Fetal Membranes, Placenta and Twinning

Jun Zhou, PhD

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-embryonic pole

•Chorion laeve-abembryonic pole

Chorion• Primary villi:

cytotrophoblast+syncytiotrophoblast

• Secondary villi:extraembryonic mesoderm enter the

primary villi

• Tertiary villi: extraembryonic mesoderm =>CT+BV

Function of Chorion1) 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

•1 liter – circulate

Amnion - Fluid• Functions

– Mechanically cushion – Protect from fetus adhesion– Movement– Temp– Growth factors

• Abnormalities1) too much (polyhydramnios) >2000 ml Abnormal digestive system

or CNS2) too little (oligohydramnios) <500 ml Abnormal urinary system

Yolk sac and Allantois

• Yolk sac– Primitive Gut– Germ Cells– Blood island

• Allantois– Allantoic A pairs– Allantoic V pairs

Umbilical vessels 2A+1V

Umbilical Cord

• Folding – purse-string closure– Amnion membrane

covered– Cord: mucous CT, 3

vessels,yolk sac ,allantois

– 50-60 cm, 2cm diameter– Long – knots– Short – abruption– 2 vessel – 20-30% other

Placentation - Overview• Functions as:

– Lungs, GI tract, Liver, Kidneys, Endocrine

• Placenta proper: Chorion frondosum+ Decidua basalis

Anatomy of the Placenta

• Fetal – Chorion– Chorion Frondosum– Chorion Laeva

• Maternal – Decidua– Decidua Basalis– Decidua Capsularis– Decidua Parietalis

Anatomy of the Placenta

• Chorionic plate• Decidual plate• Anchoring villi• Decidual septa

15-20 Cotelydons• At birth 500 g• 10-20 min “after

birth” Diameter• 3 cm thick

Placental-Fetal Circulation• Fetus: umbilical A cap. Of chorion umbilical

V• Mother: spiral A chorion space uterine V

Placental Barrier

--the structure between fetal and maternal blood

--components:

1)endothelium of chorion capillary and its basal lamina

2) CT in the core of the villus

3) trophoblast epithelium and its basal lamina

Placental Function1.Exchange of Metabolites:

nutrients antibody, waste 2.Defense barrier3.hormone production- human chorionic gonadotropin (HCG) Begin: end of 2nd week Highest: 2nd month- Estrogen and progesterone- Placental lactogen

Anne Geddes

Twinning

• Abnormal for Humans• Two types –

– One zygote (monzygotic)

– Two (or more) zygotes (dizygotic)

• Most common (70%)• 2 (or more) oocytes• 2 (or more) sperm• Non Identical

Twinning - Monozygotic• Zygote split

– 2-4 cell• 2 amn, 2 chorion

– Blastocyst • 2 amn, 1 chorion

– Germ disc• 1 amn, 1 chorion

– Later • Conjoined twins

Twins

Twinning - Monozygotic• Zygote split

– 2-4 cell• 2 amn, 2 chorion

– Blastocyst • 2 amn, 1 chorion

– Germ disc• 1 amn, 1 chorion

– Later • Conjoined twins

Conjoined Twins