Download Implantation

Second week of development
; Bilaminar Embryonic Disc
2012. 04.05
Department of Anatomy
In-je University College of Medicine
Summary of first week
Morula
solid ball of cells
Zygote
Implantation
Blastocyst
with blastocoele cavity
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.
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
Stage of two
1. Embryoblast = hypoblast + epiblast
“ bilaminar germ disc”
2. Trophoblast = cytotrophoblast + syncytiotrophoblast
3. Two cavities = blastocystic cavity (yolk sac) + amniotic cavity
4. Two layers of extraembryonic mesoderm
– somatic mesodermal layers + splanchnic mesodermal layers
– 2 layer 사이에 extraembryonic coelum 형성 = chorionic cavity
5. End of 2nd week → complete embedding
– complete implantation
– end of 1st week에 시작되어 end of 2nd week 에 완료됨.
6. Others
– 2 layers primary stem villi
– Fetal lacuna와 maternal sinusoid의 교통
–
“ uteroplacental circulation” 의 확립
Development of the Embryoblast
7-8 days
6 days
Blastocystic cavity
Exocoelomic cavity
▶ Inner cell mass → embryoblast ; “bilaminar embryonic disc”
1) Epiblast -- layer of columnar cells adjacent to the amniotic cavity
future
Ectodermal layer
Mesodermal layer
Endodermal layer
2) Hypoblast -- layer of cuboidal cells facing the exocoelomic cavity
→ form exocoelomic membrane
Trophoblast
9 days
▶ two layers (7 days)
1)
2)
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‟
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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.
9 days

Implantation can be detected by:
1. Ultasonography.
2. hCG
(human chorionic gonadotrophin
which is secreted by the
Syncytiotrophoblast) by the end of 2nd
week
Closing plug
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10 days
10 days; closing plug 형성
12 days; closing plug → 손상 회복
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
Decidual reaction
12 days
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
Two cavities
1. Amniotic cavity
2. Yolk sac
Amnion
- protection of embryo/fetus
Yolk sac
- Early site of blood cell formation
Amniotic cavity
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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
Yolk sac
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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
At this stage the Embryonic disc

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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
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
Connecting stalk
space
extraembryonic coelom
(=chorionic cavity)
Chorion
12~13 days
• Amnion: Two layers:
– Amnioblasts
– Extraembronic splanchnic mesoderm
• Wall of the yolk sac: Two layers:
– Endoderm
– Extraembronic splanchnic mesoderm
• Chorion: three layers
– Extraembryonic somatic mesoderm
– Cytotrophoblast
– Syncytiotrophoblast
Secondary Yolk Sac
 The primary yolk sac decreases in size
and becomes the secondary (definitive)
yolk sac
13 days
Primary chorionic villi
13 day
d
Primary chorionic villus
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
At the end of the second weeks
13~14days
•
•
•
•
Primary yolk sac replaced by secondary yolk sac
Formation of chorionic cavity or extraembryonic cavity
Development of connecting stalk (umbilical cord)
Trophoblast forms villous structures
- „Primary chorionic villi‟
• Prechordal plate develop as thickening of hypoblast (14days)
Completion of Implantation
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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
The second week of development
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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
Development of Human Embryonic Tissues
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
Clinical application
 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.
 Placenta previa
 Gestational trophoblastic diseases
Ectopic implantation
Gastrointestinal tract
※ Abnormal implantation sites
⇒ frequency: 1/80 ~ 1/250 (1~2% in USA)
1.
2.
3.
4.
Tubal implantation : 95 %
Ovarian implantation : 3 %
Cervical implantation : 1 %
Abdominal implantation : 1 %