Anhui Medical University

Anhui Medical University

General embryology II

Chao Liu Ph.D.

Department of Histology and EmbryologySchool of Basic Medical SciencesSchool of International Education

Anhui Medical University

The fetal membrane and placenta

1. chorion:2. amnion:3. yolk sac:4. Allantois5. umbilical cord

I. Fetal membrane

1. Chorion

Chorion -- made up of the syncytiotrophoblast, cytotrophobalst and extraembryonic mesoderm.

a. Chorionic plateb. Villi：primary, secondary, tertiary

Primary villi: a central core of cytotrophoblastcovered by a layer of syncytiotrophoblast.Adoining villi are separated by an intervillousspace.

cytotrophoblastsyncytiotrophoblast

intervillous space

extra-embryonic mesoderm

capillary

blood vessel

decidua

secondary villi :• An inner layer extra-embryonic mesoderm• An intermediate layer cytotrophoblast• An outer layer syncytiotrophoblast

syncytiotrophoblast

intervillous space

extra-embryonic mesoderm

capillary

blood vessel

decidua

cytotrophoblast

tertiary villi: are like secondary villi except thatextra-embryonic mesoderm differentiate intocapillaries and CT.

cytotrophoblastsyncytiotrophoblast

intervillous space

extra-embryonic mesoderm

capillary

blood vessel

decidua

• Chorion frondosum：adjacent to decidua basalis with numerous, large and branched villi, forming fetal component of placenta.

• chorion laeve：The chorion associated with the decidua capsularis, the villi of which become degenerated.

Excessive proliferation of the trophoblast may lead the hydatidiform mole with large amounts of vesicles and dead embryo.

Abnormalities

The malignant proliferation of trophoblast cells is called the chorion carcinoma.

2. AmnionThin, transparent membrane made up of an amniogenic cell layer and outer extraembryonic mesoderm layer.

Amnion

Amniotic fluid

• is a dialysate of blood, first secreted by the amniotic membrane. By 16 weeks of gestation, the fetal urine is also a major contributor to the amniotic fluid.

• about 1- 1.5 liter at full term.

Functions of amniotic fluid

• 1) provide a protective cushion and absorb jolts; • 2) allow for fetal movement and prevent adherence of

the fetus;• 3) maintain body temperature of the embryo;• 4) help to dilate and clean the cervical canal making

childbirth easier.• 5) Prenatal diagnosis: biochemical assay ,chromosome

or gene analysis.

Circulation of the amniotic fluid

•30ml/10wk → 350ml/20wk → 1L/38wk •Totally renewed every 3h

3. Yolk sac• Inner endoderm + outer extraembryonic mesoderm

Its significance in development is

• a. Some endoderm cells of it form primordial germ cells.

• b. Mesoderm cells of its wall differentiate into primitive blood cells.

4. Allantois• 1) Formation: caudal part of yolk sac →

diverticulum extending into body stalk • 2) Significance: blood vessels in its wall →

umbilical arteries and vein.

allantois-outpocketing of embryo’s gut, incorporated into umbilical cord, forms blood vessels of umbilical cord

5. Umbilical cord

• Formation: body stalk → connecting the embryo or fetus to the placenta, 50 cm longat birth.

umbilical vesselsContents: body stalk, yolk sac stalk, allantois,

2 arteries and 1 vein, mucoid C.T.

very long (＞80cm) → encircling neck or extremities very short (＜35cm) → prematurely detaching of placenta and bleeding

Abnormalities

II. Placenta

• the structure by which exchange of material between fetus and mother takes place.

• it consists of chorionic plate and basal plate.• size: round, disc-shaped, 15-20 cm in D, 2.5 cm

thickness, 500g in weight

• Components: fetal portion ← chorion frondosum • Maternal portion ← decidua basalis

fetal portion

Maternal portion

• fetal surface:smooth,covered byamnioticmembrane

• mothersurface: rough,15-30cotyledons

Chorionic plate→stem villi→cytotrophoblastic shell Basal plate→placental septa→cotyledonsFree villi + intervillous spaces (maternal blood)

Placental barrier: the structure between fetal and maternal blood.Components:

• a. Endothelium and its basal lamina of the villous capillary.

• b. Connective tissue in the core of the villus.

• c. Basal lamina of cytotrophoblast and cytotrophoblast.

• d. Syncytiotrophoblast layer.

• Exchange of gases, nutrients, metabolites between the maternal and fetal blood.

• Protection• Hormone production: all from syncytiotrophoblast

Human chorionic gonadotropin (HCG) Human placental lactogen (HPL)Progesterone and estrogen

Functions of placenta

Key points

1. The component and function of fetal

membrane.

2. The structure and function of placenta.

3. Placental barrier.

Homeworks?

1. What are the compositions of fetal

membranes?

2. Describe the structural features and

functions of Placental barrier.