Download - Bilaminar germ disc presentation
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.
Mudoogo Edgar
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
Mudoogo Edgar
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.
Mudoogo Edgar
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.
Mudoogo Edgar
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
Mudoogo Edgar
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.
Mudoogo Edgar
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-