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 Embryonic Cavities Trophoblasts Inner cell mass Cytotrophoblasts Syncytiotrophoblasts Uteroplacental Cir. Embryonic Cavities Amniotic cavity Yolk sac Germ Disc Epiblasts Hypoblasts

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

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

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/knot a solid rod of mesoderm Basis of the axial skeleton in the midline

Review/Background First Week Second Week Third- 8th week Gastrulation Primitive pit Primitive node Primitive streak Notochord

Expansion and Elongation Migration of Epiblasts to form Endoderm Mesoderm Ectoderm Expansion of the disk Notochord: -midline rod of mesoderm -orientation the germ disc

Differentiation of trilaminar germ disc 3rd –8th weeks embryonic period: the major body form and organ structure take Bilaminar disc: flat and roundelongated with a broad cephalic and a narrow caudal end Trilaminar disc:a pear-shaped appearance

Ectoderm - Neuralation Overlying notochord Thickens – Neural Plate Edges - Neural Folds Fuse at future neck region Zippers shut Neural Tube Ant/Post Neuropores

Ectoderm - Neuralation Overlying notochord Thickens – Neural Plate Edges - Neural Folds Fuse at neck region Zippers shut Neural Tube Ant Neuropore: 25th days Post Neuropore: 27th days

Ectoderm – Neuralation Failure of Neurotube closure Anencephaly – Ant Meningocele – post

Ectoderm – Neural Crest As neural tube closes -Neural crest Detach and migrate Melanocytes – skin Dorsal root ganglia

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.

Mesoderm Loose – condenses day 17 3 columns Paraxial mesoderm Intermediate mesoderm Lateral Plate mesoderm

Paraxial Mesoderm Segmentation Blocks of tissue along axis Somite:20th days, 3 pairs/ per day,42-44 pairs by the end of 5th weeks -Sclerotome: vertebra -Myotome:muscle of the back -Dermatome:dorsal dermis of the overlying skin

Intermediate Mesoderm Kidney and genital system “Urogenital tissue”

Lateral Plate Mesoderm Intra-embryonic coelom: →body cavity 1.Outer layer: parietal mesoderm →muscle, CT of the body wall →parietal layer of cavities 2.Inner layer: visceral mesoderm →muscle, CT of the gut →Visceral layer of cavities 3.Cavity: peritoneal, pleural and pericardial cavities mesenchyme: →CT, cardiovascular and lymph system

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

Embryonic Folding Craniocaudal folding--CNS Lateral folding—somites Endoderm-lined cavity is incorporated into the embryo properprimitive gut Yolk sacvitelline duct Oropharyngeal membrane Cloacal membrane Flat discpear, round

3-8th week

OBJECTIVES 1.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.