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WVSOM - Embryology-2

Bilaminar Stage - Formation of Extraembryonic Layers & Cavities

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Amniocentesis Procedure where sample of amniotic fluid is withdrawn from amniotic cavity and analyzed for markers (i.e. a-fetoprotein) to detect developmental defects (ex: downs, cystic fibrosis)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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