SECOND WEEK OF DEVELOPMENT (BILAMINAR

DISC

1. The trophoblast differentiates into cytotrophoblast & syncytiotrophoblast.

Cells of the cytotrophoblast divide and migrate externally. They lose their cell membranes to form the syncytiotrophoblast.

2. The syncytiotrophoblast erodes the maternal tissues, so lacunae filled with maternal blood surround columns of syncytiotrophoblast.

The syncytiotrophoblast erodes the endothelium of maternal blood forming sinusoids.Lacnunae become continuous with sinusoids forming uteroplacental circulation.

3. These columns of syncytiotrophoblast will form primary villi.

1- FURTHER DEVELOPMENT OF THE TROPHOBLAST

1. The inner cell mass is differentiated into: Epiblast (tall columnar) and hypoblast (cuboidal). Both layers will form the bilaminar germ disc.

2. Formation of cavities:• Clefts develop within the epiblast

and coalesce to form the amniotic cavity.

• The epiblast (future ectoderm) will surround the amniotic cavity so forming the amniotic membrane (amnioblast).

2- FURTHER DEVELOPMENT OF THE EMBRYOBLAST

• The hypoblast cells migrate and line the inner surface of the cytotrophoblast so forming the exocoelomic membrane which limits a space called the exocoelomic cavity (primary or primitive yolk sac).

BILAMINAR EMBRYONIC DISC

• A new layer develops consisting of loosely arranged cells derived from the yolk sac cells.

• This layer fills the space between the exocoelomic membrane and the cytotrophoblast.

• Large spaces develop in the extraembryonic mesoderm and coalesce to form the extraembryonic coelom.

3- DEVELOPMENT OF EXTRAEMBRYONIC MESODERM

EXTRAEMBRYONIC MEMBRANES& EXTRAEMBRYONIC MESODERM

• The extraembryonic coelom divides the extraembryonic mesoderm into the extraembryonic somatic mesoderm (lines the trophoblast, and covers the amnion) and extraembryonic visceral mesoderm (covers the yolk sac).

• Extraembryonic coelom surrounds primitive yolk sac & amniotic cavity except between germ disc & trophoblast (connecting stalk). First connected to amniotic sac then shifted to the tail of embryonic disc.

• Extraembryonic somatic mesoderm, cytotrophoblast, and syncytiotrophoblast constitute the chorion.

3- DEVELOPMENT OF EXTRAEMBRYONIC MESODERM

Clinical application• The syncytiotrophoblast secretes human

chorionic gonadotrophin hormone which prevents the degeneration of the corpus luteum. It also stimulates the production of progesterone which in turn is important in sustaining the placenta. By the end of the 2nd week, the amount of this hormone will be sufficient to be detected in the maternal blood and urine. This is the basis of pregnancy test.

• The trophoblast differentiates into 2 layers, cytotrophoblast & syncytiotrophoblast

• The inner cell mass differentiates into 2 layers, epiblast & hypoblast.

• The primary mesoderm splits into somatopleuric primary mesoderm & splanchnopleuric primary mesoderm.

THE SECOND WEEK OF DEVELOPMENT IS THE WEEK OF TWOS, BECAUSE OF THE FOLLOWING:

Chorion

• Chorion frondosum• Chorion laeve

ABNORMALITIES OF CHORION

• Hydatiform mole• Choriocarcinoma• Placenta accreta

• According to function:

1. 1- Anchoring villi

2. 2. Free villi

• According to structure:

1. Primary villi

2. Secondry villi

3. Tertiary villi

TYPES OF CHORIONIC VILLI

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