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Chaper 11, Opener Discoidal meroblastic cleavage in a chick egg Blastodisc resulting from cleavage Formation of the three-layered blastoderm of the chick embryo Formation of the three-layered blastoderm of the chick embryo Formation of the three-layered blastoderm of the chick embryo Cell movements of the primitive streak and fate map of the chick embryo Cell movements of the primitive streak and fate map of the chick embryo Zona pellucida has changed from a disc to a pear shape and Hensen’s node appears. Cell movements of the primitive streak and fate map of the chick embryo Cell movements of the primitive streak and fate map of the chick embryo Migration of endodermal and mesodermal cells through the primitive streak Migration of endodermal and mesodermal cells through the primitive streak Chick gastrulation 24–28 hours after fertilization 24 hrs primitive streak full extension 25 hrs two somite stage Chick gastrulation 24–28 hours after fertilization 27 hrs four somite stage 28 hrs Hensen’s node is caudal Specification of the chick anterior-posterior axis by gravity Induction of a new embryo by transplantation of Hensen’s node Formation of Hensen’s node from Koller’s sickle Green – anterior cells = Hensen’s node and chordamesoderm Red - posterior cells= posterior region of primitive streak Pre-streak embryo Gene expression in the primitive streak If Hensen’s node is the Organizer, what signals lead to the node’s induction? Expresses Vg1 and Nodal = Nieuwkoop center Both Nieuwkoop center and Organizer genes expressed Expresses chordin and Cgoosecoid = Organizer Gene expression in the primitive streak Signals required to establish A-P axis - Summary In posterior marginal zone before primitive streak Wnt-8c all of posterior marginal zone. Vg1 localized to posterior marginal zone. Both signals required to induce the expression of Nodal and set up the Organizer on the anterior portion of Koller’s sickle. Nodal and FGF, expressed by Koller’s sickle, are required for the formation of the streak and a functional Hensen’s node. Once the primitive streak is formed and Hensen’s node becomes functional, Hensen’s node expresses genes associated with The Organizer in amphibians, like chordin and noggin. Possible contribution to chick neural induction by the inhibition of BMP signaling Noggin is expressed in Hensen’s node. Its expression inhibits BMP signaling ** In chick embryos repression of BMP signal does not seem to be sufficient for neural induction – FGF signaling required. Roles of FGF signaling in a chick’s embryo: 1. Specify (induce) mesoderm 2. After mesoderm ingresses stop ingression of cells to allow neural place cells to stay outside. 3. Help bring about neurulation by making antagonists of BMP signal stronger. •FGFs are synthesized in Hensen’s node precursor cells prior to gastrulation. As far as we know FGF signaling is not involved in gastrulation in the amphibian embryo. Model by which FGFs regulate mesoderm formation and neurulation Epiblast Stage X = one layer blastoderm Fgf8 induces pre-neural genes Cerberus inhibits Nodal ERNI and Sox3 – pre neural Genes induced by Fgf8 Stage 1 = primitive streak Cerberus signal goes forward= Nodal expression = mesoderm induction Model by which FGFs regulate mesoderm formation and neurulation Stage 4 = complete primitive streak, induction of Churchill. Curchill induces Sip1 which blocks mesoderm induction. Stage 4 (end) = no more mesoderm ingression. Neuroectoderm induced. Anterior-posterior patterning in the chick embryo Retinoic acid, secreted by posterior mesoderm, induces caudalization. In amphibians the caudalizing signal seems to be Wnt. Expression of retinaldehyde dehydroxylase-2 – synthesis of retinoic acid Expression of retinoic acid degradation enzymes Hoxb1 expression marks the anterior boundary of the posterior cells Pathway for left-right asymmetry in the chick embryo Nodal = signal Pitx2 = transcription factor Unanswered question: What limits ActivinB expression to right side of the embryo? Development of a human embryo from fertilization to implantation Comparison of early cleavage in (A) echinoderms and amphibians and (B) mammals Cleavage of a single mouse embryo in vitro Compaction Morula – 16 cells Blastocyst Schematic diagram showing the derivation of tissues in human and rhesus monkey embryos Tissue formation in the human embryo between days 7 and 11 Tissue formation in the human embryo between days 7 and 11 Tissue formation in the human embryo between days 7 and 11 11.33 Tissue formation in the human embryo between days 7 and 11 (Part 4) Amnion structure and cell movements during human gastrulation Amnion structure and cell movements during human gastrulation A-P axis and notochord formation in the mouse AVE = anterior visceral endoderm expresses anterior markers like Cerberus Axis and notochord formation in the mouse Anterior-posterior patterning in the mouse embryo Evolutionary conservation of homeotic gene organization and transcriptional expression in fruit flies and mice Axial skeletons of mice in gene knockout experiments Left-right asymmetry in the developing human Left-right axis 1. frogs – starts with the localization of Vg1 on the left 2. chicks – starts with the suppression of sonic hedgehog on the right 3. mammals – starts with the ciliary movement of NVPs (nodal vesicular parcels) from the node to the left side. NVPs contain Sonic hedgehog protein. The end of the pathway, the activation of Nodal and expression of txn factor Pitx2 on the left side is common to all. Situs formation in mammals Ciliated cells in the mammalian node Situs formation in mammals