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

Fetal membrane — overview Originate from blastocyst, don’t participate in the formation of embryo Including: 1) Chorion 2) Amnion 3) Yolk sac 4) Allantois 5)Umbilical cord

Chorion Formed by trophoblast +extraembryonic mesoderm Chorion frondosum (bushy chorion)- embryonic pole Chorion laeve (smooth chorion)- abembryonic pole

Development of villi Week 2 to week 3  Primary villi: cytotrophoblast+syncytiotrophoblast  Secondary villi: extraembryonic mesoderm enter the primary villi  Tertiary villi: extraembryonic mesoderm =>CT+BV

Function of Chorion 1)Exchange of metabolite: portion of placenta (Chorion frondosum) 2) Hormone production: human chorionic gonadotropin (HCG)

Amnion Amniotic membrane : amniotic epi.+ extraembryonic mesoderm Amniotic fluid : Produce:1)amniotic cells 2) infusion of fluid from maternal blood 3) urine output from the fetus 4) pulmonary secretions Output: 1) absorbed by amniotic cells 2) fetus swallow 30 ml weeks 450 ml weeks ml weeks---circulate

Amnion - Fluid  Functions  Mechanically cushion  Protect from fetus adhesion  Movement  Maintain Temp  Abnormalities 1) too much (polyhydramnios) >2000 ml Abnormal digestive system or CNS - esophageal atresia - anencephaly 2) too little (oligohydramnios) <500 ml Abnormal urinary system - poor development of kidney - urethra atresia

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

Umbilical Cord  Folding – a purse string closure  Amnion membrane covered  Cord: mucous CT, 3 vessels,yolk sac,allantois  At birth, cm, 2cm diameter  Long – knots  Short – placenta detachment

Placenta - Overview  Functions as:  Lungs, GI tract, Liver, Kidneys, Endocrine  Placenta proper: Chorion frondosum (fetus)+ Decidua basalis (mother)

Anatomy of the Placenta  Fetal – Chorion  Chorion Frondosum  Chorion Laeva  Maternal – Decidua  Decidua Basalis  Decidua Capsularis  Decidua Parietalis

Anatomy of the Placenta  At birth 500 g  cm Diameter  3 cm thick  Anchoring villi  Decidual septa  Cotyledons

Placental-Fetal Circulation  Fetus: umbilical A  carries O 2 /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 --the structure between fetal and maternal blood --components: 1)endothelium of chorion capillary 2) CT in the core of the villus 3) trophoblast epithelium

Placental Function 1.Exchange of Metabolites: nutrients antibody, waste 2.Defense barrier 3.hormone production  human chorionic gonadotropin (HCG) Begin: end of 2 nd week Highest: 2 nd month  Estrogen and progesterone  Placental lactogen

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

Twins - Monozygotic  Zygote split  2-4 cell  2 amn, 2 chorion  Blastocyst  2 amn, 1 chorion (most common)  Bilaminar germ disc  1 amn, 1 chorion (rarely happen)  Incomplete splitting  Conjoined twins

Conjoined Twins

OBJECTIVES 1.The components of fetal membrane and their functions. 2.Structure and function of placenta 3.Composition of Placenta barrier