BILAMINAR DISC (SECOND WEEK)

Presentation byMudoogo Edgar

Mudoogo Edgar

Objectives

• Definition

• Development

• Clinical correlates

Mudoogo Edgar

Defination of bilaminar

disc

• This refers to the epiblast and hypoblast evolved from the embryoblast.

• These two layers are sandwiched between two cavities(balloons) i.e. primitive York sac and amniotic cavity.

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Development

Zygote Morula Blastocyst

Trophoblast

Embryoblast (Formative mass- 60 cells)

Embryoblast transforms into 2 distinct epithelial layers just before implantation occurs;

epiblast – outer layer , has columnar cells

Hypobalst- inner layer, has cuboidal cells

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The second week of development is significant for:

1. The formation of the bilaminar disc (two-layers) – this will give rise to all the tissues and organs of the body

2. The completion of implantation

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Syncytiotrophoblast cells displace the endometrium around the implantation site

The endometrium has undergone changes.

Cells have become filled with glycogen and lipids

The nutrients spill into the connective tissue

This is called the decidua reaction

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The syncytiotrophoblast is responsible for hormone production. hCG maintains the corpus luteum in the ovary, allowing it to continue to produce P+E.

Produces P+ E tomaintain the pregnancy

Ovary Uterus

hCG

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The cells of the embryoblast will also differentiate into 2 layers:

1. The epiblast- a layer of high, columnar cells adjacent to the amniotic cavity.

2. The hypoblast- A layer of small cuboidal cells adjacent to the blastocyst cavity.

Together these layers form a flat disc.

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Amnioblasts (derived from the epiblast) separate and form the lining of the amniotic cavity.

Cells from the hypoblast form a membrane that lines the inner surface of the cytotrophoblast. This forms the exocoelomic cavity or primitive yolk sac

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The cavities allow movement of the disc

The primordial uteroplacental circulation is established

The yolk sac contains no yolk- the embryo is nourished from the lacunar networks- but may have a role in selective transfer of nutrients

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New cells appear between the yolk sac and the cytotrophoblast

They form a layer of loose connective tissue: extraembryonic mesoderm.

Cavities or spaces appear in the extra-embryonic mesoderm

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The cavities form a new space- the chorionic cavity

The primitive yolk sac is pinched off- a secondary or definitive yolk sac is formed

The cavity divides the extraembryonic mesoderm into the

1. Extraembryonic somatic mesoderm- lining trophoblast and amnion

2. Extraembryonic splanchnic mesoderm- lines the yolk sac

Mudoogo Edgar

Mudoogo Edgar

The chorion is formed by

1. Extraembryonic somatic mesoderm

2. Cytotrophoblast3. Syncytiotropho-

blastThe chorion forms

the wall of the chorionic cavity- the amniotic cavity and yolk sac are suspended in the chorionic cavity by the connecting stalk.

1

2

3

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By day 14 the embryo has the form of a flat, bilaminar disc, ovoid in shape.

In a localized area of the hypoblast, the cells become more columnar and form a thickened circle area, the prechordal plate.

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At the end of the second week:

Trophoblast has had a period of growth- greater than the embryoblast.

The 2 layer bilaminar disc is formed and will give rise to other tissues and structures.

Mudoogo Edgar

Clinical correlates

Human chorionic gonadotrophin (HCG) hormone produced by syncytiotrophoblast can be detected by medical test.

Hydatidiform mole – little or no embryonic tissue, secrete high levels of HCG producing beneign or malignant tumor (choriocarcinoma). Trophoblast is well developed.

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Clinical correlates ct’d

Implantation sites usually posterior or anterior wall of body of uterus

Abnormal implantation sites – ectopic pregnancy

o abdominal cavity- pouch of Douglaso mesenteryo ampullary region of tube

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Clinical correlates ct’d

o tubal implantation, internal os, ovarian leading to primary ovarian pregnancy

o interstitial implantation of narrow width of uterine tube

Bleeding on the day due to increased blood with in lacunae spaces.

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2nd week of development: The week of twos WHY?

Trophoblast ↗cytotrophoblast ↘syncytiotrophoblast Embryoblast ↗hypoblast ↘epiblast Extraembronic mesoderm ↗somatopleure ↘splanchnopleure Cavities ↗amniotic ↘primitive yolk sac

Mudoogo Edgar

Mwebare munonga

“JUST BECAUSE YOU KNOW THE STUFF DOESN’T MEAN YOU ARE SMART……….. YOU HAVE TO KNOW HOW TO USE THAT INFORMATION”

-JOSH KELLER-