Embryology

1. Third week of Development:
Trilaminar Germ Disc
Dr. Rabia Inam Gandapore
Assistant Professor
Head of Department Anatomy
(Dentistry-BKCD)
B.D.S (SBDC), M.Phil. Anatomy (KMU),
Dip. Implant (Sharjah, Bangkok, ACHERS) , CHPE
(KMU),CHR (KMU), Dip. Arts (Florence, Italy)

2. Teaching Methodology
 LGF (Long Group Format)
 SGF (Short Group Format)
 LGD (Long Group Discussion, Interactive discussion with the use of models or diagrams)
 SGD (Short Group)
 SDL (Self-Directed Learning)
 DSL (Directed-Self Learning)
 PBL (Problem- Based Learning)
 Online Teaching Method
 Role Play
 Demonstrations
 Laboratory
 Museum
 Library (Computed Assisted Learning or E-Learning)
 Assignments
 Video tutorial method

3. Goal/Aim (main objective)
 To help/facilitate/augment the students about the:
 Formation of three germ layers:
1. Derivatives of Ectoderm.
2. Derivatives of Mesoderm
A. Paraxial mesoderm,
B. Intermediate mesoderm,
C. Lateral plate mesoderm.
3. Derivatives of endoderm

4. Specific Learning Objectives (cognitive)
At the end of the lecture the student will able to:
 Enumerate and sketch a labeled diagram of the stages involved in the third
week of development: Trilaminar Germ Disc

5. Psychomotor Objective: (Guided response)
 A student to draw labelled diagram of third week of development

6. Affective domain
 To be able to display a good code of conduct and moral values in the class.
 To cooperate with the teacher and in groups with the colleagues.
 To demonstrate a responsible behavior in the class and be punctual, regular, attentive and
on time in the class.
 To be able to perform well in the class under the guidance and supervision of the teacher.
 Study the topic before entering the class.
 Discuss among colleagues the topic under discussion in SGDs.
 Participate in group activities and museum classes and follow the rules.
 Volunteer to participate in psychomotor activities.
 Listen to the teacher's instructions carefully and follow the guidelines.
 Ask questions in the class by raising hand and avoid creating a disturbance.
 To be able to submit all assignments on time and get your sketch logbooks checked.

7. Lesson contents
Clinical chair side question: Students will be asked if they know what is the function of
Outline:
 Activity 1 The facilitator will explain the student's Third week of development
 Activity 2 The facilitator will ask the students to make a labeled diagram of the 3rd week of
development
 Activity 3 The facilitator will ask the students a few Multiple Choice Questions related to it with
flashcards.

8.  Students assessment: MCQs, Flashcards, Diagrams labeling.
 Learning resources: Langman’s T.W. Sadler, Laiq Hussain Siddiqui, Snell Clinical Anatomy, Netter’s
Atlas, BD Chaurasia’s Human anatomy, Internet sources links.

9. Trilaminar Germ Disc
(3rd Week)
 GASTRULATION: FORMATION OF EMBRYONIC MESODERM & ENDODERM
 FORMATION OF NOTOCHORD
 TROPHOBLAST

10. Objectives
 Describe Formation of three germ layers.
 Derivatives of Ectoderm.
 Derivatives of Mesoderm
A. Paraxial mesoderm,
B. Intermediate mesoderm,
C. Lateral plate mesoderm.
 Derivatives of endoderm

11. SEQUENCE OF EVENTS IN DEVELOPMENT OF
EMBRYO
1. Fertilization
2. Cleavage
3. Gastrulation
4. Organogenesis

12. Gastrulation
 Occurs during 3rd week of gestation
 process that establishes all 3 germ
layers:
A. Ectoderm,
B. Mesoderm
C. Endoderm

13. PRIMITIVE STREAK
 Gastrulation begins with formation of PRIMITIVE
STREAK on dorsal surface of epiblast
 Forms in beginning of 3rd week
 Faint, midline groove on dorsal surface of bilaminar
embryo in caudal region.
 Cephalic end of streak, PRIMITIVE NODE, consists of
 slightly elevated area surrounding small Primitive pit.
 Primitive Groove
 Primitive Pit,
 Primitive Node
together constitute
Primitive streak

15. Gastrulation (cont’d)
 Cells of epiblast migrate towards
primitive streak & become flask-
shaped,
 detach from epiblast & slip beneath it
 This inward movement is known as
INVAGINATION.

16. Gastrulation (cont’d)
 After invagination, some cells
displace/replace hypoblast, creating
ENDODERM
 others come to lie between epiblast &
newly created endoderm to form
MESODERM
 Cells remaining in epiblast then form
ECTODERM.

17. Gastrulation (cont’d)
 Formation of intraembryonic mesoderm converts
bilaminar germ disc into trilaminar germ disc.
 Thus,
 EPIBLAST, through process of gastrulation, is
source of all of germ layers,& cells in these
layers will give rise to all of tissues & organs in
embryo.

18. Gastrulation (cont’d)
 More & more cells move between epiblast & hypoblast layers, the begin to
spread laterally & cephalad.
 Gradually, migrate beyond margin of the disc & establish contact with the
extraembryonic mesoderm covering the yolk sac & amnion.
 In cephalic direction, they pass on each side of Prechordal plate.

19. Prechordal plate
 Prechordal plate forms between tip
of notochord & buccopharyngeal
membrane
 derived from some of first cells that
migrate through primitive node, in a
cephalic direction.
 Later, prechordal plate will be
important for induction of forebrain.
 Buccopharyngeal membrane: oral
cavity

21. Formation of Notochord
 Pre-notochordal cells invaginating in
primitive pit move forward cephalad
 Form a hollow tube called notochordal
process formed by these cells
 located between prechordal plate &
primitive pit.

23.  Notochordal process remains attached to primitive pit, which adds cells to
notochordal process
 Notochordal process first splits along its ventral surface fuses with endoderm to form
a ribbon-like notochordal plate.
 This plate then detaches from endoderm & rises dorsally into embryo as it folds &
rounds up into a solid rod-shaped notochord.

25. Formation of Notochord (cont’d)
 Because elongation of notochord is a dynamic process, cranial end forms first &
caudal regions are added as primitive streak assumes a more caudal position.

26. Formation of Notochord (cont’d)
 At the point where primitive pit forms an
indentation in epiblast, the neurenteric
canal temporarily connects amniotic & yolk
sac cavities.

28. Formation of caudal mesodermal eminence
 Final event of gastrulation involves retreating
primitive streak producing a caudal midline mass of
mesoderm called caudal eminence.
 This structure gives rise to:
 Caudal mesodermal components of embryo
including pelvic viscera
 portion of neural tube caudal to 2nd sacral level.

30. Buccopharyngeal membrane
 Is at cranial end of disc
 Consists of small region of tightly adherent ectoderm & endoderm cells that
represents future opening of oral cavity.
 No intervening mesoderm.

31. Cloacal membrane
 formed at caudal end of embryonic disc.
 similar in structure to buccopharyngeal
membrane
 consists of tightly adherent ectoderm &
endoderm cells
 no intervening mesoderm.

32. Development of Allantois
 When cloacal membrane appears, the
posterior wall of yolk sac forms a small
diverticulum - allantoenteric diverticulum,
or allantois
 extends into connecting stalk.
 appears around 16th day of development.

33. Allantois (cont’d)
 In some lower vertebrates, allantois serves as
a reservoir for excretion products of renal
system
 In humans, it remains rudimentary
 may be involved in abnormalities of bladder
development.

34. Fate Map of epiblast established During
Gastrulation
 Specific areas of epiblast give rise to specific zones
of endoderm & mesoderm.
 Epiblast cells that ingress through cranial region
of node become Notochord

35. Fate map (cont’d)
 Those migrating at lateral edges of node &
from cranial end of streak become paraxial
mesoderm

36. Fate map (cont’d)
 Cells migrating through
mid streak region become
intermediate mesoderm
 Those migrating through
more caudal part of streak
form lateral plate
mesoderm
 Cells migrating through
caudal-most part of streak
contribute to extra-
embryonic mesoderm

37. Sidedness in embryo
 Motile primary cilia protruding from surface of cells of
primitive pit rotate producing a right to left movement
of fluid in primitive pit. This fluid movement either
causes the movement of vesicles toward left side that
then release their contents initiating a cascade of
molecular events on the left, or fluid deflects non-
motile primary cilia on the left initiating a cascade of
molecular events on the left.
 In both scenarios, the molecular events initiated on the
left result in an asymmetry to gene expression
patterns on the left vs the right that sets up the
sidedness of the embryo

38. Growth of Embryonic Disc
 Initially flat & almost round,
 Gradually becomes elongated, with a
broad cephalic & narrow caudal end .
 Expansion of embryonic disc occurs
mainly in cephalic region
 Region of primitive streak remains more
or less same size.
 Growth & elongation of cephalic part of
disc are caused by continuous migration of
cells from primitive streak region in a
cephalic direction.

39. Growth of the Embryonic Disc (cont’d)
 Invagination of surface cells continues until end of 4th
week.
 Has important bearing on development of embryo.
 Cephalic part, germ layers begin their specific
differentiation by middle of 3rd week,
 Caudal part, differentiation begins by end of 4th
week.
 Formation of germ layers continues in caudal
segments while cranial structures are differentiating,
causing embryo to develop cephalocaudally.
 When process of gastrulation is complete, primitive
streak disappears.

40. Changes consequent upon gastrulation
1. Gastrulation converts embryo into a trilaminar disc
2. Epiblast & hypoblast are replaced by ectoderm, endoderm & mesoderm.
3. Mesoderm does NOT extend between epiblast & hypoblast at buccopharyngeal &
cloacal membranes
4. Intra-embryonic mesoderm merges with extra-embryonic mesoderm at periphery
of embryonic disc

41. Revision of concepts

42.  Cells from primitive node proliferate &
migrate to form a midline notochord,
 Cells from primitve streak migrate laterally
& cranially between epiblast & hypoblast to
form intra-embryonic mesoderm.
 Meosderm lies on either side of
notochord, buccopharyngeal membrane &
cloacal membrane.

44. Clinical Correlations
 Teratogenesis
 beginning of 3rd week , highly sensitive stage for
teratogenic insult.
 High doses of alcohol: kill cells in anterior
midline of germ disc, produces deficiency of
midline in craniofacial structures & resulting in
holoprosencephaly.
In such a child, forebrain is small, two
lateral ventricles often merge into a single
ventricle & eyes are close together
(hypotelorism).

45. Sirenomelia (caudal dysgenesis )
 Insufficient mesoderm is formed in caudal-most region of
embryo.
 variable range of defects:
 Hypoplasia (underdevelopment) & fusion of lower limbs,
 vertebral abnormalities,
 renal agenesis,
 imperforate anus
 anomalies of genital organs.
 condition is associated with maternal diabetes & other
causes.

46. Situs inversus
 A condition in which transposition
of viscera in thorax & abdomen
occurs.

47. Tumors Associated With Gastrulation
 Remnants of primitive streak persist in sacrococcygeal region.
 These clusters of pluripotent cells proliferate & form tumors,
known as Sacro-coccygeal teratomas
 Commonly contain tissues derived from all 3 germ layers. (i.e.
may contain hair, teeth, cartilage & fat)
 most common tumor in newborns.

48. Further Development of Trophoblast
 Beginning of third week:
trophoblast is characterized by
primary villi that consist of a
cytotrophoblastic core covered by
a syncytial layer.
 During further development,
mesodermal cells penetrate core
of primary villi & grow toward
decidua. The newly formed
structure is known as secondary
villus

51. Further Development of the Trophoblast
(cont’d)
 End of third week: mesodermal cells in core of villus
begin to differentiate into blood cells & small blood
vessels, forming villous capillary system, known as
tertiary villus or definitive placental villus.
 Capillaries in tertiary villi make contact with capillaries
developing in mesoderm of chorionic plate &
connecting stalk.
 These vessels, in turn, establish contact with
intraembryonic circulatory system, connecting
placenta & embryo.
 Hence, when heart begins to beat in 4th week of
development, villous system is ready to supply the
embryo proper with essential nutrients & oxygen.

54. Further Development of the Trophoblast
(cont’d)
 Cytotrophoblastic cells in villi
penetrate into overlying syncytium
& reach maternal endometrium.
 They establish contact with similar
extensions of neighboring villous
stems, forming thin outer
CYTOTROPHOBLAST SHELL.
 This shell gradually surrounds
trophoblast entirely & attaches the
chorionic sac firmly to maternal
endometrial tissue.

55. Further Development of
Trophoblast (cont’d)
 Villi that extend from Chorionic
plate to Decidua Basalis (decidual
plate: part of endometrium where
placenta will form) are called stem
or anchoring villi.
 Those that branch from sides of
stem villi are free (terminal) villi,
through which exchange of nutrients
& other factors will occur.

58. Chorionic cavity & connecting stalk
 Chorionic cavity becomes larger & by 19th or
20th day, embryo is attached to its trophoblastic
shell by a narrow connecting stalk.
 Connecting stalk later develops into
umbilical cord, which forms connection
between placenta & embryo.

59. Thank You