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| ==Introduction==
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| {| class="prettytable"
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| | [[File:Human_embryo_day_18.jpg|300px]]
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| | Gastrulation means "gut forming" and converts the inner cell mass which then formed the bilaminar embryo (epiblast, hypoblast) into the trilaminar embryo (ectoderm, mesoderm, endoderm).
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| The process involves the migration of cells from the epiblast layer through the primitive streak to form first the endoderm layer and then a second intermediate layer the mesoderm layer. Once all cells have left the epiblast layer it now becomes the ectoderm layer.
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| These three germ cell layers (ectoderm, mesoderm, endoderm) will form in a layer specific manner all the future tissues of the developing embryo.
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| '''Approximate cross-section of an 18 day human conceptus.'''
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| Identify the 3 layers of the trilaminar embryo: ectoderm (columnar cells), intraembryonic mesoderm (mesenchymal cells, endodermal cells (cuboidal single layer). Identify primitive groove with dense cluster of primitive streak cells below it.
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| (Image: Nishimura etal., 1977)
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| == Conceptus Cavities Week 2 and Week 3 ==
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| {| border='0px'
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| | <wikiflv width="214" height="220" autoplay="true" position="left">Chorion 001.flv|File:Chorion 001 icon.jpg</wikiflv>
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| | Animation shows the events following implantation and focuses on changes in the the spaces surrounding the embryonic disc, the extraembryonic coelom. The blastoceol cavity is converted into two separate spaces: the yolk sac and the chorionic cavity. The third space lies above the epiblast, the amniotic cavity.
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| '''blue''' - epiblast layer
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| '''yellow''' - hypoblast layer
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| '''red cells''' - extraembryonic mesoderm layer
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| '''green''' - trophoblast layer
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| '''red spaces''' - blood-filled spaces, maternal lacunae
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| '''white cells''' - (left) endometrial gland (right) endometrial epithelium
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| :[[Media:Chorion_001.mov|Quicktime version]]
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| == Stage 7 ==
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| | [[File:Stage7-sem2.jpg|300px]]
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| | [[File:Stage7_features.jpg]]
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| | '''Features:''' embryonic disc, primitive node, primative streak, primitive groove, yolk sac
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| '''Facts:''' Week 3, 15 - 17 days, 0.4 mm
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| '''View 1:''' embryonic disc, showing the epiblast viewed from the amniotic (dorsal) side.
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| '''Events:''' Gastrulation is continuing as cells migrate from the epiblast, continuing to form mesoderm.
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| Mesoderm lies between the ectoderm and endoderm as a continuous sheet except at the buccopharyngeal and cloacal membranes. These membranes have ectoderm and endoderm only and will lie at the rostral (head) and caudal (tail) of the gastrointestinal tract.
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| | '''Gastrulation:''' Through the '''primitive streak '''cells migrate continuously through week 3 into week 4. Initial cells replace hypoblast as an epithelial layer the '''endoderm'''. Later migrating cells spread between the two epithelial layers to form '''mesoderm'''.
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| | [[File:Stage7_primitive_streak_labelled.jpg]]
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| | [[File:Stage7 axes.jpg]]
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| | '''Axes:''' embryonic disc is shown rostral (head) to top and caudal (tail) to bottom. Left and right are the lateral margins of the disc as shown.
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| == Gastrulation ==
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| Carnegie Stage 7 and 8, gastrulation, migration of cells through the primitive streak to form endoderm and mesodermal layers of embryo.
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| [[File:Chicken-gastrulation3.jpg]]
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| [[File:Trilaminar_embryo.jpg]]
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| Scanning electron micrograph showing the early forming 3 layers: ectoderm, mesoderm and endoderm.
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| [[File:Mesoderm 001 icon.jpg|200px|link=Development_Animation_-_Mesoderm]]
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