third to eighth week: the embryonic period
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Third to Eighth Week: The Embryonic Period. Jun Zhou (周俊) School of Medicine, Zhejiang University 20130108. Review. End of the Week of Twos. 2 major cell masses Trophoblasts Inner cell mass Trophoblasts Cytotrophoblasts Syncytiotrophoblasts Uteroplacental Cir. - PowerPoint PPT PresentationTRANSCRIPT
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Third to Eighth Week: The Embryonic Period
Jun Zhou (周俊)
School of Medicine, Zhejiang University20130108
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Review
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End of the Week of Twos•2 major cell masses
–Trophoblasts –Inner cell mass
•Trophoblasts–Cytotrophoblasts–Syncytiotrophoblasts Uteroplacental Cir.
•Embryonic Cavities–Amniotic cavity–Yolk sac
•Germ Disc–Epiblasts–Hypoblasts
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Formation of trilaminar disc•Early of three weeks•Primitive streak: caudal end, a midline indentation on the epiblast surface•Primitive node/knot•Primitive pit•Proliferation & Invagination of epiblasts
Intra-embryonic mesoderm
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Gastrulation•Migrating epiblasts will form:–Endoderm: migrated cells displace hypoblast cells–Mesoderm: cells btw the endo and ecto–Ectoderm
(Note - epiblasts=>all layers)•Mesoderm cranial and lateral extension
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Distribution of intra-embryonic mesoderm
-spread laterally and cranially-Fused membranes
Oral: oropharyngeal membrane Cloacal: cloacal membrane
Formation of notochord- Invaginate in through primitive pit/k
not- a solid rod of mesoderm- Basis of the axial skeleton in the midl
ine
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Review/Background•First Week•Second Week•Third- 8th week
–Gastrulation•Primitive pit•Primitive node•Primitive streak•Notochord
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Expansion and Elongation
•Migration of Epiblasts to form
–Endoderm–Mesoderm–Ectoderm
•Expansion of the disk•Notochord: -midline rod of mesoderm
-orientation the germ disc
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Differentiation of trilaminar germ disc
3rd –8th weeks
embryonic period: the major body form and organ structure take
Bilaminar disc: flat and roundelongated with a broad cephalic and a narrow caudal end
Trilaminar disc:a pear-shaped appearance
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Ectoderm - Neuralation
•Overlying notochordThickens – Neural Plate•Edges - Neural Folds•Fuse at future neck region•Zippers shut
–Neural Tube
•Ant/Post Neuropores
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Ectoderm - Neuralation•Overlying notochordThickens – Neural Plate•Edges - Neural Folds•Fuse at neck region•Zippers shut
–Neural Tube•Ant Neuropore: 25th days
•Post Neuropore: 27th days
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Ectoderm – Neuralation
•Failure of Neurotube closure
–Anencephaly – Ant–Meningocele – post
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Ectoderm – Neural Crest
•As neural tube closes -Neural crest
•Detach and migrate–Melanocytes – skin–Dorsal root ganglia
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Differentiation of the Ectoderm :-the central nervous system (CNS)-Neural crest cells (melanocyte, PNS)
In general terms it may be stated that the ectoderm gives rise to those organs and structures that maintain contact with the outside world.
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Mesoderm
•Loose – condenses day 17
•3 columns
–Paraxial mesoderm
–Intermediate mesoderm
–Lateral Plate mesoderm
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Paraxial Mesoderm •Segmentation•Blocks of tissue along axis•Somite:20th days, 3 pairs/ per day,42-44 pairs by the end of 5t
h weeks
-Sclerotome: vertebra
-Myotome:muscle of the back
-Dermatome:dorsal dermis of the overlying skin
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Intermediate Mesoderm•Kidney and genital system “Urogenital tissue”
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•Intra-embryonic coelom: →body cavity1.Outer layer: parietal mesoderm→muscle, CT of the body wall→parietal layer of cavities2.Inner layer: visceral mesoderm →muscle, CT of the gut→Visceral layer of cavities3.Cavity: peritoneal, pleural and pericardial cavities
Lateral Plate Mesoderm
mesenchyme: →CT, cardiovascular and lymph system
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Endoderm•From the Yolk sac•Forms Primitive gut•Besides the GI system
-Foregut: thymus,thyroid,lung,liver,
pancreas, etc.
-Midgut: none
-Hindgut: bladder,urethra
•Surrounded by visceral lateral plate mesoderm
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Embryonic Folding
•Craniocaudal folding--CNS•Lateral folding—somites•Endoderm-lined cavity is incorporated into the embryo properprimitive gut•Yolk sacvitelline duct•Oropharyngeal membrane•Cloacal membrane•Flat discpear, round
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3-8th week
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OBJECTIVES1.Bilaminar and trilaminar germ discs
2.Undertand the origin of the three embryonic “germ” layers that make up the trilaminar disk.
3.Describe the formation and derivatives of ectoderm, mesoderm and endoderm.
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