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by: Mrs. Willis Mante


Mrs. Willis Mante

GPA 3.6

Jennifer Knight

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Jennifer Knight
Class Notes
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This 3 page Class Notes was uploaded by Mrs. Willis Mante on Thursday October 29, 2015. The Class Notes belongs to MCDB 4650 at University of Colorado at Boulder taught by Jennifer Knight in Fall. Since its upload, it has received 13 views. For similar materials see /class/231838/mcdb-4650-university-of-colorado-at-boulder in Molecular, Cellular And Developmental Biology at University of Colorado at Boulder.

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Date Created: 10/29/15
MCDB 4620 VERTEBRATE DEVELOPMENTAL BIOLOGY LECTURE NOTES 91002 Lecture 5 Induction in the early embryo Reading Gilbert 6364 214216 314319 How is the identity of each germ layer established To answer this question we need to start looking at the molecules that control development We will start by examining the dorsal region of the amphibian embryo Importance of the dorsal region in amphibians Figs 1011 1018 Sperm entry de nes the lture ventral side of the embryo The dorsal side of the embryo first seen as the dorsal lip of the blastopore thus normally forms opposite the point of sperm entry A process called cortical rotationis initiated by entry of the sperm This rotation of the cortical cytoplasm of the embryo with respect to the internal cytoplasm is required for dorsal lip formation and gastrulation at later stages Evidence 1 UV irradiation prevents cortical rotation and generates a quotventralizedquot embryo 2 Bisection of the egg before first cleavage demonstrates dorsal region is necessary for normal development Molecules involved Wnt pathway members These experiments also indicate that the normal fate of a cell and its developmental potency or potential may differ Primary embryonic induction in 39 quot 39 Pro ressive determination Spemann and quot 1920 tissues changed fate in transplantation experiments Ectoderm normally fated to become neural took on two different fates depending on WHEN it was transplanted at early gastrula transplant formed epidermis like its new neighbors at late gastrula transplant formed an ectopic neural tube Cells and tissues progressively lose their potential to regulate and gradually become determined or irreversibly committed to a specific fate Fates of cells are o en determined by induction interactions between cells Induction of mesoderm P Nieuwkoop 1970s animal and vegetal cap explants do not form mesodermal tissues eg muscle blood heart notochord on their own 1021 However if recombined in culture mesoderm can form Timing Explanted marginal zone cells from blastula can t form mesoderm but marginal zone cells from a later blastula 256 cell would Thus mesoderm is induced in the marginal zone cells before gastrulation onset by mid blastula Also mesoderm is not uniform but patterned as seen from the blastula fate map So animal hemisphere cells are induced to become mesoderm by signals from vegetal hemisphere cells Culture experiments using different blastomeres combined with animal caps showed that different signals for inducing different mesodermal tissues were coming from the underlying vegetal blastomeres as early as the 32 cell stage 1022 Molecular nature of mesoderm inducing signals When a lter with 04m pores is placed between the animal and vegetal explants mesoderm is still formed Thus actual contact is not required for the induction of mesoderm Fu1thermore when conditioned medium was placed onto ectoderm cells in culture factors in the medium could change the fate of the cells Therefore the inducing signals must be diffusible factors Many have been identified One example is Activin a member of the TGFb superfamily of growth factors Activin has different effects on cells depending on its concentration Thus it appears to act as a morphogen capable of inducing different types of mesoderm dependent upon its concentration 319 Vegetal cells produce a graded output of activin ultimately leading to a high concentration in the dorsal region and low in the ventral region 320 Activin in turn activates dorsalspecific transcription factors like goosecoid highest and brachyury 321 322 These transcription factors in turn induce the formation of specific tissue types Low concentrations of activin induce the formation of ventral mesoderm blood Intermediate levels of activin induce lateral mesoderm muscle High concentrations of activin induce dorsal mesoderm like notochord A morphogen thus patterns tissue in a concentration dependent fashion and the response of the tissue can be thought to respond at various threshold levels to the morphogen The organizer Spemann s graduate student Hilde Mangold found one region of the amphibian gastrula that seemed to be completely determined transplantation of the dorsal lip into the ventral area of a host produced a twinned embryo Fig 1020 The embryos had a second primary axis a second dorsal neural tube notochord and associated structures Transplantations showed that some tissue in the new secondary axes was derived from host having been quotinducedquot by the transplant to develop into dorsal structures Host tissue itself was changed by the transplanted organizer cells Spemann called this phenomenon primary induction He termed the dorsal lip quotthe organizer Spemann earned the Nobel Prize in 1935 for this work Interestingly the term quotprimary inductionquot turns out to be a misnomer since there are actually earlier inductive events that are required to establish the organizer The Nieuwkoop center Even before the organizer is established individual blastomeres from the amphibian egg have different properties as mentioned above Gerhalt and colleagues 1980s showed that an earlier induction occurred at the blastula stage to create the organizer If they transplanted a dorsal vegetal blastomere from the 16 or 32 cell stage to a UV irradiated embryo no cortical rotation all ventralized dorsalization was restored In addition transplantation of these cells into the ventral side of an untreated host embryo caused twinning G10 l l When the transplanted dorsal vegetal cells were marked with a ourescent dye the descendants of these cells did not give rise to any dorsal structures but only to gut tissue Therefore these dorsal vegetal cells like the organizer in Spemann39s experiments were causing dorsalization by induction This region was called the Niewkoop center The position of the Niewkoop center is just vegetal to the position of the organizer the two quotcentersquot are present at 39 times of J 39 I T39 39 p then Organizer Thus a series of successive inductions pattern the mesoderm of the early embryo The ve getal hemisphere is initially speci ed by cytoplasmic determinants activated during cortical rotation The dorsal vegetal cells of the Nieuwkoop center then induce formation of the Spemann organizer from mesodermal precursor cells just above them The organizer then initiates the movements of gastrulation at the blastopore lip The dorsal mesoderm produced during gastrulation moves toward the lture head region and forms the notochord which later induces formation of the central nervous system from the overlying ectoderm


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