SECOND WEEK OF DEVELOPMENT BILAMINAR GERM DISC

What main processes occur on day 8 of development?

1. Implantation progresses

2. Trophoblast differentiates into two layers (cytotrophoblast, syncytiotrophoblast)

3. Formation of the bilaminar germ disc (hypoblast, epiblast)

4. Development of the amniotic cavity

The bilaminar germ disc is from the inner cell mass (embryblast), which differentiates into two layers:

Hypoblast

Epiblast

In the amniotic cavity, the epiblasts adjacent to the cytotrophoblast are called:

amnioblasts

This layer of the trophoblast cells that remain cellular and proliferative.

cytotrophoblast

This layer is the outer layer of the trophoblast that is multinucleated and invasive, responsible for embedding the embryo into the endometrium.

Syncytiotrophoblast

What is the function of the syncytiotrophoblast?

Produces enzymes that erode maternalion tissues, allowing deeper implantation and the establishment of nutrient exchange

This is the upper layer of the bilaminar germ disc, composed of columnar cells

Epiblast

This is the lower layer of the bilaminar germ disc, composed of cuboidal cells.

hypoblast

What is the function of the epiblast and hypoblast?

Give rise to tissues of the embryo and its support structures

On day 8, the amniotic cavity will begin to form. the small cavity begins to form in the:

epiblast

Cells that are derived from the epiblast that line the amniotic cavity.

Amnioblasts

What are the main processes on day 9 of development?

1. Embryo further embeds into the endometrium

2. Lacunae (lake) formation begins

3. Development of the yolk sac

During lacunae formation, vacuoles appear in the:

Syncytiotrophoblast

During the formation of the lacunae, the spaces will begin to coalesce. These spaces will fuse to form larger lacunae. The lacunae will then:

fill with maternal blood, starting early nutrient and gas exchange

This is a membrane of flattened cells, forming the thin membrane that live the blastocyst cavity (yolk sac development)

Exocoelomic membrane

What two things occur on day 9 when the yolk sac is developming?

Formation of the exocoelomic membrane

Formation of primitive yolk sac

What main processes occur on day 11-12 of development?

1. Complete embedding of the blastocyst

2. Establishment of uteroplacental circulation

3. Formation of extra embryonic mesoderm

What steps occur in completing the embedding of the blastocyst (11 and 12)?

1. Syncytiotrophoblast surround embryo, ensure deep embedding/nutrient exchange

2. Maternal blood flow through lacunae (primitive uteroplacental circulation)

3. Ensures nutrient and oxygen supply to developing embryo

What are the steps of the formation of the extraembryonic mesoderm?

1. Cell layer forms between the cytotrophoblast and primitive yolk sac

2. Extraembryonic mesodem supports the embryo and contributes cavity formation

(visceral and parietal mesoderm can be seen)

What is the function of the extraembryonic mesoderm?

Support the embryo and contributes to chorionic cavity formation

What are the steps in the establishment of uteroplacental circulation?

1. Materal blood fills the lacunae, nutrient exchange system

2. Primitive circulation begins to form, supporting embryo development.

(vital for embryo survival and growth)

What processes occur on day 13 in development?

1. Development of the secondary yolk sac

2. Formation of the chorionic cavity

3. Development of primary villi

What are the steps in the development of the secondary yolk sac?

1. Hypoblast cells migrate along inside the exocoelomic membrane, forming the secondary yolk sac

2. Replacement of the primitive yolk sac

3. Provides continued nutrition to the embryo

What are the steps of the formation of the chorionic cavity?

1. Cavities appear in the extraembryonic mesoderm, forming the chorionic cavity

2. Chorionic cavity surrounds the embryo, separating it from the outer layer

What are the steps of the development of the primary villi (day 13)?

1. The cytotrophoblast cells will proliferate

2. These cells will invade the syncytiotrophoblast to form primary villi

3. Primary villi formation is the initial stages of the placental villi

(crucial for establishing the placenta and nutrient exchange)

What are the steps of the expansion of extraembryonic mesoderm?

1. Extraembryonic mesoderm surrounds key structures (amniotic cavity and yolk sac)

2. Clearly see seen as two layers (somatic/visceral and splanchnic/parietal)

Abnormal implantation can occur where:

1. Abdominal cavity (peritoneum)

2. Ampullary region of the tube

3. Tubal implantation

4. Interstitial implantation

5. Region of the internal os (placenta previa)

6. Ovarian implantation

If implantation occurs in the abdominal cavity, where does it most commonly occur?

Peritoneal lining of rectouterine cavity (pouch of Douglas)