4. Bilaminar and Trilaminar Germ Discs.pdf

4. Bilaminar and Trilaminar Germ Discs.pdf

1. Prof. Dr. Asma’ Hassan
Anatomy Unit, Faculty of Medicine
UNiSZA
Terengganu Darul Iman
Bilaminar Germ Disc
and
Trilaminar Germ Disc
1
Strictly for instructional use only

2. Objectives
•Describe the formation of amniotic cavity,
bilaminar germ disc and the yolk sac.
•Describe the development of extraembryonic
mesoderm, chorionic cavity and the
connecting stalk.
•Describe the appearance and fate of primitive
streak.
•Describe the formation of trilaminar germ disc
(gastrulation).
•Explain the formation of notochord.
2

3. stratum
functionalis
stratum
basale
Endometrium
site of cyclic
changes
Myometrium Perimetrium
Histological layers of Uterus
smooth
muscle layer
thin connective
tissue
regenerates
after pregnancy
shed off in
parturition
3
stratum
functionalis

stratum
basale

endometrium

4. Week Development in Pre-embryonic period
1 oFertilisation
oCleavage, Morula, Blastocyst
oImplantation
2

‘rule of twos’

Bilaminar
germ disc
Differentiation of Trophoblast
Bilaminar embryonic disc
Amniotic & chorionic cavities
Somatic & splanchnic extraembryonic
mesoderm
Primary & secondary yolk sacs
3

Trilaminar
germ disc

•Trilaminar disc, Notochord, Neural tube
•Intraembryonic mesoderm, Coelom
•Primitive blood vessels
•Allantois
•Secondary and tertiary chorionic villi
•Folding of embryo
4

5. Day 2 Day 3 Day 4 Day 6
Day 8 Day 9 Day 12 Day 13
Chronology of Foetal Development in the 2
nd
week of Life
Chronology of Foetal Development in the 1
st
week of Life
•Decidual
reaction occurs
•Bilaminar disc
forms
•Amniotic cavity
forms
•Zygote (2 cell)
stage
•Morula (16 cell)
stage
•Reaches
uterine cornua
•Blastocyst
stage
•Enters
uterine cavity
•Implantation
begins
•Lacunar stage
•Exocoelomic
membrane
forms
•Lacunae
formation
•Extra-embryonic
mesoderm
appears
•Extra-embryonic
coelom forms
•Implantation is
complete
•Primary chorionic
villi form
•Chorionic cavity
appears
•Uteroplacental
circulation begins

6. Week 1: Blastocyst formation
Shortly after morula enters uterus (day 4 after fertilisation),
a fluid-filled space (called blastocyst cavity) appears inside
the morula
The conceptus is now called blastocyst:
1.Outer cell mass is now called trophoblast
– forms the extra-embryonic tissues, e.g. placenta
2.Inner cell mass is now called embryoblast
– forms the embryonic tissues
Later zona pellucida (ZP) degenerates – implantation
occurs (Day 6)
embryoblast
trophoblast
ZP degenerates
blastocyst
cavity
zona pellucida

7. Maternal / Mother’s side Embryo / Baby’s side
Endometrium of uterus
Named as decidua when
the mother is pregnant
Outer layer
of conceptus
is called
trophoblast
Inner layer of
conceptus is
called
embryoblast
Conceptus
•Embryo: < 8 wks
•Foetus: > 9 wks
•Newborn: < 1 mth
afterbirth
•Infant: < 1 yr after
birth
•Placenta
•Amniotic
membrane
•Amniotic
fluid
Stratum functionalis is
shed off after parturition
Stratum basale regenerates
after parturition

8. Decidual reaction
•after implantation of an embryo in the uterus, its
endometrium layer is called decidua
•changes occur in the endometrial stromal cells which is
called decidual reaction


Endometrium layer in non-
pregnant uterus
Large decidual cells, tortuous
endometrial glands in pregnant
uterus
8

9. Week 2 – Bilaminar germ disc:
1. Differentiation of Trophoblast
2. Bilaminar embryonic disc
3. Amniotic & Chorionic cavities
4. Somatic & Splanchnic extra-embryonic
mesoderm
5. Primary & Secondary yolk sacs
9

10. 10
Bilaminar
germ disc
hypoblast
Outer cells mass
or Trophoblast
syncytiotrophoblast
cytotrophoblast
Inner cells mass
or Embryoblast
epiblast

11. Differentiation of Trophoblast:
Syncytiotrophoblast
•syncytiotrophoblast is the outer layer of trophoblast
•has a multinucleated cells with indistinct cell boundaries
•erodes the endometrium; opens up its glands & vessels
•it secretes human chorionic gonadotropin (hCG):
- to maintain the activity of corpus luteum
syncytiotrophoblast
11

12. Differentiation of Trophoblast:
Cytotrophoblast
•cytotrophoblast is the inner layer of trophoblast
•lined with simple cuboidal cells
•it is actively in mitosis
•it secretes proteolytic enzymes to break down endometrial
cells
•endometrial cells later fused with one another
•lose their cell membranes to give rise to syncytiotrophoblast
cytotrophoblast
12

13. Formation of Bilaminar Embryonic Disc
•Inner cell mass (embryoblast) splits into 2 layers:
1. Epiblast (ectoderm)
- composed of tall columnar cells
2. Hypoblast (endoderm)
- composed of cuboidal cells

•These 2 layers are in contact & form the bilaminar
embryonic disc


13

14. Epiblast: Formation of Amniotic cavity
•fluid begins to collect
between epiblast cells to
form amniotic cavity

•the inner side of cavity is
lined by epiblast cells called
amnioblasts

•amnioblasts secrete
amniotic fluid

amniotic
cavity filled
with amniotic
fluid
epiblast cells
amnioblasts
14
blastocyst
cavity

15. Hypoblast:
Formation of Primary yolk sac
•hypoblast cells begin to
migrate out, over the inner
surface of cytotrophoblast

•cells completely lined the
former blastocyst cavity

•this cells forms a thin
membrane called exocoelomic
membrane or Heuser’s
membrane

•the cavity is now called the
primary yolk sac or exo-
coelomic cavity

primary
yolk sac
cytotrophoblast
exocoelomic
membrane
hypoblast cells
15
blastocyst
cavity

16. Formation of Extra-embryonic Mesoderm (EEM)
•this layer appears between
Heuser’s membrane &
cytotrophoblast

•the mesoderm grows and
migrates to surround both:
–amniotic cavity
–primary yolk sac
(except at the connecting
stalk)

•EEM lies outside of embryonic
disc




extra-embryonic
mesoderm
cyto-
trophoblast
Heuser’s
membrane
primary
yolk sac
16

17. fluid-filled
spaces
17
Formation of Chorionic cavity
•spaces later appear in the
extra-embryonic mesoderm

•fluid begins to fill up the
spaces

•the spaces later become
confluent

•it forms extra-embryonic
coelom or chorionic cavity

•the chorionic cavity splits
the mesoderm into 2 layers

primary
yolk sac
chorionic
cavity
extra-embryonic
mesoderm

18. Splitting of Extra-embryonic Mesoderm
•the 2 layers of EEM are:
1. Somatic / parietal extra-
embryonic mesoderm
–contributes to chorion

2. Splanchnic / visceral
extra-embryonic
mesoderm
–covering amniotic cavity
and primary yolk sac

•these 2 layers are linked by
the connecting stalk


chorionic cavity
Splanchnic
EEM
Somatic
EEM
connecting stalk
18
yolk
sac

19. Formation of Secondary yolk sac
•as chorionic cavity expands,
primary yolk sac decreases
in size

•it is now called secondary
or definitive yolk sac

•splanchnic EEM covering
the embryo is connected to
somatic EEM by a
connecting stalk




secondary
yolk sac
chorionic
cavity
connecting
stalk
19
splanchnic
EEM
somatic EEM

20. Formation of Chorion
•Chorion is the outermost membrane composed of:
1)Somatic extra-embryonic mesoderm
2)Trophoblast:
i.syncytiotrophoblast
ii.cytotrophoblast




syncytiotrophoblast
chorionic
cavity
chorion
somatic extra-embryonic
mesoderm
cytotrophoblast
20

21. 21
Further development of Chorionic Cavity
•cavity expands greatly by
accumulation of fluid within it

•it becomes the dominant
cavity at this stage (< 8 wks)
compared to amniotic cavity

•chorionic cavity is lined by
somatic extra-embryonic
mesoderm

•amniotic cavity & secondary
yolk sac appear smaller at
this stage
< 8 weeks = chorionic cavity > amniotic cavity
Somatic
EEM
chorionic cavity
amniotic cavity
secondary
yolk sac

22. Further development of Amniotic cavity
•amniotic cavity later expands and
becomes larger than chorionic cavity
•> 8
th
week, it covers the entire embryo;
obliterating chorionic cavity

chorionic
cavity
amniotic
cavity
secondary yolk
sac
22
< 8 wks
> 8 wks
chorionic
cavity

23. 23
Connecting Stalk
•connecting stalk is the forerunner of umbilical cord

•it is located at the tail end of embryonic disc

•structures suspended by the connecting stalk are:
1)embryo
2)amniotic cavity
3)yolk sac
connecting
stalk

24. Trilaminar germ disc
24

25. Week Development in Pre-embryonic period
1 oFertilisation
oCleavage, Morula, Blastocyst
oImplantation
2

‘rule of twos’

Bilaminar
germ disc
Differentiation of Trophoblast
Bilaminar embryonic disc
Amniotic & chorionic cavities
Somatic & splanchnic extraembryonic
mesoderm
Primary & secondary yolk sacs
3

Trilaminar
germ disc

•Trilaminar disc, Notochord, Neural tube
•Intra-embryonic mesoderm, Coelom
•Primitive blood vessels
•Allantois
•Secondary and tertiary chorionic villi
•Folding of embryo
25

26. Prochordal plate and Cloacal plate
Prochordal plate
•a thickening of hypoblast
cells
•at cranial end of primitive
streak
•consists of columnar cells
•a primordium of
oropharyngeal membrane

Cloacal plate
•a slight thickening of
hypoblast cells
•at caudal end of
primitive streak
•adherent to overlying
epiblast layer

26
prochordal
plate
rostral end
cloacal
plate
caudal end
primitive
streak
prochordal
plate
epiblast
hypoblast

27. The Third Week: Gastrulation
•Formation of 3 germ layers

1. Primitive streak formation

2. Development of
intra-embryonic mesoderm

3. Notochord formation
from ectoderm

4. Neurulation formation from ectoderm
(including neural crest formation)

5. Further development of Mesoderm

6. Development of the Endoderm
27

28. 1. Primitive Streak Formation
•a faint groove appears on
dorsal aspect, in the midline of
embryonic germ disc

•it first appears at the caudal end

•grows cranially by addition of
cells at the caudal end

•this groove later becomes
deeper and elongates

•the groove is called the
primitive groove

•on day 16, the groove occupies
about half the length of embryo
cranial
caudal
28

29. 1. Further Development of Primitive Streak
•a deeper depression
appears at the cranial end of
groove called primitive pit

•the mound surrounding it is
the primitive node or
Hensen’s node

•the entire structure is the
primitive streak
29
cranial
caudal

30. 2. Intra-embryonic mesoderm
•epiblast cells on each side
of primitive streak begin to
proliferate

•these cells lose their
connection with each other

•they migrate through the
primitive streak between
epiblast & hypoblast to form
intra-embryonic mesoderm
30

31. 2. Intra-embryonic mesoderm
•epiblast cells also replaced
the hypoblast cells and form
the embryonic endoderm

•this process is called
invagination / ingression

•cells passed throughout the
disc except at prochordal
plate & cloacal membrane

31

32. Further Development of Primitive Streak
•epiblast cells which rest on
the midline form 2 structures:
1.prochordal plate:
- located cranial to
primitive pit
2. notochord:
- a midline tube
prochordal
plate
notochord
32

33. Outcome and Fate of Primitive Streak
•establishment of the body
axes: anteroposterior,
dorsoventral, left-right

•as gastrulation proceeds, it
regress caudally & gradually
become shorter

•by Day 22, primitive streak
represents about 10-20% of
the embryo’s length

•by day 26, it disappears

Remnant may persists to form
teratoma

33

34. Fate Map Established during Gastrulation

Part of Primitive
node/streak
Fate
Cranial region of
primitive node
prechordal plate,
notochord
Lateral edge of node
Cranial end of node
paraxial mesoderm
Mid-region of primitive
streak
lateral plate mesoderm
Most caudal region of
primitive streak
extraembryonic
mesoderm
34

35. Terminology in Primitive streak formation
•Primitive streak
- intense proliferation of epiblast
cells
- has 2 ridges on each side of
primitive streak

•Primitive node
–accumulation of cells
–most rostral region of primitive
streak

•Primitive groove
–the lower midline portion of
primitive streak

•Primitive pit - the depression
35

36. The Third Week: Gastrulation
•Formation of 3 germ layers

1. Primitive streak formation

2. Development of
intra-embryonic mesoderm

3. Notochord formation
from ectoderm

4. Neurulation formation from ectoderm
(including neural crest formation)

5. Further development of Mesoderm

6. Development of the Endoderm
36

37. 3. Notochord Formation
•cells grow rostrally from the
primitive node beneath the
ectoderm

•this is the notochordal process

•cells are add on to its cranially
until it reaches the prochordal
plate

•As the primitive streak recedes,
the primitive node also moves
caudally and notochoral process
increases in length

•it forms a hollow tube called
notochordal canal





37

38. 3. Notochord Formation
•the floor of the canal &
underlying endoderm
break down

•the roof of canal persists
as notochordal plate

•a transient opening
communicates the yolk
sac with amniotic cavity
called neurenteric canal





Neurenteric
canal
38

39. 3. Notochord Formation
•notochordal plate cells
proliferate & detach from
endoderm

•notochordal plate will then
curved and form a solid rod
called definitive
notochord

•definitive notochord plays
an important role in the
induction of vertebral
bodies




39
definitive
notochord

40. References
1.Sadler, T W. Langman’s Medical Embryology
2.Moore, Persaud & Torchia, The Developing
Human. Clinically oriented Embryology
3.Larsen, Human Embryology