Second week of development Bilaminar Embryonic Disc Department

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

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

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 2 nd week → complete embedding – complete implantation – end of 1 st week에 시작되어 end of 2 nd 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 (9 days) ; Blood-filled Lacunae appear in the Syncytiotrophoblast which communicate forming a network (12 days) → ‘ Lacunar stage’

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

Closing plug 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

Function of Trophoblast § Immune barrier § Trophoblast secretes human chorionic gonadortopin (h. CG) 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 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 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 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 u 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

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 6 th day after fertilization. ; completed by the 11 th or 12 th day.

Implantation ; embedding of blastocyst into uterine lining begins at 7 day Lacunae and primary villi formed by syncytiotrophoblast All of these form placental tissues

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

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

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 가 라앉음. 11~12 day; lacunae network 형성 →primordial uteroplacental circulation 12~13 day; closing plug → 손상 회복 13~14 day; primary chorionic villi 발생

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

v 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 2 nd 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.

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 %