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BIOL 3P28 - 2ND MIDTERM.docx

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Robert Carlone

BIOL 3P28 –2 ND MIDTERM morphogenesis - FORMATION OF ORGANIZED ANiMAL BODIES - Morphogenesis results from a limited number of cell behaviors - Direction and # of cell divisions, Shape changes, Movement, Growth, Death, Surface changes Embryo cells  epithelial cells - form tubes and sheets while remaining adhered to one another  Mesenchymal cells - migrate individually, form extensive ECM that keep individual cells separate Differential cell affinity - Re-aggregation of cells & Sorting out final positions reflect embryonic location - cells have a different set of proteins in its cell membrane, these differences are responsible for forming the structure of the tissues and organs during development Selective affinity - sorting out and reconstruction of spatial relationships in aggregates of embryonic cells - reaggregated cells become spatially segregated = instead of two mixed cell types, each sorts into its own region - the final positions of the reaggregated cells reflect their respective positions in the embryo - elective affinities change during development Cell adhesion -- Steinberg’s thermodynamic adhesion hypothesis  explain patterns of cell sorting based on thermodynamic principles (Cells move to create tissue)  Cells form a sorting Hierarchy by differential surface tensions  Controlled by cadherins  Cells with greater surface tension sorted within cells that w/less surface tension (Less adhesive outside)  surface tensions of individual germ layers are precisely those required for sorting pattern  If final position of type A is internal to type B, and type B is internal to type C, A = internal to C  Cells always form aggregates with the smallest free energy and the most stable configuration  Cells in embryo are in equilibrium until new gene activity causes changes in cell surface molecule  This results in cell movements (rearrangements) until a new equilibrium is reached. Cadherin = cell adhesion - For cell adhesion, morphogenetic movements, correct morphogenesis, signaling molecule for gene expression o Aggregate surface tension = # of cadherin molecules on cell surface, More cadherin = tighter adhesion - are transmembrane proteins that interact with other cadherins on adjacent cells - cadherins are anchored inside the cell by catenins - Interfering with cadherin function prevents formation of tissues and cause cells to disaggregate - E-cadherin (LCAM,uvomorulin) –epithelial cadherin--expressed early, later restricted to epithelium - P-Cadherin (placental)--restricted to trophoblast cells & uterine wall, helps placenta stick to uterus - N-cadherin (neural) – early = mesodermal cells, later = developing neural cells in CNS - EP-cadherin (C-cadherin) – adherence of Xenopus blastula cells and during gastrulation o Protocadherins –lack connections to cytoskeleton through catenins, keep epithelial cells together 1 differentiation event in mammalian development -- distinguishes the trophoblast cells (outer cells that bind to uterus) from inner cell mass (those cells that will generate the embryo and eventually the mature organism). Cell Migration - move during gastrulation - in epithelia the moving force is the cells at edge of the sheet - in mesenchymal cells – cells become polarized (defines front and back) and migrate Induction and Competence  One group of cells changes behavior of another group during the construction of an organ/tissue  Induction o interaction at close range b/w 2 or more cells of different histories and properties o Prime Ex. vertebrate eye --Lens can act as an inducer of other tissues, including retina 1. Inducer – produces the signal that causes change (paracrine factor) 2. Responder – the tissue induced to change  Competence = The ability to respond to inducer signal  Reciprocal induction = when 2 interacting tissues are both inducers and are competent to respond to other's signals - Instructive induction-‐signal needed to induce genes, responding cell can’t form new tissue w/o inducer - Permissive induction --‐ has already been specified, responder has all factors for making new tissue, needs appropriate env’t. Doesn’t req signals from inducer Epithelial to Mesenchymal transitions (EMT) & interactions o Represent some of the best examples of tissue induction o 1. Regional Specificity Induction o Instructive -- Tells what genes to activate o can generate different structures from the same responding tissue o 2. Genetic specificity Induction - responding epithelium can respond to instructive signals only as far as its genome permits - responses are species specific, signals can cross species barriers - in amphibians – mixing the signal from a frog and newt mouth created a mouth that was species specific Inducing signals? - Paracrine -- diffuse small distances to induce changes in neighboring cells (extracellular space) - juxtacrine factors -- proteins on one cell surface interact w/receptor proteins on adjacent cells - autocrine -- when the same cells that secrete paracrine factors also respond to them - endocrine factors -- (hormones) travel through the blood Paracrine signal transduction cascades - FGFs -- signaling through RTK then phosphorylates proteins o Important for regeneration & blood vessel formation o Fgf8 – limb development & lens induction o Can activate JAK-STAT pathway  Binding phosphorylates STAT (signal transducers and activators of transcription)  Mutations & premature activation of JAK-STAT pathway = dwarfism  gene activation pathway activated by prolactin  Thanatophoric dysplasia - severe dwarfism by failure of growth plates to form.  FGFR-3 mutation -- normally expressed in chondrocytes --‐signals cartilage to begin differentiating early = short limbed dwarfism o Ligland  RTK GNRPRAS  RAF MEK  ERK  Trans Factor  Transcription - Hedgehog o induce cell types & create boundaries b/w tissues o protein must be complexed with a molecule of cholesterol in order to function  Cholesterol required for cleavage of Hh and for diffusion & Patched function.  Without cholesterol = molecule diffuses too quickly and dissipates o responsible for patterning neural tube, limb, that portion of each somite forms the vertebrae, the feathers of the chick in proper place, pinkies are always most posterior o Sonic hedgehog (Shh)  expression in chick embryo gut, nervous system and posterior portion of limb o mutations -- inactivate = malformations, activate = cause cancers o Hedgehog  Patched  Smoothened  Ci protein made activator  transcription - Wingless o The Wnt (wingless + integrated) family of paracrine factors o Complexed to lipid – to increase concentration o Induces dorsal somite tissues, polarity of dorsal limb structures, stem cells o Wnt-4 required for urogenital formation and female sex determination o Wnt paracrine factor diffusion is affected by other proteins o activation is often accomplished by inhibiting an inhibitor. o Wnt FrizzledDisheveled GSK3  β-catenin  transcription o Noncanonical – affect actin & microtubular cytoskeleton - TGFbeta o dimers, diffusible (determined by sequences at N--‐termini) o The Smad pathway is activated by TGF-β superfamily o TGF-β superfamily ligand  Receptor II  Receptor I  Smad activation  Smad dimerization  new transcription How cells respond to paracrine inducers. o Transmembrane receptors o Binding of ligand induces conformational change in receptor o Cytoplasmic domain becomes “enzymatically active” o allows the receptor to phosphorylate other cytoplasmic proteins Maintaining Differentiation 1. Transcription factor becomes independent of the signal that induced it 2. Stabilize by synthesizing proteins that act on chromatin to keep the gene accessible 3. the cell can make both that signaling molecule and that molecule's receptor 4. A cell may interact with neighboring cells so each one stimulates the differentiation of the other ECM (extra cellular matrix) = source of important signals o Cell adhesion, migration, and formation of epithelial sheets/tubes depend on the ability of cells to form attachments to extracellular matrix o Extracellular matrix are made up of collagen, proteoglycans, and glycoprotein molecules o Proteoglycans critical in the delivery of the paracrine factors o glycoproteins are responsible for organizing the matrix and the cells into an ordered structure o Fibronectin – adhesive molecule, proper alignment of cells  expression in the Xenopus gastrula o Laminin – for adhesion of epithelial o Integrins – important receptors for ECM components & binding of glycoproteins o Binding of integrins to ECM can activate RTK pathway o Anoikis – special apoptosis – death from detachment of epithelial cells from ECM Part II: Specification -- Cell commitment and early development -- pgs.110-­‐119 Processes of Cell Commitment to a fate stages:  1. Specification – labile, autonomous differentiation in neutral environment. Still reversible!  2. Determination – stable, autonomous differentiation when transplanted to another site. Irreversible! Autonomous specification - Gives rise to a pattern of embryogenesis called mosaic development. - transcription factors already present in egg cytoplasm to regulate gene expression, directing the cell into a particular path of development. - the egg cytoplasm contains morphogenetic determinants which influence the cell's development. - the cell "knows" what it is to become very early and without interacting with other cells - Autonomous specification in the early tunicate embryo o Still forms all structures it would have had if it stayed in the embryo Conditional specification - ability of cells to achieve their respective fates by interactions with other cells - what a cell becomes depends upon its position within the embryo - also by paracrine factors secreted by its neighbors - transcription factors determined by paracrine or juxtacrine interations Syncytial specification - Uses conditional and autonomous specification - Interactions are between regions of egg cytoplasm, not b/w cells - Interactions b/w nuclei & transcription factors of individual cells in a common cytoplasm - seen mostly in insect embryos. - Axial specification in Drosophila. - Bicoid and nanos localization infruit fly embryos Germ Plasm Theory - Weissman develops first testable model of cellular specification o Sperm and egg provide equal contributions to embryo o Chromo
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