Lecture 11 Notes - Vandan Desai BIOL 442Developmental...

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Vandan Desai BIOL 442—Developmental Biology 1 Lecture #11—Axis Formation and Mesoderm Induction I. Overview :: A. During cleavage , blastula , and early gastrulation progressive differentiation of cells results in the formation of the 3 germ layers: i. Ectoderm = skin and nervous system ii. Endoderm = linings of the digestive tube, bladder, pancreas, liver, etc. iii. Mesoderm = muscle, bone, cartilage, etc. B. During cleavage , we saw how cytoplasmic localization of determinants in the oocyte (egg) gave rise to germ cells and we saw localization of determinants that will later play a role in predisposing cells to form the germ layers II. Interactions b/w Cells that Induce Formation of Mesodermal Structures and Set Up Body Axes & The Nature of Inductive Interactions :: A. In our discussion of axis formation and mesoderm induction , we’re going to focus on amphibians (frogs) b/c much of the most detailed information has come from studies on amphibians B. 3 different axes have to form: i. Anteroposterior axis = develops during gastrulation from the animal-vegetal axis, which originates during oogenesis ii. Dorsoventral axis = formed after fertilization and fixed before first cleavage iii. Left-right asymmetry = not obvious until the heart and other internal organs become asymmetrically located during later embryogenesis But, the molecular cues to left-right asymmetry are evident during blastula and early gastrula C. The Concept of Regulative Development i. An isolated blastomere has a potency greater than its normal embryonic fate That potency might have been restricted as a result of cytoplasmically localized determinants ii. A cell’s fate is further determined by interaction b/w neighboring cells --- inductions or inductive interactions iii. We’ll first look at the role of the cytoplasmically localized determinants that set up the animal-vegetal pole , then look at how these confer a “ reduced potency on cells, and then how inductive interactions b/w cells result in later steps in axis formation
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Vandan Desai BIOL 442—Developmental Biology 2 D. Animal-vegetal axis originates by oriented transport of materials during oogenesis i. As a result of cytoplasmic localization events that we discussed last time, two key molecules are localized at the vegetal pole : VegT = a transcription factor Vg1 = the transforming growth factor beta ligand , which is a paracrine factor (could transform other cells to cancerous state ) ii. Fate map of Xenopus embryo: The surface of the animal half of the egg will give rise to the ectodermal cells The cells that form from surface of the vegetal half of the egg will form the endoderm The cells that form from the internal cytoplasm of the egg will become mesoderm iii. VegT and Vg1 initially impose this fate map on the embryo: If you use antisense oligos to reduce VegT , the embryos do not make normal fate maps: Endoderm is missing and mesoderm is reduced Embryos that lack Vg1 also are missing endodermal cells and show reduced mesoderm iv.
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