Fetal Membranes, Placenta and Twinning

Jun Zhou（周俊）

School of Medicine, ZheJiang University

20140106

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 villiWeek 2 to week 3 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

•30 ml--- 10 weeks

•450 ml--- 20 weeks

•800-1000 ml --- 37 weeks---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 At birth, 50-60 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 15-25 cm Diameter 3 cm thick Anchoring villi Decidual septa

15-20 Cotyledons

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 barrier

3.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) oocytesNon Identical

Twins - Monozygotic

Zygote split 2-4 cell

2 amn, 2 chorion

Blastocyst 2 amn, 1 chorion

(most common)

Bilaminar germ disc1 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