Unit 15a Axis Specification Gastrulation in Mammals Placental mammals are modified amniotes Gilbert 10e Fig 9 1 Placental mammals are modified amniotes Gilbert 10e Fig 9 1 Early cleavages occur in the oviduct Gilbert 10e Fig 9 16 Fertilization initial divisions cleavages occur in the Fallopian tube oviduct Implantation in the uterus occurs when the embryo is a blastocyst Mammals undergo rotational cleavage Gilbert 10e Fig 9 17 Cleavage leads to the blastocyst Gilbert 10e Fig 9 20 Compaction occurs at the 8 cell stage uncompacted 8 cell compacted 8 cell compaction Tight junctions Compaction occurs at the 8 cell stage due to formation of cell cell junctions Gilbert 3e Fig 3 24 Compaction occurs at the 8 cell stage uncompacted 8 cell compacted 8 cell Compaction involves formation of adherens and tight junctions Gilbert 7e Fig 11 29 Compaction requires E cadherin normal embryos compact E cadherin antibody treated embryos fail to compact Conclusion E cadherin is required for compaction The blastocoel forms at the 16 to 32 cell stage Outer cell 32 cell stage cross section Inner cell mass Blastocyst Cavitation Blastocoel Internal cell Gap junctions Gilbert 3e Fig 3 24 Trophoblast cell Inner cell mass ICM cells come from interior cells of the 32 cell embryo They are located above the blastocoel a fluid filled space At the blastocyst stage cells are no longer totipotent but some are pluripotent From A Child Is Born Monozygotic twins show when the potency of the blastocyst is restricted Gilbert 10e Fig Early splitting leads to 2 blastocysts 2 amnions 2 chorions 9 22 Later splitting leads to two ICMs so 2 amnions but 1 chorion Monozygotic twins show when the potency of the blastocyst is restricted Very late splitting leads to a split in the ICM leading to 1 chorion and 1 amnion Conclusion the pluripotency of ICM cells is maintained until immediately prior to gastrulation Gilbert 10e Fig 9 22 Pluripotency requires oct 4 ICM cells express oct 4 Trophoblast cells do not Eventually hypoblast expresses high levels of oct 4 Oct 4 Gilbert 10e Fig 9 19 Pluripotency depends on levels of oct 4 ICM cells express oct 4 Trophoblast cells do not Eventually hypoblast expresses high levels of oct 4 Stewart C 2000 Nat Genetics 24 328 330 Pluripotency depends on levels of oct 4 high medium low High levels of oct 4 in transgenics leads to hypoblast Medium levels leads to ICM cells stem cells Low levels in knockouts leads to trophoblast Stewart C 2000 Nat Genetics 24 328 330 Pluripotency depends on a transcriptional circuit involves Oct4 Sox2 and Nanog Initial circuit gets established Mature circuit Gilbert 10e Fig 9 18 Trophoblast differentiation depends on transcription factors Tead4 and Cdx2 Gilbert 10e Fig 9 19 Trophoblast fate requires Cdx2 Cdx2 Oct4 Cdx2 Trophoblast cells express Cdx2 ICM cells express Oct4 In Cdx2 knockouts there are many more cells that express Oct4 Strumpf et al 2005 Development 132 2093 2102 Tead4 promotes Cdx2 expression in outer cells Wildtype Tead4 knockout Cdx2 Oct4 Loss of Tead4 leads to loss of expression of Cdx2 and excess Oct4 Conclusion Tead4 is required for Cdx2 expression http www cdb riken jp en 04 news articles 080128 tead4 html Tead4 and Yap work together to activate Cdx2 transcription Gilbert 10e Fig 9 19 Tead4 binds Yap This presumably leads to expression of Cdx2 in outer cells trophoblast Hippo signaling in inner cells ICM leads to destruction of Yap and no Cdx2 expression ICM and visceral endoderm sort out Gilbert 10e Fig 9 25 Nanog Nanog marks inner cell mass Gata6 marks visceral endoderm Hatching involves degrading the zona Gilbert 10e Fig 9 21 Embryos secrete a hatching enzyme protease that degrades the zona Mammals Blastocyst 7 Days Trophoblast over the ICM invades the uterine lining endometrium Gilbert 10e Fig 9 24 Mammals Blastocyst 8 Days The amnion forms from epiblast fluid accumulates between the amnion and the epiblast Syncytiotrophoblast invades the uterine lining It forms by fusion of trophoblast cells Gilbert 10e Fig 9 24 Mammals Blastocyst 9 Days Amnion forms a covering over the epiblast Syncytiotrophoblast develops cavities lacunae Gilbert 10e Fig 9 24 Mammals Gastrulation Gilbert 10e Fig 9 24 Gastrulation involves primitive streak formation Gilbert 10e Fig 9 26 The yolk sac is an extraembryonic structure Gastrulation involves primitive streak ingression Gilbert 10e Fig 9 26 Definitive endoderm and mesoderm form from epiblast The node and the AVE induce different neural structures is the Organizer AVE Anterior visceral endoderm The AVE functions like the anterior endomesoderm of amphibians to induce anterior neural ectoderm Gilbert 10e Fig 9 28 The AVE induces anterior neural structures Gilbert 9e Fig 8 27 The AVE functions like the anterior endomesoderm of amphibians to induce anterior neural ectoderm The mouse node expresses Organizer molecules The node expresses chordin shown and noggin not shown Gilbert 9e Fig 8 28 Chordin noggin are functionally redundant WT Chordin KO Chordin Noggin KO Chordin knockouts have mild phenotypes chordin noggin knockouts lose many neural structures Conclusion chordin and noggin are redundant Gilbert 9e Fig 8 28 A P Patterning involves Wnts BMPs and FGFs Gilbert 10e Fig 9 29 Wnt and BMP antagonists function as in other vertebrates Anterior neural induction involves Cerberus Gilbert 7e Fig 11 41 Vertebrate Nieukoop Center Comparison Gilbert 10e Fig 9 34 Vertebrate Nieukoop Center Comparison Gilbert 10e Fig 9 34 Vertebrate Organizer Comparison Epiboly SMO adapted Gilbert 10e Fig 9 34