人胚胎早期发生II Trilaminar Germ Disc (Third Week of Development) 周俊 浙江大学医学院 20170612

OBJECTIVES 1.Bilaminar and trilaminar germ discs 2.Describe the formation and derivatives of ectoderm, mesoderm and endoderm. 3.The components of fetal membrane and their functions. 4.Structure and function of placenta 5.Composition of Placenta barrier

End of the Week of Twos 2 major cell masses Embryonic Cavities Trophoblasts Inner cell mass Cytotrophoblasts Syncytiotrophoblasts Embryonic Cavities Amniotic cavity Yolk sac Germ Disc Epiblasts Hypoblasts

Formation of trilaminar disc Early of three weeks Primitive streak（原条）: caudal end, a midline indentation on the epiblast surface Primitive node/knot（原结） Primitive pit（原凹） 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 Fused membranes Oral: oropharyngeal membrane（口咽膜） Cloacal: cloacal membrane（泄殖腔膜）

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

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 （外胚层）- Neuralation Overlying notochord Thickens – Neural Plate （神经板） Edges - Neural Folds （神经褶） Fuse at future neck region Zippers shut Neural Tube（神经管） Ant/Post Neuropores （前/后神经孔）

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 Ant Anencephaly （无脑儿） post Meningocoele（脊膜膨出）

Ectoderm – Neural Crest As neural tube closes -Neural crest （神经嵴） Detach and migrate PNS Melanocytes – skin 肾上腺髓质

Differentiation of the Ectoderm : 神经管：CNS，神经垂体 神经嵴：PNS，melanocyte,肾上腺髓质 表面外胚层：表皮及其衍生物，感觉上皮，腺垂体 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 （轴旁中胚层） Intermediate mesoderm （间介中胚层） Lateral Plate mesoderm （侧中胚层）

Paraxial Mesoderm Segmentation Blocks of tissue along axis Somite（体节）:20th days, 3 pairs/ per day,42-44 pairs by the end of 5th weeks -Sclerotome（生骨节）: vertebra -Myotome（生肌节）:skeletal muscle -Dermatome（生皮节）:dermis

生皮节 生肌节 生骨节

Intermediate Mesoderm Kidney and genital system “Urogenital tissue”

Lateral Plate Mesoderm Intra-embryonic coelom: →body cavity 1.Outer layer: parietal mesoderm, （体壁中胚层）surrounding the intraembryonic cavity 2.Inner layer: visceral mesoderm （脏壁中胚层）→muscle, CT of the gut 3.Cavity: pericardial, pleural and peritoneal cavities Mesenchyme（间充质）: →cardiovascular and lymph system

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

Endoderm From the Yolk sac Forms Primitive gut （原始消化管） Besides the GI system -Foregut: thymus,thyroid,lung,liver, pancreas, etc. -Midgut: none -Hindgut: bladder,urethra Surrounded by visceral lateral plate mesoderm

The end of 8th week

Anne Geddes