4End of the Week of Twos 2 major cell masses Embryonic Cavities TrophoblastsInner cell massCytotrophoblastsSyncytiotrophoblastsUteroplacental Cir.Embryonic CavitiesAmniotic cavityYolk sacGerm DiscEpiblastsHypoblasts
5Formation of trilaminar disc Early of three weeksPrimitive streak: caudal end, a midline indentation on the epiblast surfacePrimitive node/knotPrimitive pitProliferation & Invagination of epiblastsIntra-embryonic mesoderm
6Gastrulation Migrating epiblasts will form: Endoderm: migrated cells displace hypoblast cellsMesoderm: cells btw the endo and ectoEctoderm(Note - epiblasts=>all layers)Mesoderm cranial and lateralextension
7Distribution of intra-embryonic mesoderm -spread laterally and cranially-Fused membranesOral: oropharyngeal membraneCloacal: cloacal membraneFormation of notochordInvaginate in through primitive pit/knota solid rod of mesodermBasis of the axial skeleton in the midline
8Review/Background First Week Second Week Third- 8th week Gastrulation Primitive pitPrimitive nodePrimitive streakNotochord
9Expansion and Elongation Migration of Epiblasts to formEndodermMesodermEctodermExpansion of the diskNotochord:-midline rod of mesoderm-orientation the germ disc
10Differentiation of trilaminar germ disc 3rd –8th weeksembryonic period: the major body form and organ structure takeBilaminar disc: flat and roundelongated with a broad cephalic and a narrow caudal endTrilaminar disc:a pear-shaped appearance
13Ectoderm – Neuralation Failure of Neurotube closureAnencephaly – AntMeningocele – post
14Ectoderm – Neural Crest As neural tube closes-Neural crestDetach and migrateMelanocytes – skinDorsal root ganglia
15Differentiation 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.
17Paraxial 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
18Intermediate Mesoderm Kidney and genital system “Urogenital tissue”
19Lateral Plate Mesoderm 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 cavitiesmesenchyme: →CT, cardiovascular and lymph system
20Endoderm From the Yolk sac Forms Primitive gut Besides the GI system-Foregut: thymus,thyroid,lung,liver,pancreas, etc.-Midgut: none-Hindgut: bladder,urethraSurrounded by visceral lateral plate mesoderm
21Embryonic Folding Craniocaudal folding--CNS Lateral folding—somites Endoderm-lined cavity is incorporated into the embryo properprimitive gutYolk sacvitelline ductOropharyngeal membraneCloacal membraneFlat discpear, round
23OBJECTIVES 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.