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WVSOM - Embryology-2
Bilaminar Stage - Formation of Extraembryonic Layers & Cavities
| Question | Answer |
|---|---|
| After implantation, the trophoblast above the epiblast produces a new layer known as the ... | Syncytiotrophblast (synctytium - multiple nuclei w/i extended, continous cytoplasm) |
| Trophoblast cells that don't become part of the syncytiotrophoblast | Cytotrophoblast |
| Syncytial nuclei of syncytiotrophoblast | Amniotic (they don't divide); nuclei proliferate by contribution from cytotrophoblast |
| 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 |
| 3 layers of embryoblast | Amnion, epiblast, hypoblast |
| Amnion | Thin membrane which forms along interior of cytotrophoblast |
| Epiblast | Sheet of high columnar cells; with exception of primordial germ cells, entire fetus will develop from this layer |
| Amniotic cavity | Fluid-filled cavity developed between amnion and epiblast; fetus will grow into space |
| Amniocentesis | Procedure where sample of amniotic fluid is withdrawn from amniotic cavity and analyzed for markers (i.e. a-fetoprotein) to detect developmental defects (ex: downs, cystic fibrosis) |
| When is amniocentesis first performed? | 14th week of gestation, amniotic cavity first appears during bilaminar stage |
| Hypoblast | Sheet of small cuboidal cells between epiblast and blastocele; produces yolk sac |
| Bilaminar germ disc | Epiblast and hypoblast together; forms sheet of cells suspended between amniotic cavity and blastocele |
| When does the blastocyst completely embed in the endometrium and fibrin coagulum closes hole that was created on surface of endometrium? | Day 9 |
| Extraembryonic endoderm (exocoelomic membrane) | Hypoblast spread around inner surface of cytotrophoblast creating think membrane of flattened cells |
| Primary yolk sac (exocoelomic cavity) | Space bound by extraembryonic endoderm and hypoblast; supplanted blastocele |
| Lacunae | Spaces in syncytiotrophoblast; period referred to as lacunar stage |
| Extraembryonic mesoderm | Layer that forms between cytotrophoblast and extraembryonic endoderm; day 11-12; derived from extraembryonic endoderm; composed of loose connective tissue |
| Extraembryonic lacunae form where? | Mesoderm |
| Chorionic cavity | Expansion and coalescence of lacunae in mesoderm; form space within extraembryonic mesoderm |
| Splanchnic extraembryonic mesoderm | Layer lining yolk sac |
| Somatic | Layer lining cytotrophoblast |
| How many layers does the chorion consist of surrounding the chorionic cavity? | 3 - somatic extraembryonic mesoderm, cytotrophoblast, syncytiotrophoblast |
| Connecting stalk | Projection of extraembryonic mesoderm that remains attached to amnion, germ disc, and secondary yolk sac; develops into umbilical cord |
| Placental circulation | Begins to develop at embryonic pole, between syncytiotrophoblast of embryo and surrounding endometrium of mother |
| What produces the sinusoids? | Penetrating syncytiotrophoblast that erodes endothelial lining of maternal capillaries; network of lacunar spaces forms in syncytiotrophoblast; sinusoids & lacunae expand = continuous |
| 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 |
| Size of blastocyst at day 14 | ~0.5cm in diamter |
| 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) |
| Exocoelomic cyst (day 13) | As chorionic cavity expands, portion of primary yolk sac pinched off and vanishes |
| Name of yolk sac after exocoelomic cyst breaks off | Secondary yolk sac (yolk sac) |
| What cells contribute to yolk sac? | Hypoblastic cells contribute by migrating along splanchnic extraembryonic mesoderm |
Created by:
JaneO
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