Bilaminar Stage - Formation of Extraembryonic Layers & Cavities
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| After implantation, the trophoblast above the epiblast produces a new layer known as the ... | Syncytiotrophblast (synctytium - multiple nuclei w/i extended, continous cytoplasm)
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| Trophoblast cells that don't become part of the syncytiotrophoblast | Cytotrophoblast
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| Syncytial nuclei of syncytiotrophoblast | Amniotic (they don't divide); nuclei proliferate by contribution from cytotrophoblast
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| What causes the syncytiotrophoblast to expand? | As cytotrophoblast cells above epiblast divide, they often fuse with synctiotrophoblast to donate nuclei; syncytiotrophoblast expands; penetrates endometrial stroma, drawing blastocyst behind it
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| 3 layers of embryoblast | Amnion, epiblast, hypoblast
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| Amnion | Thin membrane which forms along interior of cytotrophoblast
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| Epiblast | Sheet of high columnar cells; with exception of primordial germ cells, entire fetus will develop from this layer
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| Amniotic cavity | Fluid-filled cavity developed between amnion and epiblast; fetus will grow into space
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| Amniocentesis | Procedure where sample of amniotic fluid is withdrawn from amniotic cavity and analyzed for markers (i.e. a-fetoprotein) to detect developmental defects (ex: downs, cystic fibrosis)
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| When is amniocentesis first performed? | 14th week of gestation, amniotic cavity first appears during bilaminar stage
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| Hypoblast | Sheet of small cuboidal cells between epiblast and blastocele; produces yolk sac
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| Bilaminar germ disc | Epiblast and hypoblast together; forms sheet of cells suspended between amniotic cavity and blastocele
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| When does the blastocyst completely embed in the endometrium and fibrin coagulum closes hole that was created on surface of endometrium? | Day 9
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| Extraembryonic endoderm (exocoelomic membrane) | Hypoblast spread around inner surface of cytotrophoblast creating think membrane of flattened cells
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| Primary yolk sac (exocoelomic cavity) | Space bound by extraembryonic endoderm and hypoblast; supplanted blastocele
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| Lacunae | Spaces in syncytiotrophoblast; period referred to as lacunar stage
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| Extraembryonic mesoderm | Layer that forms between cytotrophoblast and extraembryonic endoderm; day 11-12; derived from extraembryonic endoderm; composed of loose connective tissue
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| Extraembryonic lacunae form where? | Mesoderm
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| Chorionic cavity | Expansion and coalescence of lacunae in mesoderm; form space within extraembryonic mesoderm
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| Splanchnic extraembryonic mesoderm | Layer lining yolk sac
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| Somatic | Layer lining cytotrophoblast
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| How many layers does the chorion consist of surrounding the chorionic cavity? | 3 - somatic extraembryonic mesoderm, cytotrophoblast, syncytiotrophoblast
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| Connecting stalk | Projection of extraembryonic mesoderm that remains attached to amnion, germ disc, and secondary yolk sac; develops into umbilical cord
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| Placental circulation | Begins to develop at embryonic pole, between syncytiotrophoblast of embryo and surrounding endometrium of mother
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| What produces the sinusoids? | Penetrating syncytiotrophoblast that erodes endothelial lining of maternal capillaries; network of lacunar spaces forms in syncytiotrophoblast; sinusoids & lacunae expand = continuous
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| Decidua reaction | Glandular secretions cause endometrium surrounding embryo to fill w/ glycogen & lipids; decidua is region of endometrium that will develop into maternal component of placenta
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| Size of blastocyst at day 14 | ~0.5cm in diamter
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| Reason for delay in diagnosis of pregnancy | Increased blood flow in lacunar space = bleeding; corresponds with 28th day of menstrual cycle (confused for menstrual bleeding)
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| Exocoelomic cyst (day 13) | As chorionic cavity expands, portion of primary yolk sac pinched off and vanishes
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| Name of yolk sac after exocoelomic cyst breaks off | Secondary yolk sac (yolk sac)
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| What cells contribute to yolk sac? | Hypoblastic cells contribute by migrating along splanchnic extraembryonic mesoderm
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Created by:
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