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lecture_zebrafish_notes - 1 Early Development in Fish...

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1 1. Early Development in Fish Cleavage : in fish, cleavage occurs only in the blastodisc (= a thin region of yolk-free cytoplasm at the animal cap of the egg) Cell divisions are meroblastic (incomplete division) and discoidal (only the blastodisc forms the embryo) Initially all cells of the blastoderm contain a connection with the large yolk cell Around the 10 th cell division zygotic transcription starts (midblastula transition) and cell movements become evident. At this time 3 cell populations can be distinguished : the yolk syncytial layer (YSL) : formed by the cells at the vegetal edge of the blastoderm that fuse with the underlying yolk cell o -> intenal YSL move under the blastoderm o -> external YSL stay at the edge of the blastoderm layer the enveloping layer (EVL) : the most superficial cells of the blastoderm o an epithelial sheet (a single layer thick) o becomes the periderm, an extraembryonic protective covering the deep cells : o give rise to the embryo proper o fate is fixed just before the onset of gastrulation Gastrulation : the first movement is the epiboly of the blastoderm cells over the yolk the deep blastoderm cells move outwardly and move over the surface of the yolk cell to envelop it completely (movement driven by the YSL) During migration, one side of the blastoderm becomes noticably thicker than the other -> dorsal side of embryo Formation of the germ layers : at 50% epiboly the marginal blastoderm thickens = the germ ring the germ ring is composed of an outer layer (= the epiblast) and an inner layer (= the hypoblast) Gastrulation starts on the dorsal side and continues on the marginal and ventral side On the dorsal side, the cells of the epi- and hypoblast intercalate to form a localized thickening = the embryonic shield (=functionally equivalent to the dorsal blastopore lip in amphibians) During epiboly, cells at the margin move towards the dorsal embryonic shield.
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2 The hypoblastic cells of the embryonic shield converge and extend anteriorly and narrow along the dorsal midline barb4right form the chordamesoderm -> precursors of the notochord The cells adjacent to the chordamesoderm barb4right the paraxial mesoderm -> precursors of the somites The epiblastic cells -> convergent extension brings more cells to the dorsal midline -> the neural keel the rest of the epiblast becomes the skin of the fish Axis formation in the fish embryo Dorsal-ventral axis formation : the embryonic shield The dorsal side of the YSL shows nuclear accumulation of β -catenin ( to Nieuwkoop Center in amphibia) the embryonic shield can act as an organizer ( the Spemann Organizer in amphybia) barb4right can dorsalize lateral and ventral mesoderm (=> notochord and somites in stead of blood and connective tissue) barb4right can turn ectoderm (epidermal precursor) into neural tissue The embryonic shield forms the prechordal plate and the notochord and these turn the overlying ectoderm into neural ectoderm and dorsalize mesoderm by secreting inhibitors ( Chordino Chordin) of BMP proteins ( BMP2B in the fish)
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