Embryology part 1

5. Ontogeny
Development of an individual is divided into:
1.Prenatal development.
2.Postnatal development.

6. I-Prenatal development

7. I-Prenatal development
It begins when the sperm and ovum fuses together to form a
zygote. It is divided in to 3 periods:
1-The proliferation period (pre-differentiation)
2-The embryonic period
3- Fetal period

8. 1.Prenatal development
3 major periods:
Proliferative period (Predifferentiation period)
Begins from fertilization till the end of 2nd week IUL.
 rapid cell proliferation.
Bilaminar disc formation.
3 germ layer.

9. 2. Embryonic period (3th-8th wIUL)
The most critical period of development because:
Intensive differentiation and development of the major
organs and systems.
The external body shape is formed during this period.
 Congenital malformations usually occur during this
period
The human is termed an embryo

10. 3-Fetal period
Extends from the end of 2nd month till birth.
 growth and further differentiation of the already established organs
and systems.
The human is called a fetus

11. Postnatal period
◦ It is divided into different periods:
Neonatal period:
◦ 2 w after birth
Infancy period:
◦ 1-2 year
◦ Deciduous teeth appear in this period.
◦ The individual is called infant or baby.
Childhood period
◦ 2-12 year.
◦ The deciduous teeth are replaced by permanent teeth (mixed dentition).

12. Puberty period
◦ 12-15 girls/ 13-16 boys.
◦ During this period secondary sexual characteristics develop and become expressed.
Adolescence period
◦ 2-3 year after puberty
◦ Extends from the earliest sign of sexual maturity till the
attainment of physical and emotional maturity.
Early adulthood
◦ Extends from 18 -25 years.
◦ Ossification and growth are completed.

13. I-Prenatal development

14. I-Prenatal development
1-The proliferation period (pre-differentiation)
Early development and formation of the germ layers

16. Early development and formation of the germ
layers (1st week iul)
1. Fertilization
Sperm + ovum= zygot (totipotent cells)
2. Zygot  mitotic division (cleavage)  2,4,8,16 cells
3. 16 cell called morula (mulbery structure) 
Blastomere
4. fluids seeps in  Blastocyst

17. Blastocyst stage
Two distinct types
Inner cell mass (embryoblasts)
forms the embryo
Outer cell mass (Trophoblast): form
the placenta

18. Bilaminar disc (2nd wiul)

19. bilaminar disc
Inner cell mass
(embryoblasts)
Hypoblast
(endoderm)
Cuboidal cells
Facing yolk cavity
Epiblast
(ectoderm)
Columnar cells
Facing amniotic
cavity

20. Formation of tri-laminar disc
three germ layers (Key players)
Cells migrate between endoderm and ectoderm to form the third
germ layer (mesoderm) 3 weeks iul.

21. The Mesoderm exist between the ectoderm and
endoderm except at
1. Oropharyngeal membrane (cranial, cephalic,
prochordal plate)
2. Cloacal membrane (caudal)
Later, these membrane rupture to form the two
opening of the alimentary canal.

26. - Monosomy
decreased number (45 chromosomes) is
usually fatal.
- Trisomy 21 or Down’s syndrome
Increased number in one
chromosome results in
a condition known
as (mongolism).

27. Trisomy 21 or Down’s syndrome
characterized by
mental retardation
flat nasal bridge
fissured protruding tongue
macroglossia
delayed tooth eruption

28. Trisomy 21 or Down’s syndrome
Remember that
different individuals
will have
different traits

29. 2- Genetic causes
They are due to a disturbance in migration and distribution
of the neural crest cells. So it leads to ''mandibulo-facial
dysostosis'' or Treacher Collin Syndrome
Characterized by
Under-developed face
Dentinogenesis imperfecta
(defect in dentin formation)

30. The end of proliferative period
Embryonic period begins

31. Organ, tissue, another
type of cells
At
certain
time &
place
Influenced
Influencer

32. The 3rd wIUL
Pear shaped embryo
-1- first inductive event (Mesoderm induce ectoderm)
Neural tube formation.
-2- Fourth germ layer formation (Neural crest cell separation)
-3- Primitive oral cavity (position).

34. Induction: is the organizing
influence of cells or their products on
the adjacent cells or tissue.
Mesoderm induce the ectoderm
to form Neuroectoderm.
Neuroectoderm thickens to form
Neural plate on the dorsal region of
the embryo.

36. The neural tube will form the spinal cord and its
anterior portions enlarge to form the primary
division of the brain.

38. At the time of neural tube closure, a group of cells
separate from the fold and is known as neural crest cells.
Neural crest cells are called also fourth germ layer or
ectomesenchymal cells.
These cells undergo extensive migration beneath the
surface ectoderm especially in the head and neck region.

42. At the 3rd wiul, the primitive oral cavity (stomodeum) lies
between the forebrain cranially and the developing heat.
At the deepest end of the stomodeum, the oral ectoderm
lies in close contact with the foregut endoderm separated by
buccopharyngeal (orophanyngeal) membrane.
During the 4th wiul, the oropharyngeal membrane ruptures
to get continuity between the two cavities (oral and
pharynx).

43. The 3rd wIUL
Pear shaped embryo
-1- first inductive event (Mesoderm induce ectoderm)
Neural tube formation.
-2- Fourth germ layer formation (Neural crest cell separation)
-3- Primitive oral cavity.

44. The 4th week
1. The oropharyngeal membrane ruptures.
2. Development of 2 endocrine glands
3. Branchial arches develop & neural crest cs migration to branchial
arches.
4. Development of maxillary process from 1st branchial arch
5. At the end the mucous membrane of tongue forms
• Thyroid gland (4-7w
IUL)
• Anterior pituitary
gland

45. Anterior pituitary gland Thyroid gland
Site Roof of the oral cavity Floor
Behind oropharyngeal
membrane
Origin Ectodermal lined pouch (Rathke’s
pouch)
Endodermal pouch
Secrets (function) Thyroid stimulating hormone (TSH )
Adreno-corticotrophic hormone
( ACTH )
Growth hormone (GH)
controls body
metabolism

48. 1. Structure & Position
2. Derivatives: a) Pouchs
b) grooves
c) Hyaline cartilage (skeletal ,neural crest cell)
3. Innervation
4. Blood supply
5. Muscle

49.  Pharyngeal arches begin to develop early in the
fourth week as neural crest cells migrate into the head
and neck region.
They are 6 cylindrical pairs of parallel bars of
mesodermal tissue.
The first four arches are well developed in man and
are numbered i, ii, iii, iv and VI.
 The 5th is transient in human.

50. They develop in sequence, and are progressively smaller
from the 1st to the last arch.
 The 1st & 2nd arches meet at the midline, the others
don’t meet & are separated by copula of His.
The pharyngeal arches contribute exclusively to the
formation the head and neck region.
 Each pharyngeal arch consists of a core of
mesenchyme.
 Covered externally by ectoderm and internally by
endoderm Except the first pharyngeal arch it is covered
by ectoderm and endoderm.

51. They are separated
externally by shallow
branchial groove
and internally by
deep pharyngeal
pouches.

52. The first pharyngeal arch (mandibular arch) develops
maxillary and mandibular prominences
The first pair of pharyngeal arches plays a major role
in facial development
The second pharyngeal arch (hyoid arch) contributes
to the formation of hyoid bone

55. The 1st groove  external auditory meatus
The ectodermal membrane in the depth
of the groove persists and together with
mesoderm and endoderm of the first pouch
forms the tympanic membrane.
The 2nd arch groove overgrow 3rd & 4th to give the smooth
side of the neck.

56. What If the cervical sinus Persist ??
Branchial cyst  Branchial fistula

58. - 1st pouch  middle ear and Eustachian
tube.
- 2nd pouch  Palatine tonsils and its
crypts.
- 3rd pouch  inferior Parathyroid &
thymus gland.
- 4th pouch  uiltimobranchial body &
superior Parathyroid

59. Parathyroid glands function throughout life in calcium
balance.
Thymus gland:
large at birth, continuous to grow only until puberty,
gradually atrophies and completely disappears later in life
(thymus posses an important endocrine function early in life.

60. Thymus gland
produces T-lymphocyte, produces thymopoietin hormone
which is important for the function of T- lymphocyte).
Ultimobranchial body; fuse with the thyroid gland and
contribute to some of its cell (para-follicular cells that store and
secret calcitonin hormone that is involved in regulation of
normal calcium level in body fluids).

61. Tonsils; function in lymphocyte development and
immunologic function

62. - First arch  mandibular nerve,
branch of Trigeminal nerve (V) and
chorda tempani nerve, branch of
Facial nerve (VII).
- Second arch  Facial nerve (VII).
- Third arch  Glossopharyngeal
nerve (IX)
- Forth arch  Vagus nerve (X).

68. (From the aortic arch)
•First arch from external and internal
carotid arteries.
. Second arch from facial artery.
•Third arch internal and common
carotid arteries.
•Forth arch from aortic arch.

71. Shallow Brancheal
groove (BAG)
Hyaline cartilage Muscles
Mesenchyme
Arteries Nerve
supply
Deep Pharyngeal Pouch
1st External auditory meatus Meckel’s cart.
Malleus & Incus
(Middle ear)
Perichondrium
Sphenomalleolar
Sphenomandibular
ligament
Muscles of
Mastication
external
internal
carotid A
Mandibular
n(Trigemina
l (V)&
chorda
tempani,
Facial N
(VII).
(with ectoderm and
endoderm) 
Tympanic membrane
Middle ear.
Eustachian tube.
2nd
Hyoid
arch
2nd BAG extend caudally
forming ectodermal
depression (cervical sinus)
obliterate 2nd,3rd,4th BAG
to give the smooth side of
the neck.
Reichert’s cart.
Stapes, styloid
process, lesser horn,
upper part of hyoid
Ms of Facial
expression &
intrinsic Ms of
tongue
facial A Facial N.
(VII).
Palatine tonsil and its
crypts
3rd greater horn, lower
part of hyoid
Stylopharynge
us
internal
&
common
carotid A
Glossophar-
yngeal N.
(IX)
Inferior part of
parathyroid & thymus
gland
4th Thyroid cartilage Pharyngeal
Ms & larynx
Ms
Vagus N
(X)
Superior part
parathyroid
Ultimobranchial body
Laryngeal cartilage
6th

72. Pharyngeal
Arch
Nerve Artery Skeletal
Structures
Muscles Ligaments
I
Mandibular
(maxillary/
mandibular
)
trigeminal
(V)
maxillary
artery
(terminal
branches)
mandible,
maxilla,
malleus,
incus (from
cartilage)
muscles of
mastication
innervated
by the arch
nerve
Shpenomall
eor&
sphenoman
dibular
ligament