3. Ancient growth and development
concepts
“From the conjugation of the blood and
semen ,embryo comes into existence. After the period of
conception, it becomes kalada (one day old embryo), after
remaining seven nights it becomes a spherical mass, after
two months the head is formed, after three months limb
region is formed”
4. Definition of 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)
5. Definition of Development
Development is a progress towards maturity” –
Todd(1931)
“Development connotes a maturational process
involving progressive differentiation at the
cellular and tissue levels” – Enlow.
6. “Development refers to all
naturally occurring
progressive, unidirectional,
sequential changes in the life
of an individual from it’s
existence as a single cell to
it’s elaboration as a
multifunctional unit
terminating in death” –
Moyers(1988)
7. Normal features of
Growth & Development
Pattern
-Differential Growth
-Cephalocaudal gradient of growth
Variability
Timing, rate & direction
8. 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.
9. DIFFERENTIAL GROWTH
Different organs grow at
different rates amount and at
different times.
Scammon’s curve of growth-
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 tissues follow a sigmoid
growth pattern.
Genital tissue grow significantly
only at puberty and achieve full
growth at about 20 yrs of age.
10. CEPHALOCAUDAL GRADIENT
OF GROWTH
This simply 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%.
11. Normal features of
Growth & Development
Pattern
-Differential Growth
-Cephalocaudal gradient of growth
Variability
Timing, rate & direction
12. 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
13. 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
14. 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)
15. Factors Implicated in the
Craniofacial Development
Growth Factors: Growth factors are peptides, which usually transmit
signals between cells and thereby modulate their activity.
GF is an inductive agent and regulate cell activity at cell to cell level.
GF have very short half life and are secreted in very small amounts.
A GF produced by one cell and acting on other cell is called paracrine
regulation, whereas the process of cell that recaptures its own
products is called autocrine regulation.
1.
2.
3.
4.
5.
6.
Transforming GF- TGF b , BMP, GDF
Epidermal GF- EGF, TGF a, amphiregulin, Heparin binding EGF
Fibroblast Growth Factor (FGF)
Insulin like Growth Factor (IGF)
Platelet derived Growth Factor (PDGF)
Neurotrophins- Nerve Growth Factor(NGF), Brain Derived
Neurotropic Factor(BDNF),Neurotrophin
16. Homeobox Genes: A group of genes with shared nucleotide segment that are
involved in bodily segmentation during embryonic development. They are
considered the key regulators of embryogenesis.
Homeobox is a stretch of DNA sequence that contains about 180 base pairs that
encode transcription factors which typically switches on cascades of other
genes.
1.HOX genes- These genes function in patterning the body axis and determine
where limbs and other body segments will grow in developing foetus.
2.Msx genes- Control cellular process of differentiation and proliferation during
development.
3.Dlx genes- Control development of ectodermal tissues derived from lateral
border of the neural plate.
-Control differentiation of a subset of GABA ergic
neurons of the basal ganglia and the cerebral cortex.
-Control patterning of the branchial arch skeleton
-Also expressed in developing bone and regulate limb
development.
4.
Shh (Sonic hedgehog) genes- Plays an important role in the early
induction of facial primordium
17. Growth 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 foetus: from 9th week onwards till birth
b. Postnatal growth
c. Maturity
d .Old age
18. Origin of Human Embryo
Human prenatal development
begins with processes involved in
the ovarian cycle and fertilization.
Fertilization occurs in the
fallopian tubes.
Fusion of the female and male
pronuclei culminate the
fertilization process.
The fertilized ovum, termed a
zygote undergoes cleavage or
division as it moves towards the
uterine cavity.
19. Morula: 4th day
The fertilized ovum, termed a
zygote, undergoes cleavage or
division as it moves toward the
uterine cavity.
The cells formed during
cleavage 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.
20. Blastocyt stage: 5th day
As the cell mass divides, it
enlarges and gains a fluid
filled inner cavity termed the
blastocele.
The blastocele separates the
cell into 2 parts:
1. An outer cell layer, the
trophoblast, and
2. An inner cell mass, the
embryoblast.
21. 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.
22. 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 the blastocele.
The amniotic cavity appears
between the ectodermal cells and
the overlying trophoblast.
23. 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.
24. 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.
25. 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.
26. The anterior plate forms the future
oropharyngeal membrane.
Finally, mesodermal cells of the
embryonic disc migrate
peripherally to join the
extraembryonic 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.
27. 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.
28. Development of Nervous System
On day 18, the developing notochord and the adjacent
mesenchyme induce the overlying ectoderm to form
the neural plate.
The neural plate then bends along its central axis to
form a groove, and the raised margin along both sides
of this groove form neural folds.
The neural folds gradually approach each other in the
midline where they fuse.
The folds remain temporarily open at the cranial and
caudal ends forming anterior and posterior
neuropores.
The neuropores close during the fourth week and the
central nervous system is established.
29. Development of Neural Crest
At the time of neural tube closure, a
unique population of cells separate from
the crest of the folds.
They are known as the neural crest
cells.
These cells undergo extensive migration
beneath the surface ectoderm, especially
in the head and neck region and give rise
to a variety of cells.
They form the sensory ganglia,
sympathetic neurons, schwann cells,
pigment cells, meninges and cartilage of
branchial arches.
They contribute to formation of the
embryonic connective tissue of facial
origin which includes connective tissue
dental structures(dentin, pulp and
30. Development of Oral Cavity
The primitive oral cavity or stomodeum appears late in
the third prenatal week as a pit or invagination of the
tissues underlying the forebrain.
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.
31. 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.
32. 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.
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.
34. 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.
35. 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.
The thymus is relatively large at birth, continues to grow till
puberty and thereafter atrophies and disappears later in life.
The ultimobranchial body fuses with the thyroid and contributes
parafollicular cells to this gland.
The parathyroid gland functions throughout life in calcium
regulation
The tonsils function in lymphocyte development and
immunologic factors.
36.
37. 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.
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.
38. The 3rd arch vessels become the common carotid
arteries which supply the neck, face and brain.
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.
39. The common carotid artery gives rise to the external
carotid and the internal carotid arteries.
The external carotid artery supplies blood to the ventral
part of the 1st and 2nd branchial arches.
The internal carotid artery supplies blood to the brain.
In the region of the ear, the internal carotid artery gives
rise to a small vessel, the stapedial artery, which supplies
most of the blood to the upper part of face and palate.
Blood supply to the face by internal carotid artery is a
characteristic of the embryo at 6 and 7 weeks.
40. Shift in Blood Supply to the 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.
41. If timing in the shift is not precise, there will be a
period when the face is deprived of oxygen and
nutrition carried by this blood supply.
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.
42. 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.
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.
43. 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.
44.
45. Development of Facial Muscles
During the 5th and 6th weeks, myoblasts within the
mandibular arch begin proliferation. They become
oriented to the sites of origin and insertion of the
muscles they will form.
By 7 weeks, the mandibular muscle mass has
enlarged and cells have begun migrating and
differentiating into the 4 muscles of mastication : the
masseter, medial and lateral pterygoid and temporal
muscles.
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
46. By the 10th prenatal week, the mandibular arch muscle masses
have become well organised bilaterally.
The muscle cells of masseter and medial pterygoid have formed a
vertical sling inserting into the site that will form the angle of the
mandible.
The temporalis muscle has differentiated in the infratemporal
fossa and is inserting in the developing coronoid process.
The lateral pterygoid muscle fibres, which also arise from the
infratemporal fossa, extend horizontally to the necks of the
condyles and insert in the articular discs.
The pharyngeal constrictor muscles have differentiated and
enclosed the pharynx.
The face will change shape considerably as it grows, and all its
muscles will develop to meet the increasing functional demand.
47. Innervation of Facial Muscles
By the 7th week, the Vth nerve has entered the mandibular
muscle mass, as has the VIIth nerve in the second arch mass.
This means that the nerves are incorporated in these muscles
very early and they follow the muscles as they migrate and
differentiate.
The trigeminal nerve (V) supplies sensory fibres to the mandible
and maxilla and motor fibres to the muscles of mastication and
to mylohyoid, tensor palatini, tensor tympani, and anterior belly
of digastric muscle.
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 muscles.
48. 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.
49.
50. 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.
51. 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).
52. 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.
53. 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.
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.
54. 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.
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
part of the face and the body of the tongue.
55. 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.
56. •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.
•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
proliferate into a solid cord which
will separate from the surface,
canalise , and form the nasolacrimal
duct
57. Formation of Upper Lip
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.
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.
58. The maxillary processes undergo considerable growth while the
frontonasal process becomes narrower bringing the anterior
nares closer together.
The mesoderm of the median part of the lip(philtrum), is
formed from the frontonasal process.
The ectoderm of the maxillary process overgrows this mesoderm
to meet that of the opposite maxillary process in the midline.
As a result, the skin of the entire upper lip is supplied by the
maxillary nerves.
59. The 3 sets of facial processes derive their nerve supply
from 3 divisions of the trigeminal nerve.
The ophthalmic division supplies the frontonasal
process, the maxillary division supplies the maxillary
process, and the mandibular division supplies the
mandibular process.
60. 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.
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. Development of the 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.
Both the primary nasal septum and the primary palate are derived
from the frontonasal process.
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.
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.
63.
64. 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.
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.
65. Palatal Shelf Elevation
Several mechanisms have been proposed to account
for the rapid 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.
66. The developing human is least susceptible to
teratogens during the proliferation period; first 2 or 3
weeks.
The embryonic period; end of 2nd or 3rd week to end of
8th week; is most critical period because it is the period
of differentiation of organs and systems.
During the foetal period; end of 8th week until birth;
the susceptibility to teratogens rapidly declines and
may cause only minor defects.
67. Hereditary causes:
1. Chromosomal abnormalities- Trisomy 21; mental retardation,
upward slanting palpebral fissures, flat nasal bridge and fissured
protruding tongue.
2. Genetic abnormalities- Mandibulofacial dysostosis
Environmental causes:
1. Infectious agents- Rubella in pregnant women causing cleft
palate, malformed teeth and congenital cataracts.
2. Radiation- Direct and indirect effect
3. Drugs- Aminopterin, tetracycline, Phocomelia by thalidomide
4. Hormones- Cortisone in experimental animals
5. Nutritional disorders- Vitamin deficiencies and
hypervitaminosis A C D
6. Teratogenic habits- Smoking, alcohol and caffeine
68. Abnormal Development
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.
69. 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. 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.
70. 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.
Mandibulofacial Dysostosis or
Treacher Collins Syndrome: This
results from failure or incomplete
migration of the neural crest cells to the
facial region.
The zygomatic bone is severely
hypoplastic . The face appears to be
drooping, and the ears are usually
malformed. The lower border of the
mandible appears concave, and cleft
palate is occasionally seen.
71. Abnormal Development
Fissural cysts: Cystic cavities which arise along the fusion of
various bones or embryonic processes and lined by epithelium.
Median Rhomboid Glossitis: It results from persistence of the
tuberculum impar and characterised by a red smooth region
anterior to the foramen caecum.
Ankyloglossia: This occurs as a result of incomplete
degeneration of cells while the body of the tongue is freed, so
that the tip of the tongue remains tied to the floor of the mouth.
Macroglossia: or abnormally large tongue is not common, but
is seen sometimes at birth when tongue slightly protrudes from
mouth. This corrects itself when the jaws grow at a rapid rate.
True macroglossia is seen in mongolism.
Bifid tongue: This is a malformation common in south
American infants and is the result of failure of the lateral lingual
swellings.
72. Developmental Anomalies
Hare lip
Oblique facial cleft
Cleft palate
Macrostomia
Microstomia
Hypertelorism
Congenital lip pits or fistulae
Double lip
Congenital tumours in relation to the face
Bifid nose
