Fetal Membranes, Placenta and Twinning

OBJECTIVES

1. The components of fetal membrane and their functions.

2. Structure and function of placenta

3. Composition of Placenta barrier

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• Primary villi: cytotrophoblast+syncytiotrophoblast

• Secondary villi: extraembryonic mesoderm enter the

primary villi

• Tertiary villi: extraembryonic mesoderm =>CT+BV

Chorion frondosum-embryonic pole Chorion laeve-abembryonic pole

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– Maintain Temp

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

or CNS - esophageal atresia - anencephaly2) too little (oligohydramnios) <500 ml Abnormal urinary system - poor development of

kidney - urethra atresia

Yolk sac and Allantois

• Yolk sac– Primitive Gut– 3rd week, Germ Cells– 3rd to 6th 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

– 50-60 cm, 2cm diameter

– Long – knots– Short – abruption

Placenta - 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

• At birth 500 g• 15-25 cm Diameter• 3 cm thick• Anchoring villi• Decidual septa

15-20 Cotelydons

Placental-Fetal Circulation• Fetus: umbilical A carries O2/nutrient depleted blood

to cap. of chorion , exchange with maternal blood of the intervillous spaceumbilical 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 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

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

– Bilaminar germ disc• 1 amn, 1 chorion

– Incomplete splitting • Conjoined twins