2. WHAT IS GROWTH?
“Developmental increase in mass.’’-
Stewart.(1982)
“An increase in size or number.” - Profitt. (1986)
“Normal changes in amount of living
substance.’’- Moyers(1988)
“Growth signifies an increase, expansion or
extension of any given tissue.” -
Pinkham.(1994)
3. WHAT IS DEVELOPMENT?
“Development is a progress towards
maturity” – Todd(1931)
“Development connotes a
maturational process involving
progressive differentiation at the
cellular and tissue levels” – Enlow.
4. INTRODUCTION
The human somatic cell contains 46
chromosomes, called as the diploid number.
Out of which 44 are autosomes and the
remaining 2 are sex chromosomes,
designated as X and Y.
The sex chromosomes in females are XX and
in males are XY.
There are two series of division of somatic
cells- MITOSIS and MEIOSIS.
5. MITOSIS produces the same number of
chromosomes in the resulting daughter cell
while MEIOSIS produces half the number i.e.
23 designated as haploid, with resultant
formation of gametes .
Development begins with FERTILIZATION,
the process in which the male gamete- the
sperm, and the female gamete- the oocyte,
unite to form a ZYGOTE.
6. THE PROCESS OF FERTILIZATION
THE MALE GAMETE (SPERM) FUSES WITH
THE FEMALE GAMETE (OVUM)
7. GROWTH IS BROADLY SUBDIVIDED AS:
a. Prenatal growth
1. Period of ovum: From time of fertilization till 1
week.
2. Period of embryo: from 2nd week till 8th week
3. Period of fetus: from 9th week onwards till
birth
b. Postnatal growth
c. Maturity
d .Old age
8. PRENATAL GROWTH
The fertilized ovum,
undergoes cleavage as it
moves toward the uterine
cavity.
The cells formed are called
blastomeres, which soon
begin to rearrange
themselves in order to
differentiate into various
groups and layers.
By the 4th day, when the
zygote reaches the uterus, it
is a many celled mass called a
MORULA
11. As the cell mass
divides, it enlarges
and gains a fluid filled
cavity termed the
blastocele(5th day).
The blastocoele
separates the cell into
2 parts:
-An outer cell layer, the
trophoblast, and
-An inner cell mass, the
embryoblast.
13. IMPLANTATION : 6TH DAY
The trophoblast
attaches to the sticky
endometrial surface on
the posterior wall of
the body of the uterus.
The surface cells of the
trophoblast produces
enzymes that digest
the uterine
endometrial cells,
which allows a deeper
penetration of the cell
mass.
15. During the second week,
the cells of the inner cell
mass of the growing
blastocyst differentiate into
2 cell types:
1. Columnar shaped
ectodermal cells and
2. Cuboidal shaped
endodermal cells
adjacent to blastocele.
The amniotic cavity
appears between the
ectodermal cells and the
overlying trophoblast.
16. Later in the
developmental process,
the amnion expands,
filling the entire extra
embryonic coelom .
Thus in its final form,
the amnion is a free
membrane enclosing a
fluid-filled space around
the embryo.
Again, cells grow from
the trophoblast and the
embryonic disc, to form
a primitive yolk sac.
17. On day 15, a groove, called
the primitive streak ,
appears on the surface of
the midline of the dorsal
aspect of the ectoderm of
the embryonic disc.
By day 16, a primitive knot
of cells, the Henson’s node,
appears at the cephalic end
of the primitive streak.
This knot gives rise to the
cells that form the
notochordal process.
19. Cells from the primitive streak and the
notochordal process migrate laterally
between the ectodermal and endodermal
layers of the embryonic shield.
These cells form the third germ cell layer
called the mesodermal layer.
By the end of the third week, the mesoderm
migrates in a lateral direction between the
ectoderm and the endoderm, except at the
anterior prochordal plate and posterior
cloacal membrane.
21. The anterior plate forms the
future oropharyngeal
membrane.
Finally, mesodermal cells of
the embryonic disc migrate
peripherally to join the
extra-embryonic mesoderm
on the amnion and yolk sac.
Anteriorly, mesodermal
cells pass on either side of
the prochordal plate to
meet each other in front of
this region.
22. FATE OF GERM LAYERS
Ectodermal cells will give rise to the nervous system;
the epidermis and its appendages (hair, nails,
sebaceous and sweat glands); the epithelium lining
the oral cavity, nasal cavities and sinuses; a part of
the intraoral glands, and the enamel of the teeth.
Endodermal cells will form the epithelial lining of the
gastrointestinal tract and all associated organs.
The mesoderm will give rise to the muscles and all
the structures derived from the connective
tissue(e.g., bone, cartilage, blood, dentin, pulp,
cementum and the periodontal ligament).
The embryonic disc will soon become altered by
bends and folds necessary for further development.
23. POST NATAL GROWTH
Pattern
-Differential Growth
-Cephalocaudal gradient of growth
Variability
Timing, rate & direction
24. PATTERN
Pattern in growth
represents proportionality
.It refers not just to a set of
proportional relationships
at a point in time, but to a
change in these
proportional relationships
over time.
The physical arrangement
of the body at any one
time is a pattern of
spatially proportioned
parts.
25. DIFFERENTIAL GROWTH
Different organs grow at
different rates and at different
times.
Scammon’s curve of growth- by
Richard Scammon.
Lymphoid tissues attain a 200%
growth by the age of ten and
then regress afterwards.
Neural tissue attains full growth
by the age of six and then stops.
General somatic tissues follow a
sigmoid pattern.
Genital tissue grow significantly
only at puberty and achieve full
growth at about 20 yrs of age.
26. CEPHALOCAUDAL GRADIENT
OF GROWTH
This means that there is an axis of
increased growth extending from the head
towards the feet.
At about 3rd month of IU life, the head
takes up 50% of total body length. By the
time of birth, the proportion of head
decreases to 30%.
This proportion steadily declines till in
adult, the proportion of head is only 12%.
27.
28. VARIABILITY
No two individuals with the exception of
siamese twins are like.
Hence it is important to have a “normal
variability” before categorizing people as
normal or abnormal
29. TIMING OF GROWTH
One of the factors for variability in growth.
Variations in timing arise, because the
biologic clock of different individuals is
different.
It is influenced by:
genetics
sex related differences
physique related
environmental influences
30. GROWTH SPURTS
Defined as periods of sudden growth acceleration
Sex-linked
Normal spurts are
Just before birth
1 year after birth
Infantile spurt – at 3 years age
Mixed dentition growth spurt – 7-9 years (females);
8-11 years (males)
Pre-pubertal spurt – 11-13 years(females); 14-16
years (males)
31. DEVELOPMENT OF OROPHARYNX
The primitive
oral cavity or
stomodeum
appears late in
the third
prenatal week as
a pit or
invagination of STOMODEUM
the tissues
underlying the
forebrain.
32. This invagination appears as a result
of the growth of the forebrain
anteriorly and of the enlargement of
the developing heart.
At the deepest end of the
stomodeum, the oral ectoderm lies in
close contact with the foregut
endoderm.
The wall between the oral and
pharyngeal cavity is termed the
oropharyngeal membrane, as it
separates the stomodeum from the
first part of the foregut.
During the fourth week of
intrauterine life, the oropharyngeal
membrane disintegrates to establish
continuity between the two cavities.
33. As the oral cavity emerges, it includes the
stomodeum and foregut and 2 important
endocrine glands develop from its roof and floor.
From the roof, an ectodermal lined pouch called
Rathke’s pouch grows dorsally into the floor of
the brain and gives rise to the anterior lobe of
the pituitary gland.
On the floor of the oral cavity, on the tongue, a
second epithelial pouch develops and grows
downward into the anterior neck to give rise to
the thyroid gland.
Both of these important endocrine glands
develop from the oral tissue.
34. BRANCHIAL ARCHES
The tissues
bordering the
oral pit inferiorly
and laterally
develop into five
or six pairs of bars
which form the
lower part of the
face and neck.
These bars are
termed branchial
arches.
35. The first four branchial arches are well
developed in humans. Only the first and
second arches extend to the midline, and each
arch is progressively smaller from first to the
last.
The mandibular branchial arch is the first to
develop. It is located just below the
stomatodeum.
The hyoid is the second arch to develop.
The IIIrd, IVth and Vth arches consist of paired
bars of epithelial covered mesoderm which are
divided in the midline by the developing heart.
36. BRANCHIAL GROOVES
• The first branchial groove deepens to form
the external auditory meatus.
• The ectodermal membrane in the first groove
persists and together with mesoderm and
endoderm from adjacent first pharyngeal
pouch, forms the tympanic membrane.
• The external features of the 2nd,3rd and4th
branchial grooves become obliterated by the
overgrowth of the second branchial arch.
• This overgrowth then provides the smooth
contour of the neck.
37. PHARYNGEAL POUCHES
The endodermal epithelium of the pharyngeal
pouches differentiate into a variety of important
organs.
From the 1st pouch ,the middle ear and the
Eustachian tube develop.
From the 2nd, the palatine tonsils originate.
From the 3rd pouch, the inferior parathyroid and
the thymus arise.
From the 4th pouch, the superior parathyroid
gland forms.
From the 5th pouch, the ultimobranchial body
develops.
38.
39. BRANCHIAL ARCH VASCULATURE
Each of the 5 branchial arches contains a pair
of blood vessels that conduct blood from the
heart to the brain and to the posterior tissues
through the arch tissues. These are called
aortic arches.
40. The anterior right and left aortic arches
develop first and, after a week, begin to
disappear as more posterior arches develop.
The most caudal arch vessels then enlarge
and mature.
The 5th arch vessels disappear next.
The 3rd, 4th and 6th arch vessels do not
disappear but are important in later
functions.
The 3rd arch vessels become the common
carotid arteries which supply the neck, face
and brain.
41. The 4th arch vessels become the dorsal aorta
which supplies blood to the entire body.
The vessels of the 6th arch supply blood to the
lungs as pulmonary circulation.
In an embryo at 4 weeks, the heart is ventral
to the arches, and the blood passes dorsally
to the brain and body.
By the 5th week, the 1st and 2nd branchial arch
vessels have disappeared, and then the blood
supply to the face is carried out by the 3rd
branchial artery which becomes the carotid
artery.
42. SHIFT IN THE BLOOD SUPPLY OF FACE
An important change in the human embryo takes
place in the 7th prenatal week as the stapedial
artery suddenly occludes and separates from the
internal carotid artery; which discontinues its
blood supply to the face and palatal tissues.
Many of its terminal branches fuse with the
peripheral branches of the external carotid.
This results in the most unusual shift in the blood
supply of the face, from the internal carotid to the
external carotid artery.
The timing of this shift is very important. The
vessels begin to degenerate at one site and
rapidly proliferate at another.
43. The 7th week is an important period of rapid
growth expansion and fusion of the facial
processes. The lip and palate are undergoing
maximal developmental changes during this
time.
Thus, a vascular deficiency at this time may
result in oxygen and nutritional deficiency
which could result in cleft lip, cleft palate or
both.
44. BRANCHIAL ARCH CARTILAGES
The initial skeleton of the branchial arches
develops from the mesenchymal tissue as
cartilaginous bars.
In the 1st arch, bilateral Meckel’s cartilages
arise. The malleus and incus develop and
ossify at the dorsal end of Meckels cartilage.
The rest of the cartilage gradually disappears,
leaving part of the perichondrium as the
sphenomalleolar ligament (ant. Ligament of
malleus) and part as the sphenomandibular
ligament.
46. In the 2nd arch, Reichert’s cartilage develops.
It gives rise to the stapes, styloid process,
lesser horn and upper part of the body of the
hyoid. The stylohyoid ligament is formed by
the perichondrium at the site of
disappearance of this 2nd arch cartilage.
The 3rd arch cartilage forms the greater horn
and lower part of the body of the hyoid.
The 4th arch cartilage forms the thyroid
cartilage.
The 5th arch cartilage has no adult derivatives.
The 6th arch cartilage forms the laryngeal
cartilages.
47. MUSCULAR AND NEURAL DEVELOPMENT
During the 5th week, myoblasts proliferate within the
mandibular arch.
By the 7th week, cells migrate and differentiate into
the 4 muscles of mastication:lateral pterygoid,
medial pterygoid, temporalis and masseter.
The muscle cells within the hyoid arch and in the
occipital myotomes undergo proliferation and
migrate anteriorly toward the floor of the mouth to
form muscles of the tongue.
Muscle cells of the 3rd and 4th arch form the
pharyngeal muscles : stylopharyngeus, cricothyroid,
levator palatini and constrictor muscles of pharynx.
48. Nerves develop in conjunction with the developing
muscle fibres. By the 7th week, the Vth nerve has
entered the mandibular muscle mass, as has the
VIIth nerve in the second arch mass.
The trigeminal nerve (V) supplies sensory fibres to
the mandible and maxilla and motor fibres to the
muscles of mastication.
The facial nerve (VII) follows the migration of the
facial muscle mass from the neck onto the face. It
also supplies the stylohyoid and stapedius muscles
and posterior belly of digastric muscle.
The glossopharyngeal nerve (IX) supplies the
stylopharyngeus and the upper pharyngeal muscles.
The vagus nerve (X) supplies the pharyngeal
constrictor and laryngeal muscle
49. DEFECTS IN BRANCHIAL ARCH
DEVELOPMENT
Cervical Cysts and Fistulae:
Caudal overgrowth of the
second arch gradually covers
the 2nd, 3rd and 4th branchial
grooves. These grooves lose
contact with the outside and
temporarily form an
ectoderm lined cavity, the
cervical sinus, which should
normally disappear.
50. Failure of complete obliteration of the
cervical sinus results in a cervical cyst. If
the cyst opens to the outside, a fistula
develops. Branchial cysts or fistulae are
found anywhere on the side of the neck
along the anterior border of the SCM
muscle.
Another cause is incomplete caudal
overgrowth of 2nd arch, which leaves an
opening on the surface of the neck
51. DEVELOPMENT OF EARLY FACE
The face develops during the 5th to 7th week of
intrauterine life from 4 primordia that surround a
central depression called the central pit.
These include the frontal process (a single
cranially located process), the 2 bilaterally located
maxillary process, and the mandibular process
derived from the first branchial arch.
52. DEVELOPMENT OF MANDIBLE
The mandibular process
appears initially as a
partially divided
bilateral structure but
soon merges at the
median line. This
process will give rise to
the mandible, the lower MERGED
ME
MANDIBLE
part of the face and the
body of the tongue.
53. By the 5th week, the nasal placodes develop
bilaterally on the lower part of the frontonasal
process where they border the oral cavity.
At the margins of the placodes, mesenchyme
proliferates and produces medial and lateral
nasal processes thus transforming the placodes
into nasal pits(nostrils).
By the 6th week of IU life, The medial and lateral
nasal processes appear as horse shoe shaped
structures with the open end of the slit in
contact with the oral cavity.
54. • The point of contact of
the epithelial covered
medial nasal and
maxillary processes is
termed the nasal fin.
• This vertically
positioned epithelial
sheet under each
nostril separates the
medial nasal and
maxillary processes;
and when the fin
disappears, the lip will
fuse.
55. • On each side, the lateral nasal process
is separated from the maxillary
process by a groove called the
nasolacrimal groove.
• This groove will eventually disappear ,
but before it disappears, the
epithelium at its depth will canalise ,
and form the nasolacrimal duct
56. DEVELOPMENT OF UPPER LIP AND
MAXILLA
During the 6th week, the 2 medial nasal
processes merge in the midline to form the
intermaxillary segment.
This will give rise to the centre of the upper lip,
the primary palate, and the part of the alveolar
process carrying the incisor teeth.
Each maxillary process grows medially and
fuses, first with the lateral nasal processes and
then with the medial nasal process.
The medial and lateral nasal processes also
fuses with each other ;thus closing the nasal
pits to the stomatodeum.
57. The mesoderm of the
lateral part of the lip is
formed from the
maxillary process. The
overlying skin is derived
from ectoderm of the
same process.
The failure of fusion of
medial nasal process
with the lateral nasal
process leads to the
formation of cleft lip.
58. DEVELOPMENT OF EYE
The eyes develop during the 5th week.
The first external sign of eye development is
the appearance of the lens placodes between
the maxillary and frontonasal processes at
the lateral sides of the face.
59. The lens placode sinks below the surface and
is eventually cut off from the surface
ectoderm.
The developing eyeball now presents as a
bulge facing laterally. With the narrowing of
the frontonasal process, they come to face
forwards.
The eyelids are derived from folds of
ectoderm that are formed above and below
the eyes, and by mesoderm enclosed within
the folds.
61. DEVELOPMENT OF EAR
The external ear is formed around the dorsal
part of the 1st ectodermal cleft.
A series of mesodermal thickenings appear
on the mandibular and hyoid arches where
they adjoin this cleft.
The pinna is formed by fusion of these
thickenings.
When first formed the pinna lies caudal to the
developing jaw. It is pushed upwards and
backwards due to later enlargement of the
mandibular process
62.
63. DEVELOPMENTAL DEFECTS OF EAR
Familial expansile osteolysis
Malleus/incus fixation
Absence of the long process
of the incus
Congenital fixation of stapes
(stapes anchored to oval
window)
Failure of annular ligament
development
Cholesteatoma
Congenital preauricular sinus.
64. DEVELOPMENT OF TONGUE
The tongue is composed of the body which is the
movable oral part and the posterior (attached) base
or pharyngeal part.
The tongue develops from the tissues of the 1st, 2nd
and 3rd branchial arches and from the occipital
myotomes.
The body of the tongue develops from 3 elevations on
the ventromedial aspect of the 1st arch: a tuberculum
impar and paired lateral lingual swellings. These
lateral lingual swellings rapidly enlarge, merge with
each other , and overgrow the tuberculum impar to
form the oral part of the tongue.
A U-shaped sulcus develops in front and on both sides
of this oral part, which allows it to be free and highly
mobile except at the region of the frenum lingulae.
65.
66. The base of the tongue develops mainly from the 3rd
branchial arch. Initially, it is indicated by 2 midline
elevations that appear caudal to the tuberculum
impar.
These are the copula of the 2nd arches and the large
hypobranchial eminence of the 3rd and 4th arches.
Later the hypobranchial eminence overgrows the 2nd
branchial arches to become continuous with the body
of the tongue.
The site of union between the base and body of the
tongue is delineated by a V-shaped groove called
sulcus terminalis.
The occipital myotomes migrate anteriorly into the
tongue during the 5th to 7th weeks.
Later, various types of papillae differentiate in the
dorsal mucosa of the body of the tongue, whereas
lymphatic tissue develop into the base of the tongue.
67. INNERVATION OF TONGUE
As the occipital muscle masses migrate
anteriorly, the IXth and XIIth nerves are carried
along into the tongue.
The Vth nerve supplies sensory fibres to the body
or anterior 2/3rds of the tongue.
The VIIth nerve supplies the taste fibres to the
same part.
The IXth nerve supplies sensory taste fibres to
the posterior 1/3rd
The hypoglossal nerve supplies the intrinsic
muscles (longitudinal, vertical and transverse)
and the extrinsic muscles (styloglossus,
hyoglossus and genioglossus).
69. DEVELOPMENT OF THYROID
In the 4th week, the thyroid gland develops as a
depression and epithelial thickening in the floor of the
pharynx.
This appears at a point between the body and base of
the tongue called the foramen caecum. From this
point, the thyroid primordium descends in the neck as
a bilobed diverticulum to reach in front of the trachea
in the 7th week.
During this migration, the gland remains connected
to the floor of the oral cavity by an epithelial cord or
duct, the thyroglossal duct which later becomes a
cord of cells.
The foramen caecum remains at the site of origin.
The thyroid gland begins to function at the beginning
of the 3rd month when colloid containing follicles
appear.
70. DEVELOPMENTAL DEFECTS OF
THYROID
Thyroglossal cyst and Fistula: Cysts and
fistulae found along the midline of the neck
usually develop from remnants of
thyroglossal duct.
Generally, thyroglossal cysts maybe found
at any point along the course of the
thyroglossal duct but it is usually found at
the level of the hyoid bone and the thyroid
cartilage.
71. DEVELOPMENT OF SALIVARY GLANDS
The major salivary glands
(parotid, submandibular
and sublingual) begin
development during 6th
to 8th week.
The parotid develops in
the lateral aspects of the
stomodeum, and the
submandibular and
sublingual develop in the
floor of the stomodeum.
72. Each gland develops through growth from a
bud of oral epithelium into the underlying
mesenchyme.
The epithelial buds differentiate into extensive
system of solid cords of cells which later form
lumen and become ducts.
Minor salivary glands develop during the 3rd
prenatal month. They remain as separate acini
scattered in the connective tissue underlying
the oral mucosa.
Failure of canalisation of ducts before acinar
secretion begins results in retention cysts.
74. DEVELOPMENT OF PALATE
By the 6th week of development, the primitive nasal
cavities are separated by a primitive nasal septum and
partitioned from the stomodeum by a primary palate.
The formation of secondary palate commences between
7 and 8 weeks and is completed around the 3rd month of
gestation.
Three outgrowths appear in the oral cavity: the nasal
septum grows downwards from the frontonasal process
along the midline, and 2 palatal shelves or processes ,
one from each side, extend from maxillary process
towards the midline.
The shelves are directed first downward on each side of
the tongue.
75. After the 7th week of
development, the
tongue is withdrawn
from between the
shelves, which now
elevate and fuse with
each other above the
tongue and with the
primary palate.
The septum and 2
shelves converge and
fuse along the midline,
thus separating the
oronasal cavity into oral
and nasal cavities.
76. For the fusion of palatine shelves to occur,
elimination of the epithelial covering of the
shelves is necessary. To achieve this fusion, DNA
synthesis ceases within the epithelium some 24 to
36 hours before the epithelial contact.
Surface epithelial cells are sloughed off as they
undergo physiologic cell death to expose the
basal epithelial cells.
These cells have the carbohydrate rich surface
coat that permits rapid adhesion and the
formation of the junctions to achieve fusion of the
processes.
77. A midline seam is thus formed of two layers
of the epithelial cells. This midline must be
removed to permit ectomesenchymal
continuity between the fused process.
The growth of the seam fails to keep pace
with the palatal growth so that the seam
first thins and then breaks down into
discrete islands of epithelial cells.
The basal lamina surrounding these cells is
lost and the epithelial cells transforms into
mesenchymal cells.
78. PALATAL SHELF ELEVATION
This process has been presumed to take place
rapidly, about as fast as the act of swallowing, as
it has never been precisely recorded.
Several mechanisms have been proposed to
account for the movement of the palatal shelves
from vertical to the horizontal position.
The closure of the secondary palate may involve
an intrinsic force in the palatine shelves the
nature of which has not been determined yet.
The extrinsic forces derived from the tongue and
jaw movements may be responsible for this.
The high content of glycosaminoglycans , which
attract water and make the shelves turgid, has
also been suggested.
80. DEVELOPMENTAL DEFECT OF PALATE
Cleft Lip: Can be unilateral, bilateral and can
vary from a notch in the vermillion border to a
cleft extending into the floor of the nostril.
Cleft palate: Less common than cleft lip. It
maybe due to lack of growth or failure of fusion
between the median and lateral palatine
processes and the nasal septum or it maybe due
to initial fusion with interruption of growth at
any point along its course. It may also be due to
interference with elevation of palatal shelves.
82. DEVELOPMENT OF TEETH
In humans, 20 primary and 32 permanent teeth
develop from the interaction of oral epithelial
cells and the underlying mesenchymal cells.
The tooth germ is derived from the dental
lamina which is formed from the ectodermal
cells of the first branchial arch.
The dental lamina connects the tooth germ to
the outer ectodermal layer.
The tooth germs gets organized into 3 parts: the
enamel organ, the dental follicle and the dental
papilla.
83.
84. STAGES OF TOOTH DEVELOPMENT
1) THE BUD STAGE:
The bud stage is
characterized by the
appearance of a tooth
bud without a clear
arrangement of cells.
The stage technically
begins once epithelial
cells proliferate into the
ectomesenchyme.
Typically, this occurs
when the fetus is around
6 weeks old.
85. 2) THE CAP STAGE:
The first signs of an arrangement
of cells in the tooth bud occur in
the cap stage. A small group of
ectomesenchymal cells stops
producing extracellular
substances, which results in an
aggregation of these cells called
the dental papilla.
At this point, the tooth bud grows
around the ectomesenchymal
aggregation, taking on the
appearance of a cap, and becomes
the enamel (or dental) organ.
86. 3) THE BELL STAGE:
The tooth histodifferentiation and
morphodifferentiation takes place in bell
stage. The dental organ is bell-shaped during
this stage, and the majority of its cells are
called stellate reticulum because of star-
shaped appearance.
Cuboidal cells on the periphery of the dental
organ here are known as outer enamel
epithelium. The columnar cells of the enamel
organ adjacent to the dental papilla are
known as inner enamel epithelium. The cells
between the inner enamel epithelium and the
stellate reticulum form a layer known as the
stratum intermedium. The rim of the dental
organ where the outer and inner enamel
epithelium join is called the cervical loop.
87. 4) ADVANCED BELL STAGE:
Hard tissues, including enamel and
dentin, develop during this stage of
tooth development. In prior stages,
all of the inner enamel epithelium
cells were dividing to increase the
overall size of the tooth bud, but
rapid dividing stops during this
stage at the location where the
cusps of the teeth form.
The first mineralized hard
tissues(enamel and dentin) form at
this location. At the same time, the
inner enamel epithelial cells change
in shape from cuboidal to columnar.
88. MINERALIZATION
Mineralization is the process of deposition of the
matrix of the hard dental structures, also called as
the appositional growth.
It is characterized by regular and rhythmic
deposition of the extracellular matrix, which is
itself incapable of further growth.
The process of formation of enamel is known as
amelogenesis, and that of dentin is known as
dentinogenesis.
The development of enamel involves two
processes: organic matrix formation and
mineralization.
89. The ameloblasts begin their secretory activity when a
small amount of dentin has been laid down. The
surface of ameloblasts facing the enamel is not smooth
but possesses projections which in corporate into the
enamel matrix termed as tomes processes.
The mineralization of enamel matrix occurs in two
stages. In the first stage interprismatic substances are
laid down, and in the second stage gradual completion
of the process occurs by total mineralization of the
structure.
90. During the formation of dentin, the
odontoblasts differentiate from a
ovoid to a columnar shape. One or
several processes arise from its apical
end in contact with the basal lamina.
The cell recedes apically and deposits
the dentinal matrix gradually, and the
several processes join into one. The
single process is termed dentinal
tubule.
The mineralization occurs in the form
of very fine layers of hydroxyapatite
deposited in the ground substance.
The crystals are arranged in an
orderly fashion, with their long axis
paralleling the fibril long axis.
91. FORMATION OF ROOT
The development of root
begins after enamel and
dentin formation has reached
the future cementoenamel
junction.
The enamel organ forms the
‘Hertwig Epithelial Root
Sheath’ which moulds the
shape of the roots and
initiates radicular dentin
formation. The differentiation
of odontoblasts and the
formation of dentin follows
the lengthening of the root
sheath.
92. DEVELOPMENT OF ADJACENT
CONNECTIVE TISSUES
1) CEMENTUM:
Cementoblasts are the cells responsible for
cementogenesis. Two types of cementum is formed:
cellular and acellular.
The cementoblasts secrete fine collagen fibrils along the
root surface at right angles before migrating away from
the tooth. As the cementoblasts move, more collagen is
deposited to lengthen and thicken the bundles of fibers.
2)PERIODONTAL LIGAMENT:
Cells from the dental follicle give rise to the periodontal
ligament.
The fibroblasts in the dental follicle secrete collagen,
which interacts with fibers on the surfaces of adjacent
bone and cementum.
93. 3) GINGIVA:
Hemidesmosomes form between the gingival epithelium
and the tooth. These are responsible for the primary
epithelial attachment.
During eruption, junctional epithelium forms from the
reduced enamel epithelium. This epithelium divides
rapidly, resulting in increased size of the junctional
epithelial layer and the isolation of the remnants of
ameloblasts devoiding them from any source of nutrition.
As the ameloblasts degenerate, a gingival sulcus is
created.
4) ALVEOLAR BONE:
Throughout the body, cells that form bone are called
osteoblasts.
These osteoblast cells form from the dental follicle.
Similar to the formation of cementum, collagen fibers are
created on the surface nearest the tooth, and deposit the
fibres to form the alveolar bone.
