This document discusses embryology topics including the bilaminar and trilaminar germ discs, and the pharyngeal apparatus. It describes how the bilaminar germ disc forms from the trophoblast and inner cell mass layers. It then explains how the trilaminar germ disc forms through gastrulation and the establishment of the three germ layers - ectoderm, endoderm, and mesoderm. The document also provides details on the development of the pharyngeal apparatus, including the pharyngeal arches, pouches, grooves, and membranes, and their adult derivatives.

1. EMBYOLOGY
SUPERVISED BY DR DILABAZ KHAN,DR
IMDADULLAH,DR JAMEEL KIFAYATULLAH
PRESENTER HIRA NISAR
KHYBER COLLEGE OF DENTISTRY

2. • Bilaminar germ disc
• Trilaminar germ disc
• Pharangeal apparatus

3. Presented to :
Dr.Imdad
Dr. Jameel
Dr. Dilabaz
Presented by:
Hira Nisar
Roll # 24
BDS 2nd year

4. IN THE NAME OF
ALLAH,
THE MOST MERCIFUL,
THE MOST BENEFICIENT!

5. CONTENTS:
•Bilaminar Germ disc
•Trilaminar Germ disc
•Pharangeal Apparatus
Pharangeal Arches
Pharangeal Pouches
Pharangeal Clefts
Pharangeal Membrane

6. Bilaminar Germ Disc

7. Definition:
Formation of epiblast and hypoblast
layers in second week of development
forms Bilaminar Germ disc.

8. HOW???

9. Development Of Trophoblast
1. The trophoblast differentiates into
cytotrophoblast & syncytiotrophoblast.
2.Cells of the cytotrophoblast divide and
migrate externally.
3.They lose their cell membranes to form the
syncytiotrophoblast

10. Inner cell mass differentiates into two layers
1:Epiblast layer
2:Hypoblast layer

11. Clinical correlates
*The syncytiotrophoblast secretes human chorionic
gonadotropin hormone which prevents the
degeneration of the corpus luteum.
By the end of the 2nd week, the amount of
this hormone will be sufficient to be
detected in the maternal blood and urine.
This is the basis of pregnancy test.

12. *occasionally implantation takes place
outside the uterus resulting in ectopic or
extra uterine pregnancy.
*In some cases trophoblast develops and
forms placental membrane although little or
no embryo tissue is present this is known as
hydatiform mole.

14. 1.Secondary yolk sac
2. Exocoelomic cyst
3. Aminotic cavity
4. Extra-embryonic coelom
5. Epiblast
6. Connecting stalk
7. Hypoblast
8. Primary villi
9. Trophoblastic lacunae
10. Extraembryonic somatic mesoderm
11. Extraembryonic splanchnic mesoderm

15. Trilaminar Germ Disc

16. Definition
Formation of three germinal layers form
the trilaminar germ disc
The three layers are
1.Ectoderm
2.Endoderm
3.Mesoderm

17. HOW???

18. Formation of Trilaminar Germ Disc
1. . Gastrulation- most characteristic
feature during 3rd week
2.3 germ layers are established- ectoderm,
endoderm and mesoderm
3.Start with formation of PRIMITIVE
STREAK

19. PRIMITIVE STREAK
1. Midline groove at epiblast
2.Cephalic end-primitive node
3.Cells of epiblast migrate towards primitive
streak
4.This inward movement is known as invagination
5.Invaginated cells displace hypoblast forming
endoderm

20. 6.Some other invaginated cells come to lie
between epiblast and newly formed
endoderm forming mesoderm
7.Remaining cells in epiblast form ectoderm
8.These 3 layers form trilaminar germ disc

24. Clinical Significance
*Teratogenesis
For example large doses of alcohol at this
stage result in holoprosencephally.
*Caudal dysgenesis(Sirenomelia)
Insufficint mesoderm is formed in caudal
most region of embryo.
*Situs inversus
Transposition of viscera in the thorax and
abdomen occurs.

26. Pharangeal apparatus

27. Pharangeal apparatus
•The head and neck region of fourThe head and neck region of four
week human embryo somewhatweek human embryo somewhat
resemble these regions of a fishresemble these regions of a fish
embryo of comparable stageembryo of comparable stage
•This explains the former use ofThis explains the former use of
designation branchial apparatusdesignation branchial apparatus

28. Pharyngeal apparatus consists ofPharyngeal apparatus consists of::
Pharyngeal archesPharyngeal arches
Pharyngeal pouchesPharyngeal pouches
Pharyngeal grooves/cleftsPharyngeal grooves/clefts
Pharyngeal membranePharyngeal membrane

29. Pharyngeal pouchPharyngeal groove
Pharyngeal
membrane
Pharyngeal arch

30. Pharyngeal ArchesPharyngeal Arches
Mesenchymal condensation on either side
of the pharynx are known as pharangeal
arches.
Definition

31. •Pharyngeal archesPharyngeal arches
begin to developbegin to develop
early in the fourthearly in the fourth
week as neuralweek as neural
crest cells migratecrest cells migrate
into the head andinto the head and
neck regionneck region

32. Pharyngeal ArchesPharyngeal Arches
• By the end of the fourth week, four pairs ofBy the end of the fourth week, four pairs of
pharyngeal arches are visible externallypharyngeal arches are visible externally
• The fifth and sixth arches are rudimentaryThe fifth and sixth arches are rudimentary
and are not visible on the surface of theand are not visible on the surface of the
embryoembryo
• The pharyngeal arches are separated fromThe pharyngeal arches are separated from
each other by fissures called pharyngealeach other by fissures called pharyngeal
groovesgrooves

33. •Each pharyngeal arch consists of a core ofEach pharyngeal arch consists of a core of
mesenchymemesenchyme
•Is covered externally by ectoderm and internally byIs covered externally by ectoderm and internally by
endodermendoderm
Pharyngeal Arch ComponentsPharyngeal Arch Components

34. Ist Pharangeal Arch Derivatives
Ist pharangeal arch consists of
•Maxillary process
•Mandibular process
Skeletal component
Ist pharangeal arch is known as mandibular arch

35. Maxillary
process
Mandibular process

36. Maxillary process
Maxillary process gives rise to
•Maxilla
•Premaxilla
•Zygomatic bone
•Part of temporal bone

37. Mandibular process
Mandibular process give rise to
• Meckle’s cartilage
•Mandible
•Malleus
•Incus
•Anterior ligament of malleus
•Sphenomandibular ligament

39. Musculature of ist arch
Musculature of ist pharangeal arch includes
•Muscles of mastication
•Anterior belly of digastric muscle
•mylohyoid
•Tensor tympani
•Tensor palatini

40. Nerve supply to the ist arch
•Nerve supply to ist arch is provided by
Mandibular branch of trigeminal
nerve
Note
•Ist arch also contribute to dermis
of the face which is innervated by
opthalmic ,maxillary and mandibular
branch of trigeminal nerve.

42. as2nd Pharangeal Arch
skeletal component
2nd
pharangeal arch gives rise to
•Stapes
•Styloid process
•Stylohyoid ligament
•Lesser horn
•Upper portion of the body of hyoid bone
2nd
pharangeal arch is also known a hyoid arch or
rechiert’s cartilage

44. Musculature of 2nd
arch
Musculature of 2nd
arch includes
•Stapedius muscle
•Stylohyoid muscle
•Posterior belly of digastric muscle
•Auricular muscle
•Muscles of facial expression

45. Nerve supplyNerve supply
•2nd
arch is innervated by facial nerve

46. 3rd
Pharangeal Arch
Skeletal component
3rd
arch includes
•Lower part of the body
•And greater horn of the hyoid bone
Musculature
Musculature is limited to stylopharangeus
muscle

48. Nerve supply
It is innervated by
glossopharangeal
nerve

49. 4th
and 6th
Pharangeal Arch
Skeletal component
Cartilage of 4th
and 6th
arch fuse together to form
•Thyroid
•Cricoid
•Aretenoid
• Corniculate
• And Cuniform cartilage of larynx

50. Musculature of 4th
Arch
It includes
•Cricothyroid
•Levator platini
•Constrictors of pharynx
Nerve supply of 4th
Arch
Innervation is provided by
Superior larangeal branch of vagus

52. Musculature of 6th
arch
•It includes intrinsic muscles of larynx
Nerve supply
•Innervation is by recurrent larangeal
branch of vagus

54. Pharangeal pouches
•The pharyngeal pouches are balloon like
diverticula that formed on the endodermal
side between the pharyngeal archespharyngeal arches
•The pairs of pouches develop in a
craniocaudal sequence between the arches

56. • There are four well defined pairs of pharyngeal
pouches
•The fifth pair is absent or rudimentary
• The first pair of pouches lies between the first
and second pharyngeal arches

57. • The first pharyngeal pouch forms stalk like
diverticulum called tubotympanic recess
•The distal portion of diverticulum widens
into sac like structure primitive tympanic Or
middle ear cavity
•The lining of tympanic cavity later aids in
the formation of tympanic membrane or ear
drum
Derivatives of Ist Pharangeal Pouch

58. •While proximal portion forms auditory tube

59. •The second pharyngeal pouch is largely
obliterated ( disappear ) as the palatine tonsils
develop
• Part of the cavity of this pouch remains as the
tonsillar sinus or fossa
Derivatives of Second Pharyngeal Pouch

60. Derivatives of 2nd
pharangeal pouch

61. Derivatives of Third Pharyngeal Pouch
Epithelium of dorsal region of 3rd
pouch
differntiates into inferior thyroid gland
While ventral region differntiates into thymus.

62. The primordia of
thymus and inferior
parathyroid glands
lose their connections
with the pharynx and
migrate into the neck
Later the inferior
parathyroid glands
separate from the
thymus and lie on the
dorsal surface of the
thyroid gland

63. Derivatives of Fourth Pharyngeal PouchDerivatives of Fourth Pharyngeal Pouch
Each dorsal part develops into a superiorEach dorsal part develops into a superior
parathyroid glandparathyroid gland
It lies on the dorsal surface of the thyroid glandIt lies on the dorsal surface of the thyroid gland
Ventral region of 4th
pouch forms ultimobranchial
body

64. Ultimobranchial body is later on incorporated
into thyroid gland.
Cells of Ultimobranchial body give rise to
parafollicular cells Or C cells of thyroid gland
which secretes calcitonin.

65. Derivatives of Fifth Pharyngeal PouchDerivatives of Fifth Pharyngeal Pouch
When this develops, this rudimentary pouchWhen this develops, this rudimentary pouch
becomes part of the fourth pharyngeal pouchbecomes part of the fourth pharyngeal pouch
and helps to form the ultimopharyngeal bodyand helps to form the ultimopharyngeal body

66. Pharyngeal GroovesPharyngeal Grooves
During the fourth and fifth weeks, head andDuring the fourth and fifth weeks, head and
neck region of the human embryo exhibit fourneck region of the human embryo exhibit four
pharyngeal grooves or clefts on each sidepharyngeal grooves or clefts on each side
These grooves separate the pharyngeal archesThese grooves separate the pharyngeal arches
externallyexternally
Only first pair persists as the external acousticOnly first pair persists as the external acoustic
meatusmeatus

67. The other grooves normally obliterated withThe other grooves normally obliterated with
the cervical sinus as the neck developsthe cervical sinus as the neck develops

68. Pharyngeal MembranesPharyngeal Membranes
• The connection between the endoderm of
the pharyngeal pouchespharyngeal pouches and the ectoderm of
the pharyngeal groovespharyngeal grooves form a double layered
membrane called the pharyngeal membrane
These membranes form where the epithelia ofThese membranes form where the epithelia of
the grooves and pouches approach each otherthe grooves and pouches approach each other

69. The endoderm of the pouches and ectoderm of theThe endoderm of the pouches and ectoderm of the
grooves are soon separated by mesenchymegrooves are soon separated by mesenchyme
Only first pharyngeal membrane becomes theOnly first pharyngeal membrane becomes the
tympanic membrane, others obliteratetympanic membrane, others obliterate