Embryology

1. Second Week of Development:
Bilaminar 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:
 Describe the formation of amniotic cavity.
 Discuss the development of embryonic disc
 Explain the development of Chorionic sac.

4. Specific Learning Objectives (cognitive)
At the end of the lecture the student will able to:
 Enlist in tabulated form steps of First week vs. Second week of Development
in tabulated or flowchart format or diagrammatic representation
 Sketch labeled diagram of the different stages of Second week of
Development

5. Psychomotor Objective: (Guided response)
 A student to draw labelled diagram of the different stages of Second 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 Second Week of Development in detail
 Activity 2 The facilitator will ask the students to make a labeled diagram of the different stages
of Second 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. Second Week of
Development
 BILAMINAR GERM DISC (DAY 8 TO DAY DAY 13)

10. Objectives
 Describe the formation of amniotic cavity.
 Discuss the development of embryonic disc
 Explain the development of Chorionic sac.

14. Day 8
 Blastocyst is partially embedded in endometrial
stroma
 Area over embryoblast i.e. Trophoblast
differentiates into:
1. Cytotrophoblast (Inner & mononucleated cells)
2. Syncytiotrophoblast (Outer & multinucleated
cells)
 Mononucleated Cells of cytotrophoblast divide &
migrate externally. They lose their cell membranes to
form multinucleated syncytiotrophoblast (without
distinct cell boundaries)
 Syncytiotrophoblast erodes the maternal tissues,
so lacunae filled with maternal blood surround
columns of syncytiotrophoblast

16. Bilaminar Germ Disc
 Inner cell mass is
differentiated into:
a- Hypoblast (cuboidal cells
close to blastocyst cavity)
b- Epiblast (Columnar cells
close to amniotic cavity)
 Both layers will form
Bilaminar germ disc.

18. Day 8 (cont’d)
 Small cavity appears within epiblast
– Amniotic cavity
 Epiblast cells adjacent to
cytotrophoblast are called
Amnioblasts
 Epiblast & amnioblast cells
together line the amniotic cavity
Hypoblast
Epiblast

19. Day 9
 Blastocyst is more deeply embedded in
endometrium
 Fibrin coagulum: closes penetration defect in
surface eipthelium
 Vacuoles: appear in trophoblast at embryonic pole
 Coalesce to form lacunae called Lacunar stage
(trophoblastic lacunae formed)
 At abembryonic pole, flattened cells probably
originating from hypoblast cells form Exocoelomic
(Heuser’s) membrane which lines inner surface of
cytotrophoblast
 Heuser membrane & hypoblast form lining of
Exocoelomic cavity or primitive yolk sac

23. Days 11 And 12
 Blastocyst is completely embedded in endometrial
stroma
 Surface epithelium covers original defect in
uterine wall
 Lacunar spaces in syncytium which form
intercommunicating network (esp at embryonic
pole)
 Syncytiotrophoblast penetrate deeper & erode
the endothelial lining of maternal capillaries which
are congested & dilated – sinusoids
 Maternal blood begins to flow through trophoblastic
system - Uteroplacental circulation becomes
established

24. Days 11 And 12 (cont’d)
 Formation Of Extra-embryonic Mesoderm &
Extraembryonic Coelom
-Cells appears between: inner surface of
cytotrophoblast & outer surface of yolk sac cavity
a. These cells are derived from: yolk sac cells &
form loose CT extra-embryonic (primary)
mesoderm.
b. They fill space between: trophoblasts, amnion
& yolk sac cavities.

27. Days 11 And 12 (cont’d)
 Large cavities develop in the
extraembryonic mesoderm &
coalesce to form extraembryonic
coelom or chorionic cavity.
 It surrounds primitive yolk sac &
amniotic cavity except where germ
disc is connected to trophoblasts by
connecting stalk or body stalk

28.  Extraembryonic somatopleuric mesoderm:
Extraembryonic mesoderm lining cytotrophoblast
& amnion
 Extraembryonic splanchnopleuric mesoderm:
Lining covering yolk sac

29.  Extraembryonic somatic
mesoderm, cytotrophoblast,
and syncytiotrophoblast
constitute the chorion.
 Growth of bilaminar germ
disc is very slow (0.1-0.2 mm)
compared with trophoblast

30. Decidua Reaction
 After implantation a generalized
reaction occurs characterized by
change of endometrial glands which
become hypersecretory whereas the
stroma becomes edematous
 Stromal cells gradually becomes
enlarged & filled with glycogen & lipids
(Appear eosinophilic as their cytoplasm
takes up pink stain due to presence of
numerous mitochondria and
intermediate filaments)

31. Day 13
 Surface defect in endometrium is
healed
 Occasional bleeding at implantation
site due to increased blood flow in
lacunae – confusion with normal
menses as it occurs near 28th day of
menstrual cycle, Hence, Inaccuracies
with determining Expected Delivery
Date

32. Day 13 (cont’d)
 Trophoblast is characterized by villous
structures
 Primary Villi (cyctrophoblast form cellular
columns surrounded by syncytioyrophoblast)
 Secondary or definite yolk sac
 Exocoelomic cysts
 Chorionic cavity
 Connecting stalk
 Umblical cord

34. Day 13 (cont’d)
 Cells of hypoblast proliferate to form
additional cells that migrate along inside of
exocoelomic membrane - form Secondary
or definite yolk sac – smaller than primitive
yolk sac (Exocoelomic cavity)
 Through this process, large portions of
primitive yolk sac cavities are pinched off -
Exocoelomic cysts which then lie in
chorionic cavity or Extraembryonic
coelom

36. Day 13 (cont’d)
 Extraembryonic coelom expands to form large
cavity - Chorionic cavity
 Only portion where extraembryonic
mesoderm traverses chorionic cavity is in the
Connecting stalk
 With development of blood vessels, the stalk
becomes Umbilical cord

38. The Second Week Of Development Is
The Week Of Twos
 Trophoblast differentiates into 2 layers,
 cytotrophoblast &
 syncytiotrophoblast
 Inner cell mass differentiates into 2 layers,
 epiblast &
 hypoblast.
 Primary/extraembryonic mesoderm splits into:
 somatopleuric primary mesoderm
 splanchnopleuric primary mesoderm.
 Starting of formation of amniotic & yolk sac cavities.

39. Clinical correlates
 Syncytiotrophoblast secretes human chorionic
gonadotrophin (hCG) hormone which prevents
the degeneration of corpus luteum.
 It stimulates production of progesterone which
helps in sustaining placenta.
 By end of 2nd week: amount of hCG will be
sufficient to be detected in maternal blood &
urine. This is basis of pregnancy test.

40. Clinical correlates: Abnormal Implantation

41. Clinical correlates
 About 35% of blastocysts are abnormal & hence
get aborted with next menstrual cycle without
producing signs of pregnancy
 Sometimes only trophoblast tissue is formed
(forming placental membrane) and little or no
embryonic tissue – Hydatidiform mole
 Benign tumor or
 Malignant tumor (choriocarcinoma/invasive mole)
Most moles arise from fertilization of an oocyte lacking
a nucleus followed by duplication of paternal
chromosomes to restore diploid number – hence
paternal genes regulate development of trophoblast

42. Assignment
 Make diagrammatic representation of
stages of embryonic weeks of development
(Sample on Right side)

43. Thank You