Fetal Membranes, Placenta and Twinning Jun Zhou (周俊） School of Medicine, ZheJiang University 20190611

OBJECTIVES The components of fetal membrane and their functions. Structure and function of placenta Composition of Placenta barrier

Fetal membrane （胎膜） Originate from blastocyst（胚泡）, don’t participate in the formation of embryo Including: 1) Chorion（绒毛膜） 2) Amnion（羊膜） 3) Yolk sac（卵黄囊） 4) Allantois（尿囊） 5)Umbilical cord（脐带）

Chorion（绒毛膜） Formed by trophoblast +extraembryonic mesoderm Chorion frondosum（丛密绒毛膜）-embryonic pole Chorion laeve（平滑绒毛膜）-abembryonic pole

Chorion Primary villi (2w): cytotrophoblast+syncytiotrophoblast Secondary villi (3w):extraembryonic mesoderm enter the primary villi Tertiary villi (end of 3w): extraembryonic mesoderm =>CT+BV

Function of Chorion Exchange of metabolite: 2) Hormone production: portion of placenta (Chorion frondosum) 2) Hormone production: human chorionic gonadotropin绒毛膜促性腺激素 (HCG)

Amnion（羊膜） Amniotic membrane: amniotic epi.+ extraembryonic mesoderm Amniotic fluid: Produce:1)amniotic cells 2) urine output from the fetus Output: 1) absorbed by amniotic cells 2) fetus swallow 1-1.5 liter – circulate

Amnion - Fluid Functions Mechanically cushion Protect from fetus adhesion Movement Maintain Temp

Abnormalities 1) too much (polyhydramnios) >1500 ml Abnormal digestive system or CNS - esophageal atresia(食管闭锁） - anencephaly（无脑儿） 2) too little (oligohydramnios) <500 ml Abnormal urinary system - poor development of kidney（肾脏发育不全） - urethra atresia（尿道闭锁）

Yolk sac and Allantois Yolk sac（卵黄囊） Allantois（尿囊） Primitive Gut 3rd week, Germ Cells 3rd to 6th week, Blood island Allantois（尿囊） Caudal extension of hindgut Allantoic A pairs Allantoic V pairs Umbilical vessels 2A+1V

Umbilical Cord（脐带） Folding – a purse string closure Amnion membrane covered Cord: mucous CT, 3 vessels,yolk sac ,allantois 50-60 cm, 2cm diameter Long (>80cm)– knots Short (<35cm)– abruption

Placenta（胎盘） - Overview Functions as: Lungs, GI tract, Liver, Kidneys, Endocrine Placenta proper: Chorion frondosum（丛密绒毛膜）+ Decidua basalis（底蜕膜）

Anatomy of the Placenta Fetal – Chorion Chorion Frondosum Chorion Laeva Maternal – Decidua Decidua Basalis（底） Decidua Capsularis（包） Decidua Parietalis（壁）

Anatomy of the Placenta At birth 500 g 15-25 cm Diameter 3 cm thick Anchoring villi（固定绒毛） Placental septa  15-20 Cotelydons（胎盘小叶）

Placental-Fetal Circulation Fetus: umbilical A carries O2/nutrient depleted blood to cap. of chorion , exchange with maternal blood of the intervillous spaceumbilical V Mother: spiral A intervillous space uterine V

Placental Barrier（胎盘屏障） --the structure between fetal and maternal blood --components: 1)Endothelium /basement membrane of chorion capillary 2) CT in the core of the villus 3) trophoblast epithelium /basement membrane

Placental Function 1.Exchange of Metabolites: nutrients antibody, waste 2.Defense barrier 3.hormone production human chorionic gonadotropin (HCG) （绒毛膜促性腺激素） Begin: end of 2nd week Highest: 2nd month Estrogen and progesterone（雌激素和孕激素） Placental lactogen（胎盘催乳素）

Twins Two types: One zygote (monozygotic) Two (or more) zygotes (dizygotic) 2 (or more) oocytes Non Identical

Twins - Monozygotic Zygote split 2-4 cell Blastocyst 2 amn, 2 chorion Blastocyst 2 amn, 1 chorion Bilaminar germ disc 1 amn, 1 chorion Incomplete splitting Conjoined twins

Conjoined Twins