth hormone, important regulator of calcium absorption from the
intestine and renal tubules. Recruitment.
8. Sex steroids
• Testosterone-High levels increase maturation of cells: decrease
• Oestrogens- Elevated levels of oestrogen increase maturation of
cells and decrease bone resorption by osteoclasts.
Bone disorders of relevance to an orthodontist
1. Congenital and hereditary disorders
2. Bone infections
3. Metabolic bone diseases
5. Other non-neoplastic disorders of bone
6. Bone tumours
Examples of Metabolic bone diseases
• Excess vitamin D3 leads to increased absorption of calcium from the gut,
hypercalcemia, increased bone production and tissue mineralisation and
• Osteomalacia and Vit D dependent rickets, is a disorder involving
softening and weakening of the bones. The condition is most common in
children. Cause: The condition is caused primarily by a lack of vitamin D
and/or calcium and phosphate. Treatment: Vitamin D deficiency is
treated with supplementation.
• Excessive secretions of PTH lead to primary hyperparathyroidism. This
relatively common condition is associated with elevated serum and
• Chronically elevated levels of growth hormone, frequently caused by an
adenoma within the anterior pituitary, lead to acromegaly. This disease is
associated with continued growth of the bones of the jaw, hands, and feet.
• A decline in sex steroids, as found in post-menopausal females, is thought
to be one of the major factors responsible for osteoporosis. Osteoporosis
is associated with either a loss of bone or a decrease in bone formation.
Example of bone infections
• Paget's disease (Osteitis deformans), characterised by excessive bone
turnover in which bones become enlarged and deformed. Complications
include fractures, neoplasia, nerve compression and high output cardiac
• Cause: Paget's disease may be caused by a slow virus infection, present
for many years before symptoms appear. There is also a hereditary factor
since the disease may appear in more than one family member.
• Treatment: Treatment surgery may include; medication that inhibits
abnormal bone resorption such as bisphosphonates, Frequency: ~5 in 100
people over 50 years of age in the UK
Congenital and Hereditary Disorders
• Cleidocranial dysplasia (CCD)
Phenotype: CCD is characterised by skeletal anomalies such as opened
fontanels, late closure of cranial sutures with Wormian bones (isolated bones
within the suture), rudimentary clavicles, and short stature.
Dental features: The maturation of the primary dentition is normal, but
permanent teeth are delayed from 1 to 4 years. Most patients have
supernumerary permanent teeth.
Inheritance: Autosomal dominant disorder
A term used to describe a collection of approximately 70 congenital disorders
that are caused by a premature fusion of the cranial sutures.
Some of the most prevalent craniosynostosis include Aperts , Crouzon , and
Most of these syndromes are due to gain of function mutations in the fibroblast
growth factor receptor 2 (FGFR2)
Inheritance: autosomal dominant trait.
Population frequency: ~0.4/1000
Phenotype: Aperts syndrome is characterised by craniosynostosis of several
sutures, symmetric syndactyly of the hands and feet
dental features: a retrusive middle third of the face, and a V-shaped maxillary
dental arch with crowded teeth. Cleft palate (25%) and mental handicap (mild in
31%, severe in 7%) are also associated
Phenotype: Crouzon syndrome is characterised by craniosynostosis, bulging
eyeballs, shallow orbits, and maxillary hypoplasia. They do not have syndactyly
or cleft palate
Phenotype: the commonest form of skeletal dysplasia, leading to a mean final
height of 132 cm for males and 123 cm for females.
Inheritance: Autosomal dominant trait
Mutation: Due to point mutations within fibroblast growth factor receptor
(FGFR) 3 genes.
Birth rate: Frequency 1 in 15000-77000 live births in different survey
Phenotype: Osteopetroses are rare disorders caused by a marked decrease in
bone resorption characterised by a lack of osteoclast function. There is
excessive bone formation but the bones have an increase in density and are
mechanically weak and prone to fracture.
Dental complications may include: delayed eruption of teeth, missing and/or
smaller teeth. There may also be evidence of enamel hypermineralisation and
hypoplasia. The jaws are composed of dense bone and the mandible is more
frequently affected than the maxilla
Inheritance: Autosomal recessive.
• Osteogenesis imperfecta (OI)
Type I: Childhood type. This is the most common form of OI and is associated
with brittle bones composed of immature woven bone. The sclerae are thin and
may appear blue due to the visible pigmented choroids. Other abnormalities
may include: conductive deafness due to otosclerosis, discoloured teeth due to
dentinogenesis imperfecta (DI), hypermobility with lax ligaments, Life span is
normal and the condition is often mild in severity.
Type II: Congenital/Perinatal type, lethal with multiple fractures at birth.
Newborns have soft calvarial bones, distinctive triangular face, bluish sclerae,
beaked nose, narrow thorax and short and deformed limbs
Type III: Infants born with fractures. Dental related abnormalities may include
reduced craniofacial size measurements and a posteriorly inclined maxilla.
Type IV: Similar to type I OI except that the sclerea are white, stature is slightly
shorter and DI is common.
Birth rate: Combined birth frequency of 1 in 20,000
• Vitamin D Resistant Rickets (familial hypophosphataemic rickets)
Phenotype: Growth retardation and childhood rickets, reduced serum phosphate.
Treatable with large doses of vitamin D (or its active metabolite calcitriol) and
Mutation: Mutations to the vitamin D receptor (VDR)
Birth Rate: 1 in 20000
Early Tooth Development and Basic Histology
Prenatal development of the dentition
• Teeth form on the frontonasal process and on the paired maxillary and
mandibular processes of the first pharyngeal arch.
• The basic histology of tooth development suggests that this process
derives from two principle cell types,
1. the oral epithelium (gives rise to ameloblasts and the enamel of the tooth
2. the underlying neural crest-derived ectomesenchyme of the first
branchial arch (contributes to the formation of the dental papilla and
follicle and therefore, to the odontoblasts, dentine matrix, pulp tissue,
cementum and periodontal ligament of the fully formed tooth).
Embryological primary tooth formation
In the human embryo, development of the deciduous dentition begins at around
6 weeks with the formation of a continuous horseshoe-shaped band of thickened
epithelium around the lateral margins of the primitive oral cavity.
The free margin of this band gives rise to two processes, which invaginate into
the underlying mesenchyme:
1. The outer process or vestibular lamina is initially continuous, but soon
breaks down to form a vestibule that demarcates the cheeks and lips from
the tooth-bearing regions.
2. The inner process or dental lamina gives rise to the enamel organs of
the future developing teeth which is called later tooth bud.
1. The dental papilla is formed by localized condensation of neural crest-
derived ectomesenchymal cells around the dental lamina. Then the dental
papilla extend around the enamel organ to form the dental follicle or tooth
bud. Together, these tissues constitute the tooth germ and will give rise to
all structures that make up the mature tooth. This is called bud stage.
2. At the cap stage, the tooth bud folds to demarcate the early morphology
of the crown, which is modified by further folding at the bell stage.
3. During the bell stage, the innermost layer of cells within the epithelial
component of the tooth organ, the inner enamel epithelium, induce
adjacent cells of the dental papilla to differentiate into odontoblasts,
responsible for the formation and mineralization of dentine. Dentine
formation is preceded by the formation of predentine. The first layer of
predentine acts as a signal to the overlying inner enamel epithelial cells to
differentiate into ameloblasts and begin secreting the enamel matrix. At
the margins of the enamel organ, cells of the inner enamel epithelium are
confluent with the outer enamel epithelial cells at the cervical loop.
Growth of these cells in an apical direction forms a skirt-like sheet called
Hertwig’s epithelial root sheath, which maps out the future root
morphology of the developing tooth and induces the further
differentiation of root odontoblasts. Degeneration of this root sheath leads
to exposure of the cells of the dental follicle to the newly formed root
dentine and differentiation into cementoblasts, which begin to deposit
cementum onto the root surface. Surrounding the enamel organ, the cells
of the dental follicle produce the alveolar bone and collagen fibres of the
Formation of permanent teeth
• Successional teeth have deciduous predecessors and consist of the
incisors, canines and premolars. Successional teeth form as a result of
localized proliferation within the dental lamina associated with each
deciduous tooth germ .
• Accessional teeth have no deciduous predecessors and consist of the
three permanent molars. In contrast, accessional teeth form as a result of
backward extension of the dental lamina into the posterior region of the
Tissue and Molecular Interactions
The genes that participate in different stages of tooth formation:
Pax; Msx; Barx , Shh; FGF and BMPs signalling protein
Tissue Recombination Experiments (important Mohammed)
A series of highly informative recombination experiments carried out by
Andrew Lumsden at Guys Dental Hospital in the 1980's demonstrated for the
first time that not only does cranial neural crest participate in mammalian
odontogenesis, it only expresses its odontogenic potential when combined with
• By recombining early first arch (oral) epithelium with cranial neural
crest cells, trunk neural crest cells or non-neural crest-derived limb
mesenchyme and then allowing these explants to continue their
development in vivo, he demonstrated that tooth development only
occurred when the oral epithelium was combined with cranial or
trunk neural crest.
When limb epithelium was combined with mandibular arch ectomesenchyme,
no teeth formed.
• Recombination experiments have also been performed to suggest the
dominance of ectomesenchyme in the specification of tooth shape,
once tooth development has been initiated.
After the bud stage, the recombination of molar epithelium with incisor dental
papilla results in the formation of an incisiform tooth.
Similarly, at the same stage the recombination of incisor epithelium with molar
dental papilla produces a molariform tooth.
Therefore, after the tooth has reached the bud stage, the ectomesenchyme of the
dental papilla is responsible for dictating what type of tooth will develop.
Conclusion: Taken together, all these early recombination experiments have
indicated that during:
• the initial stages of odontogenesis, the inductive capacity for tooth
development resides within the oral epithelium which under the
influences of specific signalling molecules (such as Bmp-4, Fgf-8 and
• later stages (from the bud stage of development) the underlying
ectomesenchyme retains the capacity to dictate shape.
How does the Genetic Control of Tooth Patterning Come About?
1. Either by clone theory as mentioned in the above including
• the initial stages of odontogenesis, the inductive capacity for tooth
development resides within the oral epithelium.
• later stages (from the bud stage of development) the underlying
ectomesenchyme retains the capacity to dictate shape..
2. the regional field theory, is that the shape of the tooth is determined from
the moment its development has been initiated
Abnormalities of tooth structure
1. Endocrine disorders
4. Nutritional deficiency
5. Haematological disorders
6. Neonatal illness
7. Postnatal illness
8. Fluoride ingestion
2. Ehlers-Danlos syndrome
4. Nutritional deficiency
5. Drugs (Tetracycline)
Amelogenesis imperfecta (AI) is a collective term for a group of inherited
conditions characterized primarily by abnormal enamel formation in either
AI can be inherited as an autosomal dominant, autosomal recessive or sex-
It has a prevalence that can range from 1 : 1,000 to 1 : 14,000, depending upon
The predominant enamel phenotype is either:
• Hypomineralized (either hypomature or hypocalcified, or a combination
of the two).
can be classified as:
• Dentine dysplasia;
• Dentinogenesis imperfecta.
Dentinogenesis imperfecta (DGI)
represents the most common group of inherited dentine disorders and there are
three essential subgroups:
1. Type I is associated with osteogenesis imperfecta with type I collagen
abnormaility. The deciduous and permanent teeth are affected by
discolouration, attrition and pulp canal obliteration.
2. Type II is seen in around 1 : 7,000 Caucasians causing translucent, amber
and bluish grey discolouration, enamel chipping and marked attrition.
The crowns are bulbous and the pulp canals also become obliterated.
3. Type III affects certain subpopulations of people, including Native
American Indians and European Caucasians. Both dentitions can be
affected by so-called ‘shell teeth’, which lose enamel and have poorly
mineralized dentine, leading to multiple pulp exposures.
Orthodontic management of AI and DGI
• Children affected by AI or DGI will require long-term multidisciplinary
• early loss of enamel leading to dentine exposure and subsequent
sensitivity, which in turn can result in poor oral hygiene and a significant
• there is known association between AI and the presence of anterior open
• When considering orthodontic treatment for the more severe cases:
1. Removable appliances should be used where possible;
2. Care needs to be taken if direct bonding is undertaken because bracket
failure or removal can lead to enamel fracture;
3. Orthodontic bands can be used where possible; and
4. Oral hygiene and diet control must be carefully monitored during
Questions and answers
From your knowledge of the functions of osteoblasts and osteoclasts why do
osteoblasts have prominent endoplasmic reticulum (ER) whereas osteoclasts
have few ER
Osteoblasts have prominent endoplasmic reticulum (ER) because their role is to
actively produce large amounts of protein, particularly type I collagen and other
bone matrix proteins. In contrast, osteoclasts do not require active ER as they
produce little protein. The main function of the osteoclast is to resorb bone by
acid and enzymic hydrolysis.
Which structures in the mature tooth are formed from the enamel organ, dental
papilla and dental follicle?
The enamel organ is derived from epithelium and forms the ameloblasts, which
produce the enamel of the tooth crown.
The dental papilla and dental follicle are derived from neural crest cells and
form the remainder of the tooth. The papilla forms the odontoblasts, which
produce dentine and the pulp.
The follicle produces the periodontium, consisting of the odontoblasts that form
root dentine, cementoblasts that produce cementum and cells that produce the
Give an outline of the early mechanisms involved during hard tissue formation
in the developing tooth.
Hard tissue formation in the tooth relies upon inductive interactions between the
internal enamel epithelium and adjacent ectomesenchymal cells of the dental
papilla. Morphological changes in the cells of the internal enamel epithelium
precede cues from these cells to the adjacent dental papilla cells to differentiate
into odontoblasts. Odontoblasts begin secreting predentine, which itself seves as
a cue for the internal enamel epithelium to differentiate into ameloblasts.
Odontoblasts and ameloblasts migrate away from each other and produce the
enamel and dentine of the tooth crown.
Why are ectomesenchymal cells so important during early tooth development?
Ectomesenchymal cells are essentially a specialised form of embryonic
connective tissue. They are derived from the neural crest, which are really a
form of embryonic stem cells. They can therefore be induced to differentiate
into a variety of cell types. In the case of the developing tooth, this includes
odontoblasts, pulp cells and cementoblasts. In other words, the connective tissue
components of the tooth