chapter 6.9-6.17 363

6.9 The sea urchin develops into a free-swimming larva

·      Bilaterally symmetrical, free-swimming larva=pluteus

·      Sea urchin egg divides by radial cleavage

·      Macromeres: 95% of the cytoplasm. Micromeres: whatever is left

·      Invagination is the inward movement of cells in a sheet-finger into a balloon style

6.10 The sea urchin egg is polarized along the animal-vegetal axis

·      Oral-aboral axis is defined by the position of the mouth in the larva and defined the larva’s plane of bilateral symmetry

6.11 The sea urchin fate map is finely specified, yet considerable regulation is possible

·      Endomesoderm (aka mesendoderm) specification hasn’t occurred when veg2 cells become mesoderm

·      Single Veg1 cell progeny labeled after the eighth cleavage give rise to either endodermal or ectodermal tissues, but not both

6.12 The vegetal region of the sea urchin acts as an organizer

·      Endomesoderm gives rise to the sea urchin spicules (skeletal rods), muscle, and gut

6.13 The sea urchin vegetal region is demarcated by the nuclear accumulation of β-catenin

·      As a result of activation of part of the canonical Wnt signaling pathway. Β-catenin concentrations are high in the micromere nuclei, but the protein is almost absent from nuclei in the most animal region of the future ectoderm

·      Maternal β-catenin and the transcription factor otx activate the gene pmar1 in the micromeres at the fourth cleavage

·      Pmar1 encodes a transcription factor that is exclusively in micromeres during cleavage and early blastula stages and is required for micromeres to express their organizer function + development as primary skeletogenic mesenchyme

6.14 The animal-vegetal axis and the oral-aboral axis can be considered to correspond to the antero-posterior and dorso-ventral axes of other deuterostomes

·      Similar to Xenopus

6.15 The pluteus skeleton develops from the primary mesenchyme

·      HesC protein represses genes essential for the micromeres to develop as primary mesenchyme, and so the spatially localized repression of hesC transcription by Pmar1 allows the activation of these genes in the micromeres only

·      Endo-16 gene codes for a secreted glycoprotein of unknown function. Endo-16 is similar to the pair-rule genes in drosophila

6.16 The oral-aboral axis in sea urchins is related to the plane of the first cleavage

·      Nodal

6.17 The oral ectoderm acts as an organizing region for the oral-aboral axis

·      Nodal is an early zygotic gene to be expressed

·      If Nodal activity is blocked, then it remains radially symmetrical and differentiation of ectoderm into oral and aboral ectoderm is blocked -> all oral fate

·      Nodal appears to have no role in specifying and patterning the mesoderm, which is one of its primary roles in vertebrates

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