1. Second week of development
; Bilaminar Embryonic Disc
Department of Anatomy
In-je University College of Medicine

2. Summary of first week Morula Zygote Blastocyst Implantation
solid ball of cells
Zygote
Blastocyst
with blastocoele cavity
Implantation

3. Stage of two Embryoblast = hypoblast + epiblast “ bilaminar germ disc”
Trophoblast = cytotrophoblast + syncytiotrophoblast
Two cavities = blastocystic cavity (yolk sac) + amniotic cavity
Two layers of extraembryonic mesoderm
somatic mesodermal layers + splanchnic mesodermal layers
2 layer 사이에 extraembryonic coelum 형성 = chorionic cavity
End of 2nd week → complete embedding
complete implantation
end of 1st week에 시작되어 end of 2nd week 에 완료됨.
Others
2 layers primary stem villi
Fetal lacuna와 maternal sinusoid의 교통
“ uteroplacental circulation” 의 확립

4. Development of the Embryoblast
6 days
7-8 days
Blastocystic cavity
Exocoelomic cavity
▶ Inner cell mass → embryoblast ; “bilaminar embryonic disc”
1) Epiblast -- layer of columnar cells adjacent to the amniotic cavity
2) Hypoblast -- layer of cuboidal cells facing the exocoelomic cavity
→ form exocoelomic membrane
Ectodermal layer
Mesodermal layer
Endodermal layer
future

6. Trophoblast ▶ Lacuna formation (9days) 9 days ▶ two layers (7 days)
Cytotrophoblast : inner layer of mononucleated cells
: form syncytiotrophoblast
Syncytiotrophoblast : derived from cytotrophoblast
: outer layer of multinucleated cells without distinct boundaries.
▶ Lacuna formation (9days)
; Blood-filled Lacunae appear in the Syncytiotrophoblast which communicate forming a network (12days) → ‘ Lacunar stage’

7. 9 days ; engulf these degenerating cells for nutrition of the embryo.
Endometrial cells
; undergo apoptosis (programmed cell death) to facilitates invasion of endometrium by the Syncytiotrophoblast.
Syncytiotrophoblast
; engulf these degenerating cells for nutrition of the embryo.
Implantation can be detected by:
1. Ultasonography.
2. hCG
(human chorionic gonadotrophin which is secreted by the Syncytiotrophoblast) by the end of 2nd week
9 days

8. Closing plug 10 days 10 days; closing plug 형성

9. Lacunar network 11~12 days Syncytiotrophoblast
; lacunae fusion ⇒ “ Lacunar network ”
; erodes the endothelial lining of maternal sinusoids.
; blood of maternal sinusoids reaches the lacunar network
⇒ Uteroplacental circulation

10. Function of Trophoblast
Immune barrier
Trophoblast secretes human chorionic gonadortopin (hCG) that helps the corpus luteum maintain the uterine lining
Endometrium → secretory phase
Decidual reaction
Lacunar network → maternal sinusoid
⇒ uteroplacental circulation

11. Two cavities 1. Amniotic cavity 2. Yolk sac Amnion Yolk sac
- protection of embryo/fetus
Yolk sac
- Early site of blood cell formation

12. Amniotic cavity 8 days a small cavity within the epiblast
appears separating embryoblast from the trophoblast
soon becomes lined by amnioblasts derived from inner cell mass
gradually increases in size and is filled with amniotic fluid

13. Yolk sac 10 days Umbilical vesicle
The blastocystic cavity becomes lined with exocoelomic membrane and is called exocoelomic cavity
The hypoblastic cells soon replace the exocoelomic membrane and the cavity is then named as the primary (primitive) yolk sac

14. At this stage the Embryonic disc
A circular bilaminar disc, that lies between the amniotic cavity and the primitive yolk sac
The epiblast forms the floor of the amniotic cavity & the hypoblast lies in the roof of the primitive yolk sac

15. Development of Extraembryonic Mesoderm
12~13 days
▶ The extraembryonic mesoderm splits into two layers
: between cytotrophoblast and exocoelomic membrane
an outer extraembryonic somatic mesoderm
an inner extraembryonic splanchnic mesoderm
extraembryonic coelom
(=chorionic cavity)
Connecting stalk
space

16. Chorion Amnion: Two layers: Wall of the yolk sac: Two layers:
12~13 days
Amnion: Two layers:
Amnioblasts
Extraembronic splanchnic mesoderm
Wall of the yolk sac: Two layers:
Endoderm
Chorion: three layers
Extraembryonic somatic mesoderm
Cytotrophoblast
Syncytiotrophoblast

17. Secondary Yolk Sac 13 days
The primary yolk sac decreases in size and becomes the secondary (definitive) yolk sac

18. Primary chorionic villi
13 day d
Primary chorionic villus

19. Implantation Definition of implantation:
; the Blastocyst penetrates the superficial (Compact) layer of the endometrium of the uterus.
Site:
; the posterior wall of uterine body near the fundus.
Time:
; begins about the 6th day after fertilization.
; completed by the 11th or 12th day.

20. ; embedding of blastocyst into uterine lining begins at 7 day
Implantation
; embedding of blastocyst into uterine lining begins at 7 day
Blastocyst - with blastocoele cavity
Trophoblast - outer layer of cells
Inner cell mass - will form embryo
Trophoblast forms
syncytiotrophoblast
- erodes into endometrium
cytotrophoblast
- carries nutrients to inner cell mass
Lacunae and primary villi formed by syncytiotrophoblast
All of these form placental tissues

21. Endometrium at the time of implantation
secretory phase
5mm in thickness by progesterone
2 step of implantation
1) contact between blastocyst & endometrium
- embryonic pole
2) infiltration between endometrial cells

22. Completion of Implantation
1주 말에 Embryonic pole부터 착상하며 2주 말에 착상 완료
착상과정; syncytium의 down growth와 proteolytic emzyme 관여
정상적 착상 장소 : body of uterus (secretory phase)
10 days : 거의 다 묻히고 ‘closing plug’가 생김
12 days: closing plug가 떨어져 나오고 complete implantation 이 일어남.
10 ~ 12 days : implantation bleeding
→ Bleeding due to increase blood flow into the lacunar space
→ Minimal in amount compared to normal menstrual flow

23. The second week of development
5 day; ZP 퇴화
6 day; Blastocyst 의 endometrium 부착
7 day; trophoblast differantiation
8 day; syncytiotrophoblast가 endometrium tissue를 부식하며 파고들어가 blastocyst가 endometrium 에 파묻힘.
; amniotic cavity 형성.
9 day; syncytiotrophoblast 안에 lacunae 출현
10 day; blastocyst가 endometrium 밑으로 가라앉음.
11~12day; lacunae network 형성
→primordial uteroplacental circulation
12~13 day; closing plug → 손상 회복
13~14 day; primary chorionic villi 발생

24. Highlights of two weeks of development
Complete implantation of blastocyst
- anterior or posterior wall of uterine corpus
Embryoblast has 2 layers(bilaminar germ disc)
- epiblast, hypoblast
Trophoblast consists of 2 layers
- cytotrophoblast, syncytiotrophoblast
2 cavities formed
- amniotic cavity, yolk sac,
extraembryonic coelomic cavity (chorionic cavity)
2 layers of extraembryonic mesoderm
- somatopleure, splanchnopleure
Start of uteroplacental circulation

25. Clinical application

26. Pregnancy test
The syncytiotrophoblast secretes human chorionic gonadotrophin hormone which prevents the degeneration of the corpus luteum. It also stimulates the production of progesterone which in turn is important in sustaining the placenta. 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.

27. Ectopic implantation ※ Abnormal implantation sites
Gastrointestinal tract
※ Abnormal implantation sites
⇒ frequency: 1/80 ~ 1/250 (1~2% in USA)
1. Tubal implantation : 95 %
2. Ovarian implantation : 3 %
3. Cervical implantation : 1 %
4. Abdominal implantation : 1 %
2) Ectopic pregnancy:
Implantation outside the uterus: the uterine tube, abdominal cavity, or ovary.
Abortion and severe hemorrhage during the 2nd month of gestation.