Gastrulation – Chick, Fish, Mammal Developmental Biology – Biology 4361 November 15, 2005Haffter et al., 1996 Zebrafish gastrulation blastodiscFigure 10.21 Zebrafish gastrulation hypoblast formation – ingression and/or involution of deep cells embryonic shield ENL periderm (extraembryonic protective membrane) epiblast ectoderm hypoblast mesendoderm (mesoderm and endoderm precursors) YSL endodermFigure 10.22 Zebrafish embryonic shield driven by YSL Embryonic shield functionally equivalent to amphibian dorsal lip of the blastopore can organize 2° axis when transplantedHaffter et al., 1996 Zebrafish gastrulation blastodisc yolk syncytial layer epiblast hypoblast somite notochord somiteFigure 5.16 epiblast forms all three germ layers (plus extraembryonic membrane) hypoblast forms extraembryonic endoderm Chick cleavage delamination migration area pellucida area opaca different from fish hypoblast forms primordial germ cells does not form any embryonic endoderm or mesodermFigure 10.23 Gastrulation chick area pellucida = inner transparent portion of the blastoderm above subgerminal space area opaca = ‘opaque’ peripheral area; cells in contact with yolk gastrulation starts with extensive cell rearrangements in posterior epiblast cells move to midline and the forward primitive streakGastrulation chick major gastrulation events occur at the primitive pit & primitive groove elongation of primitive streak and formation of primitive groove & primitive pit anterior end of primitive ridges are thickened = Hensen’s node functional equivalent of amphibian blastoporeFigure 10.24 involution ingression future endoderm ingress first; displace hypoblast Chick gastrulationChick gastrulation future mesoderm (axial, paraxial, lateral) cells ingress from anterior end of primitive pit epithelial to mesenchymal transformation { extraembryonic membranes surround yolk ectoderm – spread by epiboly hypoblast cells spread along yolk surface mesoderm cells in betweenFigure 10.26 Gastrulation fate map chickFigure 10.27 Chick – 24 hChicken development 1 head region starts to lift off the yolkChicken development 2 ventral closure of the body proceeds from anterior to posteriorChicken development 3 ventral closure of the body is completedChicken development extraembryonic membranes and extraembryonic blood vessels completely surround the yolk yolk sac is connected to the gut by a narrow bridge after completed ventral closure, embryo rests on top of the yolk sacEarly mammalian development not very well studied: small eggs difficult to maintain in culture beyond blastocyst stage ethical issues Figure 5.11 inner cell mass & trophoblast are equivalent to epiblast & hypoblast of the chicken embryo Mammalian cleavage holoblastic cleavage results in a blastocyst: hatching from zona implantation in uterus placenta formation trophoblast embryo inner cell massEarly development in mammals Figure 5.8 embryo enters the uterus in the ‘morula’ stage blastocyst stage makes contact with uterus before implantation into uterus, the embryo sheds zona pellucida Human: 0 6 daysGilbert SF, Developmental Biology, 6 th ed, Sinauer, 2000 hatching of a mouse blastocyst from the zona pellucida Early development in mammals mouse blastocyst entering the uterus initial implantation of the blastocyst (rhesus monkey)Early development in mammals Gilbert SF, Developmental Biology, 7 th ed, Sinauer, 2003 amniotic cavity begins to form results in epiblast & hypoblast inner cell mass delaminates hypoblast cells that line the blastocoel, forming extraembryonic endoderm trophoblast divides into: syncytiotrophoblast furthers progression of the embryo into uterus wall by digesting uterus tissue cytotrophoblast anchors embryo to uterus tissue Human: 7 – 11 daysEarly development in mammals Gilbert SF, Developmental Biology, 7 th ed, Sinauer, 2003 extraembryonic mesoderm forms; it will develop into the extraembryonic coelom embryo is nourished by maternal blood uterus sends additional blood vessels cytotrophoblast enzymatically remodels maternal blood vessels (trophoblastic lacunae) twolayered blastodisc similar to that in birds embryonic epiblast & hypoblast epiblast delaminates the amniotic ectoderm that surrounds the amniotic cavity Human 9 – 11 daysFigure 10.31 Human blastocyst – 12 dcells ingress and involute along the primitive groove and at Hensen’s node Gilbert SF, Developmental Biology, 6 th ed, Sinauer, 2000 Gastrulation in mammals form endoderm (replaces hypoblast) and mesodermFigure 10.32 Human gastrulation – 16 dFigure 10.30 Primate germ layer derivatives embryo
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