09.9.24 1 CHAPER 11, EARLY DEVELOPMENT OF VERTEBRATES: FISH, BIRDS AND MAMMALS Phylogeny of Vertebrates Meyer&Zardoya,Annu.Rev.Ecol.Evol.Syst.2003.34:311‒38 Different cleavages in four vertebrate classes Discoidal meroblastic cleavage Discoidal meroblastic cleavage Radial Holoblastic cleavage Rotational Holoblastic cleavage Discoidal meroblastic cleavage in a zebrafish egg 1-cell embryo 2-cell embryo 4-cell embryo 8-cell embryo 32-cell embryo 64-cell embryo The blastodisc can be seen atop the yolk cell. Discoidal meroblastic cleavage in a chick egg View from the animal pole, future dorsal side • Fertilization occurs in the oviduct. • Most of the actual embryo are from area pellucida. • Some of marginal zone cells become very important in determining cell fate. Development of a human embryo from fertilization to implantation • Fertilization occurs in the oviduct.09.9.24 2 Comparison of early cleavage in echinoderms, amphibians and mammals Radial Holoblastic cleavage Rotational Holoblastic cleavage Nematodes also have a rotational form of cleavage, but not forming blastocyst. Summary-1  Fish, reptiles and birds undergo discoidal meroblastic cleavage.  the early cell divisions do not cut through the yolk of the egg.  These early cells form a blastoderm. Although methods of both cleavage and gastrulation differ among the vertebrate class, certain underlying principles are common to all vertebrates. Cell movements during gastrulation of the zebrafish Blastoderm at 30% of epiboly (4.7 hrs) The deep cells of blastoderm form between yolk syncytial layer and the enveloping layer Cell movements during gastrulation of the zebrafish Formation of the hypoblast (9 hrs) Deep cells migrate over the top of yolk and form the hypoblast and epiblast layers by involution at the margin or by ingression from epiblast. Cell movements during gastrulation of the zebrafish Marginal region On the future dorsal side, these layers intercalate to from the embryonic shield, a structure homologous to amphibian organizer. Cell movements during gastrulation of the zebrafish At 90% of epiboly (9 hrs) Completion of gastrulation (10.3 hrs) 09.9.24 3 The embryonic shield as organizer in the fish embryo Transplantation of the embryonic shield into the ventral side of another embryo will cause the formation of a second embryonic axis. About 100 cell stage of gastrula Shh mRNA expressed ventral midline Induced by activating nuclear catenin Summary-2  In fish, the deep cells of blastoderm form between yolk syncytial layer and the enveloping layer.  Deep cells migrate over the top of yolk and form the hypoblast and epiblast layers.  On the dorsal side, these layers intercalate to from the embryonic shield, a structure homologous to amphibian organizer.  Transplantation of the embryonic shield into the ventral side of another embryo will cause the formation of a second embryonic axis. Axis formation in the zebrafish embryo Prior to gastrulation During gastrulation Ventral Dorsal BMP2B induces the ventral & lateral mesoderm and epidermal differentiation. Chordino blocks BMP2B and dorsalize the mesoderm and ectoderm. Possible contribution to chick neural induction by the inhibition of BMP signaling Hensen's node is homologous to the amphibian blastopore. Neurulating embryo Noggin in notochord and pharyngeal endoderm Bmp7 entire epiblast Phosphorylated Smad1 not in neural plate FGFs (firoblast growth factors) synthesized in Hensen’s node block BMP signaling and induce neural plate. Noggin, chordin and Nodal block BMP signals. Expression of BMP antagonists in the mammalian node Expression of chordin during gastrulation Anterior primitive streak, node and axial mesoderm No jaw, single eye chordin KO Wild Chordin & Noggin double KO No ear Noggin and chordin block BMP signals. Anterior-posterior patterning in the mouse embryo Late Gastrula Gradients of Wnts, BMPs and FGFs signals induce anterior-posterior axis 09.9.24 4 Summary-3  In each of vertebrates, neural ectoderm is permitted to form where the BMP-mediated induction of epidermal tissue is prevented. No BMP signals = neural ectoderm Formation of the three-layered blastoderm of the chick embryo Animal pole Stage X Hypoblast islands In Cleavage Margin by Koller’s sickle Formation of the three-layered blastoderm of the chick embryo Primary Hypoblast layer is formed Secondary hypoblast cells migrate Stage XII & XIII Formation of the three-layered blastoderm of the chick embryo Stage 2 The primitive streak cells from a layer between the hypoblast and epiblast Stage 3 The primitive streak has become a definitive region of epiblast, future mesoderm and endoderm. Summary-4  In chick embryos, early cleavage forms an area opaca and an area pellucida.  The region between them is marginal zone.  Gastrulation begins at the posterior marginal zone.  Gastrulation begins at the posterior marginal zone, as the hypoblast and primitive steak both start there. Cell movements of the primitive streak and fate map of the chick embryo Dorsal view 3-4 hrs after fertilization 7-8 hrs after fertilization The primitive streak is derived from anterior epiblast cells and the central cells of the posterior marginal zone.09.9.24 5 Cell movements of the primitive streak and fate map of the chick embryo 15-16 hrs after fertilization The primitive streak extends rostrally, Hensen’s node is formed. Cell movements of the primitive streak and fate map of the chick embryo 19-22 hrs after fertilization Cell movements of the primitive streak and fate map of the chick embryo Cells migration through Hensen’ node become prechordal mesoderm and is followed by the head process and notochord cells. 23-24 hrs after fertilization Summary-5  The primitive streak is derived from anterior epiblast cells and the central cells of the posterior marginal zone.  As the primitive streak extends rostrally, Hensen’s node is formed.  Cells migration through Hensen’ node become prechordal mesendoderm and is followed by the head process and notochord cells. Migration of endodermal and mesodermal cells through the primitive streak Tracks of GFP-labeled cells Anterior Posterior Hensen’s node Travel anteriorly: prechordal plate and notochord Travel laterally: notochord and somites Primitive streak Primitive streak Intermediate mesoderm and lateral plate mesoderm