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
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 3 rd –8 th 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
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.
Paraxial Mesoderm Segmentation Blocks of tissue along axis Somite:20 th days, 3 pairs/ per day,42-44 pairs by the end of 5 th weeks - Sclerotome: vertebra -Myotome:muscle of the back -Dermatome:dorsal dermis of the overlying skin
Intermediate Mesoderm Kidney and genital system “Urogenital tissue”
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 Lateral Plate Mesoderm 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
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.